WEG Induction Motors Fed by Pwm Frequency Converters Technical Guide 028 Technical Article English

8/8/2019 WEG Induction Motors Fed by Pwm Frequency Converters Technical Guide 028 Technical Article English

1/36

Motors | Energy | Automation | Coatings

Induction motors ed byPWM requency inverters

g Technical guide


2/36

www.weg.net

Technical guide Induction motors ed by PWM requency inverters2

Table o contents1 Introduction........................................................................................................................................................................................... 4

2 Normative Aspects.............................................................................................................................................................................. 52.1 NEMA MG1 - Motors and generators / United States............................................................................................................... 52.2 NEMA - Application Guide or AC Adjustable Speed Drive Systems........................................................................................ 52.3 IEC 60034 - Rotating Electrical Machines / Internacional........................................................................................................ 52.4 Other technical documents o re erence......................................................................................................................................... 53 Induction machines speed variation.................................................................................................................................................54 Characteristics o PWM requency inverters............................................................................................................................... 74.1 General................................................................................................................................................................................................... 74.2 Control Types....................................................................................................................................................................................... 85 Interaction between inverter and AC power line............................................................................................................................ 8

5.1 Harmonics............................................................................................................................................................................................. 85.1.1 Normative considerations about the harmonics............................................................................................................................ 95.2 Line reactor / DC bus choke.............................................................................................................................................................. 96 Interaction between inverter and motor...................................................................................................................................... 106.1 Harmonics in uencing motor per ormance...................................................................................................................................106.1.1 Normative considerations about the inverter output harmonics.......................................................................................... 106.2 Considerations regarding energy e fciency.................................................................................................................................. 116.2.1 The in uence o the speed variation on the motor e fciency.................................................................................................... 126.2.2 Normative considerations about the e fciency o inver ter ed motors................................................................................ 126.3 In uence o the inverter on the temperature rise o the windings......................................................................................... 13

6.4 Criteria regarding the temperature rise o WEG motors on VSD applications....................................................................... 136.4.1 Torque derating...................................................................................................................................................................................136.4.2 Breakaway torque.............................................................................................................................................................................. 146.4.3 Breakdown torque............................................................................................................................................................................. 156.5 In uence o the inverter on the insulation system.................................................................................................................... 156.5.1 Rise Time............................................................................................................................................................................................ 156.5.2 Cable length........................................................................................................................................................................................ 166.5.3 Minimum time between successive pulses (MTBP).................................................................................................................... 176.5.4 Switching requency ( s).................................................................................................................................................................... 186.5.5 Multiple motors................................................................................................................................................................................... 18

6.6 Criteria regarding the insulation system o WEG motors on VSD applications......................................................................186.7 Normative considerations about the insulation system o inverter ed motors.................................................................. 186.8 Recommendations or the cables connecting WEG motors to inverters............................................................................ 196.8.1 Cable types and installation recommendations.......................................................................................................................... 206.9 In uence o the inverter on the motor sha t voltage and bearing currents......................................................................... 206.9.1 Common mode voltage.....................................................................................................................................................................216.9.2 Equivalent circuit o the motor or the high requency capacitive currents............................................................................ 216.9.3 Methods to reduce (or mitigate) the bearings currents in inverter ed motors................................................................... 226.10 Criteria regarding protection against bearing currents (sha t voltage) o WEG motors on VSD applications................. 236.11 Normative considerations about the current owing through the bearings o inverter ed motors............................... 23

6.12 In uence o the inverter on the motor acoustic noise............................................................................................................. 236.13 Criteria regarding the noise emitted by WEG motors on VSD applications.......................................................................... 236.14 Normative considerations about the noise o inverter ed motors....................................................................................... 24


3/36

www.weg.net

Technical guide Induction motors ed by PWM requency inverters 3

6.15 In uence o the inverter on the mechanical vibration o the motor....................................................................................... 24

6.16 Criteria regarding the vibration levels presented by WEG motors on VSD applications......................................................246.17 Normative considerations about mechanical vibration o inverter ed motors................................................................... 247 Interaction between motor and driven load...................................................................................................................................257.1 Load types......................................................................................................................................................................................... 257.1.1 Variable torque loads........................................................................................................................................................................ 257.1.2 Constant torque loads...................................................................................................................................................................... 257.1.3 Constant horsepower loads............................................................................................................................................................ 267.2 Speed duties.......................................................................................................................................................................................267.2.1 Variable speed duty........................................................................................................................................................................... 267.2.2 Continuous speed duty.................................................................................................................................................................... 26

8 Dimensioning and analysis o actual drive system applications Practical examples........................................................ 268.1 Constant torque application - compressor...................................................................................................................................268.1.1 Example.............................................................................................................................................................................................. 268.1.2 Solution................................................................................................................................................................................................ 268.2 Squared torque application - centri ugal pump.......................................................................................................................... 278.2.1 Example............................................................................................................................................................................................... 278.2.2 Solution................................................................................................................................................................................................ 278.3 Special application - long cable......................................................................................................................................................298.3.1 Example.............................................................................................................................................................................................. 298.3.2 Solution................................................................................................................................................................................................ 29

8.4 Variable torque / variable speed application - textile industry.................................................................................................. 308.4.1 Example...............................................................................................................................................................................................308.4.2 Solution................................................................................................................................................................................................ 318.5 Example considering the use o WEG Optimal Flux................................................................................................................... 328.5.1 Example............................................................................................................................................................................................... 328.5.2 Solution................................................................................................................................................................................................ 329 Recommendations or the measurement o PWM wave orms................................................................................................ 329.1 Warning................................................................................................................................................................................................329.2 Instrumentation.................................................................................................................................................................................. 329.3 Parameter measurements.............................................................................................................................................................. 33

9.4 Grounding considerations................................................................................................................................................................339.4.1 Grounding o control......................................................................................................................................................................... 339.4.2 Grounding o motor........................................................................................................................................................................... 339.5 Measurement procedures............................................................................................................................................................... 339.5.1 Wave orm visualization..................................................................................................................................................................... 339.5.2 Oscilloscope scale setting.............................................................................................................................................................. 339.5.3 Triggering............................................................................................................................................................................................ 3310 Conclusion.......................................................................................................................................................................................... 3411 Bibliography.........................................................................................................................................................................................35


4/36

www.weg.net


The number o industry applications in which inductionmotors are ed by static requency inverters is growing astand, although much has already been done within this feld,there is still a lot to be studied/understood regarding suchapplications. The advance o variable speed drives systemsengineering increasingly leads to the need o specifctechnical guidance provision by electrical machines anddrives manu acturers, so that such applications can besuitably designed in order to present actual advantages interms o both energy e fciency and costs.

This technical guide aims to clari y the main aspectsconcerning applications o low voltage ( 690 V) induction

motors with static requency inverters supply, or rames IEC 355 (NEMA 587), in a didactic and concise approach.

First o all the principal and most broadly ollowedinternational standards about the subject are mentioned.

Then the theoretical basis o speed variation on inductionmachines by means o indirect static inverters is presented,as well as the undamental characteristics o electronicinverters.

Once the basics o adjustable speed drives are known, thebehavior o the whole power system is analyzed. Each

component o the power system (AC power line - requencyinverter - induction motor - load) is ocused, as well as theoverall interactions between them, resulting rom speedvariation. In this manner the whole drive system can be wellunderstood.

At last examples o VSD systems designs are presented, or abetter understanding o the matters exposed throughout thedocument.

Always looking out or a technical elucidation as complete aspossible along this guide, some controversial points areemphasized. Divergences existing among distinct

standardization organisms are discussed and WEGs point o view is explained.

1 Introduction


5/36

www.weg.net


For an induction motor, rotor speed, requency o the voltagesource, number o poles and slip are interrelated accordingto the ollowing equation:

where:n : mechanical speed (rpm)

1: undamental requency o the input voltage (Hz)

p : number o poles s : slip

The analysis o the ormula above shows that the mechanicalspeed o an induction motor is a unction o threeparameters. Thus the change o any o those parameters willcause the motor speed to vary as per the table below.

