7154/7156 Variable Speed Drives7154/7156 Variable Speed Drives

Paul WeingartnerPaul Weingartner

569-1776569-1776

OverviewOverview

Variable Frequency drives (VFD)Variable Frequency drives (VFD)

Application of VFDsApplication of VFDs

Power quality issuesPower quality issues

Human Machine Interface (HMI)Human Machine Interface (HMI)

Standards organizationsStandards organizations

NEMANEMA

IEEEIEEE

IECIEC

NEMANEMA

EnclosuresEnclosures

Motor characteristic curvesMotor characteristic curves

History of adjustable speed History of adjustable speed systemssystems

Variable pitch pulleyVariable pitch pulley

Motor-Generator (MG) setMotor-Generator (MG) set

Eddy current clutchEddy current clutch

Solid state drivesSolid state drives

ProblemsProblems

ExpensiveExpensive

Electrical (utility) issuesElectrical (utility) issues

Motor wear/tearMotor wear/tear

Solid State drivesSolid State drives

DC drivesDC drives

AC soft startAC soft start

AC Variable frequency drivesAC Variable frequency drives

AC vector drivesAC vector drives

DC drivesDC drives

High torqueHigh torque

Large speed ratiosLarge speed ratios

Regenerative brakingRegenerative braking

DC motors – high maintenanceDC motors – high maintenance

BasicsBasics

SpeedSpeed

TorqueTorque

HorsepowerHorsepower

EfficiencyEfficiency

Power factorPower factor

Real powerReal power

Apparent powerApparent power

Leading power factorLeading power factor

Inductive reactanceInductive reactance

Capacitive reactanceCapacitive reactance

Electric utilitiesElectric utilities

Commerical customers are defined as Commerical customers are defined as users above 15KVAusers above 15KVA

Electric chargeElectric charge

Demand chargeDemand charge

Power factor penaltiesPower factor penalties

BrakingBraking

None – let load coast to stopNone – let load coast to stop

Dynamic breaking – resistive load, uses Dynamic breaking – resistive load, uses generator effectgenerator effect

Plugging – reverse polarity across motorPlugging – reverse polarity across motor

DC injection – DC voltage is applied across two DC injection – DC voltage is applied across two phases of an AC induction motor. Current must phases of an AC induction motor. Current must be limited and timing is critical for proper usebe limited and timing is critical for proper use

RegenerativeRegenerative

Mechanical brakeMechanical brake

GoalsGoals

Ability to vary speedAbility to vary speed

Limit power factor issuesLimit power factor issues

Sensitive to electric demand issuesSensitive to electric demand issues

Often need “soft start”Often need “soft start”

Cost savingsCost savings

Ways to start a motorWays to start a motor

Full voltage – Across the line startingFull voltage – Across the line starting

Reduced voltage startingReduced voltage starting

Soft start – limit current and rate of startupSoft start – limit current and rate of startup

VFD – great latitude over motor controlVFD – great latitude over motor control

Relative cost difference for 1 HP Relative cost difference for 1 HP motormotor

Full voltage - $120Full voltage - $120

Reduced V - $200Reduced V - $200

Soft state - $250Soft state - $250

VFD - $400VFD - $400

MotorsMotors

3 phase squirrel cage induction motor3 phase squirrel cage induction motor

Principle of operationPrinciple of operation

Synchronous speedSynchronous speed

SlipSlip

Starting characteristicsStarting characteristics

NEMA classificationsNEMA classifications

Motor Insulation classMotor Insulation class

Motor VFD issuesMotor VFD issues

Volts/Hertz ratioVolts/Hertz ratio

Constant volts rangeConstant volts range

VFD principle of operationVFD principle of operation

3 phase rectifier3 phase rectifier

DC busDC bus

3 phase inverter3 phase inverter

VFDs – 1VFDs – 1stst Generation Generation

VVI – Variable Voltage InvertersVVI – Variable Voltage Inverters 6 step drive6 step drive Uses SCRs on rectifier front endUses SCRs on rectifier front end Variable voltage DC busVariable voltage DC bus

Problems with VVI drivesProblems with VVI drives

Motor signal – not very sinusoidal, causes Motor signal – not very sinusoidal, causes problemsproblemsSensitive to source voltage flucuations – Sensitive to source voltage flucuations – 5-10% change will fault the drive5-10% change will fault the driveAt low speed the drive will “cog” creating At low speed the drive will “cog” creating stresses on shafts, etc – freq should be stresses on shafts, etc – freq should be above 15 Hzabove 15 HzDrive will reflect harmonics back to the lineDrive will reflect harmonics back to the lineShort power loss is badShort power loss is bad

CSI – Current Source InverterCSI – Current Source Inverter

Similar to VVI, but adds a line reactor on Similar to VVI, but adds a line reactor on the DC busthe DC bus

Supports regenerative braking without Supports regenerative braking without needing extra hardwareneeding extra hardware

Creates harmonicsCreates harmonics

PWMPWM

Operating frequency – carrier frequencyOperating frequency – carrier frequency Increasing the carrier frequency decreases Increasing the carrier frequency decreases

the efficiency of the drive electronicsthe efficiency of the drive electronics

Duty cycleDuty cyclet-ont-ont-offt-offTransistor exampleTransistor example Linear operation vs. PWMLinear operation vs. PWM Power dissipationPower dissipation

