Wound Rotor Im

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Renewable Energy

Three-Phase Wound-RotorInduction Machines

Courseware Sample86367-F0

A


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RENEWABLE ENERGY

THREE-PHASE WOUND-ROTOR INDUCTION MACHINES

Courseware Sample

bythe staff

of

Lab-Volt Ltd.

Copyright 2011 Lab-Volt Ltd.

All rights reserved. No part of this publication may bereproduced, in any form or by any means, without the prior

written permission of Lab-Volt Ltd.

Printed in CanadaMarch 2011


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AThree-Phase Wound-Rotor Induction Machines v

Foreword

The primary difference between three-phase wound-rotor induction machinesand three-phase squirrel-cage induction machines is the design of the machinerotor. The rotor of three-phase squirrel-cage induction machines is made ofconducting bars short-circuited by rings at both ends, while the rotor of three-phase wound-rotor induction machines consists of wire windings similar to the

machine stator windings. These rotor windings are accessible through slip ringsand brushes, allowing the connection of electrical components to the machinerotor.

Three-phase wound-rotor induction machines offer a number of advantages overother types of induction machines, most notably the ability to produce highstarting torques at lower starting currents. Large three-phase wound-rotorinduction machines are also easier to assemble than comparatively sized three-phase squirrel-cage induction machines. Due to these advantages, three-phasewound-rotor induction machines are commonly used in industry for anyapplication requiring a large rotating machine (0.75 MW, or 1000 hp, and more).In such cases, the ability to produce a high starting torque with a reasonablestarting current is crucial.

Another particularity of three-phase wound-rotor induction machines is thepossibility to adjust the machine operating speed by controlling the rotor currents.Due to this feature, three-phase wound-rotor induction machines weretraditionally used in applications requiring a rotating machine to run over a widerange of speeds. Today, however, the ability to adjust the speed of three-phasewound-rotor induction machines is used less frequently because the sameresults can be achieved more efficiently by controlling the speed of three-phasesquirrel-cage induction machines using variable-frequency motor drives.

Typical wound-rotor found in three-phase wound-rotor induction machines.


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AThree-Phase Wound-Rotor Induction Machines vii

Table of Contents

Introduction Three-Phase Wound-Rotor Induction Machines .................... 1Introduction to three-phase wound-rotor induction machines.

Exercise 1 Three-Phase Wound-Rotor Induction Machine with aShort-Circuited Rotor ................................................................ 5Three-phase wound-rotor induction machine with a short-circuited rotor windings.

Exercise 2 Three-Phase Wound-Rotor Induction Machine with RotorResistance ................................................................................ 13Three-phase wound-rotor induction machine starting currentand torque versus the rotor resistance. Varying the speed ofa three-phase wound-rotor induction machine through therotor resistance.

Appendix A Equipment Utilization Chart ................................................... 31Appendix B Glossary of New Terms ........................................................... 33Appendix C Impedance Table for the Load Modules ................................ 35Appendix D Circuit Diagram Symbols ........................................................ 37Index of New Terms ............................................................................................. 39Bibliography ......................................................................................................... 41We Value Your Opinion! ...................................................................................... 43


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Sample Exercise

Extracted from

Student Manual


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AThree-Phase Wound-Rotor Induction Machines 13

When you have completed this exercise, you will know the effects of varying therotor resistance of a three-phase wound-rotor induction machine on the machinestarting current and torque, as well as on the machine rotation speed andefficiency.

The Discussion of this exercise covers the following points:

Three-phase wound-rotor induction machine starting current and torque

versus the rotor resistance Varying the speed of a three-phase wound-rotor induction machinethrough the rotor resistance

Three-phase wound-rotor induction machine starting current andtorque versus the rotor resistance

Three-phase wound-rotor induction machines have an important advantage overthree-phase squirrel-cage induction machines during start-up: it is possible toreduce the starting current while increasing the starting torque. The ability ofwound-rotor induction machines to reduce the starting current is crucial for large-size induction machines (0.75 MW, or 1000 hp, and more) because these

machines operate with large currents, especially during start-up. During thisperiod, the current requirements of large squirrel-cage induction machines canbe as high as four times the machine nominal current. Such high currentrequirements put a great strain on the ac power network and make large squirrel-cage induction machines impractical or even impossible to start.