2 Normative Aspects

3 Induction machines speed variation

2.1 NEMA MG1 - Motors and generators / UnitedStates

g Parte 30 - Application considerations or constant speedmotors used on a sinusoidal bus with harmonic contentand general purpose motors used with adjustable-

requency controls or both (2006)g Parte 31 - Defnite-purpose inverter- ed polyphase motor

(2006)

2.2 NEMA - Application Guide or AC AdjustableSpeed Drive Systems (2001) 2.3 IEC 60034 - Rotating Electrical Machines /International

g Parte 17 - Cage induction motors when ed rom inverters application guide (2006)

g Parte 25 - Guide or the design and per ormance o cageinduction motors specifcally designed or inverter supply(2007)

2.4 Other technical documents o re erence

g GAMBICA/REMA Technical Guides or Variable SpeedDrives and Motors

g GAMBICA/REMA Technical Reports or Variable SpeedDrives and Motors

g CSA C22.2 No.100-2004 Item 12 (Canada) Motors andGenerators Industrial Products

g JEM-TR 148-1986 (Japan) Application guide or inverterdrive (general-purpose inverter)

g IEC 60034-18-41 Qualifcation and design tests or Type Ielectrical insulation systems used in rotating electricalmachines ed rom voltage inverters

g Papers and books related to this subject

Speed variation

Parameter Application characteristics

Number o polesDiscrete variation

Oversizing

Slip

Continuous variation

Rotor losses

Limited requency range

Voltage requencyContinuous variation

Utilization o STATIC FREQUENCY Inverters!

n = 120 f 1 (1-s) p


6/36

www.weg.net


The utilization o static requency inverters comprehendscurrently the most e fcient method to control the speed o induction motors. Inverters trans orm a constant requency-constant amplitude voltage into a variable (controllable)

requency-variable (controllable) amplitude voltage. Thevariation o the power requency supplied to the motor leadsto the variation o the rotating feld speed, which modifes themechanical speed o the machine.

The torque developed by the induction motor ollows theequation below:

Despising the voltage drop caused by the stator impedance,the magnetizing ux is ound to be:

where:T : torque available on the sha t (N.m)f m : magnetizing ux (Wb)

I 2 : rotor current (A) depends on the load!V 1 : stator voltage (V)

k 1 e k 2 : constants depend on the material and on themachine design!

Considering a constant torque load and admitting that thecurrent depends on load (there ore practically constantcurrent), then varying proportionally amplitude and requencyo the voltage supplied to the motor results in constant ux

and there ore constant torque while the current remainsunchanged. So the motor provides continuous adjustmentso speed and torque with regard to the mechanical load.Losses can be thus minimized in accordance with the loadconditions by keeping the slip constant at any speed, or agiven load.

The curves below are obtained rom the equations above.

The ratio V1/ 1 is kept constant up to the motor base (rated)requency. From this requency upwards the voltage is kept

constant at its base (rated) value, while the requency appliedon the stator windings keeps growing, as shown next.

Thereby the region above the base requency is re erred to asfeld weakening, in which the ux decreases as a result o requency increase, causing the motor torque to decrease

gradually. The typical torque versus speed curve o an

inverter ed induction motor is illustrated below.

The number o variable speed applications controlled bymeans o a requency inverter has increased signifcantly overthe recent years. This may be explained by the many beneftsprovided by such applications:g Aloo control the control can be installed remotely at a

suitable location, keeping just the motor in the processingarea on the contrary o hydraulic and mechanical varyingspeed systems.

g Aloo control the control can be installed remotely at asuitable location, keeping just the motor in the processingarea on the contrary o hydraulic and mechanical varyingspeed systems.

g Cost reduction direct on line startings o induction motorscause current peaks that harm the motor as well as otherelectric equipments linked to the electrical system. Static

requency inverters provide so ter startings, resulting incost reduction with regard to maintenance.

g Gain o productivity industrial systems are o ten oversizeddue to an expectation o uture production increase. Staticinverters allow the proper regulation o the operationalspeed according to the equipments available and theproduction needs.

g Energy E fciency the power system global e fciencydepends not only on the motor, but also on the control.Static inverters are high e fciency apparatuses, reachingtypically 97% or more. Induction motors also present highe fciency levels, reaching up to 95% or even more in larger

f m = k 2 .V 1

1

T = k 1 . f m . I 2

It comes out that torque is kept constant up to the baserequency and beyond this point it alls down (weakening

feld). Since the output is proportional to torque timesspeed, it grows linearly up to the base requency and romthat point upwards it is kept constant. This is summarizedby the graph beside.


7/36

www.weg.net


machines operating at rated conditions. When speedvariation is required, the output changes in an optimizedway, directly a ecting the energy consumption and leadingto high e fciency levels per ormed by the system (inverter +motor).

g Versatility static requency inverters suit both variable andconstant torque loads. With variable torque loads (lowtorque demand at low speeds) the motor voltage isdecreased to compensate or the e fciency reductionnormally resultant rom load reduction. With constanttorque (or constant power) loads the system e fciencyimprovement comes rom the easibility o continuousadjustment o speed, with no need to use multiple motorsor mechanical variable speed systems (such as pulleys andgears), which introduce additional losses.

g High quality the accurate speed control obtained withinverters results in process optimization, providing a fnalproduct o better quality. Aloo control the control can be installed remotely at asuitable location, keeping just the motor in the processingarea on the contrary o hydraulic and mechanical varyingspeed systems.

4 Characteristics o PWM requency inverters

4.1 General PWM voltage source static requency inverters presentlycomprehend the most used equipments to eed low voltageindustrial motors in applications that involve speed variation. They work as an inter ace between the energy source (ACpower line) and the induction motor.

In order to obtain an output signal o desired voltage andrequency, the input signal must accomplish three stages

within a requency inverter:

g Diode bridge - Rectifcation o the AC input voltage -

constant amplitude and requency - coming rom thepower grid;

g DC link or lter - Regulation/smoothing o the rectifedsignal with energy storage through a capacitor bank;

g IGBT power transistors Inversion o the voltage comingrom the link DC into an alternate signal o variable

amplitude and requency.

The ollowing diagram depicts the three stages o an indirectrequency inverter.

Rectifer

Input:50/60 Hz (1f or 3 f ) Output: Variable voltageand requency

AC AC

DC

Filter Inverter| motor

VPWM

Motor3 f

VDC 1.35 Vinor 1.41 Vin

Vin


8/36

www.weg.net


NOTES:g Under light load (or at no load) conditions, the DC link

voltage tends to stabilize atHowever, when the motor drives heavier loads ( orinstance, at ull load), the DC link voltage tends to the value

g The criteria used to defne the insulation system o WEGmotors ed by inverters, presented urther on, consider thehighest o those values (1.41Vin), which is more critical tothe motor. In this way WEG motors attend both situationssatis actorily.

2 Vrede @1,41 Vrede

2 Vrede @1,35 Vrede(3/ P )

4.2 Control Types There are basically two inverter control types: scalar (openloop) and vector (open or closed loop).

The scalar control is based on the original concept o arequency inverter: a signal o certain voltage/ requency ratio

is imposed onto the motor terminals and this ratio is keptconstant throughout a requency range, in order to keep themagnetizing ux o the motor practically unchanged. It isgenerally applied when there is no need o ast responses totorque and speed commands and is particularly interestingwhen there are multiple motors connected to a single drive. The control is open loop and the speed precision obtained isa unction o the motor slip, which depends on the load,since the requency is imposed on the stator windings. Inorder to improve the per ormance o the motor at lowspeeds, some drives make use o special unctions such asslip compensation (attenuation o the speed variation as

unction o load) and torque boost (increase o the V/ ratio to

5.1 HarmonicsFor the AC power line, the system ( requency inverter +motor) is a non-linear load whose current include harmonics( requency components multiples o the power line

requency). The characteristic harmonics generally producedby the rectifer are considered to be o order h = np1 on the AC side, that is, on the power line (p is the number o pulseso the inverter and n =1,2,3). Thus, in the case o a 6 diode (6pulses) bridge, the most pronounced generated harmonicsare the 5th and the 7th ones, whose magnitudes may vary

rom 10% to 40% o the undamental component, dependingon the power line impedance. In the case o recti yingbridges o 12 pulses (12 diodes), the most harm ul harmonicsgenerated are the 11th and the 13th ones. The higher theorder o the harmonic, the lower can be considered itsmagnitude, so higher order harmonics can be fltered moreeasily. As the majority o drives manu acturers, WEGproduces its low voltage standard inverters with 6-pulserectifers.

compensate or the voltage drop due to the statorresistance), so that the torque capacity o the motor ismaintained. This is the most used control type owing to itssimplicity and also to the act that the majority o applicationsdo not require high precision or ast responses o the speedcontrol.

