dc motor drives, 3 speed feedback and current … current control in pwm dc motor drives using...

© Festo Didactic 88553-00 37

When you have completed this exercise, you will know how to improve the regulation of speed in PWM dc motor drives. Methods to control the armature current using either voltage rate-of-change limitation or a current feedback loop and a current limiter in PWM dc motor drives will also be studied. You will understand the block diagrams of the circuitry and the operation mechanisms related to both speed regulation and armature current control.

The Discussion of this exercise covers the following points:

Improving speed regulation in PWM dc motor drivesThe A-B shaft encoder. Operation of the speed feedback loop.

Armature current control in PWM dc motor drives using voltage rate-of-change limitation

Armature current control in PWM dc motor drives using current feedback loop and a current limiter

Improving speed regulation in PWM dc motor drives

The previous exercises illustrated that varying the torque of the mechanical load applied to the motor in a PWM dc motor drive (for a given duty cycle) causes the rotation speed to change significantly. This can be problematic in applications where the speed of rotation must be tightly controlled (i.e., maintained to a constant value).

Speed regulation in a PWM dc motor drive can be greatly improved by adding a speed feedback loop which measures the motor rotation speed and automatically adjusts the dc voltage which the chopper applies to the dc motor (by automatically adjusting the duty cycle of the chopper) to maintain the motor speed at the desired value (i.e., to the speed command value). Such a PWM dc motor drive including a speed feedback loop is shown in Figure 16. In this example, a four-quadrant chopper is used to make the drive bidirectional, but a buck-boost chopper could also have been used if a unidirectional drive were required or sufficient.

Speed Feedback and Current Control in PWM DC Motor Drives

Exercise 3

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Exercise 3 – Speed Feedback and Current Control in PWM DC Motor Drives Discussion

38 © Festo Didactic 88553-00

Figure 16. A PWM dc motor drive (a bidirectional PWM dc motor drive with regenerative braking in this case) with a speed feedback loop.

A speed feedback loop consists of a speed sensor, a speed measurement circuit, an error detector, and a proportional-integral (PI) amplifier. The speed sensor in the case shown in Figure 16 is an A-B type shaft encoder mounted on the dc motor shaft.

The A-B shaft encoder

A shaft encoder is an electromechanical device that translates position and/or rotation speed into electrical signals which can then be interpreted. An A-B shaft encoder can be made of an annular permanent magnet fixed to one end of the motor shaft and of two Hall-effect sensors located at fixed positions along the course of the magnet. The two sensors are separated by a 90° angle as shown in Figure 17.

Battery DC Motor

PWM Generator

Four-Quadrant Chopper

Duty Cycle ( ) Control

Speed Feedback Loop

Speed Sensor(A-B Shaft Encoder)

A

B

Speed Measurement

Circuit

PI Amplifier

SpeedCommand

( )

Error Detector



Figure 17. A simple A-B encoder and its signals waveforms.

The signals produced by sensors A and B in the encoder are typically square waves whose frequency is proportional to the rotation speed of the shaft. There is a 90° phase shift between the signal of sensor A and the signal of sensor B. The polarity of this phase shift indicates the direction of rotation (clockwise if signal A leads signal B, counterclockwise otherwise).

Operation of the speed feedback loop

The speed feedback loop in Figure 16 is able to regulate the speed of rotation of the motor by varying the duty cycle of the four-quadrant chopper. A speed command (i.e., a speed setpoint) is given by an operator and enters the loop. This value is compared to the actual speed of rotation of the motor, determined by the speed measurement circuit using the A-B shaft encoder signals, to

generate an error ( ). The error is then amplified by the PI

amplifier and used to adjust the chopper duty cycle in order to bring the error to

zero (and consequently, make equal to ). For instance, when the

rotation speed is less than the speed command, the error detector signal is positive, thereby increasing the duty cycle of the four-quadrant chopper. This increases the dc voltage which the four-quadrant chopper applies to the dc motor until the speed error is corrected. At this point, the error detector signal is zero and the PI amplifier produces a fixed signal that sets the duty cycle of the four-quadrant chopper to the exact value required to maintain equilibrium. A well-tuned PI amplifier allows the speed feedback loop to adjust the duty cycle promptly in order to maintain the desired motor speed.