Large-size induction machines are also required to produce high starting torquesas they are generally used to drive high-inertia loads (e.g., large pumps, cranes,grinders). In such cases, the ability of three-phase wound-rotor inductionmachines to increase the machine starting torque is beneficial because itreduces the amount of time required for the machine to reach the nominal speedand shortens the machine start-up time (and thus, the amount of time duringwhich the current drawn by the machine is many times the nominal current).

The following two sections explain in more detail the ability of three-phasewound-rotor induction machines to reduce the machine current and increase themachine torque during start-up.

Three-phase wound-rotor induction machine starting current

The starting current of a three-phase wound-rotor induction machine can belowered by increasing the machine rotor resistance. The relationship between the

Three-Phase Wound-Rotor Induction Machine with Rotor

Resistance

Exercise 2

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance Discussion

14 Three-Phase Wound-Rotor Induction MachinesA

amount of current drawn by a three-phase wound-rotor induction machine as afunction of the machine speed for different rotor resistance values is shown inFigure 4.

Figure 4. Three-phase wound-rotor induction machine starting current as a function of themachine speed for different rotor resistance values.

As Figure 4 shows, the higher the rotor resistance of a three-phase wound-rotorinduction machine, the lower the machine starting current. For instance, passingfrom a short-circuited rotor condition to a high rotor resistance condition reducesthe machine starting current from about 400% of the machine nominal current to

about 200% of the machine nominal current. As large-size machines require alarge amount of current during start-up, such a gain can make an enormousdifference. Figure 4 also shows that, as the machine nears the synchronousspeed, the effect of the machine rotor resistance on the machine currentbecomes less and less important until, at the synchronous speed, the machinecurrent is the same for any rotor resistance.

Three-phase wound-rotor induction machine starting torque

The starting torque of a three-phase wound-rotor induction machine, like itsstarting current, can be adjusted by changing the rotor resistance. Increasing therotor resistance of a three-phase wound-rotor induction machine has the effect ofshifting toward the left the breakdown torque region on the torque-versus-speed

curve. The relationship between the torque and speed of a three-phase wound-rotor induction machine for different rotor resistance values is shown in Figure 5.

0 25 50 75 100

100

200

300

400

500

M

achinecurrent(%o

fnominalcurrent)

Short-circuited rotor

Low rotor resistance

Moderate rotor resistance

High rotor resistance

Machine speed (% of synchronous speed)


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Figure 5. Three-phase wound-rotor induction machine torque-versus-speed curves fordifferent rotor resistance values.

As Figure 5 shows, the higher the rotor resistance of the three-phase wound-rotor induction machine (up to a certain value), the higher the machine startingtorque. However, as the machine speed increases, a high rotor resistancecauses the machine torque to decrease more rapidly than when lower rotorresistance values are used. For this reason, after the machine initial start-up, therotor resistance is usually progressively lowered, or simply short-circuited, inorder to reduce the rotor resistance to a minimum and optimize the machine

torque.

Figure 6 shows on the same graph both the current-versus-speed and torque-versus-speed curves of a three-phase wound-rotor induction machine fordifferent rotor resistance values. As you can see, when the machine rotorresistance is high, the starting current is minimal and the starting torque ismaximal. This is perfectly adapted to applications where large inductionmachines are started under heavy mechanical loads.

0 25 50 75 100

40

80

120

160

200

Machinetorque(%o

fnominaltorq

ue)







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Figure 6. Three-phase wound-rotor induction machine current-versus-speed and torque-versus-speed curves for different rotor resistance values.

Varying the speed of a three-phase wound-rotor induction machinethrough the rotor resistance

Another advantage three-phase wound-rotor induction machines have overthree-phase squirrel-cage induction machines is the ability to adjust the machinespeed. Traditionally, this ability was used in applications requiring the machine to

run over a wide range of speeds. Nowadays, however, the ability to adjust thespeed of three-phase wound-rotor induction machines is less and less used asthe same results can be achieved more efficiently by controlling three-phasesquirrel-cage induction machines using variable-frequency motor drives.

It is possible to adjust the speed of three-phase wound-rotor induction machinesby varying the rotor resistance. For any given torque (within the normal operatingrange of the machine), the higher the rotor resistance, the lower the machinerotation speed. This relationship is illustrated in Figure 7. In this example, athree-phase wound-rotor induction machine is coupled to a constant-torquebrake that produces a constant opposing torque. As you can see, for a givenmachine torque during motor operation, the machine speed decreases as themachine rotor resistance increases.