The vector control enables ast responses and high level o precision on the motor speed and torque control. Essentiallythe motor current is decoupled into two vectors, one toproduce the magnetizing ux and the other to producetorque, each o them regulated separately. It can be openloop (sensorless) or closed loop ( eedback).

g Speed eedback a speed sensor ( or instance, anincremental encoder) is required on the motor. This controlmode provides great accuracy on both torque and speed

o the motor even at very low (and zero) speeds.g Sensorless simpler than the closed loop control, but itsaction is limited particularly at very low speeds. At higherspeeds this control mode is practically as good as the

eedback vector control.

The main di erence between the two control types is that thescalar control considers only the magnitudes o theinstantaneous electrical quantities (magnetic ux, current andvoltage) re erred to the stator, with equations based on theequivalent electrical circuit o the motor, that is, steady stateequations. On the other hand, the vector control considersthe instantaneous electrical quantities re erred to the rotor

linkage ux as vectors and its equations are based on thespatial dynamic model o the motor. The induction motor isseen by the vector control as a DC motor, with torque and

ux separately controlled.

5 Interaction between inverter and AC power line

The power system harmonic distortion can be quantifed bythe THD (Total Harmonic Distortion), which is in ormed by theinverter manu acturer and is defned as:

where: Ah are the rms values o the non- undamental harmoniccomponents A1 is the rms value o the undamental component

The wave orm above is the input measured current o a6-pulse PWM inverter connected to a low impedance powergrid.

THD =h = 2

8

2 An

A1


9/36

www.weg.net


3.2.p. line .I rated

5.1.1 Normative considerations about the harmonics The NEMA Application Guide or AC ASD Systems re ers toIEEE Std.519 (1992), which recommends maximum THDlevels or power systems 69 kV as per the tables presentednext. This standard defnes fnal installation values, so thateach case deserves a particular evaluation. Data like thepower line short-circuit impedance, points o commonconnection (PCC) o inverter and other loads, among others,in uence on the recommended values.

5.2 Line reactor / DC bus chokeHarmonic currents, which circulate through the power lineimpedances and depend on the rectifer input/outputimpedance values, cause harmonic voltage drops that distortthe power supply voltage o the inverter and other loadsconnected to this line. These harmonic current and voltagedistortions may increase the electrical losses in theinstallation, lowering the power actor and overheatingcomponents such as cables, trans ormers, capacitor banks,motors, etc.

The addition o a line reactor and/or a DC bus choke reducesthe harmonic content o the current and increase the power

actor. The DC bus choke has the advantage o notintroducing a motor voltage drop but depending on thecombination o its value with the power line impedance andthe DC link capacitance values it may result in undesirableresonances within the overall system. On the other hand, theline reactor decreases the medium voltage o theintermediate circuit but attenuates more e ectively powersupply voltage transients. Besides that, it extends thesemiconductors and the DC link capacitor bank li etimes, as

The maximum harmonic current distortion recommended byIEEE-519 is given in terms o TDD (Total Demand Distortion)and depends on the ratio (ISC / IL), where:ISC = maximum short-current current at PCC.IL = maximum demand load current ( undamental requencycomponent) at PCC.

The documents mentioned rom IEC, however, do not setlimits or the harmonic distortion injected by inverters into thepower line.

L =(voltage drop) %. Vline H

Voltage harmonics

Even components 3,0%

Odd components 3,0%

THDvoltage 5,0%

Individual Odd Harmonics

(Even harmonics are limited to 25% o the odd harmonic limits)

Maximum harmonic current distortion in percent o ILISC / IL < 11 11 h

17

17 h

23

23 h

35

35 h TDD

< 20* 4.0 2.0 1.5 0.6 0.3 5.0

20 < 50 7.0 3.5 2.5 1.0 0.5 8.0

50 < 100 10.0 4.5 4.0 1.5 0.7 12.0

100 < 1000 12.0 5.5 5.0 2.0 1.0 15.0

> 1000 15.0 7.0 6.0 2.5 1.4 20.0

* All power generation equipment is limited to these values o current distortion, regardless o actual I SC / IL.

(a)

(a)

(b)

(b)

a result o the decrease o both the rms current o therecti ying diodes and the current ripple through the middlecircuit capacitors.

The value o the line reactor needed or the desired voltagedrop to be obtained can be calculated as ollows:

[ ]

Current and voltage wave orms with (b) and without (a) linereactor. It can be seen that line reactors so ten the peaks,thus reducing the harmonic content and the rms value o theinput current. Additionally, diminution o the supply voltagewave orm distortion is thereby caused. A minimum line impedance that introduces a voltage drop

rom 1 to 2%, depending on the inverter size, isrecommended in order to ensure the inverter li etime. As rule o thumb, it is recommended to add a line reactor tothe existing power supply impedance (including trans ormersand cables) so that a total voltage drop o 2 to 4% isachieved. This practice is considered to result in a goodcompromise between motor voltage drop, power actorimprovement and harmonic current distortion reduction.

The (a) line reactor and (b) DC bus choke electricalinstallations are shown next.

(a) Input line reactor connection

Converter input current

Converter input voltage


10/36

www.weg.net


Then the motor ed by requency inverter sees a pulsating(PWM) voltage and a practically sinusoidal current, so thatthe voltage harmonics generally present higher magnitudesthan the current harmonics.

* All requency inverters manu actured by WEG employ Space Vector Modulation.

6.1.1 Normative considerations about the inverteroutput harmonics There is no international standardization defning maximumacceptable values or voltage and current harmonicdistortion. However, the international standards do considerthe increase o motor losses due to the non-sinusoidalsupply.

IEC 60034-17 provides an example o motor losses increaseowing to PWM supply. Motor in o: 315 IEC rame, ratedtorque and speed values.

6.1 Harmonics in uencing motor per ormance The induction motor, when under PWM voltage coming romthe inverter, is subjected to voltage harmonics ( requencycomponents above the undamental requency). Dependingon the type o PWM employed, the switching requency andother peculiarities o the control, the motor may presente fciency decrease and losses, temperature, noise andvibration levels increase.

Furthermore other e ects may appear when inductionmotors are ed by inverters. Insulation system dielectric stressand sha t voltages allied with potentially damaging bearingcurrents are well known side e ects. Although not producedspecifcally by harmonics but by other matters that will soonbe approached, these are important e ects and should notbe neglected. The motor current and voltage wave ormswhen under PWM supply are illustrated below.

