ge vfd motors

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MotorsMotors

Application of InductionMotors with Variable

Frequency Drives

1-1

Ney E. T. MerhebJune 1999

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MotorsMotorsIntroduction

l It is becoming very popular the utilization of squirrel cageinduction motors with variable frequency drives (VFD’s) in theindustry.

l The advance of power electronics technology and the cost of theVFD’s are making this utilization possible.

l The many advantages of this type of application are:Ù Speed variation.Ù Heavy load inertia starting.Ù High starting torque requirements.Ù Low starting current requirements.Ù High efficiency at low speed.Ù High power factor.

l The induction motor with VFD’s replaces with advantage todaythe DC motors, and wound rotor motors.

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The Specificationl The utilization of VFD’s may affect the design, performance

and reliability of induction motors. Some effects are positivelike the less fatigue to the mechanical components and rotatingelements due to soft speed variation.

l The life of the motors can be influenced positively whenadequately applied.

l There are some important factor to be considered whenspecifying motors with VFD’s application:

ÙThe “Common Mode” VoltageÙHarmonicsÙSwitching Frequencies and Stationary WavesÙSpeed RangeÙStarting Aspects

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MotorsMotors

The Speed Torque Curve

® The synchronous speed of the motor varies linearly with the electricalfrequency and inversely with the number of poles.

® For a given Torque the Shaft Power varies linearly with the speed (n).

n s =

120 fe

p

T =P (Hp)

2 π fm

=5250 x P(Hp)

nm

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The Voltage over Frequency Curve(V/Hz)

® The Maximum torque of the motor varies with the square of the flux(or V/Hz relation).

® On the constant voltage region the maximum torque decreases withhe square of the V/Hz relation.

v (t) =d φ

Nd t

φ =V

ω Ncos ω t

M⇔⇔

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The Induction Motor Speed Torque Curve240

220

200

180

160

140

120

100

80

60

40

20

00 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Tor

que

N.m

Speed rpm

Maximum Torque

Starting Torque Full Load Torque

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Defining the Motor - The Operating Pointsl The motor design is based on the load speed torque curve.

l Check the operating points.

l Motor voltage should be limited to its nominal volts.

l Maximum current occurs at base speed. (VFD sizing andselection).

l Scalar type inverters allows the adjustment of the V/Hz curve.

l Use of voltage boosting at low speed to increase the startingtorque.

l The use of vector control VFD’s allows the control of the torquein all speed range automatically (requires tachometers)

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Some Examples

l Constant Torque applicationsl Steel mills machines, Paper machines (winder, tension reels, mill

stands)

l Cement mills, rubber mills, mixers, crushersl Conveyors

l Cranes and elevators cars

l Variable Torque applications:l Centrifugal pumps

l Centrifugal fans

l Compressors

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Some Examples

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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1.0

0.8

0.6

0.4

0.2Top Speed

1.2

Base SpeedMotor Maximum Current

Load Torque Curve

Constant Hp Line

Mo

tor

Vo

ltag

e in

p.u

. T

orq

ue

in p

.u.

VFD Lower Frequency Limit

Motor speed in p.u. of synchronous speed

Constant Voltage Region

Consta

nt V

olts/H

z Line

Motor Voltage

ð reduced DC motor field voltage region

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To

rqu

e in

p.u

.

Mo

tor

Vo

ltag

e in

p.u

.

1.25

0.75

0.5

0.25

1.50

Constant V

olts/H

z Line

Load Torque

0.2 0.4 0.6 0.8 1.0

Motor Speed in p.u.

1.0

Base SpeedMotor Torque

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To

rqu

e in

p.u

.

Mo

tor

Vo

ltag

e in

p.u

.

1.66

1.0

0.66

0.33

2.0

0.2 0.4 0.6 0.8 1.0

Motor Speed in p.u.