Armature current control in PWM dc motor drives using voltage rate-of-change limitation

As observed in the previous exercises, the dc armature current ( ) is likely to

exceed the nominal value (i.e., the nominal armature current of the motor) whenever the duty cycle of the chopper in a PWM dc motor drive is changed significantly to vary the motor speed. The larger the change in the duty cycle, the higher the dc armature current increase. It must also be stated that the higher the inertia of the mechanical load, the slower the rate at which the motor speed can change and, consequently, the longer the time during which the nominal armature current of the motor can be exceeded. This becomes problematic when the period during which the nominal armature current is exceeded lasts long enough to cause the motor overload protection to trip.

North pole of magnet Shaft

end

Sensor A

Sensor B

Sensor signals

A

B

Clockwise rotation Signal A leads signal B

Counterclockwise rotation Signal B leads signal A

South pole of magnet



The above observations about the operation of PWM dc motor drives rapidly bring the following question to mind: Why is the magnitude of the dc motor armature current ( ) increasing considerably whenever the duty cycle of the

chopper is changed significantly to vary the motor speed? In brief, this is due to a large momentary increase in the difference between the dc voltage applied to the armature of the dc motor and the counter-electromotive force voltage produced by the dc motor, as explained below.

The dc voltage applied to the armature of the dc motor ( ) varies almost

instantaneously when the chopper duty cycle is varied. On the other hand, the

motor counter-electromotive force voltage ( ) can only vary as fast as the rate at which the motor speed ( ) can change, this rate being inversely proportional to the load inertia. Consequently, the dc voltage across the motor armature resistance ( ), which is equal to , increases greatly

whenever the duty cycle of the chopper is changed, thereby causing the dc motor

armature current ( ) to increase considerably. The dc motor armature current

( ) remains high as long as the motor counter-electromotive force voltage

( ) has not virtually caught up with the dc motor armature voltage ( ).

This is shown in Figure 18.



Figure 18. The magnitude of current increases considerably whenever the chopper duty

cycle is changed significantly because the difference between voltage EA, dc and voltage ECEMF (i.e., voltage EAR, dc) increases greatly.

In PWM dc motor drives, the increase in the dc motor armature current ( )

during accelerations and decelerations can be controlled by limiting the rate at which the dc voltage applied to the motor by the chopper (i.e., the dc motor armature voltage ) can change. By properly setting the maximum rate of

change of the dc voltage ( ) applied to the motor armature by the chopper, it

is possible to prevent the value of this voltage from significantly diverging from the motor counter-electromotive force voltage ( ) during accelerations and

Duty cycle ( ) anddc motor armature

voltage ( )

Motor speed ( )and motor CEMF

voltage ( )

DC armatureresistance voltage

( )

DC motor armature

current ( )Time 0

Time 0

Time 0

Time 0

Nominal armature current of the motor



decelerations, thereby preventing the dc motor armature current ( ) from

increasing considerably.

In PWM dc motor drives with no speed feedback loop, rate-of-change limitation of the dc voltage applied to the motor armature ( ) is achieved by directly

limiting the rate at which the chopper duty cycle ( ) can change. This is shown in Figure 19 below.

Figure 19. Rate-of-change limitation of the dc armature voltage (EA, dc) achieved via direct

limitation of the rate of change of the chopper duty cycle ( ).

In PWM dc motor drives with a speed feedback loop, rate-of-change limitation of

the dc voltage applied to the motor armature ( ) is achieved by limiting the

rate at which the speed command ( ) can change, as shown in Figure 20

below.

Chopper

PWM Generator

Rate-of-Change Limiter


Time

Time

Time

DC

moto

r arm

atu

re

cu

rre

nt

()

Nominal armaturecurrent of the motor

and

0

0

0

Du

ty c

ycle

()

Battery

DC motor



Figure 20. Rate-of-change limitation of the dc armature voltage (EA, dc) achieved via limitation of the rate of change of the speed command ( ).