6 Interaction between inverter andmotor

PWM voltage at theinverter output

Inverter ed motor current

There are basically the ollowing solutions to mitigate theharmonics generated by a PWM requency inverter:

Methods o r educ tion o ha rmon ics Solut ion cha rac ter ist ic s

Installation o output passive lters(L, LC (sinusoidal), dV/dt)

Installation costs increase

Restrictions or vector control operation Voltage drop (motor horsepower

reduction)

Use o multi-level inverters

Costs increase

Inverter reliability decrease

Control complexity increase

Pulse Width Modulation quality

improvement (optimization o pulse

patterns)

Space Vector Modulation (SVM)*

Do not increase costs

Voltage control upgrade

Higher system (inverter + motor)

e ciency

Switching requency increase

Inverter e ciency decrease (higher

switching losses)

- Common mode leakage current fow

increase

(b) DC bus choke connection


11/36

www.weg.net


6.2 Considerations regarding energy e fciency The lack o international standards that speci y test

procedures to evaluate the system (motor + inverter)e fciency allows such tests to be carried out in many di erentand non contestable ways. There ore, the results obtainedshould not in uence the acceptance (or not) o the motor,except under mutual accordance between customer andmanu acturer. Experience shows the e ectiveness o theconsiderations below.g An induction motor ed by PWM voltage presents a lower

e fciency level than when ed by purely sinusoidal voltage,due to the losses increase caused by harmonics;

g Anyway, when induction motors are ed by static inverters,the e fciency o the overall system, rather than the motore fciency only, should be evaluated;

g Each case must be properly analyzed, taking into accountcharacteristics o both the motor and the inverter, such as:operating requency, switching requency, speed range,load conditions and motor power, THD, etc.

g The measuring instrumentation is extremely important orthe correct evaluation o electrical quantities on systemsunder PWM duty. True RMS meters must be used, in orderto permit reliable measurements o power;

g Higher switching requencies increase the motor e fciencyand decrease the inverter e fciency (due to the increase o commutation losses).

g High e fciency motors keep their e fciency higher,compared to standard motors, when both are ed byinverters.

Losses caused by undamental requency

Losses caused by harmonics

A Stator winding lossesB Rotor winding lossesC Iron lossesD Additional load lossesE Frictional losses

F Stator winding lossesG Rotor winding lossesH Iron lossesI Additional load lossesJ Commutation losses

NOTE: rame 315 (IEC) motor operating at rated speed andtorque.

IEC 60034-25 illustrates the motor losses increase due toPWM supply by means o the ollowing curves:

NEMA MG1 Part 30 considers a derating actor (torquereduction) to avoid excessive overheating o a generalpurpose motor ed by converter, compensating or thecirculation o harmonic currents due to the PWM voltageharmonic content:

Where:n: order o the odd harmonic, not including those divisible bythree Vn: per unit magnitude o the voltage at the nth harmonic

requency

HVF =n = 5

8

2 Vn

n


12/36

www.weg.net


6.2.1 The in uence o the speed variation on the motore fciency The e ects o speed variation on the motor e fciency can beunderstood rom the analysis o the behavior o the inverter

ed motor output power as a unction o its operation speed.

6.2.2 .1 Numerical example

6.2.2 Normative considerations about the e fciency oinverter ed motors

Supposing, or instance, a 60 Hz requency base or thesituations outlined above:

Some practical values ound by means o the input-outputmeasurement method are shown below or standard motors:

g NEMA MG1 Part 30 E fciency will be reduced when amotor is operated on a bus with harmonic content. The

harmonics present will increase the electrical losses which,in turn, decrease e fciency. This increase in losses will alsoresult in an increase in motor temperature, which urtherreduces e fciency.

Motor 75 HP (55 kW) 6 poles 400 V 50 Hz

conv = P 2 /P 1

conv = P 3 /P 2 sist = P out /P in = P 3 /P 1 = conv . mot

And, according to the exposed above,

Then the ollowing situation results rom speed reduction:

Considering that the motor losses are essentially comprisedo Joule losses (PJ) and iron losses (PI) and assuming that theJoule losses prevail, then the motor e fciency all at lowspeeds, where the motor output power is reduced and,despite the slight decrease o the iron losses ( requencydependant), the Joule losses (current square dependant) arekept nearly constant or a constant torque load, so that a terall there is no signifcant variation o the overall losses.

The equations next explain that. Defning e fciency as:

P 60Hz = P u

P 30Hz = P u = 0,5 P u6030

h % =

h %

= P out P out

P out

P out + Losses

Losses @ P J + P iron

Losses @constant P iron + P J @constant (P J >> P iron )

(P J > P iron )

P in

}

}

Motor 15 HP (11 kW) 4 poles 400 V 50 Hz


13/36

www.weg.net


6.4 Criteria regarding the temperature rise o WEGmotors on VSD applications

6.4.1 Torque deratingIn order to keep the temperature rise o WEG motors, whenunder PWM supply, within acceptable levels, the ollowingloadability limits must be attended (observe the motor line

and the fux condition ).NOTE: Applications with motors rated or use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

6.4.1.1 Para motores do mercado NEMA

6.3 In uence o the inverter on the temperature rise othe windingsInduction motors may heat up more when ed by requencyinverter than when ed by sinusoidal supply. This highertemperature rise results rom the motor losses growth owingto the high requency components o the PWM signal and theo ten reduced heat trans er resulting rom speed variation.

The voltage harmonic distortion contributes to increase themotor losses, once that creates minor hysteretic loops in thelamination steel, increasing the e ective saturation o themagnetic core and giving rise to high requency harmoniccurrents, which bring about additional Joule losses.Nevertheless, these high requency components do notcontribute to the production o torque at steady operation o the motor, since they do not increase the airgap undamental

ux, which rotates at synchronous speed. The operation atlow speeds causes the ventilation over the (sel -ventilated)motor rame to decrease, consequently lowering the motorcooling and raising in this way the thermal stabilizationtemperature.

g NEMA MG1 Part 31 Per ormance tests, when required,shall be conducted on a sinusoidal power supply unlessotherwise specifed by mutual agreement between themanu acturer and the user.

g NEMA Application Guide or AC ASD Systems The overall

e fciency o an ASD is based on the total losses o thecontrol, the motor, and any auxiliary equipment. (...) Themotor e fciency when operated on a control is slightly lessthan when operated on sinewave power. Overall systeme fciency is o ten increased when used an ASD. Traditionalmethods o changing speed such as gears or beltsintroduce additional losses which reduce e fciency.

g IEC 60034-17 The per ormance characteristics andoperating data or drives with inverter- ed cage inductionmotors are in uenced by the complete system, comprisingsupply system, inverter, induction motor, mechanicalsha ting and control equipment. Each o these componentsexists in numerous technical types. Any values quoted inthis technical specifcation are thus indicative only. (...) There is no simple method to calculate the additionallosses and no general statement can be made about theirvalue. Their dependence upon the di erent physicalquantities is very complex. Also there is a great variety botho inverters and o motors.

g IEC 60034-25 The recommended methods to determinethe motor e fciency are given in IEC 60034-2 (summation-o -losses method or motors > 150 kW and input-outputmeasurement or motors 150 kW). The no-load losses(including the additional losses) should be measured at thesame pulse pattern and pulse requency that the inverterwill produce at rated load. The determination o the overalle fciency o the system (motor + inverter) by means o input-output measurement or motors > 150 kW is alsoapplicable under agreement between manu acturer anduser. In this case, however, the motor e fciency shall not bedetermined separately.

There ore, when operating with requency inverters, both thee ects mentioned above must be considered. There arebasically the ollowing solutions to avoid excessiveoverheating o the inverter ed motor:g Torque derating (oversizing o the sel ventilated motor

rame);g Utilization o independent cooling system (separate

ventilation);g Utilization o the Optimal Flux Solution (exclusive to

applications using WEG drives and motors).

TEFC W21 and W22 (High E fciency) motors

Frame SizeConstant

Torque

Variable

Torque

Constant

PowerDrive Comments

143

587(***)

12:1 1000:1 60 120 Hz AnyConstant

fux

100:1(*) - 60 120 Hz WEG(**) Optimal fux

587(****)4:1 1000:1 60 120 Hz Any

Constant

fux

10:1 - 60 120 Hz WEG(**) Optimal fux

TEFC NEMA PREMIUM EFFICIENCY motors

Frame Size

Constant

Torque

Variable

Torque

Constant

Power Drive Comments

143

587(***)

20:1 1000:1 60 120 Hz AnyConstant

fux

1000:1(*) - 60 120 Hz WEG(**) Optimal fux

587(****)6:1 1000:1 60 120 Hz Any

Constant

fux

12:1 - 60 120 Hz WEG(**) Optimal fux

(*)Satis actory motor per ormance depends on proper drive setup please contact WEG(**)WEG drive CFW-09 version 2.40 or higher, operating in sensorless (open loop) vector

mode(***)Motors with rated power 250 hp. Criteria also valid or motors o the rame sizes 447

and 449(****)Motors with rated power > 250 hp. Criteria also valid or motors o the rame sizes 447

and 449


14/36

www.weg.net


1. The speed ranges stated above are related to the motorthermal capability only. Speed regulation will depend on VFDmode o operation and proper adjustment.