0 25 50 75 100

40

80

120

160

200

Machinetorque(%o

fnominaltorq

ue)



100

200

300

400

500

Machinecurrent(%o

fnominalcurr

ent)





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Figure 7. Three-phase wound-rotor induction machine speed variation for a given torque fordifferent rotor resistance values.

However, varying the three-phase wound-rotor induction machine speed in sucha way has drawbacks. Firstly, it reduces the machine efficiency and mechanicalpower output in comparison to a similarly sized three-phase squirrel-cageinduction machine operating at the same speed. This is due to the fact thatadding resistance to the machine rotor increases the machine copper ()losses and thus lowers the machine efficiency and mechanical power output.

In addition, the higher the rotor resistance of a three-phase wound-rotor inductionmachine, the greater the variation in the rotation speed as the torque varies. This

relationship is illustrated in Figure 8. As the figure shows, for a given torquevariation, the resulting machine speed variation is much more important when themachine rotor resistance is high than when the machine rotor is short-circuited.This means that three-phase wound-rotor induction machines having a high rotorresistance are more susceptible to speed variations as the load torque changes.

0 25 50 75 100

40

80

120

160

200

Machinetorque(%o

fnominaltorque)






Machine speed variationfor a given torque


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance Procedure Outline


Figure 8. Three-phase wound-rotor induction machine speed variation as the machine torquevaries for different rotor resistance values.

The Procedure is divided into the following sections:

Setup and connections Effect of the rotor resistance on the starting current and torque of a

three-phase wound-rotor induction machine Varying the speed of a three-phase wound-rotor induction machine

through the rotor resistanceCAUTION!

High voltages are present in this laboratory exercise! Do not make or modify

any banana jack connections with the power on unless otherwise specified!

Setup and connections

In this section, you will set up a circuit containing a three-phase wound-rotor

induction machine coupled to a brake. You will then set the measuring equipmentrequired to study the three-phase wound-rotor induction machine operationduring start-up.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list ofequipment required to perform this exercise.

Install the required equipment in the Workstation.

PROCEDURE OUTLINE

PROCEDURE

0 25 50 75 100

40

80

120

160

200

Machinetorque(%o

fnominaltorque)






Machine speedvariation as themachine torque varies

Machine torque variation


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance Procedure


Make sure you use the same Four-Quadrant Dynamometer/Power Supplymodule as in Exercise 1 of this manual by confirming that the modules serialnumber is the same as the serial number you recorded in the first step ofExercise 1.

Mechanically couple the Three-Phase Wound-Rotor Induction Machineto theFour-Quadrant Dynamometer/Power Supplyusing a timing belt.

2. Make sure that the ac and dc power switches on the Power Supplyare set tothe O (off) position, then connect the Power Supply to a three-phase acpower outlet.

Make sure that the main power switch on the Four-Quadrant Dynamometer/Power Supplyis set to the O(off) position, then connect its Power Inputto anac power wall outlet.

Connect the Power Inputof the Data Acquisition and Control Interfaceto a24 V ac power supply.

3. Connect the USB port of the Data Acquisition and Control Interfaceto a USBport of the host computer.

Connect the USB port of the Four-Quadrant Dynamometer/Power Supplytoa USB port of the host computer.


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4. Connect the equipment as shown in Figure 9.

As the machine starting current will reach a value that is higher than 4 A, usethe 40 A input terminal for current input I1on the Data Acquisition and ControlInterface.

Local ac power network Rotorresistance, ,

()

Voltage(V)

Frequency(Hz)

120 60 171

220 50 629

240 50 686

Figure 9. Three-phase wound-rotor induction machine coupled to a brake.

5. Make the necessary switch settings on the Resistive Loadmodule in order toobtain the rotor resistance value required.

Short-circuit the resistors in the Resistive Loadmodule in order to temporarilyreduce the rotor resistance to zero.

6. Turn the Four-Quadrant Dynamometer/Power Supply on, then set theOperating Mode switch to Dynamometer. This setting allows the Four-Quadrant Dynamometer/Power Supplyto operate as a prime mover, a brake,or both, depending on the selected function.

7. ConnectAnalog Output nof the Four-Quadrant Dynamometer/Power Supplyto Analog Input 8/n of the Data Acquisition and Control Interface using aminiature banana plug lead.