Control of the dc armature current in PWM dc motor drives using rate-of-change limitation of the dc armature voltage works well as long as nothing prevents the motor speed from varying as expected upon a change of the chopper duty cycle or a change of the speed command. It does not work well (i.e., it does not prevent the dc armature current from exceeding the nominal value) when the motor speed can no longer vary as expected, such as when the load inertia becomes much higher than normal or when motor rotation is prevented by a frozen bearing (either on the motor or on the load). In such a situation, a current feedback loop and a current limiter can be added to the PWM dc motor drive to prevent excessive motor armature current from causing the motor overload protection to trip. This is discussed in the next section of this discussion.

Armature current control in PWM dc motor drives using current feedback loop and a current limiter

Adding a current feedback loop and a current limiter to a PWM dc motor drive is a reliable means of controlling the dc armature current so as to prevent the motor overload protection from tripping under any circumstances (even when abnormal operating conditions are experienced, such as those mentioned above). Also, a current feedback loop is commonly used in conjunction with a speed feedback loop in PWM dc motor drives because using a current feedback loop alone does not provide good speed regulation.

Chopper

PWM Generator



Time

Time

Time

Nominal armaturecurrent of the motor

0

0

0

Battery

DC motor

Speed

Command ( )

Speed Feedback Loop

, , and



Such a drive is shown in Figure 21. A generic chopper is shown in the block diagram of the PWM dc motor drive. A buck-boost chopper or a four-quadrant chopper is generally used depending on whether a unidirectional drive or a bidirectional drive is required.

Figure 21. A PWM dc motor drive with speed and current feedback loops and a current limiter.

The speed feedback loop in the PWM dc motor drive above operates as explained in the first section of this discussion. However, the output of the speed feedback loop is not used to directly control the duty cycle of the chopper. Instead, it is used as the current command of the current feedback loop in what is known as a cascade configuration. In a cascade control scheme, the output of a

BatteryDC Motor

Speed Sensor(A-B Shaft Encoder)

A

B

Speed Measurement

Circuit

Speed Error Detector

Speed Feedback Loop

Current Feedback Loop

Current Error Detector


Chopper

PI Amplifier

Limiter

Speed Command

( )

PWM Generator

Current Command

( )

PI Amplifier

Lowpass Filter




first feedback loop (the master loop) becomes the setpoint (command) of a second feedback loop (the slave loop). The process variable controlled by the slave loop must vary much more quickly than the process variable controlled by the master loop. In the case of the PWM dc motor drive above, the slave loop variable is the dc armature current while the master loop variable is the motor rotation speed. Also, the output of the speed feedback loop is passed through a limiter to bound the maximum value of the current command, and thus, ensure that the dc armature current never exceeds this limit. In general, the current limit is commonly set to a value close to the nominal value of the motor armature current to avoid motor overload. It is possible to set the current limit to a value that significantly exceeds the nominal armature current of the motor for short periods of time in order to increase the performance (notably the acceleration and deceleration) of the dc motor drive. This, however, requires some additional control to ensure that momentary increases of the current limit do not increase the risk of having a motor overload significantly.

The current feedback loop in the PWM dc motor drive above is able to regulate the armature current of the motor by varying the duty cycle of the chopper. A current command (i.e., a current setpoint) is given by the speed feedback loop, goes through a limiter, and enters the current feedback loop. The value of the limited current command is compared to the actual dc armature current of the

motor to generate an error ( ). Notice that the dc value of

the armature current is obtained by lowpass filtering the armature current measured with a current sensor. The error is then amplified by the PI amplifier and used to adjust the chopper duty cycle in order to bring the error to zero (and

consequently, make equal to ). For instance, when the dc armature

current is higher than the limited current command, the error detector signal is negative, thereby decreasing the chopper duty cycle. This decreases the dc voltage which the chopper applies to the dc motor until the error is corrected. At this point, the error detector signal is zero and the PI amplifier produces a fixed signal that sets the chopper duty cycle to the exact value required to maintain equilibrium.

