ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 Lab 12: The universal motor.
Objective: to examine the construction of the universal motor; to determine its no-load and full-load
characteristics while operating on AC; to determine its no-load and full-load characteristics while
operating on DC; to compare the starting torque on both AC and DC; to observe the effect of
removing the compensating winding Equipment: Power Supply, DAI, Universal motor (8254), Electrodynamometer (8960), timing belt.
Theory:
The AC/DC universal motor is found in portable tools such as electric drills, saws, sanders, etc.,
and in home appliances such as vacuum cleaners, electric mixers, blenders, etc., where high speed,
power and small size are an advantage. However, it is closer in concept to the DC motor than to
the AC motor and, therefore, has some inherent disadvantages, which could be avoided in
purely AC induction motors; chiefly, the need for commutation and brushes.
The universal motor is basically a series DC motor which is specially designed to operate on
AC as well as on DC. A standard DC series motor has very poor characteristics when operated on AC,
mainly due to two reasons:
a) The high reactance of both the armature and field windings limits AC current to a much
lower value than DC current (for the same line voltage).
b) If solid steel is used for the stator frame, AC flux will produce large eddy currents in the
frame with consequent heating.
To insure satisfactory operation of the universal motor from an AC power source, some
modifications are necessary. The reactance of the series field and armature windings must be
reduced as much as practicable. The reactance of the series field winding can be somewhat reduced
by using fewer turns of heavier wire. However, it would not be practical to eliminate the reactance
voltage drop due to the series field since that would also eliminate the magnetic field. The reactance
voltage drop due to the armature winding can be practically eliminated by use of a compensating
winding. The compensating winding is connected in series with the armature winding (conductive
compensation) and arranged such that the ampere-turns of the compensating winding oppose and
neutralize the ampere-turns of the armature. To realize this compensation, the compensating winding
is displaced by 90 electrical degrees from the field winding. Since the motor used in this experiment
us a 4-pole motor, the mechanical displacement is 450. The compensating winding also improves
commutation considerably. This is a great adventure since the field of a universal motor is weakened
by lowering the reactance of the series field winding. If the compensating winding is short circuited
(inductive compensation), the alternating currents in the armature are induced by transformer action
into the shorted compensating winding, thus, effectively cancelling the reactive armature currents.
To reduce losses due to hysteresis and eddy currents, the field structure is laminated. Few universal
motors operate at the same speed on AC as on DC. Whether it runs faster on AC or DC is a matter of
design.
The reactance of the armature winding can be lowered by placing a compensating winding on the
stator so that the fluxes oppose or "cancel" each other. This same compensating winding can be
Page | 1 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 connected in series with the armature winding. In this case, the motor is said to be conductively
compensated. Under these conditions, the universal motor will have similar operating characteristics
whether on AC or DC power.
The compensating winding may be simply shorted upon itself, so that it behaves like a shortcircuited secondary of a transformer (the armature winding acting as the primary). The induced AC
current in the compensating winding again opposes or "bucks" the armature current and the motor
is said to be inductively compensated. The reactance of the field winding can be kept low by
limiting the number of turns.
The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current
will always be less than the DC starting current (with equal supply voltages). Consequently,
the starting torque on AC power will be lower than the starting torque on DC power.
The compensating winding has the important role of reducing the overall reactance of the motor.
However, it also has an equally important part in opposing armature reaction, thereby improving
commutation. An uncompensated universal motor will lose most of its power. Sparking at the
brushes will also be markedly worse.
The power output (in horsepower) of the motor delivered to the load is defined as follows:
Pout ,hp =
1.4 ⋅ ωrpm ⋅ TNm
10 000
(12-1)
where ωrpm is the motor speed in revolutions per minute, TNm is its torque in Newton-meters.
Keep in mind that one horsepower equals approximately to 746 W. The reactive power [var] can be
computed as:
Q = S 2 − P2
(12-2)
where S is the apparent power [VA], P is the real power [W] consumed by the motor. The
efficiency of the motor is:
efficiency =
Pout ,W
P
⋅100 %
(12-3)
where Pout,W is the output power delivered to the load in Watts. The motor losses, therefore, are
estimated as:
Losses = P − Pout ,W
(12-4)
Page | 2 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 Experiment:
1) Examine the construction of the Universal motor (use Figure 12-1 for reference). Identify the
armature, the main series, and the compensating windings and their terminals. Identify the
commutator and brushes. Note that the neutral position of the brushes is indicated by the red
mark on the housing. The brushes can be positioned on the commutator by moving the lever
to the right or to the left.