Three-phasewound-rotor

inductionmachine

Brake

40 A input

L1

L2

L3


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Connect the common (white) terminal of the Analog Outputs on the Four-Quadrant Dynamometer/Power Supply to one of the two analogcommon (white) terminals on the Data Acquisition and Control Interfaceusing a miniature banana plug lead.

8. Turn the host computer on, then start the LVDAC-EMSsoftware.

In the Module Selector window, make sure that the Data Acquisition andControl Interface and the Four-Quadrant Dynamometer/Power Supply aredetected. Select the Computer-Based Instrumentation function for the Data

Acquisition and Control Interface. Also, make sure that the selected networkfrequency corresponds to the frequency of your local ac power network, thenclick the OKbutton to close the Module Selectorwindow.

9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supplywindow, then make the following settings:

Set the Pulley Ratioparameter to 24:24.

Set the Function parameter to Two-Quadrant, Constant-TorqueBrake.

Set the Torqueparameter to 3.0 Nm (26.55 lbfin).

Start the Two-Quadrant, Constant-Torque Brake.

In LVDAC-EMS, set the Rangesetting of current input I1to high.

10. In LVDAC-EMS, open the Oscilloscope, then make the appropriate settings

in order to observe the machine current (stator current) waveform measuredusing input I1, as well as the machine speed measured using AnalogInput 8/n.

It is recommended to set the sensitivity of the channel used to observe themachine current waveform to 5 A/div, the sensitivity of the channel used toobserve the variation of the machine speed to 200 r/min / div, and the timebase to 50 ms/div.

On the Oscilloscope, set the trigger type to Hardware, the trigger source tothe channel used to observe the machine current waveform, and the triggerlevel to 2 A. Adjust the horizontal position of the trigger point to about 1division of the left-hand side of the oscilloscope screen.

These settings ensure that the Oscilloscopebegins to record data only whenthe machine stator current reaches a value of 2 A, i.e., when the machinestarts.

On the Oscilloscope, click the Single Refreshbutton.


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Effect of the rotor resistance on the starting current and torque of athree-phase wound-rotor induction machine

In this section, you will start the three-phase wound-rotor induction machine andrecord on the Oscilloscope the machine current waveform and speed signalwhen the machine rotor is short-circuited. You will then increase the resistance at

the rotor and record on the Oscilloscope the machine current waveform andspeed signal. You will compare the machine current waveforms you obtainedwith a short-circuited rotor and with rotor resistance, and analyze the results. Youwill also compare the machine speed signals you obtained with a short-circuitedrotor and with rotor resistance, and analyze the results.

11. On the Three-Phase Wound-Rotor Induction Machine, press and hold theProtection Override push-button in order to override the overvoltageprotection then, on the Power Supply, turn the three-phase ac power sourceon. Wait for about three seconds, then turn the three-phase ac power sourceoff. Release the Protection Override push-button in order to reactivate theovervoltage protection device.

Lab-Volts Three-Phase Wound-Rotor Induction Machine is fitted with anovervoltage protection device preventing the rotor voltage from reachingvalues higher than about 110% of the machine nominal rotor voltage. Sincethe rotor voltage shortly exceeds this limit during start-up, it is necessary totemporarily override the overvoltage protection in order to observe themachine characteristics during start-up.

12. On the Oscilloscope, store in Memory 1the data you just recorded, then clickthe Single Refreshbutton to reset the Oscilloscope.

If the speed signal of the three-phase wound-rotor induction machine is of anegative polarity, invert the connections at two of the three phase terminals of

the machine stator windings, then repeat the previous step.

13. On the Resistive Load module, remove the leads that short-circuit theresistors connected to the rotor. The machine rotor resistance should now beequal to the resistance value indicated in Figure 9.

14. On the Three-Phase Wound-Rotor Induction Machine, press and hold theProtection Overridepush-button in order to override the overvoltage safetydevice then, on the Power Supply, turn the three-phase ac power source on.Wait for about three seconds, then turn the three-phase ac power source off.Release the Protection Override push-button in order to reactivate the

overvoltage protection device

In the Four-Quadrant Dynamometer/Power Supply window, stop the Two-Quadrant, Constant-Torque Brake.

15. On the Oscilloscope, show Memory 1 on screen. Show also the horizontalcursors.


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On the Oscilloscope, use the horizontal cursors to determine the amplitude ofthe three-phase wound-rotor induction machine current during start-up, witha short-circuited rotor and with rotor resistance.

