ADJUSTABLE-SPEED DRIVESjesman/BigSeti/ftp/Actu... · an adjustable-speed drive, but per-forms only one function — slowly in-creasing the voltage applied to an ac motor. In essence,

1998 PT Design A39

A djustable-speeddrives adjust thespeed of a driven

shaft to a speed selected byan operator or by an auto-matic speed selection device.These automatic devices in-clude speed reference sig-nals generated by otherdrives, process controllers,programmable controllers,or other devices.

In addition to changing the speed ofa machine, adjustable-speed drivesare also used to maintain the speed ofa machine, regardless of load, to veryclose tolerances: 6 0.1%, is not un-common. This is more accurate thanthe speed regulation offered by a con-ventional ac motor, which can vary3%, from no load to full load.

There are many applications thatrequire adjustable-speed capability.Just a few include:

• Large paper machines and print-ing presses must accelerate smoothlyfrom standstill to running speed with-out breaking the webs.

• Drill presses often use a widerange of speeds for a single job: lowspeeds for drilling large holes andhigh speeds for small holes.

• Pump and fan efficiencies greatlyincrease by adjusting their speedsrather than regulating flow by throt-tling.

• Metering pumps require differ-ent speeds to produce different com-pounds.

• Speeds and positions of robotsmust be precise and change with eachphase of a task.

SOFT STARTOne type of unit closely resembles

an adjustable-speed drive, but per-forms only one function — slowly in-creasing the voltage applied to an acmotor. In essence, it is a solid-statereduced-voltage starter — oftencalled “soft start,” because it smoothlyaccelerates the motor to runningspeed. This soft acceleration preventsbreaking belts, damaging conveyors,

torque. How close this setspeed remains constant de-pends on the type of driveand its capabilities, and on“other variables.” (See“Speed regulation.”)

Armature control — shortterm for “armature voltagecontrol” of a dc motor, thisdescribes the usual methodof changing the speed of a dcmotor — controlling the ar-

mature voltage. When this method isused, most dc motors are capable ofdelivering constant torque.

Base speed — used primarily inspecifying dc shunt-wound motors.Base speed is the motor speed whiledelivering rated torque with field cur-rent and armature voltage at ratedvalues.

Brushless dc, brushless ac, andbrushless PM servo motor — generallyrefer to motors with permanent mag-net rotors and wound stators that arewound similar to 3-phase inductionmotors. Often abbreviated BLDC,BDC, and BLAC, this design is alsosimilar to an ac permanent-magnetsynchronous motor. (See “Positioningdrives.”)

Constant horsepower operation —see “Field control” and Figures 1 and 2.

Drift — speed change at constantload caused by many factors, most of

etc. It also reduces high peak electri-cal currents usually encounteredwhile starting an ac motor across theline. These electrical units, covered inthe Controls and Sensors departmentin this handbook, under “Motor Con-trollers,” are similar in appearanceand use much of the same technologyas electrical adjustable-speed drives.

There are other methods for softlystarting loads, such as fluid couplingsbetween the motor and the drivenload. These are discussed in the Cou-plings and U-Joints department.

COMMON TERMSMost of the following terms apply to

all types of adjustable-speed drives.Additional information is given forsome terms as indicated.

Adjustable-frequency drive (AFD)— common term for ac, adjustable-frequency drive,inverter, and acdrive. (See “ACdrives.”)

Adjustablespeed — wherespeed is adjustedmanually or auto-matically. The setspeed (desired op-erating speed) isrelatively constantregardless of load,as opposed to“variable speed.”

Adjustable-speeddrive — a unit thatadjusts the speed of a shaft to a setspeed that remains relatively constantregardless of changes in required load

ADJUSTABLE-SPEED DRIVESSOFT START .........................................................A39COMMON TERMS .................................................A39 APPLICATION CONSIDERATIONS.........................A41 TYPES OF DRIVES ................................................A41 ELECTRICAL ADJUSTABLE-SPEED

DRIVES ..............................................................A41MECHANICAL ADJUSTABLE-SPEED

DRIVES ..............................................................A52ADJUSTABLE-SPEED DRIVE ADVERTISING .........A54

Figure 1— Output power of dc motor witharmature-voltage and field-currentcontrol.

which are undefined.Dynamic braking — method of

stopping a dc motor faster than coast-ing to rest. This method requires a re-sistor connected across the motor ar-mature. During stopping the DBresistor dissipates the rotating en-ergy, Figure 3. Dynamic braking doesnot offer controlled stopping as doesregeneration, discussed later.

Electrical adjustable-speed drive —includes motor and drive controller.However, in some cases, “drive” isused to denote only the controller thatconverts plant ac power to adjustable-voltage dc or to adjustable-frequency,adjustable-voltage ac.

Electrically commutated motor(ECM) — same as brushless dc(BLDC). (Also see later discussion,“Brushless dc drives.”)

load overhauls the motor, which putspower back to the drive controller.Most mechanical adjustable-speeddrives and motor-generator types ofelectrical adjustable-speed drives in-herently offer regenerative capability.Therefore, replacement drives mustbe carefully selected so they have re-generative capability if the applica-tion requires it.

Regenerative drive — in SCR-typedc drives, regenerated power is usu-ally put back to the ac power sourcethrough a second set of SCRs, Figure4. In transistorized dc drives andmost AFDs, this power is dissipatedby a resistor across the dc bus that isbetween the input rectifier and thepower amplifier, Figure 5.

Designed to control current flow in

Field control — ashortened term, thismeans adjusting thespeed of a dc motorby controlling themotor field current.

When the field current is reduced, themotor tends to increase speed and de-crease in torque capability. Thus the

motor power outputis essentially con-stant, Figure 1.

Near-constant-horsepower capabil-ity can also be ob-tained from ac drivesby operating the mo-tor at rated voltageand above rated fre-quency. This also in-creases motor speedand decreases motor

torque capability, Figure 2.Inverter — usually refers to an elec-

trical adjustable-speed drive thatuses an ac motor. Actually, this termapplies only to the inverter sectionthat inverts dc to controlled-fre-quency ac in an ac adjustable-fre-quency drive controller.

Other variables— changes in linevoltage, line fre-quency, ambienttemperature, hu-midity, transientson the ac powerline, and drift,which is caused byundefined factors.

Preset speed —one or more speedsat which the drive should operate. Relay contacts or programmable-con-troller outputs select which presetspeed is operational.

Pulse-width modulation (PWM) —inverter technique for chopping (mod-ulating) a dc voltage usually at 2,000to 20,000 Hz to produce adjustable-frequency, adjustable-voltage ac, oradjustable-voltage dc.

Regeneration — occurs when the

A40 1997 Power Transmission Design

Figure 2 — Typical output power of an acmotor operating above and below basespeed.

Figure 3 — DC motor with conventionaldynamic-braking (DB) circuit. Whencontroller powers motor, the normallyopen (NO) M contact is closed. Uponstopping, this NO contact opens, and thenormally closed (NC) contact closes,connecting the DB resistor across themotor armature. The DB resistor thendissipates the rotating mechanical energyas heat.

Figure 4 — Regenerative power modulefor SCR-type dc drive with single-phaseinput. (Field supply not shown.) Theforward section controls power duringmotoring, and the reverse section controlspower while it is regenerating to the acpower source. Power module for three-phase power is similar, except eachsection has six SCRs.

