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Department of Electrical and Computer Engineering

The University of British Columbia

EECE 374 Experiment 2

Permanent Magnet DC Machine

Objective

You will make some measurements enabling you to derive some equivalent circuit parameters for

a 48V HP permanent magnet DC machine. This experiment will be available for 2 weeks, but it

should not take more than one.

Preparation

Revise your notes on DC machines and make sure that you understand the equivalent circuit and

related equations. There are some simple models in PSIM which may be of use.

Tests

Every student should use a standard laboratory notebook for calculations and recording results.

Students using loose leaf paper will be penalized 5% of the mark.

The test bench consists of a permanent magnet machine mounted in a cradle so that the shaft torque

can be measured. There is also an incremental shaft encoder allowing position and speed to be

determined. Various test machines can be mechanically coupled to this dynamometer, including an

identical DC machine. These machines have been aligned in numbered sets and should be used in

the same sets. Care should be taken when changing the test machines so that the dynamometer shaft

is not subjected to knocks, which can affect the alignment and subsequent readings.

1. Set up the transformer box as a 12 pulse diode bridge. The teaching assistant will

demonstrate this. An internal three ohase variac will allow you to produce a variable DC

voltage on the range of about 0 50V.

2. Connect the fan of the resistive load bank and connect this in series with the output of thediode bridge and the armature of the dynamometer (note that the green plug is a ground and

should not be connected to the power circuit).

3. Arrange to measure the current in this circuit . This can be done either directly with a meter

or by using a Hall Effect current transducer or small resistor to provide a voltage output

proportional to the current.

4. Without any other machine coupled to the dynamometer use a wooden dowel resting against

the bench and between the prongs of the metal dynamometer coupling to prevent the

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dynamometer shaft from rotating.

5. Keep the dowel firmly in place and by using a combination of variac and external resistor

settings take a set of readings at about rated armature current and half rated current. This

should be done quickly to avoid overheating the stationary armature. You should record the

voltage across the armature terminals, the armature current and the torque. Tapping the

dowel lightly will ensure that there is no stiction effect in the torque readings.6. Reduce the current to zero and record the zero offset in the torque reading. You should also

use the calibration weight to check the full scale torque reading. The teaching assistant will

demonstrate this.

7. Remove the dowel and the external resistor box so that the bridge is now connected directly

to the armature terminals and the shaft is free to turn. You should still include a means of

measuring the armature current.

8. For about rated and half rated armature voltage record the speed and armature current. The

torque reading should be zero (why?).

9. From the above readings estimate values of armature resistance, kand friction and windage

torque for this machine. Note that kcan be estimated in two ways and the friction and

windage torque can be estimated from the no load current.10. Connect the electrolytic capacitor provided across the input terminals of the chopper unit,

making sure you use the correct polarity. With the variac set to produce minimum voltage

connect the input of the chopper to the output of the diode bridge, again ensuring the correct

polarity.

11. Use one leg of the chopper as a two quadrant chopper and connect the output to the armature

terminals, including a means of observing the armature voltage waveform.

12. Use the oscilloscope to set the signal generator output to produce pulses of about 5% duty

cycle with an amplitude of 5 - 10V and a frequency of 10 kHz.

13. Connect this pulse output to the appropriate BNC control input of the chopper module and

gradually bring the variac voltage up to maximum value. You should observe the armature

voltage and current during this process to ensure that the chopper is working properly.14. Change the duty cycle slowly to 50% and then record the armature voltage and current

waveforms and machine speed. Note the qualitative effect of increasing and decreasing the

chopper frequency.

15. Returning the chopper frequency to 1 kHz increase the duty cycle to the maximum possible.

16. Reduce the variac voltage to zero and couple the second DC machine to the dynamometer.

Connect the armature terminals of this directly to the output of the diode bridge, including

a means of measuring this current.

17. It is important to ensure that both machines are tending to run in the same direction so unplug

the dynamometer armature temporarily and increase the variac until the two machines start

turning. Check the direction and if it is not the same as before reverse the connection at the

armature of the new machine.

18. With the variac set to minimum reconnect the dynamometer armature as before and then

gradually increase the variac output, observing the current in the dynamometer. This should

not be taken above rated value.

19. Increase the variac output as high as possible and then if necessary reduce the chopper pulse

width gradually until the dynamometer armature current is at about its rated value. The

dynamometer should now be acting as a generator with a negative average current.

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20. Measure both armature currents and of possible the shaft torque. Measure the average

armature voltages and from these readings estimate the electrical power flow in each

machine and the mechanical power exchange. Try to estimate the electrical and mechanical

losses and show the total power balance.

Report

A brief report is due from every student one week after the second laboratory session. It should

include both the simulation and measured results with comments on the comparisons. What

maximum current and voltage ratings would you recommend for this converter?

16 October 2004