Even though a current feedback loop and a current limiter are used to achieve proper armature current control, this is generally not sufficient to prevent the motor speed from varying too rapidly in applications requiring gradual speed variation. Consequently, even if a current feedback loop and a current limiter are used, a rate-of-change limiter is often included at the input of the speed feedback loop to limit the rate at which the motor speed can vary (i.e., to limit the rate of motor acceleration or deceleration).

The PWM dc motor drive shown in Figure 21 presents a dynamic performance which is slower than an equivalent drive (i.e., a PWM dc motor drive with a speed feedback loop) without limitations. Therefore, it requires more time to attain a new speed. On the other hand, the rate of motor acceleration or deceleration and the dc armature current are tightly controlled, thereby ensuring reliable operation of the drive in almost any situation.

Exercise 3 – Speed Feedback and Current Control in PWM DC Motor Drives Procedure Outline


The Procedure is divided into the following sections:

Set up and connections

Operation of a PWM dc motor drive with a speed feedback loop

Armature current control using voltage rate-of-change limitation

Armature current control using a current feedback loop and a current limiter

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

banana jack connections with the power on unless otherwise specified.

Set up and connections

In this part of the exercise, you will set up and connect the equipment.

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

Install the equipment in the Workstation.

a Make sure that the Permanent Magnet DC Motor is installed to the right of the Four-Quadrant Dynamometer/Power Supply.

a Before beginning this exercise, measure the open-circuit voltage across the Lead-Acid Battery Pack, Model 8802, using a multimeter. If the open-circuit voltage is lower than 51.2 V, ask your instructor for assistance as the Lead-Acid Battery Pack is probably not fully charged. Appendix D of this manual in-dicates how to prepare (fully charge) the Lead-Acid Battery Pack before each laboratory period.

2. Mechanically couple the Four-Quadrant Dynamometer/Power Supply to the Permanent Magnet DC Motor using a timing belt.

Before coupling rotating machines or working on them, make absolutely sure that

power is turned off to prevent any machine from starting inadvertently.

3. Make sure that the main power switch on the Four-Quadrant Dynamometer/Power Supply is set to the O (off) position, then connect its Power Input to an ac power wall outlet.

4. Connect the Power Input of the Data Acquisition and Control Interface (DACI) to a 24 V ac power supply.

Connect the Low Power Input of the Chopper/Inverter to the Power Input of the DACI. Turn the 24 V ac power supply on.

PROCEDURE OUTLINE

PROCEDURE

Exercise 3 – Speed Feedback and Current Control in PWM DC Motor Drives Procedure


5. Connect the USB port of the DACI to a USB port of the host computer.

Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer.

6. Turn the Four-Quadrant Dynamometer/Power Supply on, then set the Operating Mode switch to Dynamometer.

7. Turn the host computer on, then start the LVDAC-EMS software.

In the LVDAC-EMS Start-Up window, make sure that the DACI and the Four-Quadrant Dynamometer/Power Supply are detected.

Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the DACI are available. Also, select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window.

8. Connect the Digital Outputs of the DACI to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable.

On the Chopper/Inverter, set the Dumping switch to the O (off) position. The Dumping switch is used to prevent overvoltage on the dc bus of the Chopper/Inverter. It is not required in this exercise.

9. Set up the circuit shown in Figure 22, which is a bidirectional PWM dc motor drive with regenerative braking and speed feedback. Use the Lead-Acid Battery Pack as a fixed-voltage dc power source for the bidirectional PWM dc motor drive.

Make sure to use the 40 A terminal of current input I2 of the DACI. Set the range of current input I2 to High (40 A) in the Data Acquisition and Control Settings window of LVDAC-EMS.

Connect Shaft Encoder Outputs A and B of the Four-Quadrant Dynamometer/Power Supply to Encoder Digital Inputs A and B, respectively, of the DACI using miniature banana plug leads. Connect the Shaft Encoder Outputs common (white terminal) of the Four-Quadrant Dynamometer/Power Supply to the digital (D) common (white terminal) of the DACI using a miniature banana plug lead.