Figure 5 — Basic diagram of the powersection of transistorized adjustable-speeddrive, including PWM ac and BLDC. PWMdc has the same configuration, but withonly four power transistors in the output(power amplifier) section. Units thatoperate from single-phase power havefour instead of six rectifiers in the inputrectifier section. Although the power-section configurations are similar for thevarious drive types, the regulator andfiring circuits are completely different.

1997 Power Transmission Design A41

both directions, these drives are usedfor controlled stopping (regenerativebraking) of high-inertia loads. Thisdesign also offers contactorless re-versing, which is beneficial for high-cyclic-duty applications.

Set speed — the desired operatingspeed. When a regulation value isquoted for set speed, the same per-centage applies at all speeds. Suchregulation is generally available onlywith drives equipped with complex ordigital control systems.

Slip — the difference between theelectrical and mechanical speeds of amotor. For example, slip is the differ-ence between synchronous field speed(typically 1800 rpm) and rotor speed(often 1750 rpm) of an induction mo-tor. This gives a 50 rpm (3%) slip. Slipis always manifested as a power loss,which appears as heat.

Speed regulation — the percentageof change in speed between full loadand no load.

Where: R = Speed regulation, %NNL = No load speed, rpmNFL = Full load speed, rpm

Unless otherwise stated, regulationis assumed to be plus-or-minus a per-centage of base speed, not of set speed.

Variable speed — often consideredsynonymous with adjustable speed,but variable speed is where speedvaries widely with load. Speed is deter-mined by the intersection of the speed-torque curves of load and drive. Exam-ples include uncontrolled units such asseries-wound motors, Wound-rotor acmotors, eddy-current clutches withouttachometer feedback, and fluid drives.Any of these may be equipped with au-tomatic speed controls, and then oper-ate as controlled-speed drives.

Vector control — controller for stan-dard ac induction motors that deliv-ers response equivalent to dc servos.(See “Positioning drives.”)

Voltage boost — adjustment to en-able an adjustable-frequency drive togive a larger-than-normal voltage atlow frequencies (normally 0 to 10 Hz)so the motor can start with extrashort-term torque for breakaway andinitial acceleration.

V/Hz (Volts per Hertz) — commonadjustment in adjustable-frequency

flected load inertia (see Basic Engi-neering department in this handbook,Formulas 9-13) is several times themotor armature inertia, or if the dutycycle involves several starts and stopsa minute, a regenerative drive may berequired.

• Communication requirements.Will it be necessary, now or in the fu-ture, to send instructions to the driveor receive information from the unit?Will network communications be re-quired? These capabilities are oftenneeded in coordinated systems con-nected to a master or host computer.

• Diagnostic capabilities. Shouldthe drive or system malfunction, willa remote computer interrogate thedrive to determine the cause of thetrouble and status of the system dur-ing the last moments of operation?Such capabilities require a communi-cation channel.

TYPES OF DRIVESThere are three basic types of ad-

justable-speed drives, and each hasits strengths and weaknesses:

Electrical drives — ac adjustablefrequency, dc adjustable voltage, slip-ring (wound-rotor) motor, eddy-cur-rent, and special purpose types, in-cluding servo and step.

Fluid drives — hydrostatic, hy-drokinetic, and hydroviscous drivesare discussed in the Fluid-PowerDrives department of this handbook.

Mechanical drives — metallic-con-tact traction type and adjustable-sheave type using belts or chains.

ELECTRICAL ADJUSTABLE SPEED

Both ac and dc drives receive ac plantpower and convert it to an adjustableoutput for controlling motor operation.DC drives power dc motors, and acdrives control ac motors. Both usuallyuse solid-state conversion systems.

A dc drive is the simpler of the twoand is frequently used as part of an acdrive controller. Therefore, we willdiscuss the dc design first.

DC driveUsually, a solid-state power unit

converts incoming ac plant power toadjustable-voltage dc. This voltage isfed to the dc motor armature. The dc

drives that establishes the voltage re-lationship to the frequency. For exam-ple, the controller for a 240-V, 60-Hzmotor normally has a V/Hz adjust-ment set at 4-V per Hertz.

APPLICATIONCONSIDERATIONS

Since there are many types of ad-justable-speed drives available, it isessential to establish the requiredfunctions and the application con-straints. The following lists many ofthe major factors that should be con-sidered:

• Maximum horsepower require-ments of the driven load, and how therequired power varies with speed.

• Torque. Constant or variablewith speed?

• Minimum and maximum speeds.• Unidirectional or bidirectional?• Acceleration. Fast, slow, or ad-

justable? Also, are any special accel-eration or deceleration characteristicsrequired, such as constant jerk sothere is no change in accelerationrate, or an S-curve? These are typi-cally required for controlling convey-ors carrying tall bottles.

• Drive response to changes inspeed and direction commands.

• Programmability needed. Will itbe necessary to change frequently theoperating parameters, thereby re-quiring a microprocessor-based drive?

• Minimum and maximum speedadjustments. Is it necessary to limitthe minimum and maximum speed ca-pabilities if the drive has a wider speedrange than required by the load?

• Speed regulation due to loadchanges and other variables, such astemperature, humidity, line-voltagefluctuation, and drift.

• Maintenance capabilities. Whattypes of equipment can the mainte-nance personnel service — mechani-cal, simple electrical, or complex elec-trical? What are service capabilities ofsuppliers?

• Control method. Will manualcontrol be used or will a motion or sys-tem controller give commands to thedrive controller?

• Environments. Will all or part ofthe drive be subjected to hazardous,abrasive, moisture, or other harsh en-vironments?

• Duty cycle, including number ofstarts and stops per hour. If the re-

RN N

NNL FL

FL

= − ×100

motor speed varies proportionally tothe dc armature voltage assuming:

1. The motor is large enough topower the connected load.

2. The motor field current is constant.

DC motors, discussed in detail in theMotor department, contain two majorcomponents — armature and field. In-teracting magnetic fields from thesetwo components turn the armature.

Two basic methods are used forconverting ac to controlled dc — SCRs(thyristors) and transistors.

SCR drives — silicon controlledrectifiers (SCRs) in the power unitconvert ac to controlled voltage dc.The SCR conducts current when asmall voltage is applied to the SCRgate, Figure 6.

Most SCR-type dc drives designed tooperate from a single-phase input havefour SCRs. Units operating from three-phase ac are frequently built with sixSCRs, Figure 7. One variation of thesedesigns includes replacing the SCRson the bottom row with diodes (recti-

Transistor dc drives — Largelybecause of the capabilities offered bymicroprocessors and the availabilityof power transistors, transistor dcdrives are now rated from fractionalto over 100 hp.

Unlike SCR-type drives, transistor-ized units contain three major powersections, Figure 5. The first convertsac plant power to fixed-voltage dc forthe dc bus. The second section filtersand regulates the dc bus voltage. Ifthe dc bus voltage goes outside theproper values, this section signals afault and shuts the drive down. Thethird section contains the power tran-sistors and chops (modulates) thefixed voltage dc to form adjustable-voltage dc for powering and control-ling the dc motor.

If the load overhauls the dc motor,it regenerates by sending power backto the power amplifier section. This,in turn, sends power to the dc buswhere the dc bus control dissipatesthe power in a resistor connectedacross the bus. If the resistor does nothave the capacity to dissipate thepower, the bus control section willsense a bus over-voltage and shut thedrive down.