Figure 12-1
2) We start our study of the Universal motor from determining the neutral brush position by
using AC excitation. Construct the circuit as shown in Figure 12-2. Do not apply power at
this time!
Page | 3 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 Figure 12-2
Couple the motor with the dynamometer by the timing belt. Do not close the motor’s front
panel. Using thin red wires, connect the “torque” and “speed” outputs of the dynamometer to
the “T” and “N” terminals of the DAI; connect “ground” terminals of dynamometer and DAI.
Set the “MODE” switch of the dynamometer to the “DYN” position and the dynamometer
load control switch to the “MAN”. Set the dynamometer control knob to its utmost counterclockwise position for a minimum load for the motor. Move the brush positioning lever to its
utmost clockwise position.
3) Turn ON the PS and adjust the output voltage to approximately 50 V. The motor should not
be rotating. Note that the AC voltage that appears across the compensating winding is
induced by the current flowing through the armature. Carefully and slowly adjust the brush
position such that the induced voltage as measured by E2 is at its maximum. This is the
neutral point of your universal motor. Do not readjust the brush position during the
experiment. Each time using the Universal motor, the brushes should be placed in the neutral
position. Return the voltage to zero, turn OFF the PS, and close the front panel of your
motor.
4) Connect the armature and compensating windings in series as shown in Figure 12-3.
Figure 12-3
Turn ON the PS and adjust it for approximately 30 V. If the line current measured by I1 is
less than 1 A, the compensating winding is producing a flux in the same direction as the
Page | 4 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 armature winding, thereby increasing the inductance (and reactance). If this is the case,
interchange the leads connected to the armature or to the compensating winding. Measure
and record the line current. Note: if the armature revolves, the brushes are not exactly at the
neutral position. Return the voltage to zero and turn OFF the PS.
5) Construct the circuit shown in Figure 12-4.
Figure 12-4
Turn ON the PS and adjust for 120 V AC. Measure and record in a Data table the values for
line current, line voltage, real power consumed by the motor, its speed, and the torque
applied to the motor. Repeat the same measurements for the load torques of 0.2, 0.4, 0.6, 0.8,
1.0, 1.2, and 1.4 Nm while recording the values in a Data table. Do not keep the load values
of 1.2 and 1.4 Nm longer than it is necessary to make measurements! Return the voltage to
zero and turn OFF the PS.
6) In your Metering window, set your meters to operate in DC mode. Instead of AC output of
the PS, connect the motor to the DC output as indicated in Figure 12-5.
Figure 12-5
Turn ON the PS and adjust the voltage to approximately 120 V DC. Note: use caution while
adjusting the DC voltage since the PS is capable for supplying DC voltages higher than 120
Page | 5 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 V. Measure and record in a Data table the values for line current, line voltage, real power
consumed by the motor, its speed, and the torque applied to the motor. Repeat the same
measurements for the load torques of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 Nm while recording
the values in a Data table. Do not keep the load values of 1.2 and 1.4 Nm longer than it is
necessary to make measurements! Return the voltage to zero and turn OFF the PS.
7) Set the dynamometer control knob to its utmost clockwise position for a maximum starting
torque. Turn ON the PS and adjust for approximately 30 V. Measure and record the motor
current and the torque developed. Return voltage to zero and turn OFF the PS.
8) Connect the motor to the AC output of the PS again as shown in Figure 12-4. Set your meters
to AC mode. Keep the dynamometer control knob in its utmost clockwise position for a
maximum torque. Turn ON the PS and adjust for approximately 30 V. Measure and record
the motor current and the torque developed. Return voltage to zero and turn OFF the PS.
9) Next, we assess the dependency of the universal motor speed on the voltage while operating
on AC. Having your circuit unchanged, set the dynamometer control knob to its utmost
counter-clockwise position for a minimum starting torque. Turn ON the PS and adjust the
voltage to 20 V. Measure the motor speed and record its value together with the value of the
line voltage in a new Data table. Repeat the same measurements while increasing the voltage
to 120 V with increments of 10 V while recording your measurements in the Data table. You
will collect total of 11 measurements to plot a smooth curve later.