1.33

Load Torque

Constant Volts/Hz Volts Boost

Base Speed

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180 360 540 720 900 1080 1260 1440 1620 1800

7292

5833

4375

2917

1458 Top Speed

8750

540 rpmMax. Motor Current at 345 V

Constant Torque region

750 HP Constant

HP region

To

rqu

e in

lb-f

t

9 18 27 36 45 54 63 72 81 90

Speed in rpm

Constant Volts = 575 V

Consta

nt V

olts/H

z Line

Volts

750 HP Motor - 6 Poles - 540 rpm base speed - 575 V

Frequency in Hz

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To

rqu

e in

lb-f

t

2278

1367

911

455

2734

Constant V

olts/Hz

Load Torque

288 576 864 1152 1440 14.4 28.8 43.2 57.6 72

Speed in rpm

1823

500 HP Motor - 6 P 1440 rpm 2300 V

Frequency in Hz

2300 V72 Hz

Motor Torque

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5KAF400HP1200

368575 575

400.9567 0.85

1.15 1.0F

80CONTINUOUS

IGBT-PWM1000

KAF TEAO6811

SKF 6319GREASE

GREASESKF 6319

40

120 12001200 1800

1200

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The Overloadl Normally the overload is specified at base speed.

l The overload occur in short period of time (60 seconds).

l Standard overload requirements is in the range of 150 to 200%.It depends on the application.

l The motor must have sufficient maximum torque to hold theoverload conditions.

l The motor will give the maximum current at the base speed. Itis based on this current that the VFD must be selected.

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The Motor Heating

l The VFD normally produces harmonics that can causeadditional overheating to the motor. The engineer will have toconsider them when designing the motor.

l Some VFD manufactures guarantee that their VFD do notproduces harmful harmonics to the motor.

l In constant torque application it is necessary to provideadditional ventilation to the motor to provide cooling at lowspeeds. A motor-blower can be used or external forcedventilation.

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MotorsMotorsThe Motor Heating

l Variable torque applications like centrifugal fans and pumps itis not required to use additional ventilation as the power isreduced by the cubic of the speed.

l The harmonics can also induce additional heat to the rotor cageso, the across the line torque can be affected (double cage shouldbe avoided).

l Frequencies too high should be avoided due to increase of motorlosses (higher than 100 Hz). The number of poles should beselected accordingly.

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MotorsMotorsThe Insulation System

l The VFD’s in general produces voltage spikes that will affect theinsulation of windings if they are not prepared for it.

l The NEMA MG1 - Part 31 Std. , requires that the low voltage motorwinding must be capable to withstand up to 1600 peak Volts with arise time of 0.1 µµs.

l Requirements for medium voltage motors is still not normalized butGE follows IEEE 522.

l The high speed switching VFD’s as the (IGCT ou IGBT) normallyproduces high frequency dV/dt that may fatigue the windinginsulation. The Multi-Level PWM system or filters will prevent highinduced voltage to the winding.

l The insulation system should be adequate to this voltage.l Example: Nominal Voltage 4160 V, Insulation System Voltage 5000 V.l Cabling length can affect the terminal voltage (RC Filters).

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The Insulation System

Typical Low Voltage Winding

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The Waveforms and Harmonics

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The Voltage Spikes

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The Mechanical System

l The operation with VFD’s will require the examination of thecritical features of the motor, as such:

èCritical speedsèMaximum operation speedèBearing lubricationèVentilation and coolingèInsulation of the bearings against current circulationèShaft grounding brushes

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l Mechanical protection and cooling arrangements most utilizedin VFD’s applications (specially constant torque):

l DPBV (Drip Proof Blower Ventilated) IP23 - IC06l DPSV (Drip Proof Separately Ventilated) IP23 - IC06l TEFC (Totally Enclosed Fan Cooled) IP54 - IC411l TEAO ( Totally Enclosed Air Over) IP54 - IC416l WPII - with blower IPW24 - IC06l TEWAC - with blower IP54 - IC86Wl TEFV (Totally Enclosed Forced Ventilated) IP54 - IC37

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TEAO ( Totally Enclosed Air Over) IP54 - IC416

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DPBV (Drip Proof Blower Ventilated) IP23 - IC06