DC drive regulator—Instruc-tions from an operator’s control sta-tion, or other input, feed into the reg-ulator, Figure 9. The regulatorcompares the instructions with thevoltage and current feedbacks andsends the appropriate signal to the

fiers) and adding aback or commutat-ing diode across thedc output.

To physicallydisconnect the mo-tor from the solid-state power mod-ule, a motorcontactor (M) isconnected betweenthe power moduleand the motor, Fig-ure 3. When thestart button ispushed, the M con-tactor picks up,closing the nor-mally open (NO)contact; then the SCRs are turned on

in sequence, sup-plying power to themotor.

Should the appli-cation require themotor to stop fasterthan coasting torest, a normallyclosed (NC) contacton the M contactoris used to connect adynamic brakingresistor across themotor armature,Figure 3. Thus, therotating mechani-cal energy in themotor armature is

dissipated as heat in this DB resistor.Although this is effective for occasionalstopping, each stop requires operatingthe M contactor, which eventuallywears out contacts. Plus, the motorslow-down rate is not controlled, as itis with regenerative braking.

Regenerative SCR dc drives —By adding another set of SCRs (labeled“Reverse section”) that are connectedin the reverse polarity, Figure 4, the

drive has regenera-tive capabilities,and can operate inall four quadrants,Figure 8.

This configura-tion offers bidirec-tional operationwithout using re-versing contactorsand controlled re-

generative stopping. (See “Regenera-tion with ac/dc drives” later in thissection).


Figure 6 — SCR conducts current from thetime of the gate pulse to the end of thathalf cycle.

Figure 7 — Typical full-wave SCR bridgefor dc drive operating from three-phase acpower.

Figure 8 — Four quadrants of motoroperation. Most systems operate in thefirst quadrant where both torque and speedare positive. During dynamic braking andregeneration, speed is positive and torqueis negative — the second quadrant.


firing circuit. This section suppliesthe gate pulses that fire the transis-tors or SCRs, causing them to con-duct. In some designs, the regulatorand firing circuit are combined in onedigital circuit.

The voltage feedback gives an indi-cation of the motor speed, and currentindicates relative motor torque.

Typical basic adjustments in theregulator include minimum speed,maximum speed, current (torque)limit, IR (load) compensation, and ac-celeration rate adjustment.

For more precise speed control, anencoder or a tachometer-generator —usually called a tach — is mounted onthe motor to give a feedback signal,Figure 10, that is proportional to ac-tual motor speed. The quality of thistach and of the regulator determinethe total drive accuracy.

Applications — DC drives with-out tach feedback frequently offer a20:1 speed range. If tach feedback isused, the speed range is usually in-creased to at least 100:1.

If the motor has sufficient coolingcapacity, the drive can usually deliverconstant torque over the full speedrange. TENV (totally enclosed, non-ventilated) and some TEFC (totallyenclosed, fan cooled) motors rated toabout 5 hp often have sufficient heatdissipating capacity to deliver ratedtorque during low-speed operation.

On larger mo-tors, a common practice is to use aseparately powered blower to force airthrough the motor so the motor isproperly cooled,even when it isrunning at lowspeeds and deliver-ing full torque. Thealternative is tospecify a largermotor that can dis-sipate the heat.

If a motor fieldregulator is addedto control the motorfield current, themotor can deliverconstant horse-power above motorbase speed, Figure1. Some motors canrun at four to sixtimes the motorbase speed. Thesedrives have both ar-mature voltage andfield control. Theyare applied on ma-chine tool spindles,center winders, andsimilar applicationswhere the torquerequirements de-crease with an in-crease in speed.

DC drives canregulate motorspeed (the most fre-quent type of appli-cation), motor cur-rent, which isproportional to mo-tor torque, and po-sition. Drives withcurrent regulatorsare used for un-

Figure 9 — DC drive with voltage (forapproximate speed) and current (fortorque) feedbacks. In most drives, theregulator and firing circuit are in the samephysical unit. These can be analog,digital, or a combination of both. Formore precise speed feedback, atachometer-generator (tach) or encodermust be driven by the motor shaft, Figure 10A.

Figure 10A — DC drive with speedfeedback in place of voltage feedback.The motor-driven tach or encoder givesmore accurate signal than estimatingspeed based on applied voltage.

Figure 10B — Follower drive, which is often confused with speedregulated drive (Figure 10A). Above voltage-regulated drivefollows a speed reference signal, usually produced by a tach onanother motor or by a digital command from the master drive’sspeed feedback.

Figure 10C — Combination of a follower drive (Figure 10B) and aspeed-regulated drive (Figure 10A). The above configurationaccurately maintains speed established by the reference signalproduced by a source outside the drive.


Silicon controlled rectifiers (SCR)were introduced in the 1950s. Gateturn-off devices (GTO) became avail-able for widespread usage in the late1970s, then power (giant) transistors(GTR) in the early 1980s and, most re-cently, insulated gate bipolar transis-tors (IGBT).

Under normal operating condi-tions, they function as one-wayswitches; current flows in one direc-tion, from the higher voltage potentialto a lower voltage potential. Connec-tions include terminals for inputpower, output power and a controlsignal to turn the device on and off.

Silicon controlled rectifier(SCR). In a typical dc drive, SCRscontrol dc motor speed and torque byconverting ac line voltage to ad-justable dc voltage. The controller de-

termines when in the ac half cycle toturn the SCR on so it conducts to theend of that half cycle. The earlier inthe half cycle the SCR turns on, thehigher the average output voltage isto the motor. To turn the SCR on, thecontroller applies a current to theSCR gate, Figure A.

SCRs can handle current from5,000 to 10,000 A. The time it takes anSCR to turn fully on after receiving agate pulse is in the order of 10s of µsec.

In the early 1960s, SCRs were usedto control the frequency of ac drives.Now, GTOs, GTRs and IGBTs havereplaced SCRs, particularly in low-voltage ac drives, because of theirgreater efficiency and controlability.

For dc drives, SCRs are the leastexpensive power semiconductor forapplications below a power range of

200 to 500 hp. In the larger powerranges, GTOs become practical. Forac drives, SCRs are the least expen-sive power semiconductors, but thedrives require many capacitors tocommutate the SCRs off to produce anac output.

Gate turn off device (GTO). AGTO, like an SCR, can handle highcurrent levels — in the thousands ofamperes. A GTO, Figure B, is similarto an SCR except that its control con-nection can turn the device on and off.Turn on occurs when a positive volt-age is applied to the gate. Applying anegative voltage to the gate turns theGTO off and interrupts current flowin the main circuit. Once turned on, aGTO will stay on and does not requiresustaining pulses. To turn a GTO off,the negative or turn-off control signalmust be greater than the currentflowing through the GTO. Typically,turn-off time of a GTO is about 50

µsec. GTOs require fast-acting, current-limitingfuses to protect againstovercurrent situations.

Efficiency of a GTO isrelatively low due to thepower source required tosupply the negative turn-off bias. Because GTOsturn on relatively slowly,high switching losses re-duce efficiency.

Giant transistor(GTR). In a GTR, thecontrol or base signalmust be maintained tokeep the device turnedon. How much control sig-nal is needed depends onhow the GTR is operated.For large-gain, low-lossoperation, the control sig-nal must be large. Forlarge-gain, large-loss op-eration, the control signalcan be small.