Figure 22. Bidirectional PWM dc motor drive with regenerative braking (four-quadrant chopper dc motor drive) and speed feedback.

10. In LVDAC-EMS, open the Chopper/Inverter Control window, then make the following settings:

Set the Function parameter to Four-Quadrant DC Motor Drive without Current Control. This function allows a four-quadrant PWM dc motor drive with a speed feedback loop to be implemented.

Set the Switching Frequency parameter to 20 kHz.

Make sure the Command Input parameter is set to Knob. This allows the speed command of the dc motor drive to be adjusted manually using a control knob.

Set the Speed Command parameter to 0 r/min.

Make sure the Pulley Ratio parameter is set to 24:12.

Set the Controller Proportional Gain Kp parameter to 60.

Set the Controller Integral Gain Ki parameter to 5000.

The rate of change of the speed command must not be limited at this time. Consequently:

Make sure the Acceleration Time parameter is set to 0 s.

Make sure the Deceleration Time parameter is set to 0 s.

Battery Pack (48 V)

IGBT Chopper/Inverter

Switching control signals from the

DACI

PermanentMagnet

DC MotorMechanical Load

40 A

Shaft Encoder

To Encoder Digital Inputs A and B of

the DACI

A B



11. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window. In the Tools menu of this window, select Friction Compensation Calibration, which will bring up the Friction Compensation Calibration dialog box. Click OK in this box to start the calibration process. Observe that the prime mover starts to rotate at high speed, thereby driving the permanent magnet dc motor. The prime mover speed is then automatically decreased by steps to perform the calibration process. Once the calibration process is completed (which takes about two minutes), the prime mover stops rotating, then the Friction Compensation Calibration dialog box indicates that the calibration process is finished. Click OK in the Friction Compensation Calibration dialog box to close this box. Restart the Four-Quadrant Dynamometer/Power Supply to apply the changes (i.e., the newly calibrated friction compensation curve) by setting the main power switch of this module to O (off), and then I (on).

12. In the Four-Quadrant Dynamometer/Power Supply window, make the following settings:

Set the Function parameter to Mechanical Load. This makes the Four-Quadrant Dynamometer/Power Supply operate like a configurable mechanical load.

Set the Load Type parameter to Flywheel. This makes the mechanical load emulate a flywheel.

Set the Inertia parameter to 0.010 kg m2 (0.237 lb ft

2). This sets the

inertia of the emulated flywheel.

Set the Friction Torque parameter to 0.3 N m (2.66 lbf in). This sets the torque which opposes rotation of the emulated flywheel.

Set the Pulley Ratio parameter to 24:12.

Start the mechanical load. The dc motor is now coupled to a flywheel emulated by the mechanical load.

Operation of a PWM dc motor drive with a speed feedback loop

In this part of the exercise, you will use the PWM dc motor drive with a speed feedback loop to power the dc motor and you will observe its behavior as the load torque is changed.

13. In LVDAC-EMS, open the Metering window. Set six meters to measure the dc battery voltage (input E1), the dc battery current (input I1), the battery power (measured from inputs E1 and I1), the dc armature voltage (input E2), the dc armature current (input I2), and the electric power at the dc motor (measured from inputs E2 and I2).

Click the Continuous Refresh button to enable continuous refresh of the values indicated by the various meters in the Metering window.

Note that in the Four-Quadrant Dynamometer/Power Supply window you can observe the dc motor speed, torque, and mechanical power. Click the



Continuous Refresh button to enable continuous refresh of the values indicated by the various meters in the Four-Quadrant Dynamometer/Power Supply window.

14. In the Chopper/Inverter Control window, start the PWM dc motor drive with a speed feedback loop by clicking the Start/Stop button. Gradually increase the speed command from 0 r/min to 2000 r/min.

Does the motor speed stabilize at 2000 r/min?

Yes No

Record the value of the duty cycle of the four-quadrant chopper as well as the dc armature voltage:

Duty cycle (torque = 0.3 N m (2.66 lbf in)) %

DC armature voltage (torque = 0.3 N m (2.66 lbf in)) V

15. In the Four-Quadrant Dynamometer/Power Supply window, increase the Friction Torque parameter to 0.5 N m (4.43 lbf in).