Amplitude of starting current with a short-circuited rotor = A

Amplitude of starting current with rotor resistance= A

16. Calculate the rms values of the starting currents recorded in the previousstep.

RMS value of starting current with a short-circuited rotor = A

RMS value of starting current with rotor resistance = A

17. Convert the rms values of the starting currents calculated in the previousstep into percentage values related to the nominal stator current of the

Three-Phase Wound-Rotor Induction Machine.

The nominal value of the stator current is indicated on the front panel of theThree-Phase Wound-Rotor Induction Machine.

Starting current with a short-circuited rotor = % of nominal current

Starting current with rotor resistance = % of nominal current

18. Do the results you obtained confirm that increasing the rotor resistance of athree-phase wound-rotor induction machine lowers the machine startingcurrent?

Yes No

Do the machine speed signals you recorded confirm that the machinestarting torque obtained with rotor resistance is higher than the startingtorque obtained with a short-circuited rotor? Explain.


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Varying the speed of a three-phase wound-rotor induction machinethrough the rotor resistance

In this section, you will set up a circuit consisting of a three-phase wound-rotorinduction machine with rotor resistance coupled to a prime mover/brake. You willstart the machine and increase the machine speed by steps. At each step, you

will record in the Data Table the machine speed, torque, mechanical power, linecurrent, active power, reactive power, and power factor. You will calculate themachine efficiency using the recorded machine mechanical power and activepower values. On the same graph, you will plot the torque versus speed curvewhen the rotor is short-circuited (using the results you obtained in the previousexercise), as well as when resistance is added to the rotor, and interpret theresults. Finally, you will plot the curves of the machine line current, active power,reactive power, power factor, and efficiency as a function of the machinemechanical power when the rotor is short-circuited (using the results youobtained in the previous exercise), as well as when resistance is added to therotor, and interpret the results.

19. Connect the equipment as shown in Figure 10.

Local ac power network Rotorresistance

,

,

()

Voltage

(V)

Frequency

(Hz)

120 60 171

220 50 629

240 50 686

Figure 10. Three-phase wound-rotor induction machine coupled to a prime mover/brake.

In LVDAC-EMS, set the Rangesetting of current input I1to low.

Three-phasewound-rotor

inductionmachine

Prime mover/brake

L1

L2

L3


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20. Make sure the switch settings on the Resistive Loadmodule correspond tothe rotor resistance values required (see table in Figure 10).

21. In the Four-Quadrant Dynamometer/Power Supply window, make thefollowing settings:

Set the Function parameter to CW Constant-Speed PrimeMover/Brake.

Set the Speed parameter to the synchronous speed of the Three-Phase Wound-Rotor Induction Machine.

The synchronous speed of the Three-Phase Wound-Rotor Induction Machineis 1500 r/min at a local ac power network frequency of 50 Hz and 1800 r/min ata local ac power network frequency of 60 Hz.

22. In LVDAC-EMS, start the Meteringapplication. Make the required settings inorder to measure the rms values (ac) of the three-phase wound-rotorinduction machine line current (input I1). Set two other meters tomeasure the machine active power and reactive power using the two-wattmeter method (meter function PQS1 + PQS2). Finally, set a meter tomeasure the machine power factor from inputs E1, I1, E2, and I2.

23. On the Power Supply, turn the three-phase ac power source on.

Before starting the prime mover/brake, make sure that the Three-PhaseWound-Rotor Induction Machineis rotating in the clockwise direction. If so,proceed directly to the next step. Otherwise, turn the three-phase ac powersource off, invert the connections at two of the three phase terminals of themachine stator windings, then repeat this step from the beginning.

24. In the Four-Quadrant Dynamometer/Power Supply window, start theCW Constant-Speed Prime Mover/Brake.

In LVDAC-EMS, open the Data Tablewindow.

Set the Data Tableto record the three-phase wound-rotor induction machinespeed , torque , and mechanical power indicated in the Four-QuadrantDynamometer/Power Supplywindow.

Also, set the Data Table to record the three-phase wound-rotor inductionmachine line current (input I1), active power , reactive power , and

power factor indicated in the Meteringapplication.

25. In the Four-Quadrant Dynamometer/Power Supply window, decrease theSpeed parameter until the three-phase wound-rotor induction machinemechanical power is approximately equal to 250 W.