PWM drivesPWM drives

Uses diodes for the rectifer, creating a Uses diodes for the rectifer, creating a Constant voltage DC busConstant voltage DC bus

Constant power factor – due to diode front Constant power factor – due to diode front endend

Full operating torque at near zero speedFull operating torque at near zero speed

No coggingNo cogging

Can ride thru a power loss from 2 Hz to 20 Can ride thru a power loss from 2 Hz to 20 secondsseconds

VFD drivesVFD drives

ScalarScalar

VectorVector

3 3 phase phase motormotor

NEMA NEMA Motor Motor

CurvesCurves

1336 picture1336 picture

1336 – Description of L7E option1336 – Description of L7E option

1336 Drive literature link1336 Drive literature link

http://www.ab.com/drives/1336PlusII/literathttp://www.ab.com/drives/1336PlusII/literature/index.htmlure/index.html

PWM PWM inverterinverter

Motor selection criteriaMotor selection criteria

Synchronous speedSynchronous speed

AC motors have a sync design speed that AC motors have a sync design speed that is a function of the number of poles and is a function of the number of poles and the line frequencythe line frequency

At sync speed ZERO torque is generatedAt sync speed ZERO torque is generated

Therefore, motors cannot run at sync Therefore, motors cannot run at sync speedspeed

Motor slipMotor slip

Since motors cannot run at sync speed, Since motors cannot run at sync speed, the will run at slightly less than this speed.the will run at slightly less than this speed.

““Slip” is the term used to describe the Slip” is the term used to describe the difference between the sync speed and difference between the sync speed and the maximum rated speed at full loadthe maximum rated speed at full load

Motor slip calcMotor slip calc

This formula includes a characteristic called slip. In a This formula includes a characteristic called slip. In a motor, slip is the difference between the rotating motor, slip is the difference between the rotating magnetic field in the stator and the actual rotor speed. magnetic field in the stator and the actual rotor speed. When a magnetic field passes through the rotor's When a magnetic field passes through the rotor's conductors, the rotor takes on magnetic fields of its own. conductors, the rotor takes on magnetic fields of its own. These induced rotor magnetic fields will try to catch up to These induced rotor magnetic fields will try to catch up to the rotating fields of the stator. However, there is always the rotating fields of the stator. However, there is always a slight speed lag, or slip. For a NEMA-B motor, slip is 3-a slight speed lag, or slip. For a NEMA-B motor, slip is 3-5% of its base speed, which is 1,800 rpm at full load. For 5% of its base speed, which is 1,800 rpm at full load. For example, example,

Volts/HertzVolts/Hertz

Drive frequencyDrive frequency

The speed at which IGBTs are switched on and The speed at which IGBTs are switched on and off is called the carrier frequency or switch off is called the carrier frequency or switch frequency. The higher the switch frequency, the frequency. The higher the switch frequency, the more resolution each PWM pulse contains. more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 Typical switch frequencies are 3,000 to 4,000 times per second (3-4 kHz). As you can imagine, times per second (3-4 kHz). As you can imagine, the higher the switch frequency, the smoother the higher the switch frequency, the smoother (higher resolution) the output waveform. (higher resolution) the output waveform. However, there is a disadvantage: Higher switch However, there is a disadvantage: Higher switch frequencies cause decreased drive efficiency. frequencies cause decreased drive efficiency. The faster the switching rate, the faster the The faster the switching rate, the faster the IGBTs turn on and off. This causes increased IGBTs turn on and off. This causes increased heat in the IGBTs. heat in the IGBTs.

High motor voltagesHigh motor voltages

http://www.mtecorp.com/solving.htmlhttp://www.mtecorp.com/solving.htmlHigh peak voltagesHigh peak voltagesFast rise timesFast rise timesStandard Motor Capabilities established by the National Standard Motor Capabilities established by the National Electrical Manufacturers Association (NEMA)and Electrical Manufacturers Association (NEMA)and expressed in the MG- I standard (part 30), indicate that expressed in the MG- I standard (part 30), indicate that standard NEMA type B motors can withstand standard NEMA type B motors can withstand 1000 volts 1000 volts peakpeak at a minimum rise time of 2 u-sec (microseconds). at a minimum rise time of 2 u-sec (microseconds). Therefore to protect standard NEMA Design B motors, Therefore to protect standard NEMA Design B motors, one should limit peak voltage to one should limit peak voltage to 1KV1KV and reduce the and reduce the voltage rise to less than 500 volts per micro-second. voltage rise to less than 500 volts per micro-second.

Constant torque loadsConstant torque loads

Conveyor systemsConveyor systems

Constant horsepower loadsConstant horsepower loads

grinders, winders, and lathes grinders, winders, and lathes

Variable torque loadsVariable torque loads

fans and pumps fans and pumps

Motor ventilationMotor ventilation

TENVTENV

TEFCTEFC

ODPODP

High Altitude considerationsHigh Altitude considerations

Motor soft startMotor soft start

Limit inrush currentLimit inrush current

Linear rampLinear ramp

S-curveS-curve

Skip freqSkip freq

Flux vector drivesFlux vector drives

http://www.mikrokontrol.co.yu/sysdrive/Whhttp://www.mikrokontrol.co.yu/sysdrive/WhatInv.htm#FVatInv.htm#FV