Does the motor drive maintain a speed of 2000 r/min?

Yes No




16. In the Four-Quadrant Dynamometer/Power Supply window, decrease the Friction Torque parameter to 0.1 N m (0.89 lbf in).

Does the motor drive maintain a speed of 2000 r/min?

Yes No




17. In the Chopper/Inverter Control window, gradually decrease the speed command to 0 r/min, then stop the PWM dc motor drive with a speed feedback loop. In the Four-Quadrant Dynamometer/Power Supply window,



stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating.

18. Does adding a speed feedback loop to a PWM dc motor drive improve speed regulation?

Yes No

Explain the operation of the speed feedback loop from the observations and measurements performed so far.

Armature current control using voltage rate-of-change limitation

In this part of the exercise, you will use the PWM dc motor drive with a speed feedback loop to compare the dc armature currents during motor accelerations and decelerations measured with and without limitation of the rate of change of the dc armature voltage.

19. In the Four-Quadrant Dynamometer/Power Supply window, set the Inertia parameter to 0.050 kg m

2 (1.187 lb ft

2) and set the Friction Torque parameter

to 0.06 N m (0.53 lbf in). Start the mechanical load.

20. Open the Data Table window. Set the timer to make 300 records with an interval of 1 second between each record. This corresponds to a 5 minute period.

Set the data table to record the speed command, the chopper duty cycle, the motor speed, the dc armature voltage and the dc armature current. Also, set the data table to record the time associated with each record.

21. In the Chopper/Inverter Control window:

Make sure the Controller Proportional Gain Kp parameter is set to 60.

Make sure the Controller Integral Gain Ki parameter is set to 5000.

Start the PWM dc motor drive with a speed feedback loop by clicking the Start/Stop button. Gradually increase the speed command from 0 r/min to 1250 r/min and wait for the speed to stabilize.

Start the timer in the Data Table window to begin recording data.



22. Suddenly increase the speed command of the PWM dc motor drive from 1250 r/min to 2500 r/min and wait for the motor speed to stabilize. Once the motor speed has stabilized, suddenly decrease the speed command from 2500 r/min to 1250 r/min. Wait again for the motor speed to stabilize.

In the Data Table window, stop the timer, then save the recorded data.

In the Chopper/Inverter Control window, set the speed command to 0 r/min, then stop the PWM dc motor drive. In the Four-Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating.

23. Plot to graphs the evolution of the speed command, chopper duty cycle, motor speed, dc armature voltage, and dc armature current during the motor acceleration and deceleration using the data you saved to a file.

Observe the evolution of the dc armature current. Is the nominal armature current of the dc motor exceeded in this process?

Yes No

Why does the dc armature current increase so much when the speed command is changed significantly?

From the recorded data, determine the time durations required to accelerate and decelerate the load.

Acceleration time s

Deceleration time s

24. In the Chopper/Inverter Control window:

Set the Acceleration Time parameter set to 15.0 s.

Set the Deceleration Time parameter set to 15.0 s.

This limits the rate of change of the speed command, and ultimately, the rate of change of the dc armature voltage.



Start the PWM dc motor drive with a speed feedback loop by clicking the Start/Stop button. Gradually increase the speed command from 0 r/min to 1250 r/min and wait for the speed to stabilize.

In the Data Table window, add the limited speed command ( ) to

the parameters to be recorded. Start the timer in the Data Table window to begin recording data.




26. Plot to graphs the evolution of the speed command, limited speed command

( ) chopper duty cycle, motor speed, dc armature voltage, and dc

armature current during the motor acceleration and deceleration using the data you saved to a file.

Observe the evolution of the dc armature current. Is the nominal armature current of the dc motor exceeded during the motor acceleration?

Yes No

Why is that so?


Acceleration time (with rate-of-change limitation) s

Deceleration time (with rate-of-change limitation) s



Are the times required to accelerate from 1250 r/min to 2500 r/min and to decelerate from 2500 r/min to 1250 r/min the same as they were before the use of rate-of-change limitation of the dc armature voltage? How do they compare?