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DPSV (Drip Proof Separately Ventilated) IP23 - IC06

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The Accessories

l The most common accessories on motors for VFD’s operationare:

2 Tachometers ( specially for vector control drives) to be usedwhen speed control is critical.

2 Winding temperature sensors as such RTD’s or Thermistorfor temperature monitoring.

2 Shaft grounding brushes (depending on VFD’s type)

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The Accessories

A TEFC Motor with DigitalTachometer

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Specifying the Motor

l In the next page we summarize all the main informationto be sent to the motor supplier when specifying ainduction motor in use with a VFD application. Thisdata will help the motor design or application engineerto correctly select the motor for the application.

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MotorsMotorsKAF INDUCTION MOTOR BASIC SPECIFICATIONShaft Power (in HP, or KW) ___________________Enclosure _______ ; Insulation F; Ambient Temperature ____ °CTemperature rise _____ no Fator de Serviço _____RPM base _____________ RPM max (top speed)_____________RPM minimum in operation________Application (Driven Machine) ______________________Torque below base speed:Variable Torque _____ Constant Torque ______Volts __________ Starting across the line capability: Yes or No _____VFD Type:_____ PWM ______ IGBT or IGCT ______ GTO _____ CSIName of VFD Manufacturer (if available) _______________________Overload at base speed:_____ 150% current (standard)_____ 200% currentService Factor with the VFD __________ (1.0 standard or 1.15)Bearings: Anti-friction _______ Sleeve ______ Manufacturer standard _________Tachometer: Yes_____ No _______Model _____________ Provision for __________Thermal Sensor on windings :RTD Pt 100 ohms____ Cu 10 ohms _____Thermistor ___________ Other accessories _________________________GE - GEVISA S.A.

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MotorsMotorsThe GEIS- GEVISA Experience

• GEIS participate on all markets which the motors with VFD is present

• GE-GEVISA as a world wide supplier for the GEIS system hasmanufactured more than 700 units of low voltage and medium voltagemotors for many different VFD applications.

• Experience with all applications and configurations with many differentdrives types and manufactures.

• The regular medium and high voltage (2300 to 13800 V) insulationsystem is adequate for all systems. There is no need for modifications foruse with VFD’s.

• The low voltage windings are special to meet the requirements of NEMAMG1 - 31 for use with VFD’s.

• GE-GEVISA has an Application Engineering team all prepared to givethe assistance to customer to help with the specification and selection of themost adequate motor for the application.

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Some KAF motors furnishedCUSTOMER COUNTRY TYPE HP VOLTS POLES ENCLOSURE

JEN AN STEEL TAIWAN KAF 800 575 6 TEAOJEN AN STEEL TAIWAN KAF 804 575 6 TEAOJEN AN STEEL TAIWAN KAF 1207 575 6 TEAOUSIMINAS BRAZIL KAF 603 460 12 WPIIUSIMINAS BRAZIL KAF 480 460 6 WPIIAK STEEL USA KAF 1178 440 6 TEFVAK STEEL USA KAF 800 440 6 TEFVNUCOR USA KAF 1125 575 4 WPIINUCOR USA KAF 950 575 4 WPIINSM THAILAND KAF 1350 575 6 TEFVNSM THAILAND KAF 500 575 6 TEFVCYPRESS CREEK USA KAF 1250 4160 8 TEFVHOOGOVENS HOLAND KAF 1126 3300 12 TEFVHOOGOVENS HOLAND KAF 80 400 24 WPIIHOOGOVENS HOLAND KAF 635 400 6 WPIICHAPARRAL STEEL USA TSAF 1500 3300 4 TEFVCHAPARRAL STEEL USA TSAF 1200 3300 8 TEWACCHAPARRAL STEEL USA TSAF 3352 3300 26 TEWACCHAPARRAL STEEL USA TSAF 3352 3300 12 TEWACCHAPARRAL STEEL USA TSAF 7375 3300 20 TEWAC

TSAF = Synchronous MotorKAF = Squirrel Cage Induction Motor

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