Current-carrying capa-bility is in the range of1,000 to 2,000 A. Switch-ing speed is about 5 µsec.Because GTRs are low-gain devices (as are alltransistors), they are typ-ically mounted with onedevice feeding another ina Darlington configura-tion, Figure C.

Power semiconductors in a-s drives

Relative power-semiconductor characteristics

Type Speed Gain Efficiency Control method Max. rating, A

SCR Slow Medium Low Current 10,000GTO Medium Medium Low Current 10,000GTR Fast Low Medium Current 2,000IGBT Fastest High High Voltage 2,000

Figure A — Silicon controlled rectifier (SCR), also known as athyristor. It conducts current during a positive half cycle fromthe time sufficient voltage is applied to the gate to the end ofthat half cycle.

Figure B — Gate turn off thyristor (GTO). Similar to an SCR,except a GTO can be turned on and off by the voltage to thegate. Positive voltage turns the GTO on and a negative voltageturns it off.

Figure C — Giant transistor (GTR) in a Darlington configuration so the transistor amplifies the signal to the base of the GTR.

Figure D — Insulated-gate bipolar transistor (IGBT) is the mostcommon power device used in current general-purpose andservo drive controllers. It can operate with carrier (also calledchopping and modulation) frequencies to more than 20 kHz.

Base

Gate


encoder. The sine wave offers reducedtorque variations (ripple) during eachshaft rotation. For example — accord-ing to some manufacturers — abrush-type dc motor may have 2% to6% torque ripple; PMDC drives withsinusoidal voltage waveforms mayhave 3% ripple; and the trapezoidalvoltage can run 4% to 15%.

Another factor that determinestorque ripple is the motor windingconfiguration. Some manufacturersoffer distributed windings that reducethe torque ripple produced by a trape-zoidal voltage.

However, methods of measure-ment, load inertia, and many othervariables determine this ripple foreach application.

AC drivesAC adjustable-speed drives are

growing in popularity, mainly be-cause ac motors are simpler than dcmotors. Moreover, recent advances ininverter technology have reduced con-troller cost and improved perfor-mance and reliability.

AC drives operate by adjusting thefrequency and voltage to ac motors.Frequency determines motor speed.To maintain constant torque, it is nec-essary to keep voltage and frequencyin a constant relationship. CalledVolts per Hertz, this is adjustable onmost ac drive controllers. There is oneexception — voltage at low frequen-cies. In this operating range, voltagemay be boosted to give the motor ex-tra torque for breakaway and initialacceleration.

General-purpose, ac adjustable-fre-quency drive controllers are made infour types: variable-voltage input(VVI), pulse-width modulated (PWM),current-source input (CSI), and load-commutated inverters (LCI). In addi-tion, wound-rotor motors and eddy-current couplings offer adjustablespeed with an ac motor connectedacross the line. Each has specificcharacteristics and advantages.

Variable-voltage input (VVI) —Although this design was common inthe 1970s and early ‘80s, it is nowgenerally limited to special applica-tions such as high-speed drives thatdeliver 400 to 3,000 Hz. The VVI de-sign, Figure 5, receives plant acpower, rectifies and controls it, anddelivers variable-voltage dc to thepower amplifier (inverter section).

winders, rewinders, and other webhandling functions.

Drives with position regulators areinstalled on machine tools, robots,winders and unwinders with storageloops, electric line shafts, and otherapplications where precise componentposition must be controlled.

Brushless dc driveConsidered both an ac drive and a

dc drive — depending on who is speak-ing — this family of drives is fre-quently called brushless dc (BLDC),brushless ac, and brushless PM servo.To distinguish this type from a drivewith a conventional ac motor that isusually also brushless, we use “brush-less dc” to denote this drive type. Itcan be applied to both general-purposeand precision motion control (position-regulated) installations.

The BLDC motor is constructedmuch like a permanent magnet acsynchronous motor with a PM rotorand a wound stator. This BLDC de-sign is often termed an “inside-out”construction, because most dc motorswith permanent magnets have themagnets stationary in the stator, andthe wound armature rotates. How-ever, the BLDC construction puts theheat-generating windings on the out-side for fast heat dissipation.

Most BLDC drives need an accu-rate method of sensing rotor speed.An encoder, or Hall-effect transistor,usually does this. For more preciseapplications, some use a resolver anda resolver-to-digital (R/D) converter.The improvement in precision de-pends on the components selected.

The power sections of a BLDC drivecontroller closely resemble a standardPWM ac drive, Figure 5. However, thecontroller portions (regulator and fir-ing circuits) are completely different.

The industry is constantly improv-ing the ability to control the shape ofthe current wave form. Generally, themore closely the current approaches asine wave, the better the motor performance.

Originally, the drive controllersproduced a square-wave voltage to theBLDC motor. With technological ad-vances, the drive manufacturers de-veloped controllers that commonlyproduce a trapezoidal voltage wave-form, and some offer a sine wave. Of-ten the sine-wave units require a re-solver, rather than the more common

Today, GTRs are gradually being re-placed by IGBTs. The faster switchingspeeds of an IGBT produce a so-called“quiet ac motor controller.” Commer-cial pressure for smaller controllerpackages and a simpler control-to-power interface is forcing the early re-tirement of GTRs for ac motor control.

Insulated-gate bipolar transis-tors (IGBT). IGBTs, Figure D, differfrom SCRs, GTOs and GTRs by thecontrol scheme. To turn on an IGBT, avoltage is applied to its gate. TheMOSFET-type input converts thevoltage to the current required to turnon the output portion (transistor) ofthe IGBT.

Thus, the interface from control topower is simplified because it re-quires little control current to handlelarge amounts of power current.Small control current also avoids thetime delays associated with large val-ues of control current. A fast controlchange produces a fast power change.Current-carrying capability of anIGBT is 1,000 to 2,000 A.

An IGBT’s fast response to signalchanges — less than 1 µsec — reducesaudible levels in an ac motor whilecontrolling torque and speed. And, itshigh switching frequency (carrier fre-quency) provides a responsive currentcontrol. Also, an IGBT’s low losses re-sult in compact packaging of the acmotor controller.

Since the introduction of IGBTs,concerns have arisen about the effectsof high-frequency switching and har-monic-rich waveforms during trans-mission of power to an ac motor. Insome cases, transient voltages candamage motor winding insulation,and cause motor failure. Two factorsin particular contribute to this phe-nomenon:

• 460-V ac drives that use thePWM power circuit and use modula-tion (chopping) frequencies of morethan 5 kHz.

• Long cable distances (over 100 ft) be-tween the drive controller and the motor.

These reflected waves, along withRFI and grounding issues, are currenttechnical hot buttons. See the “Reduc-ing motor problems caused by voltagetransients” section in Motors for somesolutions to these potential problems.

Excerpted from an article by Allen-Bradley Co., Inc. in the February,1996 issue of PTD.

The power amplifier inverts the vari-able-voltage dc to variable-frequency,variable-voltage ac. It can be builtwith power transistors or SCRs.

The output voltage from a VVI unit isfrequently called a six-step wave form.

The VVI was one of the f irstsolid-state ac drives to gain generalacceptance.