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In the Data Table, click on the Record Databutton to record the current valueof the machine speed , torque , mechanical power , line current ,active power , reactive power , and power factor .

26. In the Four-Quadrant Dynamometer/Power Supply window, increase the

Speed parameter so that the machine mechanical power decreases toapproximately -250 W in steps of about 25 W. For each machine mechanicalpower value , record the machine parameters in the Data Table.

27. When all data has been recorded, turn the three-phase ac power source inthe Power Supplyoff.

In the Four-Quadrant Dynamometer/Power Supply window, stop theCW Constant-Speed Prime Mover/Brake.

28. In the Data Table window, save the recorded data, then export it to a

spreadsheet application.

In the spreadsheet application, add a new parameter to the results: the three-phase wound-rotor induction machine efficiency . To calculate the machineefficiency , use the following equations:

When the machine mechanical power is of positive polarity (i.e.,when the machine operates as a motor), divide each machinemechanical power value by the corresponding machine activepower value , then multiply the result by 100 to express theefficiency as a percentage.

When the machine mechanical power is of negative polarity (i.e.,

when the machine operates as a generator), divide each machineactive power value by the corresponding machine mechanicalpower value , then multiply the result by 100.

Do not calculate the machine efficiency when the machine mechanicalpower is lower than about 50 W (positive or negative).

29. Plot on the same graph curves of the three-phase wound-rotor inductionmachine torque as a function of the machine speed when the rotor isshort-circuited, as well as when resistance is added to the rotor. Use theresults you recorded in the previous exercise and in this exercise to do so.

Observe the graph. Does adding resistance to the rotor of a three-phasewound-rotor induction machine have any significant effect on the machinetorque versus speed curve? If so, explain.


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Do your results confirm that increasing the resistance at the rotor of a three-phase wound-rotor induction machine changes the machine speed for agiven machine torque ?

Yes No

30. Plot on the same graph curves of the three-phase wound-rotor inductionmachine line current as a function of the machine mechanical power when the rotor is short-circuited, as well as when resistance is added to therotor. Use the results you recorded in the previous exercise and in thisexercise to do so.

Observe the graph. Does adding resistance to the rotor of a three-phasewound-rotor induction machine have any significant effect on the machineline current ? If so, explain.

31. Plot on the same graph curves of the three-phase wound-rotor inductionmachine active power and reactive power as a function of the machinemechanical power when the rotor is short-circuited, as well as whenresistance is added to the rotor. Use the results you recorded in the previousexercise and in this exercise to do so.

Observe the graph. Does adding resistance to the rotor of a three-phasewound-rotor induction machine have any significant effect on the machineactive power and reactive power ? If so, explain.

32. Plot on the same graph curves of the three-phase wound-rotor inductionmachine power factor as a function of the machine mechanical power when the rotor is short-circuited, as well as when resistance is added to therotor. Use the results you recorded in the previous exercise and in thisexercise to do so.

Observe the graph. Does adding resistance to the rotor of a three-phasewound-rotor induction machine have any significant effect on the machinepower factor ? If so, explain.


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance Conclusion


33. Plot on the same graph curves of the three-phase wound-rotor inductionmachine efficiency as a function of the machine mechanical power witha short-circuited rotor, as well as with rotor resistance, when the machineoperates as a motor ( 50 W). Use the results you recorded in theprevious exercise and in this exercise to do so.

Plot on another graph curves of the three-phase wound-rotor inductionmachine efficiency as a function of the machine mechanical power witha short-circuited rotor, as well as with rotor resistance, when the machineoperates as a generator ( -50 W). Use the results you recorded in theprevious exercise and in this exercise to do so.

Observe the graphs. Does adding resistance to the rotor of a three-phasewound-rotor induction machine have any significant effect on the machineefficiency ? If so, explain.

34. Close LVDAC-EMS, then turn off all the equipment. Disconnect all leads andreturn them to their storage location.

In this exercise, you learned the effects of varying the rotor resistance of a three-

phase wound-rotor induction machine on the machine starting current andtorque, as well as on the machine rotation speed and efficiency.

1. Describe the effect of increasing the rotor resistance of a three-phasewound-rotor induction machine on the machine starting current.

2. Describe the effect of increasing the rotor resistance of a three-phasewound-rotor induction machine on the machine starting torque.

