Brushless MotorsSpring 2007

1. Introduction

2. Brushless DC Motor Characteristics

3. Experimental Procedures

4. Report

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Control Current

VM1

Load Box +

–

Armature Current, I

Wattmeter

VM2 , I

are correctly connected. You must have your connections checked by the instructor before proceeding. The 50-Ω rheostat, R1, must initially be set to

maximum resistance. R2 should remain at 50 Ω throughout the experiment∂.

b) We will first view the waveforms of the voltages that are applied to the

motor power leads. With the DC wall voltage at 40 VDC as measured by VM1,

apply power to the circuit. Adjust R1 to obtain a steady speed of 3000 rpm.

Obtain printouts of the voltages between points A, B, C, and chassis ground; also

obtain printouts of the A-to-B and A-to-C line-to-line voltages. In both cases,

note the relative time displacements and magnitudes of the waveforms.

c) Next we will verify that the no-load motor speed is proportional to applied

voltage. Vary the voltage to the motor in 5 volt steps from 0 to 30 volts by using

several settings of R1. Record the motor speeds and the VM2 voltages.

d) Finally, we will measure the motor’s speed-torque characteristic. With R1

initially set at 50 Ω, adjust R1 to keep VM2 = 15 VDC as you load the motor with

the dynamometer. Record motor speed, torque, VM1, VM2, armature current I,

and power for armature currents from minimum to 5 Amps. Do not exceed 5 A. Repeat for VM2 = 30 VDC.

e) With all power connections removed, measure all 3 DC line-to-line motor

resistances.

∂ The “A” student will have noted the absence of an Equipment List. By now you should be able to collect a reasonable set of experimental tools with but minimal coaching.

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4. Report

a) Present graphs of the waveforms that you observed in part 3b. Explicitly

indicate the relative time displacement and voltage magnitude at each point on

the waveform where it makes a significant change. Comment cogently.

b) Present in tabular form the data from part 3c with no-load speed in rpm.

Using the data in this table, plot no-load speed (rpm) versus applied voltage.

From this plot/data deduce, an approximate functional interrelationship between

no-load speed and applied voltage.

c) From the data taken for part 3d, plot the torque (ft-lb) vs. speed (rpm) for

the motor with 15 VDC applied. On the same axes, plot also the data for 30 VDC

applied. Infer the stall torque of the motor and the no-load speed at both

voltages. Comment on their characteristics. Do the graphs match theoretical

expectations? A simple “yes” or “no” answer here does not suffice.

d) How would you reverse the direction of rotation of this motor?

e) Calculate the number of magnetic poles that this motor has.

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