27. In the Chopper/Inverter Control window, start the PWM dc motor drive with a speed feedback loop by clicking the Start/Stop button. Gradually increase the speed command from 0 r/min to 1500 r/min and wait for the speed to stabilize.

28. In the Four-Quadrant Dynamometer/Power Supply window, momentarily increase the Friction Torque parameter to 0.65 N m (5.75 lbf in) to simulate an abnormal opposition to the rotation of the motor. Observe the armature current as you do so.

Is the nominal value of the armature current exceeded significantly?

Yes No

Is voltage rate-of-change limitation a reliable means of preventing excessive armature currents in abnormal operating conditions?

Yes No

29. In the Chopper/Inverter Control window, gradually decrease the speed command to 0 r/min, then stop the PWM dc motor drive with a speed feedback loop. In the Four-Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating.

Armature current control using a current feedback loop and a current limiter

In this part of the exercise, you will use a PWM dc motor drive with both a speed feedback loop and a current feedback loop including limitation of the dc armature current. The response of the system will be studied and compared to the previous results. Finally, the response for a more restrictive current limit will also be studied.

30. Modify your circuit as shown in Figure 23, which is a four-quadrant PWM dc motor drive with a speed feedback loop and current feedback loop.

Make sure to use the 40 A terminal of current input I4 of the DACI. This input is required for the current feedback loop. Set the range of current input I4 to High (40 A) in the Data Acquisition and Control Settings window of



LVDAC-EMS. Note that input I4 cannot be used for metering or observing the signal of the armature current because it is used as the current sensor of the feedback loop.

Figure 23. Bidirectional PWM dc motor drive with regenerative braking (four-quadrant chopper dc motor drive), speed feedback, and current feedback.

31. In the Chopper/Inverter Control window, make the following settings:

Set the Function parameter to Four-Quadrant DC Motor Drive. This function allows a four-quadrant PWM dc motor drive with a speed feedback loop and a current feedback loop to be implemented.

Set the Switching Frequency parameter to 20 kHz.

Speed Control

Make sure the Command Input parameter is set to Knob.

Set the Speed Command parameter to 0 r/min.

Make sure the Pulley Ratio parameter is set to 24:12.



Make sure the Acceleration Time parameter is set to 0 s.

Make sure the Deceleration Time parameter is set to 0 s.

Battery Pack (48 V)

IGBT Chopper/Inverter

Switching control signals from the

DACI

PermanentMagnet

DC Motor

Mechanical Load

40 A

Shaft Encoder

To Encoder Digital Inputs A and B of

the DACI

A B

40 A



Current Control



Set the Feedback Range parameter to 40 A.

Set the Feedback Filter Cutoff Frequency parameter to 250 Hz.

Set the Current Command Limit parameter to 6.7 A (i.e., the nominal armature current of the Permanent Magnet DC Motor).

32. In the Four-Quadrant Dynamometer/Power Supply window, make sure the Inertia parameter is set to 0.050 kg m

2 (1.187 lb ft

2) and the Friction Torque

parameter is set to 0.06 N m (0.53 lbf in). Start the mechanical load.

33. In the Data Table window, make sure the timer is set to make 300 records with an interval of 1 second between each record. This corresponds to a 5 minute period. Also, add the current command and limited current

command ( ) to the parameters to be recorded.

34. In the Chopper/Inverter Control window, start the PWM dc motor drive with speed and current feedback loops by clicking the Start/Stop button. Gradually increase the speed command from 0 to 1250 r/min and wait for the speed to stabilize.

Start the timer in the Data Table window to begin recording data.




36. Plot to graphs the evolution of the speed command, current command, limited current command, chopper duty cycle, motor speed, dc armature voltage, and dc armature current during the motor acceleration and deceleration using the data you saved to a file.



Observe the evolution of the dc armature current. Is the nominal armature current of the dc motor exceeded in this process? Why?