CONCLUSION

REVIEW QUESTIONS


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance Review Questions


3. Why is the rotor resistance of a three-phase wound-rotor induction machineusually short-circuited after the machine initial start-up?

4. How is it possible to vary the speed of a three-phase wound-rotor inductionmachine?

5. What is the main drawback of lowering the speed of a three-phase wound-rotor induction machine using the machine rotor resistance?


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Sample

Extracted from

Instructor Guide


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Exercise 2 Three-Phase Wound-Rotor Induction Machine with Rotor Resistance


Exercise 2 Three-Phase Wound-Rotor Induction Machine

with Rotor Resistance

15.The following oscilloscope screen shows the obtained results.

Three-phase wound-rotor induction machine starting current and speed with short-circuitedrotor and with rotor resistance.

Amplitude of starting current with a short-circuited rotor =8.11 A

Amplitude of starting current with rotor resistance=4.42 A

16.RMS value of starting current with a short-circuited rotor =5.73 A

RMS value of starting current with rotor resistance =3.13 A

17.Starting current with a short-circuited rotor=458% of nominal current

Starting current with rotor resistance =250% of nominal current

18.Yes

Yes. The machine speed increases more rapidly during start-up with rotorresistance than with a short-circuited rotor. Consequently, the machinestarting torque is higher with rotor resistance than with a short-circuited rotor.

ANSWERS TOPROCEDURE STEPQUESTIONS

Oscilloscope SettingsChannel-1 Scale ............................ 5 A/divChannel-2 Scale .................. 200 r/min/divShow memories ........................Memory 1Show cursors ........................... HorizontalTime Base ................................. 50 ms/divTrigger type ............................... HardwareTrigger source ................... ................. Ch1Trigger level .................... ...................... .. 2

Motor speed withrotor resistance

Motor speed with short-circuited rotor

Motor current withshort-circuited rotor

Motor current withrotor resistance


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28.The results obtained are presented below.

Three-phase wound-rotor induction machine speed , torque , mechanical power , line current , active power ,reactive power , power factor , and efficiency when resistance is added to the rotor.

Speed

(r/min)

Torque

(Nm) [lbfin]

Mechanicalpower

(W)

Linecurrent

(A)

Active power

(W)

Reactivepower

(var)

Powerfactor

Efficiency

(%)

1343 1.79 (15.8) 251.7 1.54 415.6 334.6 0.78 60.6

1438 1.49 (13.2) 223.7 1.39 345.0 325.5 0.73 64.8

1495 1.28 (11.3) 200.8 1.29 298.5 322.6 0.68 67.3

1547 1.09 (9.64) 176.4 1.21 255.7 333.1 0.61 69.0

1598 0.90 (7.94) 150.1 1.16 214.6 326.6 0.55 69.9

1643 0.71 (6.32) 122.9 1.10 176.5 329.3 0.47 69.6

1678 0.57 (5.01) 99.5 1.08 145.7 332.8 0.40 68.3

1706 0.43 (3.84) 77.6 1.06 120.9 336.3 0.34 64.1

1741 0.28 (2.47) 50.9 1.05 90.4 342.1 0.26 56.3

1772 0.14 (1.22) 25.6 1.05 62.6 350.3 0.18

1801 -0.01 (-0.04) -1.0 1.06 36.4 354.3 0.10

1826 -0.13 (-1.14) -24.7 1.06 11.7 360.8 0.03

1853 -0.25 (-2.24) -49.2 1.08 -12.1 369.1 0.03 24.6

1877 -0.38 (-3.34) -74.1 1.10 -35.2 374.6 0.09 47.5

1902 -0.51 (-4.48) -100.7 1.13 -57.8 384.7 0.15 57.4

1923 -0.62 (-5.47) -124.4 1.16 -77.9 392.6 0.20 62.6

1944 -0.73 (-6.47) -148.7 1.19 -98.2 400.0 0.24 66.1

1968 -0.86 (-7.57) -176.2 1.22 -120.0 408.6 0.28 68.1

1987 -0.96 (-8.46) -198.8 1.25 -137.2 406.7 0.32 69.0

2010 -1.07 (-9.50) -225.9 1.29 -158.8 421.0 0.35 70.3

2028 -1.18 (-10.4) -249.8 1.32 -175.9 430.4 0.38 70.4

29.The resulting graphs are shown below.

When the machine torque is expressed in Nm:

Three-phase wound-rotor induction machine torque (expressed in Nm) as a function of themachine speed when the rotor is short-circuited, as well as when resistance is added to therotor.