2. W21 and NEMA PREMIUM EFFICIENCY WEG MOTORS o all rame sizes can also be blower cooled under request. Insuch case, the motor will be suitable or variable andconstant torque applications rated up to 1000:1 with anydrive.3. W21 and NEMA PREMIUM EFFICIENCY WEG MOTORScomply with those maximum sa e operating speedsestablished in NEMA MG1 Parts 30 and 31 (2003).

NOTE:

The relations set above describe operation speed ranges.Supposing or instance a 60 Hz base requency, the ollowingequivalence is valid:

6.4.1.2 IEC marketConstant fux condition:Encompassed motor lines: Totally enclosed o -the-shel motors attending IE1 (as per IEC 60034-30) or highere fciency levels.

6.4.2 Breakaway torque According to NEMA MG1 Parts 30 and 31, the motor shouldbe capable o producing a breakaway torque o at least140% o rated torque requiring not more than 150% ratedcurrent. WEG motors when ed by inverters attend suchrecommendation.

Optimal fux condition:Encompassed motor lines: Totally enclosed o -the-shel motors attending IE2 (as per IEC 60034-30) or highere fciency levels.

The patented WEG Optimal Flux solution was developed orthe purpose o making WEG induction motors able to operateat low speeds with constant torque loads still keeping anacceptable temperature rise level, without the need o neitheroversizing the machine nor blower cooling it.

It is based on the continuous minimization o the motor

Relation Frequency range

4:1 15 60 Hz10:1 6 60 Hz

12:1 5 60 Hz

20:1 3 60 Hz

100:1 0,6 60 Hz

1000:1 0,06 60 Hz

losses (heat sources) by means o the optimization o itsmagnetic ux, parameter controlled by the CFW09. From thestudy o the composition o the overall motor losses and theirrelation with the requency, the magnetic ux and the current,as well as the in uence o the ventilation system on the motortemperature rise, it was ound an optimal ux value or each

requency, allowing or a continuous minimization o theoverall motor losses through the whole speed range. Thesolution obtained was implemented within the CFW09, inorder that the motor magnetic ux optimal condition can beachieved automatically by the drive, su fcing or that a simpleadjustment o the inverter properly made.

The motor iron losses strongly depend on the requency. Asthe operation requency is varied downwards, the iron lossesare gradually reduced. There ore it is interesting at low speedoperation to increase the magnetic induction ( ux density) o the motor, so that the torque can be kept constant with areduced current, which causes reduced Joule losses. Thusas the speed alls, it is possible to reduce the voltageproportionally less than the requency, resulting in an optimal V/Hz ratio (greater than the rated value), which minimizes themotor losses altogether. It is considered thereby that themajor motor losses occur due to Joule e ect on thewindings.

This solution was especially conceived or low speedapplications with constant torque loads and must be used inno way with variable torque loads or above the motor base

requency. Besides, the Optimal Flux WEG solution isapplicable only when:g

the motor is ed by WEG inverter (CFW09) version 2.40 orhigher;g sensorless vector type control is used.


15/36

www.weg.net


6.4.3 Breakdown torque Above base speed the motor voltage must be kept constant

or constant power operation, as already shown. NEMA MG1Part 31 prescribes that the breakdown torque at any requen-cy within the defned requency range shall be not less than

150% o the rated torque at that requency when rated volt-age or that requency is applied. WEG motors when ed byinverters satis y such criterion up to 90 Hz.

The maximum torque capability o the motor (breakdowntorque) limits the maximum operating speed in which con-stant power operation is possible. Attending NEMA recom-mendations, one can approximately fnd this limit rom the ol-lowing equation:

6.5 In uence o the inverter on the insulation system The evolution o the power semiconductors have led to thecreation o more e fcient, but also aster, electronic switches. The high switching requencies o the IGBT transistorsemployed in modern requency inverters bring about someundesirable e ects, such as the increase o electromagneticemission and the possibility o voltage peaks, as well as highdV/dt ratios (time derivative o the voltage, that is, rate o electrical potential rise), occurrence at the inverter ed motorterminals. Depending on the control characteristics (gateresistors, capacitors, command voltages, etc.) and the PWMadopted, when squirrel cage induction motors are ed by

requency inverters, those pulses combined with theimpedances o both the cable and the motor may causerepetitive overvoltages on the motor terminals. This pulsetrain may degrade the motor insulation system and mayhence reduce the motor li etime.

The cable and the motor can be considered a resonantcircuit, which is excited by the inverter rectangular pulses.When the values o R, L and C are such that the peak voltageexceeds the supply voltage (VDC 1.41 Vin), the circuitresponse to this excitation is a so called overshoot. Theovershoots a ect especially the interturn insulation o randomwindings and depend on several actors:rise time o thevoltage pulse, cable length and type, minimum time

between successive pulses, switching requency and multimotor operation.

6.5.1 Rise Time The PWM voltage takes some time to rise rom its minimumto its maximum value. This period is o ten called rise time.Due to the great rapidity o switching on the inverter stage,the growth o the voltage wave ront takes place too ast and,with the power electronics advance, these transition timestend to be more and more reduced.

6.5.1.1 Normative considerations about rise time The defnitions o rise time (tr) according to NEMA and to IECStandards di er, as shown below, allowing or interpretationdivergences and con icts between manu acturers and userso motors and drives.

tr: time needed or the voltage to rise rom 10 to 90% o theDC link voltage (1.41Vrated)

RPM max = 2 Tmax RPM base3

Then the inverter ed motor is subjected to extremely high dV/ dt rates, so that the frst turn o the frst coil o a single phaseis submitted to a high voltage level. There ore variable speeddrives can considerably increase the voltage stress within amotor coil, though owing to the inductive and capacitive

characteristics o the windings, the pulses are damped onthe subsequent coils.

So the rise time (tr) has a direct in uence on the insulation li e,because the aster the pulse wave ront grows, the greater thedV/dt ratio over the frst coil and the higher the levels o voltage between turns, causing the insulation system to wearmore quickly away. Thus the motor insulation system shouldpresent superior dielectric characteristics in order to standthe elevated voltage gradients occurring on PWMenvironment.

NEMA MG1 Part 30

Tbase


16/36

www.weg.net


Supposing the motor voltage Vrated = 460 V VlinkDC 1,41 x 460 = 648,6 V

V = 0,8 x 648,6 = 518,9 V

Assuming that rise time = 0,1 st = 0,1 s

Supposing the motor voltage Vrated = 460 Vwith incidence o 1200 V peaks

V = 0,8 x 1200 = 960 V

Assuming tr = 0,25 s:

NEMA de nition o dV/dt

IEC 60034-25

IEC de nition o dV/dt

dV

dV

V

V

V

V

518,9

960

5189

3840

dt

dt

t

t

0,1

0,25

=

=

=

=

=

=

s

s

[

[

[

[

tr: time needed or the voltage to rise rom 10% to 90% o thepeak voltage at motor terminals

The signal arriving at the motor through the cable is partiallyre ected, causing overvoltage, because the motor high

requency impedance is greater than the cable impedance.Excessively long leads increase the overshoots at the motorterminals. According to the NEMA Application Guide or AC ASD Systems, with the modern IGBT controls overshootsbegin to occur with a cable length o a ew eet and canreach 2 times the control DC bus voltage at a length lessthan 50 eet. In some cases, however, very long cables (inexcess o 400 eet, or example) can result in a situationwhere the overshoot does not decay quickly enough. In thiscase the voltage peak at the motor terminals can ring up wellbeyond 2 times the inverter DC link voltage. This behavior is a

unction o the PWM pulse pattern, the rise time and the very

NOTE: Due to the cable, the rise time is higher at the motorterminals than at the inverter terminals. However, a verycommon mistake in the dV/dt calculation is to consider therise time at the inverter terminals and the voltage peak at themotor terminals, resulting in an unlikely dV/dt value. Forinstance, considering tr = 0.1 s (typical value ound at theinverter) in the case above it would result dV/dt = 9600 V/ s!

