COMMUTATOR MACHINES
A.
B.
PREPARATION
1.
Introduction
2.
Fundamental Physics (The Right-Hand Rule)
3.
The Simple Commutator
4.
Field Generation and Introduction to More Advanced D.C. Commutation
5.
Equivalent Circuit of D.C. Machines
6.
What Happens if You Plug into A.C. by Mistake?
7.
Universal Motors
8.
References
EXPERIMENT
1.
Equipment List
2.
The Universal (Series) Motor
3.
a.
Preliminaries
b.
Direction of Rotation
c.
DC Starting Current
d.
DC Torque versus Speed
e.
AC Torque versus Speed
f.
AC Speed versus Voltage
Report
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Figure 10.2
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Chapman, S. J., 2002. Electric Machinery and Power System
Fundamentals (1st ed.) McGraw Hill
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B.
EXPERIMENT – Commutator Machines
1.
Equipment List
1
Instrument rack containing the usual equipment
1
Boehm 1/3 hp, 5000 rpm, Type 73 universal motor mounted on a test
plate with force transducer and rpm transducer
1
variable autotransformer (Variac)
1
wattmeter
?
such other items as may profitably be employed in making the mandated
measurements
2.
The Universal (Series) Motor
a.
Preliminaries. The set-up for this experiment requires the following: a
wattmeter to measure power, a current measuring device to determine motor
current, a voltmeter to measure motor voltage, and a dynamometer. Record all
pertinent nameplate data. Before applying any power or configuring your
apparatus, determine the DC resistance of the motor.
The motor is mounted in a test rig that is mated to a dynamometer. The
dynamometer allows one to control the torque applied to the motor shaft and to
measure both rpm and torque. The rpm sensor operates on a Hall effect
principle and consists of a disk bearing two magnetic pole-pairs on its periphery
and a detector that pulses once each time a pole-pair is rotated past it. Observe
that, by counting the output of the detector, you will get a frequency, φ [Hz], that
is twice the actual number of revolutions per second of the motor. Therefore,
motor rpm = n = 60×(½φ) = 30φ.
To observe the rpm sensor in operation, proceed as follows. Energize the
detector with a voltage equal to 20 VDC and attach its output to the
oscilloscope, the torque controller, and frequency counter via coaxial cables.
You should now be able to appropriately configure your apparatus.
WARNING: BE CAREFUL !!
It can hurt to tangle with a rotating motor.
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b.
Direction of Rotation. Plug the motor control load box to 115 VDC. Set
the dvnamometer torque to a minimum. Turn on the power and observe the direction of
rotation. Turn off the power, reverse the power leads, turn on the power, and again
observe the direction of rotation. Turn off the power, reverse the FWD/REV switch, turn
on the power, and yet again observe the direction of rotation. Turn off the power and
set the FWD/REV switch to produce clockwise rotation when viewed looking toward
the shaft end (front) of the motor.
c.
DC Starting Current. With V set to 115 VDC and the dynamometer torque
at a minimum, observe the starting current using single sweep option on the scope and
make a hardcopy. Use the Hall Effect Clamp-On Probe to measure starting current.
d.
DC Torque versus Speed. With V set to 115 VDC, measure the input
power to the motor P(n), dynamometer torque T(n), and the motor current I(n) versus
speed n in rpm (30φ). Avoid exceeding the wattmeter current limit of 5 Amp.
CAUTION ! DO NOT OVERHEAT THE DYNAMOMETER.
CAUTION ! DO NOT STALL THE MOTOR.
CAUTION ! USE EAR PROTECTION AS WARRANTED.
e.
AC Torque versus Speed. With Variac voltage V set to 115 VAC, 60 Hz,
measure P(n), T(n), and I(n) versus speed n in rpm (30φ). This is an exact analogue of
Part B.2.d. At the conclusion of this part, let the dynamometer cool thoroughly.
f.
AC Speed versus Voltage. With Variac voltage V set to 120 VAC,
increase the dynamometer control current until motor current I is roughly one-half its
rated value (i.e., about 2.5 A); do not adjust the dynamometer control current again
during this part of the experiment. Then obtain data sets for n(V), P(V), T(V), and I(V)
for a range of values of voltage V.
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3.
Report
a.
Explain how you measured the DC resistance and why you did it this way.
b.
Discuss and explain your observations on direction of rotation.
c.
Present the scope hardcopy of the starting transient and determine the
experimental peak value of current. Also, calculate the theoretical peak
value of the starting current and compare it to the experimental peak
value. Discuss any reasons for differences between the two.
d.
Plot P(n) vs. VI(n) for DC operation. Comment tersely and cogently#.
e.
For operation at 115 VDC, make a table of the following parameters:
speed n in rpm (30φ), P(n) in Watts, T(n) in both ft-lb and n-m, I(n) in Amp,
PDEL in both Hp and Watts, and efficiency η(n) in %. Note that efficiency
η(n) = PDEL/P where both PDEL and P are in Watts. In a second table,
show speed n in rpm and normalized values of the above parameters
where they have been normalized to their values at the smallest value of
n (slowest speed). Now on a SINGLE graph sheet, plot the normalized
parameters versus speed n in rpm. Comment tersely and cogently.
f.
Repeat Part 3.e above for operation at 115 VAC. Add phase angle θ(n) in
degrees inferred from the data to the tables, but do not normalize it. On a
single graph sheet, plot all normalized parameters and also phase angle θ
verses speed n in rpm. Comment cogently.
g.
Make a plot of non-normalized T(n) ft-lb and η(n) versus speed n in rpm for
DC and AC operation at 115 V and comment on these differences in
behavior.
h.
For Part B.2.f, make a table including V, T(V) in ft-lb, I(V), n(V), and η(V)
and then plot T(V), I(V), n(V), and η(V) versus V on a single graph sheet.
Comment tersely and cogently.
#
It is the aim of the instructors to make this course piffle-free. So strive for deep insight in your comments.
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