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0 300 600 900 1200 1500 1800 2100

Machine speed (r/min)

Machinetorque

(Nm)

With a short-circuited rotor

With rotorresistance


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When the machine torque is expressed in lbfin:

Three-phase wound-rotor induction machine torque (expressed in lbfin) as a function of themachine speed when the rotor is short-circuited, as well as when resistance is added to therotor.

Yes. When the rotor resistance of a three-phase wound-rotor inductionmachine is increased, the machine torque versus speed curve becomesmuch less steep, in both motor and generator operation. This means that,when the rotor resistance is increased, the machine torque increases muchless rapidly as the machine slip increases.

Yes

30.The resulting graph is shown below.

Three-phase wound-rotor induction machine line current as a function of the machine

mechanical power when the rotor is short-circuited, as well as when resistance is added tothe rotor.

No

-20

-15

-10

-5

0

5

10

15

20

25

0 300 600 900 1200 1500 1800 2100

0

0.5

1

1.5

2

2.5

-450.0 -350.0 -250.0 -150.0 -50.0 50.0 150.0 250.0 350.0 450.0

Machine speed (r/min)

Machinetorque

(lbfin)



Machine mechanical power (W)

Machinelinecurrent




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Three-phase wound-rotor induction machine active power and reactive power as afunction of the machine mechanical power when the rotor is short-circuited, as well aswhen resistance is added to the rotor.

No


Three-phase wound-rotor induction machine power factor as a function of the machinemechanical power when the rotor is short-circuited, as well as when resistance is added tothe rotor.

No

-300.0

-200.0

-100.0

0.0

100.0

200.0

300.0

400.0

500.0

600.0

-450.0 -350.0 -250.0 -150.0 -50.0 50.0 150.0 250.0 350.0 450.0

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

-450.0 -350.0 -250.0 -150.0 -50.0 50.0 150.0 250.0 350.0 450.0

Machine mechanical power(W)

Machineactivepow

er(

W)

Machinereactivepower(

var)

Reactive power witha short-circuited rotor

Active power with ashort-circuited rotor

Active power withrotor resistance

Reactive power with rotor resistance


Machinep

owerfactor

With rotor

resistance



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33.The resulting graphs are shown below.

Three-phase wound-rotor induction machine efficiency as a function of the machinemechanical power with a short-circuited rotor, as well as with rotor resistance, when themachine operates as a motor.

Three-phase wound-rotor induction machine efficiency as a function of the machinemechanical power with a short-circuited rotor, as well as with rotor resistance, when themachine operates as a generator.

When resistance is added to the rotor of the three-phase wound-rotorinduction machine, the machine efficiency is approximately equal to orlower than when the rotor is short-circuited, and stabilizes at a lower value.

1. Increasing the rotor resistance of a three-phase wound-rotor inductionmachine reduces the amount of current drawn by the machine during start-up.

2. Increasing the rotor resistance of a three-phase wound-rotor inductionmachine shifts toward the left the breakdown torque region on the machinetorque versus speed curve. Thus, the higher the machine rotor resistance (upto a certain value), the higher the machine starting torque.

0

10

20

30

40

50

60

70

80

90

100

0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0 450.0

0

10

20

30

40

50

60

70

80

90

100

-400.0 -350.0 -300.0 -250.0 -200.0 -150.0 -100.0 -50.0 0.0

ANSWERS TO REVIEWQUESTIONS


Machineefficiency

(%)




Machineefficiency

(%)




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3. After the initial start-up of a three-phase wound-rotor induction machine, ahigh rotor resistance causes the torque produced by the machine todecrease rapidly as the machine speed increases. This is why the machinerotor resistance is usually short-circuited after initial start-up to optimize thetorque produced by the machine.

4. It is possible to vary the speed of a three-phase wound-rotor inductionmachine by varying the machine rotor resistance. The higher the machinerotor resistance, the lower the machine speed for a given torque.

5. The main drawback of lowering the speed of a three-phase wound-rotorinduction machine by increasing the machine rotor resistance is the reductionof the machine efficiency. This is because increasing the machine rotorresistance increases the machine copper () losses.


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Bibliography

Wildi, Theodore, Electrical Machines, Drives, and Power Systems, 6th Edition,

Upper Saddle River, Prentice Hall, 2005, ISBN 978-0131776913.

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