With the input voltage of 120 V, gradually increase the load to 1.0 Nm. Measure the motor
speed and record its value together with the value of the line voltage in your Data table.
Decrease the voltage by 10 V and repeat the same measurement while recording the results
for line voltage and motor speed. Keep decreasing the voltage until the motor stalls. Record
the values of line voltage and motor speed at each step.
10) To study the universal motor’s uncompensated operation, eliminate the compensating coil
winding by reconnecting your circuit as shown in Figure 12-6.
Figure 12-6
Page | 6 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 Set the dynamometer control knob to its utmost counter-clockwise position for a minimum
starting torque. Torn ON the PS and adjust for 120 V. Carefully increase the motor load to
0.4 Nm. Measure and record the motor current and speed. Return the voltage to zero and turn
OFF the PS.
11) Reconnect your circuit for DC operation as shown in Figure 12-7.
Figure 12-7
Set the dynamometer control knob to its utmost counter-clockwise position for a minimum
starting torque. Torn ON the PS and adjust for 120 V. Note: use caution while adjusting the
DC voltage since the PS is capable for supplying DC voltages higher than 120 V. Carefully
increase the motor load to 0.4 Nm. Measure and record the motor current and speed. Return
the voltage to zero and turn OFF the PS.
12) To observe effects of the universal motor inductive compensation, reconnect your circuit
for AC operation as shown in Figure 12-6. Short out the compensating coil by connecting a
lead directly across its terminals. Turn ON the PS and adjust for 120 V. Set the motor load to
1.0 Nm. Measure and record the motor current and its speed. While the motor is operating,
remove the short across the compensating coil. Make notes on your observations (you will be
asked to comment on them later on). Caution: hold the lead by the insulated connectors; do
not touch the exposed terminals! Return the voltage to zero and turn OFF the PS.
13) Reconnect your circuit for DC operation as shown in Figure 12-7. Short out the
compensating coil by connecting a lead directly across its terminals. Turn ON the PS and
adjust for 120 V. Set the motor load to 1.0 Nm. Measure and record the motor current and its
speed. While the motor is operating, remove the short across the compensating coil. Make
notes on your observations (you will be asked to comment on them later on). Return the
voltage to zero, turn OFF the PS, and disassemble your circuit.
Page | 7 ELEN 3441 – Fundamentals of Power Engineering Lab # 12 Spring 2008 In your report:
1) From your observations in Part 3 and material discussed in the class, explain the purpose of
determining the brush neutral position.
2) Report the line current you have measured in Part 4. Was the armature revolving?
3) Using Matlab and the data collected in Part 5, plot the speed of the universal motor as a
function of its load while operating on AC.
4) Using Matlab and the data collected in Part 6, plot (on the same axes as in Part 3) the speed
of the universal motor as a function of its load while operating on DC.
5) For the AC operation of the universal motor (data collected in Part 5), compute its full-load
(1 Nm) power factors, power delivered to the load, motor efficiency, and motor losses under
the full-load conditions.
6) For the DC operation of the universal motor (data collected in Part 6), compute the power
delivered to the load, motor efficiency, and motor losses under the full-load conditions.
7) Compare the universal motor operating characteristics (speed, delivered power, efficiency)
on AC and DC and list the major differences.
8) Compare the starting torque of the universal motor while working on DC and AC as assessed
in Parts 7 and 8. Explain these results.
9) Using Matlab and the data collected in Part 9, plot (on the same axes) the dependence of the
universal motor speed on the line voltage for the no-load and full-load AC operations. Make
two curves distinguishable. Comment on their shape.
10) For the uncompensated operation of the universal motor, report the line current drawn by the
motor and its speed while operating on AC and DC as measured in Parts 10 and 11.
Comment on your observations and compare these results with the corresponding values
measured previously for the normal operation of the universal motor.
11) For the inductive compensation of the universal motor studied in Parts 12 and 13, report and
compare the values of the motor current and speed while operating on AC and DC. Report
your observations on those experiments. Does the inductively compensated DC motor works
as well as the AC inductively compensated motor? Explain.
12) Explain the difference between an inductively compensated and a conductively compensated
universal motor. Why a compensating winding is necessary in an AC series motor?
Page | 8