Acceleration time (with current limitation) s

Deceleration time (with current limitation) s

Are the times required to accelerate from 1250 r/min to 2500 r/min and to decelerate from 2500 r/min to 1250 r/min the same as they were before the use of any kind of limitation? How do they compare?

What would happen to the dynamic performance of the PWM dc motor drive if you were to lower the Current Command Limit parameter to 3.0 A? Try it out to validate your answer.

37. In the Chopper/Inverter Control window, set the Current Command Limit parameter to 6.7 A, then start the PWM dc motor drive with speed and current feedback loops by clicking the Start/Stop button. Gradually increase the speed command from 0 r/min to 1500 r/min and wait for the speed to stabilize.

38. In the Four-Quadrant Dynamometer/Power Supply window, increase the Friction Torque parameter to each of the values given in the table below. This large increase in friction torque is performed to simulate a strong and abnormal opposition to the motor rotation. For each value of torque, wait for the motor speed to stabilize and record the values of the parameters listed in the table.



Table 1. Experimental results as a function of the friction torque of the load.

Friction torque

(N m [lbf in])

DC armature current

(A)

DC armature voltage

(V)

Motor speed

(r/min)

Motor torque

(N m or lbf in)

Current command

(A)

Limited current

command (A)

Chopper duty cycle (%)

0.1 [0.89]

0.2 [1.77]

0.3 [2.66]

0.4 [3.54]

0.5 [4.43]

0.6 [5.31]

0.7 [6.20]

0.8 [7.08]

0.9 [7.97]

1.0 [8.85]

1.1 [9.74]

1.2 [10.62]

39. In the Chopper/Inverter Control window, gradually decrease the speed command to 0 r/min, then stop the PWM dc motor drive. In the Four-Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating.

40. Observe the values you recorded. Is the dc armature current still limited to the nominal armature current of the dc motor?

Yes No

Based on this observation, is using a current feedback loop and a current limiter a reliable means to prevent excessive dc armature currents, even under abnormal operating conditions?

Yes No

Explain how the limiter and current feedback loop limit the maximum value which the dc armature current can reach.

Exercise 3 – Speed Feedback and Current Control in PWM DC Motor Drives Conclusion


41. In the Tools menu of the Four-Quadrant Dynamometer/Power Supply window, select Reset to Default Friction Compensation. This will bring up the Reset Friction Compensation dialog box. Click Yes in this window to reset the friction compensation to the factory default compensation.

42. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables.

Make sure the Lead-Acid Battery Pack is recharged promptly.

This exercise introduced the use of a speed feedback loop to improve the behavior (i.e., speed regulation) of the PWM dc motor drive when the mechanical load changes. Rate-of-change limitation of the dc armature voltage was also presented to help in reducing the dc armature current spikes occurring during motor accelerations and decelerations, at the expense of some responsiveness of the dc motor drive. The use of a current feedback loop and current limitation was also introduced to demonstrate how tight control of the dc armature current under any operating conditions can be achieved. The resulting PWM dc motor drive constitutes a versatile solution which eliminates all the shortcomings of the basic PWM dc motor drive described at the beginning of this manual.

1. The speed feedback loop in a dc motor drive requires knowledge of the actual speed of the motor. Which device is commonly used to measure the motor speed?

2. The speed feedback loop in a dc motor drive generally consists of a speed sensor, a speed measurement circuit, an error detector, and _________________________________?

3. Why is the magnitude of the dc armature current ( ) increasing

considerably whenever the duty cycle of the chopper in a PWM dc motor drive is changed suddenly to vary the motor speed?

CONCLUSION

REVIEW QUESTIONS

Exercise 3 – Speed Feedback and Current Control in PWM DC Motor Drives Review Questions


4. Explain why rate-of-change limitation of the dc voltage applied to the motor armature by the chopper in a PWM dc motor drive is an efficient means of preventing the dc armature current from increasing considerably during motor accelerations and decelerations.

5. Which shortcomings of the basic PWM dc motor drive are overcome by adding a speed feedback loop, a current feedback loop, and a current limiter?

dc motor drives, 3 speed feedback and current … current control in pwm dc motor drives using...

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