Owing to the di erences existing between the rise timedefnitions given by NEMA and IEC, misunderstandings o tenhappen when calculating the voltage gradient (dV/dt).

According to NEMA criterion the DC link voltage (1.41 Vin)must be taken as 100% voltage re erence or thedetermination o rise time and the calculation o dV/dt. According to IEC criterion, however, the peak voltage arrivingat the motor terminals is what must be taken as 100%voltage re erence. Due to the cable, the rise time to beconsidered in IEC criterion will be normally higher than theone considered in NEMA criterion (which is the valuein ormed by the inverter manu acturer). Thus depending onthe criteria considered throughout the calculations, prettydi erent values o dV/dt are likely to be attributed to the samesituation.

The insulation criteria defned or WEG motors are based onNEMA, in order not to depend on the fnal customerinstallation. Furthermore the NEMA criterion seemsappropriate or considering just the linear stretch o the curveto approximate the derivative (dV/dt V/ t). The IECcriterion considers the peak voltage at the motor terminals,something extremely complicated to be predicted orestimated a priori. The rise time at the motor terminals isincreased by the cable high requency impedance. The dV/dtratio at the motor terminals (milder than at the drive terminals)can be also calculated, but it requires a reliable measuremento the voltage pulses at the motor leads and most o timesthis is not easily accomplished or not even easible,demanding a technician amiliar with such applicationsequipped with a good oscilloscope.

6.5.2 Cable lengthBeside the rise time, the cable length is a predominant actorin uencing the voltage peaks occurrence at the inverter edmotor terminals. The cable can be considered a transmissionline with impedances distributed in sections o inductances/ capacitances series/parallel connected. At each pulse, theinverter delivers energy to the cable, charging those reactiveelements.


17/36

www.weg.net


Converter terminals 65.5 t cable

98.5 t cable 328 t cable

6.5.2.1 Corona e ectDepending on the quality/homogeneity o the impregnationthe impregnating material may contain voids (cavities), inwhich the ailure mechanism o the interturn insulationdevelops. The deterioration o the motor insulating systemdue to the voltage overshoots occurs by means o PartialDischarges (PD), a complex phenomenon resulting romCorona.

Between adjacent charged conductors there is relativevoltage, which gives rise to an electric feld. I the establishedelectric feld is high enough (but below the breakdownvoltage o the insulating material), the dielectric strength o the air is disrupted, that is, i there is su fcient energy, oxygen(O2) is ionized in ozone (O3). The ozone is highly aggressiveand attacks the organic components o the insulation systemdamaging it. For this to happen though the voltage on theconductors must exceed a threshold value, the so calledCorona Inception Voltage, that is the local breakdownstrength in air (within the void). The CIV depends on thewindings design, insulation type, temperature, superfcialcharacteristics and moisture.

6.5.3 Minimum time between successive pulses(MTBP) The voltage measurements presented above show that thereis a succession o peaks in the voltage wave orm delivered bythe drive and arriving at the motor terminals. This signalpropagates trough the cable at a determined velocity.Depending on the winding characteristics and, with respectto the wave orm, on the minimum time between successivepulses, the voltage appearing between turns may varysensibly.

The average voltage applied at the motor terminals iscontrolled by the width o the pulses and by the timebetween them. The overshoots get worse with shorter timesbetween pulses. This condition is most likely to occur at highpeak or high output voltages and during transient conditions,such as acceleration or deceleration. I the time betweenpulses is less than three times the resonant period o thecable (typically 0.2 to 2 s or industrial cable), then additionalovershoot will occur. The only way to be sure that thiscondition does not exist is by measuring the pulses directlyor by contacting the control manu acturer.

cable type. Voltage measurements realized at the inverterterminals (0 t cable) and at the motor (Vrated = 400 V)terminals with di erent cable lengths are presented next. Theovershoots also depend on the type o cable used in theinstallation; there ore the wave orms shown below areillustrative only.

Vpeak = 560 V

Vpeak = 750 V

Partial discharge e ect on the motor insulation system

Damaged insulation due to PD activity

PD is thus a low energy discharge which, a ter long termactivity, prematurely degrades the motor insulation. Theerosion reduces the thickness o the insulating material,resulting in a progressive reduction o its dielectric properties,until its breakdown voltage capability alls below the level o the applied voltage peak, then the insulation breakdownoccurs.

Vpeak = 630 V

Vpeak = 990 V


18/36

www.weg.net


When the time between successive pulses is less than 6 s,particularly when the frst and the last turns o a single coil o a random winding are side by side, it may be assumed thatthe voltage between adjacent conductors is the peak to peakvalue between pulses. This act results rom the rapidity o

the pulse propagation within a coil, because while the frstturn stands a peak to peak voltage value, the voltage on thelast turn is very low, probably zero.In the case o the example shown above the MTBP wasbelow 6 s and there were actually motor ailures due toshort circuit between turns.

6.5.4 Switching requency ( s )Beside the e ects caused by the rise time and the MTBP,there is also the requency at which they are generated.Di erently rom eventual impulses caused by line handles, itis about a pulse train supported at a certain requency.Owing to the ast developments on power electronics,presently this requency reaches easily values such as 20kHz. The higher the switching requency, the aster thedegradation o the motor insulation takes place. Studies bearout that there is no simple interrelation between the insulationli e and the switching requency, in spite o that experienceshave shown interesting data:g I s 5 kHz the probability o insulation ailure occurrence

is directly proportional to the switching requencyg I s > 5 kHz the probability o insulation ailure occurrence

is quadratically proportional to the switching requency.

High switching requencies can cause bearing damages. Onthe other hand, switching requency increase results in themotor voltage FFT improvement and so tends to improve themotor thermal per ormance besides reducing noise.

6.6 Criteria regarding the insulation system o WEGmotors on VSD applicationsWhen WEG low voltage induction motors are used withinverters, the ollowing criteria must be attended in order toprotect the insulation system o the motor. I any o theconditions below are not satisfed, flters must be used.

NOTE: Applications with motors rated or use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

6.5.5 Multiple motorsI more than one motor is connected to a control, there canbe additional overshoot due to re ections rom each motor. The situation is made worse when there is a long length o lead between the control and the common connection o motors. This length o lead acts to decouple the motor romthe control. As a result, re ections which would normally beabsorbed by the low impedance o the control can be carriedto another motor and add to the overshoot at its terminals.

6.7 Normative considerations about the insulationsystem o inverter ed motorsg NEMA MG1 i the voltage at the inverter input does no

exceed the motor rated voltage and i the voltage observedat the motor terminals does not exceed the limits shownbelow, it may be assumed that there will be no voltagestress reducing signifcantly the li e o the insulation system.

When connecting multiple motors to a single inverter, L mustbe as short as possible.

The maximum recommended switching requency is 5 kHz.

Moisture is detrimental to insulating materials and there oremust be avoided or a longer motor li e to be guaranteed. Inorder to keep the motor windings dry, it is recommended theuse o heating resistors. The insulation system to be used in each case depends onthe motor rated voltage range and on the rame size.

Motor rated voltage

VoltageSpikes

motor

terminals

dV/dt

inverter

terminals

Rise

Time do

conversor*

MTBP*

VNOM 460 V 1600 V 5200 V/ s0,1 s 6 s460 V VNOM 575 V 1800 V 6500 V/ s

575 V VNOM 690 V 2200 V 7800 V/ s

* In ormed by the inverter manu acturer

INVERTER


19/36

www.weg.net


6.8 Recommendations or the cables connecting WEGmotors to inverters As already mentioned the maximum peak voltage appearingat the terminals o the inverter ed motor depends on many

actors, predominantly the cable length.

When supplying WEG motors with inverters, the ollowingpractical rules are suggested or the evaluation o the need o using flters between motor and inverter.

g IEC 60034 or motors up to 500 V the insulation systemmust stand voltage peak levels as shown below. Formotors above 500 V, rein orced insulation systems must beapplied or flters shall be installed at the inverter output,aiming to increase the rise time and to limit voltage peaks.