Pulse-width modulated (PWM)— Many PWM units (also frequentlytermed “volts per Hertz drives” to dis-tinguish them from vector drives) of-fer operation to zero speed. Some pro-duce frequency ranges near 200:1.This wide range is possible becausethe controller converts ac supplypower to fixed-voltage dc for thepower amplifier. In this amplifier,the dc is “chopped” (modulated) toproduce different widths of pulses,thus varying the effective voltage. Al-though the voltage is chopped, thecurrent wave form is closer to sinu-soidal than any other system.

PWM units use several types ofpower transistors (including insu-lated gate bipolar transistors (IG-BTs), and gate-turn-off SCRs (GTOs).

The PWM power circuit requires amore complex regulator than does theVVI design. However, the increasinguse of microprocessors has nearly elim-inated significant economic differences.

Because a fixed-voltage dc bus sup-plies the inverter section, a large sin-gle dc supply can be used to powerseveral inverter sections.

Types of regulators — The PWMpower circuit is commonly used withthree basic types of regulators. It isthese regulators that largely deter-mine the drive capabilities, includingresponse, speed regulation due totransient load changes, and low-speedtorque capabilities.

Volts/Hertz — The most commonand lowest cost configuration, it isusually available with or without aspeed feedback capability. This de-sign generally offers the basic driveadjustments — including set speed,torque limit, V/Hz, voltage boost atlow speeds, minimum and maximumspeeds, acceleration and decelerationrates, and other similar adjustments,which meet the requirements for mostapplications.

Basic vector — Introduced in themid-80s, this regulator was a signifi-cant advancement over the Volts/Hz de-sign. Such units use an approximationmethod to control the stator and rotor

feedback signal will improve thedrive’s performance.

Current-source inverter (CSI)— Usually applied to drives of 50 hpand larger, CSI units are well suitedfor powering pumps and fans as anenergy-saving alternative to throt-tling for flow control.

Capable of operating with efficien-cies close to dc drives, the CSI designoffers economies over both the VVIand PWM units for pump, fan, andsimilar applications.

The CSI inherently offers regenera-tive capabilities. With an overhaulingload, the controller feeds power backinto the ac power system.

flux angles to optimize motor operation. Some vector drives were expected

to have open-loop speed regulation(no speed feedback signal from themotor) equivalent to a dc drive withspeed feedback. Many units didn’tquite live up to the expectations. Re-gardless, the basic vector does offerimproved performance, which in somecases may be suitable for simple coor-dinated systems.

Vector — More recently, in the mid-90s, much improved vector regulatorswere introduced. These have more ad-vanced microprocessors and DSPsthat significantly enhance drive oper-ation, including response and positionregulation capa-bilities. One rea-son for the vec-tor’s capabilityto perform sowell is its abilityto “see” thecounter EMFproduced by themotor, then thecircuitry adjuststhe start of eachPWM pulsetrain and thespecific durationof each pulse.

Such vectordrives are power-ing high-speedprinting presses,paper machines,winders andother coordi-nated engi-neered systems.Vector drives arealso used onservo positioningsystems, such asmachine toolspindle and feeddrives. Some, de-pending on therating, can accel-erate from stand-still to full speedin 1 to 200 msec.

For more onthese position-ing drives, see afollowing sec-tion, “Position-ing Drives.”

With all typesof these drives, aspeed or position



Load-commutated inverter(LCI) — Designed for controlling syn-chronous motors rated in hundreds orthousands of horsepower, these unitsuse the leading power-factor capabili-ties of synchronous motors to commu-tate the power SCRs off. These invert-ers, some rated up to 6,000 Vac,control large blowers and induced-draft fans in power plants.

Due to the higher cost of syn-chronous motors over conventional in-duction motors, LCI systems becomeeconomical in the 400-600 hp range,and larger.

Wound-rotor motor control —Another system for controlling ac

currents in nearby metal. These cur-rents, in turn, create their own mag-netic fields. Properly used, the eddy-current magnetic fields can interactwith the primary magnetic field.

The eddy-current coupling is builtwith two parts: one is coupled to theinput shaft, and, in many applica-tions, powered by an ac motor; theother is connected to the output shaft.There is no mechanical connection be-tween the two parts; rather, the con-nection is purely magnetic. An elec-tromagnet coil generates the couplingmagnetic force, Figure 12. The mag-netic strength, (coil current) deter-mines the slip between the input andoutput parts. There must be some slipbetween these two parts, usually aminimum of 3 to 5%, depending uponthe design, for the system to work.(Eddy-current clutches and brakesare discussed in the Clutch and Brakedepartment in this handbook.)

In some smaller ratings, the mag-net coil rotates with the output shaft.

motors rated inhundreds of horse-power, a wound-ro-tor motor con-troller appliesconstant voltageand frequency tothe primary of awound-rotor mo-tor. Speed is ad-justed by varyingthe resistance of the motor secondary,Figure 11. As resistance is increased,rotor losses increase, thus reducingthe motor shaft speed.

A solid-state controller effectivelychanges the resistance of the rotor sec-

ondary; and,rather than dis-sipate the en-ergy as heat, itfeeds this en-ergy back to theac supply. Thiss l ip-recoverysystem greatlyincreases motore f f i c i e n c y .W o u n d - r o t o rmotor controlserves manypump and faninsta l la t ionsthat need aspeed range of2:1 or 3:1.

Eddy-cur-rent coupling— A movingmagnetic fieldinduces eddy

Notes: 1. These are typical values for each drive type and

do not reflect state-of-the-art products or special designs.

2. Brush wear becomes critical on high duty-cycle applications where the motor constantly accelerates and decelerates. Although the absence of brush wear is not as important in other applications, many specifiers select brushless units to eliminate brush maintenance completely.

3. Many times, load rotary inertia reflected to the motor shaft can be reduced by installing a speed reduction means (gears or timing belt) between the motor and driven load. (The reflected inertia is reduced by a square of the reduction ratio.) This technique is frequently used when the load inertia significantly exceeds the motor rotor inertia.

Figure 11 — Wound-rotor ac motor andcontroller. Increasing the resistance inthe motor secondary decreases motorspeed.

Slip rings carry the current from thesupply wires to the rotating coil. Inlarger units, the coil is stationary andinduces magnetic fields in both the in-put and output members.

In operation, the operator selectsthe desired speed, which establishesthe speed reference signal. The ex-citer control converts this signal tothe proper dc voltage for the coil, Fig-ure 13. The tachometer-generatorsends an output speed signal back tothe exciter, which makes any neededchange in the coil voltage to maintainthe set speed.

Since any slip generates heat, eachcoupling must be designed for ade-quate cooling. Usually, the smallercouplings rated from fractional toover 100 hp are air cooled, and largerunits that can control thousands ofhorsepower are water-cooled.

Selection factors include load(speed-torque) characteristics, rat-ings, efficiency when slip is consid-ered, and cooling method.

protection, selectable modulating fre-quency from 1.25 to 18 kHz, stall pro-tection, and many others.

Within the module, a mathematicalmodel of the motor accurately calcu-lates motor temperature and im-proves motor operational stability, es-pecially for larger motors.

A crucial design requirement is toeliminate the heat generated by themotor and the control electronics.Some solutions use a multifinned as-sembly that mounts on the motor sothe air forced by the external motorfan goes through the module coolingfins as well as over the motor shell.

Regeneration with ac/dc drivesLike dc drives, ac drives can regen-

erate when a load pulls the motorfaster than its synchronous speed. Be-cause the synchronous speed of an acmotor is proportional to the appliedfrequency, a four-pole motor operat-

ing on 60-Hz power could encounterregeneration at speeds over 1,800rpm, its synchronous speed. At 30Hz, regeneration could occur at aspeed over 900 rpm.