WEG motors ully attend NEMA MG1 Parts 30 and 31.

Nema MG1 - Part 30

General purpose motors

Nema MG1 - Part 31

Defnite purpose inverter ed motors

Vrated 600 V : Vpeak 1kV

Rise time 2 s Vrated > 600 V : Vpeak 3,1 Vrated

Rise time 0,1 s

Vrated 600 V : Vpeak 2,04 Vnom

Rise time 1 s

Vrated 600 V : Vpeak 2,04 Vrated

Rise time 1 s

IEC 60034-17 General purpose motors

IEC 60034-25De nite purpose motors

Valid or standard motors.

It is remarkable the similarities existing between IEC andGAMBICA criteria, as well as their disparity with respect toNEMA criteria. This results rom the particular defnitions o rise time and dV/dt according to each institution. One cannotice that the insulation criteria rom both IEC and GAMBICAtake into account the cable length, in ormation which WEGalso considers relevant.

The output reactor is necessary or the eddy current thatows rom inverter to earth to be limited. The input (line)

reactor prevents the inverter ground ault rom tripping.

The output reactor design must take account o additionallosses occurring due to current ripple and current leakage toearth, which increases as cable length rises. For long cablesand reactors designed or small currents there will be greatin uence o the leakage currents on the reactor losses (andheating). The cooling system o the inverter panel must alsotake the reactors additional losses into account or a sa etemperature operation to be assured.

The output reactor must be installed near the inverter, asshown below.

A: Valid or motors up to 500 Vac (without flters)B: Valid or motors up to 690 Vac (without flters)C: Measured results at 415 Vac supply with di erent cablelengths

g GAMBICA/REMA the European association o motors(REMA) and inverters (GAMBICA) manu acturers set thecriteria shown next based on its members experience.

Cable length L Output flters

L 100 m Not needed

100 m < L 300 mOutput reactor needed

(at least 2% voltage drop)

L > 300 m Special lters needed (contact WEG)


20/36

www.weg.net


L1 = Line reactor selection criteria according to clause 5.2L2 = Output reactor must be installed next to the inverter.

Cable shield must be grounded at both ends, motor andinverter. Good EMC practices such as 360 bonding o theshields are recommended, in order or low impedance or

high requency to be provided.For the shield to operate also as protective conductor, itshould have at least 50% o the phase conductorsconductance. I the shield does not have enough cross-section or that, then a separate earth conductor is neededand the shield provides EMC and physical protection only. The shield highrequency conductance should be at least10% o that o the phase conductors.

PE = protective earth conductorSCU = concentric copper (or aluminum) screen

Symmetrical Shielded Cables: three-core cable (with orwithout conductors or protective earth) symmetricallyconstructed + a concentric copper or aluminum protectiveshield/armour

A e = steel or galvanized iron

6.8.1 Cable types and installation recommendations The characteristics o the cable connecting motor and

requency inverter, as well as its interconnection and physicallocation, are extremely important to avoid electromagneticinter erence in other devices.

6.9 In uence o the inverter on the motor sha t voltageand bearing currents The advent o static inverters aggravated the phenomenon o induced sha t voltage/current, due to the unbalancedwave orm and the high requency components o the voltagesupplied to the motor. The causes o sha t induced voltageowing to the PWM supply is thus added to those intrinsic to

6.8.1.2 Shielded cablesg They help to reduce the radiated emission through the

motor cables in the Radio Frequency range (RF).g They are necessary when the installation must comply with

the EMC Directive 89/336/EEC as per EN 61800-3.g They are also necessary when using Radio Frequency

Inter erence Filter (whether built-in or external) at inverterinput.

g Minimum distances between motor cables and otherelectrical cables ( or instance, signal cables, sensor cables,etc.) must be observed in the fnal installation, as per tablebelow.

6.8.1.3 Installation recommendationsIEC 60034-25 presents cable types and construction details.

6.8.1.1 Unshielded cablesg Three-core unshielded motor cables can be used when

there is no need to ulfll the requirements o the EuropeanEMC Directives (89/336/EEC).

g Certain minimum distances between motor cables andother electrical cables must be observed in the fnalinstallation. These are defned in the table below.

g Emission rom cables can be reduced i they are installedtogether on a metallic cable bridge which is bonded to theearthing system at least at both ends o the cable run. Themagnetic felds rom these cables may induce currents innearby metalwork leading to heating and increasing losses.

Recommended separation distances between motor cable

(shielded or not) and other cables o the installation

Cable Length Minimum separation distance

30 m 10 cm

> 30 m 25 cm

The basic given recommendations are summarized in thetable below. For more details and updated in ormation thecurrent standard version shall be consulted.

The grounding system must be capable to provide good

connections among equipments, or example, betweenmotor and inverter rame. Voltage or impedance di erencesbetween earthing points can cause the ow o leakagecurrents (common mode currents) and electromagneticinter erence.

Examples o shielded cables recommended by IEC60034-25

Alternate motor cables or conductors up to 10 mm 2

L1

L2L3

Scu

L1 PE

L2L3

Scu

L1L2L3

AFe

L1

L2L3

Scu

PEPE

PE

PEs


21/36

www.weg.net


The sum o the instantaneous voltage values at the (threephase) inverter output does not equal to zero

This high requency common mode voltage may result inundesirable common mode currents. Existing straycapacitances between motor and earth thus may allowcurrent owing to the earth, passing through rotor, sha t andbearings and reaching the end shield (earthed).

Practical experience shows that higher switching requenciestend to increase common mode voltages and currents.

These discontinuous electric discharges wear the racewaysand erode the rolling elements o the bearings, causing smallsuperimposing punctures. Long term owing dischargecurrents result in urrows ( uting), which reduce bearings li eand may cause the machine to ail precociously.

6.9.1 Common mode voltage The three phase voltage supplied by the PWM inverter,di erently rom a purely sinusoidal voltage, is not balanced. That is, owing to the inverter stage topology, the vector sumo the instantaneous voltages o the three phases at theinverter output does not cancel out, but results in a high

requency electric potential relative to a common re erencevalue (usually the earth or the negative bus o the DC link),hence the denomination common mode.

6.9.2 Equivalent circuit o the motor or the highrequency capacitive currents

The high requency model o the motor equivalent circuit, inwhich the bearings are represented by capacitances, showsthe paths through which the common mode currents ow.

The rotor is supported by the bearings under a layer o non-conductive grease. At high speed operation there is nocontact between the rotor and the (earthed) outer bearingraceway, due to the plain distribution o the grease. Theelectric potential o the rotor may then rise with respect to theearth until the dielectric strength o the grease flm isdisrupted, occurring voltage sparking and ow o dischargecurrent through the bearings. This current that circulateswhenever the grease flm is momentarily broken down iso ten re erred to as the capacitive discharge component. There is still another current component, which is induced bya ring ux in the stator yoke and circulates permanentlythrough the characteristic conducting loop comprising thesha t, the end shields and the housing/ rame, that is o tencalled the conduction component.

Cer : Capacitor ormed by the stator winding and the rotorlamination (Dielectric = airgap + slot insulation + wireinsulation)

Crc : Capacitor ormed by the rotor and the stator cores(Dielectric = airgap)

Cec : Capacitor ormed by the stator winding and the rame(Dielectric = slot insulation + wire insulation)

Cmd e Cmt : Capacitances o the DE (drive end) and the NDE(non-drive end) bearings, ormed by the inner andthe outer bearing raceways, with the metallicrolling elements in the inside. (Dielectric = gapsbetween the raceways and the rolling elements +bearing grease)

ICM : Total common mode current

Ier : Capacitive discharge current owing rom the stator tothe rotor

Ic : Capacitive discharge current owing through the bearings

the motor ( or instance, electromagnetic unbalance causedby asymmetries), which as well provoke current circulationthrough the bearings. The basic reason or bearing currentsto occur within an inverter ed motor is the so called commonmode voltage. The motor capacitive impedances becomelow in ace o the high requencies produced within theinverter stage o the inverter, causing current circulationthrough the path ormed by rotor, sha t and bearings back toearth.