A typical example of a regenera-tive application is a fully loaded ele-vator. The motor is loaded as it liftsthe elevator car and its contents.When traveling downward, the mo-tor “holds back” on its descent (nega-tive torque) and must have someplace to deposit this regenerated en-ergy. Other examples of regenerativeapplications include winders and un-winders, machine tools, dynamome-ters, centrifuges, hoists and cranes,downhill conveyors, press feeders,

and rolling mill run-out tables.

Motor-mounted drivecontrollers

Although motors with built-on con-trollers have been made, they are gen-erally limited to dc subfractional andfractional-horsepower motors and ap-pliance motors. For integral ratings,the controller has been too big. But,that is changing.

Technological breakthroughs en-able integral horsepower ac motors toinclude sophisticated controls. Sev-eral companies have developed an in-tegrated module that contains thecontrol and power circuits for an acadjustable-speed drive, Figure 14.Moreover, these modules are smallenough to fit on ac motors rated 1-hpand larger. Benefits of these drives in-clude:

• Save space by eliminating theneed for a separate controller (whichis often as big as the motor).

• Reduce installed costs becausepower wiring between the motor andcontroller is eliminated.

• Eliminate motor problemscaused by high-voltage transients.

• Wipe out the finger pointing be-tween the motor and controller manu-facturers when a problem comes up.

• Enable using adjustable-speeddrives in applications that previouslycould not justify the expenses of a sep-arate drive controller.

The controller modules includemost functions: Start, stop, forward,reverse, speed control, preset speeds,adjustable acceleration and decelera-tion rates and curves (S-curve orother), readouts of speed and motorcurrent in rpm and amperes or in en-gineering units, motor and controller


Figure 12 — Typical eddy-currentcoupling.

Figure 13 — Control scheme for eddy-current coupling.

Figure 14 — More adjustable-speedcontrollers are mounted in a housing thatis often attached to the top of the motor.


Many seemingly innocuous appli-cations can turn regenerative duringcertain parts of their operation. Everyload should be examined to determinewhat an overhauling load can be dur-ing any part of the operating cycle.

To dissipate excess power, userscan:

• De-tune high-inertia loads bylengthening deceleration time.

• Add a resistive snubber acrossthe dc bus to handle short-term, inter-mittent regenerative loads. In theseunits, sensors measure the dc busvoltage. When it exceeds specific lim-its, a power circuit connects a resistoracross the dc bus to dissipate the ex-cess energy.

• Use a line-regenerative AFC oradd-on regenerative module for moresevere regenerative situations. AFCsequipped with line regeneration han-dle intermittent and continuous over-hauling loads.

Three recent technological ad-vances have brought the regenerativeAFC into vogue: better power semi-conductors, faster microprocessorcontrol, and PWM vector AFCs.

Synchronous rectifier drivescontain two complete IGBT bridges,both PWM controlled. The input acpower currents are nearly sinusoidaland devoid of the 5th and 7th harmon-ics produced by AFCs that use diodebridge rectifiers.

In the motoring mode, the IGBTrectifier section works with a reso-nant-tuned input line reactor and dcbus capacitor to create dc bus voltage.A complex switching pattern enablesa higher dc bus voltage than that cre-ated by a conventional diode bridgerectifier. This ability to regulate dcvoltage can be beneficial duringbrown-out conditions.

During regeneration, the IGBTbridge feeds pulses of the excess dcbus voltage into the ac power line,minimizing harmonic distortion andmaximizing input power factor.

Benefits and features include:• Bidirectional power flow to and

from the ac power line.• Controllable input power factor.• Lowest ac line harmonics.• Ability to create higher motor out-

put voltages than the input voltage.• Adaptable to common dc bus

designs.• Power-dip ride-through when

line voltage sags.Bi-directional transistor recti-

• Power dip ride-through usingphase-back, phase-forward control.

Common dc bus configuration.To conserve energy and reduce the re-quired capacity of the input ac-to-dcrectifier, multiple AFCs can share acommon dc bus. This enables regener-ating drives to supply energy to mo-toring drives.

If the net power is always awayfrom the ac line (more drives motoringthan regenerating), a regenerativeconverter isn’t needed. In this case,the regenerated power is immediatelyre-used by the motoring drives.

One major advantage with multipleAFCs on common dc bus is that youcan use a single regenerative module,sized for worst case energy return.Thus, the regenerative module musthave the capacity to handle only thesurplus energy that is unused by themotoring AFCs.

The engineer designing to a commondc bus must coordinate individual AFCprotection. Most systems use manycombinations of fusing, circuit break-ers, and reactors. Plus, electrolytic ca-pacitors in the dc bus store tremen-dous amounts of energy. Therefore, itis essential to prevent an isolated AFCfailure from causing a catastrophicfault to the entire system.

Excerpted from an article by Re-liance Electric in the June 1995 issueof PTD.

Positioning drivesAs automation increases, the need

for special-purpose electrical drivesconstantly expands. Many of theseapplications require not only control-ling speed and direction of shaft rota-tion, but also controlling position of apart. This part can be an item beingmachined or sorted, or it can be a partof a machine that is performing thework. Examples include cutters ofcontour milling machines and thearm of a robot.

Two of the most common types ofthese special positioning drives in-clude servo and step.

Servo — A servo positioning drivereceives signals from a controller —computer numerical control (CNC),programmable logic controller (PLC),or computer — that tells the drive thedirection, speed, and time to move apart from Location A to Location B.

Plus, a sensor continuously deter-mines the exact position of the part,

fier. Designed as an add-on moduleto PWM AFCs, this backward-con-ducting bridge is similar to the basicpower bridge in a synchronous recti-fier bridge. Bi-directional modulescan be used independently on stand-alone, single-section PWM AFCs; orthey can be used to augment a multi-ple-AFC, common-bus system.

Self-contained modules monitorthe dc bus voltage. When a predeter-mined threshold is exceeded, theIGBT bridge switches on at the peakof the input power sine wave. Becausepower returns to the line at the exactcenter of the sine wave cycle, there islittle angular displacement betweencurrent and voltage.

Current source inverter (CSI).Based on mature SCR technology,current source inverters (CSI) cantranspose the ac line source and loadduring regeneration.

This AFC is a current source thatregulates motor terminal voltage. Atthe heart of the CSI is a large dcchoke. Its inductance is 10 times themotor inductance. During regenera-tion, the voltage across the dc bus in-verts polarity. This action enables theunidirectional SCR converter to putpower back into the ac line.

PWM with regenerative six-pulse SCR rectifier. Typically re-ferred to as an S-6R, the six-pulsecontrolled-bridge rectifier features asecond reverse-connected rectifier. Acommon practice with regenerative dcdrives is to connect both rectifierbridges in parallel to the ac line. Onan AFC, the rectifier must be pro-tected from inverting faults. Thesefaults occur when the dc bus voltageexceeds the peak of the sine-wave, acline voltage. This voltage differencewill usually cause one of the SCRs inthe reverse-bridge to remain in con-duction while a forward-bridge SCRalso conducts. This dual conductionproduces a phase-to-phase short cir-cuit, turning expensive drives intosmoke generators.

To avoid this problem, connect thereverse-rectifier to a higher voltagesource (usually a transformer with ahigher secondary voltage).

Regenerative PWM inverters withS-6R rectifiers offer:

• Reliable SCR power semiconduc-tors.

• Reverse six-pulse power circuitthat is adaptable to common-bus applications.

and sends this information to the con-troller. The controller then deter-mines the difference between the in-tended position at any given time andthe actual position. Using this errorsignal, the controller corrects thepower to the drive motor to maintainthe needed position-time relation-ship. Thus, the drive is controlling po-sition, because it is using a positionfeedback loop.

Offering extremely fast responseand precise control, some servos ac-celerate from standstill to 1000 rpmin a few milliseconds. Most servos arebuilt with at least two feedback loops— speed (or current) and position.

Speed feedback can be supplied bya tachometer-generator (tach). An en-coder, or similar sensor, can give posi-tion feedback. In other drives, one de-vice—usually an encoder—suppliesboth speed and position feedback sig-nals. (These sensors are described indetail in the Controls and Sensors De-partment of this Handbook.) In somedesigns, the position feedback is con-nected to the system controller; and inothers, it feeds to the drive controller.

Many multiaxis drive controllersinclude system control functions plusa power supply and several servopower amplifiers in one unit.

For many years, dc was the onlytype of high-response servo driveavailable. However, microprocessorsand significant developments inpower transistors and magnets havemade several alternatives available.

Conventional dc—For some, themotor follows the general design ofconventional dc motors. However, ser-vomotors have low armature inertiaand offer extremely fast response (ac-celeration of 4,000 radians/sec2).

Printed-circuit or disc armature de-signs, operate above 4,000 rpm andoffer acceleration rates of 15,000 radi-ans/sec2.

Servo power amplifiers usually havepower transistors to produce a pulse-width-modulated (PWM) dc output.This design offers faster response andbetter commutation between thebrushes and commutators than offeredby power units with SCR designs.

Brushless dc (BLDC) — Consid-ered both an ac and a dc design, theBLDC servo is discussed previously inthis department under “Brushlessdc.” Nowadays, the BLDC servo is themost frequently installed type ofservo, especially for those rated less

able this type of drive to offer re-sponse equal to or better than dcunits. Plus, these ac systems have nomaintenance problems associatedwith dc brushes and commutators.

AC vector servos are increasingly theservo of choice for applications requir-ing more than about 20 hp. For more in-formation on ac vector drives, see theprevious department Vector under the

than about 20 hp.AC vector — Built with high-re-

sponse, two-phase or three-phase acinduction or permanent-magnet syn-chronous motors, these ac servos alsooperate on adjustable frequency to ob-tain adjustable speed. The capabili-ties of these servo drives are con-stantly increasing. Recent advancesmade possible by microprocessors en-


Until recently, power line pollution— voltage and current harmonics —caused few problems and was often ig-nored. However, the increasing use ofnonlinear-load devices — the causes

of power-line pollution — makes re-ducing harmonics now a necessity.Examples of nonlinear-load devicesinclude ac and dc adjustable-speeddrives, static uninterruptable powersupplies, programmable controllers,rectifiers, switching-type power sup-plies, static lighting ballasts, comput-ers, and x-ray devices.

Power-line pollution can cause suchproblems as power factor reduction,power system overloading, system res-onance, and failure of motors, trans-

formers, and electronic equipment. There are three approaches to meet

the limits of current and voltage distor-tion as specified in IEEE Std. 519-1992:

Drive filters. A specific filter foreach drive trapsand eliminatesunwanted har-monics as closeto the source aspossible so theydo not affectother equip-ment, Figure E.

An alterna-tive, less-costlyapproach usesone filter to re-duce the har-monics of agroup. Such afilter is con-nected at thepoint of commoncoupling (PCC)or inboard of theplant maintransformer.The group filter

enables drives within that group to af-fect the operation of other driveswithin the same group. For example,harmonics produced by one drive maycause erratic operation of anotherdrive or other electronic devices.

If a filter is required and if electriccosts require adding power-factor cor-recting capacitors, plan both at thesame time, because part of the filterdesign may require capacitors. Also,be sure that the filter design does notproduce a system resonance.

Simulating 12-pulse systems.Individual drive transformers con-nected delta-wye for half the load andother transformers connected indelta-delta can produce a combinedharmonic content that approaches the

Cleaning up power-line pollution

Main transformer

Other loads

DC adjustable-speed drivecontroller

DC motor

DC motor

DC motor

DC motor

Filter

Filter

Filter

Filter

PCC

500-hp

500-hp

200-hp

200-hp

Figure E — Filter at each drive eliminatesunwanted harmonics at the source. Thisfilter placement reduces the chance of theharmonics produced by one drive fromaffecting the operation of another device.


heading “Types of regulators.”Step-motor drive — Unlike a

closed-loop servo drive that requires aposition feedback signal, a step-motordrive usually operates open-loop. Thedrive control pulses the motor with aspecific number of pulses to achievethe desired position change. In addi-tion to determining the number ofpulses required for a specific move

cussed in the Motor Product Depart-ment of this Handbook.)

Step drives can be used with posi-tion feedback for increased accuracy,because these motors can get out ofstep during unusual load conditions.

Servo-step trade-offs — Decidingwhich precision motion control sys-tem to select is usually not clear cut.Major factors include torque-speed

set, the drive also determines the rateof these pulses and hence the speedduring the transition.

These logic control devices offercomplex programming capabilities,and the ability to step motors in frac-tions of a step for greatly improvedresolution and repeatability. Some of-fer better than 65 arc sec/rev un-loaded. (Step-motor details are dis-

content of a 12-pulse drive, Figure F. This method reduces harmonics at

the PCC, if the sum of delta-wye loadsequals the delta-delta loads.

This transformer configuration canbe cost-effective if present drive-isola-tion transformers have the same pri-mary and secondary voltages, and thesecondary is reconnectable to a deltaconfiguration and maintains theproper output voltage.

Dedicated 12-pulse system. Forlarger drives (over 500 hp) considercreating a dedicated 12-pulse systemfrom two six-pulse drives that haveload-sharing capability or buying a 12-pulse drive. The resultant harmonicsremain fairly constant. This approachwill reduce the amount of required fil-tering and its associated cost.

There are two basic configurations:

• Two separateisolation trans-formers, one con-nected delta-wyeand the second delta-delta.

• A single transformer with twosecondaries. In this case, the primaryis in delta and one secondary is in wyeand the second is in delta.

In the first method, should either ofthe two transformers fail, replace-ment is relatively easy, because bothare standard model number items,Figure G. However, the installed costof this system is higher, because twosets of transformer primary protec-tion are required. Also, harmonic re-duction takes place at the PCC, Fig-ure H, so the original harmonicscaused by each six-pulse drive will be

Main transformer

Delta Wye

Other loads

PCC

500-hp

500-hp

200-hp

200-hp

DC motor

DC motor

DC motor

DC motor

seen by any other loads connected tothe plant main transformer.

With the alternate arrangement,harmonic reduction takes place in thetransformer primary. Also, installedcost is lower, because the single trans-former requires only one set of trans-former primary protection. But, be-cause the transformer has twosecondary windings — one delta, theother wye — if something should hap-pen to the transformer, repair or re-placement would take time.

Excerpted from an article byTheodore J. Bryda, Consultant, in theApril 1995 issue of PTD.