22/36

www.weg.net


Crater occasioned by electroerosion on theinner raceway o the bearing

Bearing raceway damaged by bearingcurrents ow

Fluting caused by electric discharges withinthe bearing

Without bearing protection:

With protected bearing:

Without bearing protection:

6.9.3 Methods to reduce (or mitigate) the bearingscurrents in inverter ed motorsFor the motor bearing currents to be impeded to circulate,both the conduction (induced on the sha t) and the capacitivedischarge (resultant rom common mode voltage)components must be taken into account. In order to eliminatethe current owing through the characteristic conductingloop it is enough to isolate the motor bearings (only one o them, in the case o a single drive end, or the both o them, inthe case o two drive ends). However, or the capacitivecomponents to be withdrawn it would be also necessary toisolate the bearings o the driven machine, in order to avoidthe migration o electric charges rom the motor to the rotoro the driven machine through their sha ts, which areelectrically connected in the case o direct coupling. Anotherway o extinguishing the capacitive discharge currentcomponent consists o short circuiting the rotor and themotor rame by means o a sliding graphite brush. This way,the inductive current component owing through thecharacteristic conducting loop can be eliminated byinsulating just a single bearing o the motor, while thecapacitive current component, as well as the trans er o capacitive charges to the driven machine, can be eliminatedby use o a short circuiting brush.

Motor with two drive ends

Motor with one drive end

With bearing protection:


23/36

www.weg.net


6.10 Criteria regarding protection against bearingcurrents (sha t voltage) o WEG motors on VSDapplications

6.11 Normative considerations about the currentowing through the bearings o inverter ed motors

6.12 In uence o the inverter on the motor acousticnoise

g NEMA MG1 Part 31 with sinusoidal supply sha t voltagesmay be present usually in motors o rame 500 and larger.(...) More recently, or some inverter types and applicationmethods, potentially destructive bearing currents haveoccasionally occurred in much smaller motors. (...) Thecurrent path could be through either or both bearings toground. Interruption o this current there ore requiresinsulating both bearings. Alternately, sha t groundingbrushes may be used o divert the current around thebearing. It should be noted that insulating the motorbearings will not prevent the damage o other sha tconnected equipment.

g NEMA Application Guide or AC ASD Systems thecirculating currents caused by common mode voltage maycause bearing problems in rame sizes smaller than 500(most likely in the 400 and larger rames).

g IEC 60034-17 or machines with rame numbers above315 it is recommended either to use an inverter with a flterdesigned to reduce the zero-sequence component o thephase voltages (so called common mode voltages) or toreduce the dV/dt o the voltage or to insulate the motorbearing(s). The need to insulate both motor bearings isseldom necessary. In such a case, the examination o the

whole drive system by an expert is highly recommendedand should include the driven machine (insulation o thecoupling) and the grounding system (possibly use o anearthing brush).

NOTE: Applications with motors rated or use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

The rotating electrical machines have basically three noisesources:g The ventilation systemg The rolling bearingsg Electromagnetic excitation

Bearings in per ect conditions produce practically despicablenoise, in comparison with other sources o the noise emittedby the motor.In motors ed by sinusoidal supply, especially those withreduced pole numbers (higher speeds), the main source o noise is the ventilation system. On the other hand, in motorso higher polarities and lower operation speeds o ten standsout the electromagnetic noise.However, in variable speed drive systems, especially at lowoperating speeds when ventilation is reduced, theelectromagnetically excited noise can be the main source o noise whatever the motor polarity, owing to the harmoniccontent o the voltage.Higher switching requencies tend to reduce the magneticallyexcited noise o the motor.

Plat orm Frame Size Standard Optional

W21

W22

mod < 315 IEC

mod < 504 NEMA No protec tion Please con tac t WEG

W21

W22

315 and 355 IEC

504/5 and 586/7 NEMA No protection *

Insulated bearing in

any or both motor

endsEarthing system with

slip ring and graphite

brush between rame

and sha t

HGF

315 mod 630 (IEC)

500 mod 1040

(NEMA)

Insulated NDE bearing

Insulated DE bearing

Earthing system with


brush between rame

and sha t

M

280 mod 1800 (IEC)

440 mod 2800

(NEMA)

Insulated NDE bearing

Insulated DE bearing

Earthing system with


brush between rame

and sha t

g IEC 60034-25 do not speci y a minimum rame size onwhich bearing protection must be applied. Within theclause broaching the e ects o magnetic asymmetries assha t voltages/bearing currents cause, it is mentioned thatbearing currents commonly occur in motors above 440

kW. For other causes, no mention is made concerningrame sizes. According to the document, the solutionadopted to avoid bearing currents depends on whichcurrent component is to be avoided. It may be made eitherby means o insulated bearings or sha t grounding systemthough.

g CSA 22.2 N100 Item 12 sha t earthing brushes must beused in motors o rame above IEC 280 (NEMA 440).

g Gambica/REMA Technical Guide or motors o ramesbelow IEC 280 the e ects o bearing currents are seldomappreciable and there ore no extra protection is needed. Insuch cases, adhering strictly to the motor and drivemanu acturers recommendations regarding theinstallation, cabling and grounding is enough. For ramesabove IEC 280, the e ects o bearing currents may besignifcant and or security special protection is advisable. This may be obtained by means o insulated NDE bearingand sha t grounding system use. In such case, care mustbe taken not to bypass the bearing insulation.* For Inverter Duty li ne motors, the earthing system is standard.

6.13 Criteria regarding the noise emitted by WEGmotors on VSD applications

Results o laboratory tests (4 point measurementsaccomplished in semi-anechoic acoustic chamber with theinverter out o the room) realized with several motors andinverters using di erent switching requencies have shownthat the three phase induction WEG motors, when ed by

requency inverters and operating at base speed (typically 50or 60 Hz), present and increment on the sound pressure levelo 11 dB(A) at most.


24/36

www.weg.net


6.14 Normative considerations about the noise oinverter ed motorsg NEMA MG1 Part 30 the sound level is dependent upon

the construction o the motor, the number o poles, thepulse pattern and pulse requency, and the undamental

requency and resulting speed o the motor. The responserequencies o the driven equipment should also beconsidered. Sound levels produced thus will be higher thanpublished values when operated above rated speed. Atcertain requencies mechanical resonance or magneticnoise may cause a signifcant increase in sound levels,while a change in requency and/or voltage may reduce thesound level. Experience has shown that (...) an increase o up to 5 to 15 dB(A) can occur at rated requency in thecase when motors are used with PWM controls. For other

requencies the noise levels may be higher.g IEC 60034-17 due to harmonics the excitation

mechanism or magnetic noise becomes more complexthan or operation on a sinusoidal supply. (...) In particular,resonance may occur at some points in the speed range.(...) According to experience the increase at constant ux islikely to be in the range 1 to 15 dB(A).

g IEC 60034-25 the inverter and its unction creates threevariables which directly a ect emitted noise: changes inrotational speed, which in uence bearings and lubrication,ventilation and any other eatures that are a ected bytemperature changes; motor power supply requency andharmonic content which have a large e ect on themagnetic noise excited in the stator core and, to a lesserextent, on the bearing noise; and torsional oscillations dueto the interaction o waves o di erent requencies o themagnetic feld in the motor airgap. (...) The increment o noise o motors supplied rom PWM controlled inverterscompared with the same motor supplied rom a sinusoidalsupply is relatively small (a ew dB(A) only) when theswitching requency is above about 3 kHz. For lowerswitching requencies, the noise increase may betremendous (up to 15 dB(A) by experience). In somecircumstances, it may be necessary to create skip bandsin the operating speed range in order to avoid specifcresonance conditions due to the undamental requency.

6.15 In uence o the inverter on the mechanicalvibration o the motorInteractions between currents and ux harmonics may resultin stray orces actuating over the motor causing m

Documents

WEG Induction Motors Fed by Pwm Frequency Converters Technical Guide 028 Technical Article English