Figure H — Improvement on the configuration in Figure 3. In thissingle-transformer scheme, the harmonics are reduced in theprimary on the transformer rather than at the PCC.

Main transformer

Other loads

PCC

DC motor

50 % I

50 % I

6-pulse SCR drive

6-pulse SCR drive

Load shareI

Main transformer

PCC

Other loads

6-pulse SCR drive

6-pulse SCR drive

DC motor

50 % I

50 % I

ILoad share

Figure F — By connecting half the load to transformersconnected delta-wye and the other half to transformers withdelta-delta configuation, the resulting harmonics approach thoseproduced by 12-pulse drives.

Figure G — Common two-transformer configuration for reducingharmonics produced by high horsepower six-pulse drive.

capabilities, especially at the lowerspeeds; operating speed range; re-sponse; maximum torque; repeatabil-ity; space constraints; and cost.

MECHANICAL ADJUSTABLE-SPEED DRIVES

Two basic types of constructioncomprise mechanical adjustable-speed drives — belt or chain, and trac-tion. Although electrical adjustable-speed drives have replacedmechanical units in some applica-tions, design and specifying engineersstill favor mechanical designs formany applications. The major rea-sons: simplicity, ease of maintenance,desirable operating characteristics,and cost.

Belt and chain drivesThe drive is based on the principle of

and the resultant output speed. Theseunits are available with hand wheelsand automatic powered devices toturn the drive control screw and movethe motor.

This same principle, with added so-phistication, is supplied in packaged

adj u s t a b l e - d i a m e -t e r sheaves. As the di-ameter of one sheave in-creases, the otherdecreases—maintainingnearly constant belt orchain length—and ad-justs the ratio of the in-put-to-output sheave di-ameters, Figure 15. Theseunits offer ratios from 2:1to the more common 6:1,with some small-powerunits capable of 16:1.

Many different typesof operating mechanismsand belts or chains are used for thisdesign. One of the simplest mecha-nisms involves sliding the drive motorto change the center distance betweensheaves. One or both sheaves arespring loaded. As the motor moves,the spring keeps nearly constant belttension by changing sheave diameter.This action changes the sheave ratio


Figure 15 — Basic concept of belt-typemechanical adjustable-speed drive.

Advances in technology are en-hancing the capabilities and lower-ing the costs of adjustable-speed acand dc controllers. Here’s a look atsome of the tradeoffs for these twotypes of drives rated to 5 hp.

Cost. General-purpose dc pack-ages are still less expensive than accontrollers due to their lower level ofcomponent complexity and fewerpower controlling devices. An acdrive has a rectifier section to convertac to dc, and an inverter section to in-vert dc to controlled ac. By contrast, alow-cost dc controller uses SCRs torectify and control the power.

Customization. An ac controller isusually a digital device. Its on-boardmicroprocessor, EPROM (electricallyprogrammable read only memory),and ASIC (application specific inte-grated circuit) are easier to customizeto application requirements.

Speed regulation. With simpleslip compensation (similar to IR com-pensation on a dc drive), an ac pack-age is capable of 1% speed regulationbased on a standard NEMA Design Bsquirrel cage induction motor.

Speed range. Speed range is afunction of the motor’s ability to dis-sipate heat, to operate without cog-ging at low speeds, and to avoid fly-ing apart at high speeds. A totally

enclosed, nonventilated (TENV) dcmotor is often selected for low-speedapplication. It is common to oversizethe ac motor or add a separately pow-ered blower. However, oversizing amotor may require selecting a largerdrive controller.

High-efficiency ac motors can usu-ally operate over a 6:1 speed range,and dc over at least 20:1 without us-ing a speed feedback device.

Power factor. A dc drive with itsSCR section has a power factor thatdecreases at light loads and lowspeeds, its measurable range isabout 20% to 95%. An ac drive with afixed-diode converter section has adisplacement power factor of 95%,regardless of load or speed.

Increasing speed. DC motorspeed is proportional to applied ar-mature voltage. AC motor speed is afunction of applied frequency andvoltage from the ac drive, which istypically capable of 400 Hz operation(12,000 rpm on a 4-pole motor).Check with the motor manufacturerwhen operating above nameplatespeed, regardless of whether it’s anac or dc motor.

Single-phase. Single-phase dcdrives are designed and rated for op-erating from a single-phase powersource. Many ac drives are designed

for single or three-phase power forlow-horsepower applications. Some,when operating from single-phasepower, require derating the outputcurrent by about 50%. Other acdrives incorporate an oversized dcbus capacitor to filter the higher cur-rent ripple to allow for single-phaseoperation without derating. But re-member, the output of an ac drive isalways three phase, regardless of in-put power supply.

Running multiple motors. Withdc drives, it is one controller per mo-tor. AC drives can run multiple mo-tors as long as they all run at thesame speed.

Rapid stopping. Bringing a mo-tor and load to a quick stop tends toturn the motor into a generator,which means the regenerative energydeveloped must be dumped usuallyinto a dynamic-braking resistor bankor back to the ac line. Dc drives (evensmall, single-phase units) are avail-able to regenerate back to the line.An ac drive requires an additionaltransistor and logic to monitor the dcbus-voltage level to turn the transis-tor on at the proper point to dump theregenerative energy into an externaldynamic-braking resistor.

Excerpted from an article bySaftronics Inc. in the April, 1996 is-sue of PTD.

The best small-motor drive: ac or dc?


drives. Most of these units include anelectric motor, adjustable-speed sec-tion, and geared reducer. The speedadjustment method can be a handwheel or an air or electric motor forremote control.

Complex belt and sheave charac-teristics affect the torque and powerthat belt drives can transmit, Figure16. The major factors include sheave

Traction drives

Direct mechanical contact ofsmooth surfaces characterizes trac-tion drives. These units operate onthe principle that smooth surfacestransmit torque without pulsation,and that speed can be adjusted by ad-justing the ratio of input-to-outputcontact diameters, Figure 17.

Other traction drives require a spe-cial lubricant that, in effect, hardensunder high pressure, such as that cre-ated between metal surfaces in heavycontact.

Traction drives are manufacturedwith speed ratios from 5:1 to specialunits with 25:1. Power ratings varyfrom fractional to about 100 hp. ■

pressure at variousspeeds, belt strength,and belt centrifugalforce.

Several types of beltsoptimize the drive forthe application. Rubberbelts, among the mostfrequently used, in-clude standard V-beltsfor applications requir-ing up to 2:1 speed ad-justment. Also, wide-thin industrial rubberbelts are designed forup to 8:1 adjustable-speed applications.

Wood block belts arestill used for high torque, low-speed ap-plications (below about 1,200 rpm). Thisdesign has a set ofwood blocks boltedto a thick composi-tion belt.

Metal chainsprovide no-slip con-nection betweeninput and output.These units offer several other advan-tages: life of 10,000 hr, small size, andeasy chain replacement.

Figure 17 — Common design of metal-to-metal traction drive. A hardened ring on theoutput shaft disc rides on the driving cone.

Figure 16 — Typical output torque andpower vs. speed curve for belt-typeadjustable-speed drive.

Documents

ADJUSTABLE-SPEED DRIVESjesman/BigSeti/ftp/Actu... · an adjustable-speed drive, but per-forms only one function — slowly in-creasing the voltage applied to an ac motor. In essence,