Week 7
Practical Work
3.4 Measuring the electrical quantities of 3-phase induction motor

Introduction
The torque is directly proportional to rotor current, IR, and the cosine of the phase
angle between the rotor and stator fields (cos θ). Another way of expressing this
relationship is that torque is directly proportional to the in-phase component of rotor
current, IR cos θ.
At the instant of start, IR is high but the in-phase components is low because of the
poor power factor (cos θ). As the rotor picks up speed, both the induced rotor voltage and
the inductive reactance decrease. Basically IR is going down while cos θ is going up. You
can see that there is not very much difference in the value of cos θ when θ is 0° (cos 0 = 1)
and when θ is 20° (cos 20 = 0.94).
Therefore, over the operating range of the motor, the rotor power factor does not play
an important part in the torque output. More important is rotor current. Rotor current falls
off sharply as the rotor approaches synchronous speed (i.e., slip approaches zero). Speed
doesn't have to drop back very much to increase the rotor current, stator power factor, and
torque.
When you are running an induction motor without load, it draws almost as much
current as it does fully loaded. This no-load current, however, is made up of two
components. The in-phase component supplies electrical and mechanical losses. The
quadrature (90 degrees out of phase) component is the magnetizing current. It is quite large
in comparison with the in-phase part. As the motor is loaded, it is like putting a resistive
load on the secondary of a transformer. The in-phase component gets larger. The stator's
power factor improves accordingly. The increased rotor current does not necessarily add to
the total current being drawn by the motor. It simply uses more of that current for useful
work.

Apparatus
1. One Squirrel cage three phase induction motor
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Week 7
Practical Work
2. One prony brak
3. One power supply
4. One AC instrumentation group
5. Two wattmeter
6. One tachometer

Procedure
1. Couple and clamp the machine with tachometer and prony brake
2. Connect the motor and instruments as shown in figure 3- 12
3. Start the motor under no load and make sure its rotation is in the "right direction for
the scale of the Prony brake.
4. Load the motor in six steps to 125% of rated capacity.
5. Record your readings in Table in your worksheet for each step.
6. Turn off the power supply.
A
L1
Supply L2
3Ø
0-415V
Ac
N
Prony
brake
W1
Stator
W2
V1
Tach
U1
U2
V2
V
W2
W1
L3
Figure (3-12): 3-phase induction motor circuit diagram
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Rotor
Week 7
Practical Work
Worksheet 07
Solve the following questions:
1. Connect the circuit as shown in figure 3-12, Operate, record your results in Table 3-5
Torque (T)
(N.m)
Line voltage
(VL)
(Volts)
Line
current(IL )
(Ampere)
Speed (N)
(RPM)
Power(P)
P =W1 +W2
(KW)
Table 3-5
Torque (T)
(N.m)
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Power Factor (P.F)
Week 7
Practical Work
2. Calculate the power factor and record
your results in Table 3-6
Table 3-6
3.plot the graph (3-5) of the torque versus motor current
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Week 7
Practical Work
Graph 3-5 : the relationship (T VS Motor Current)
4.plot the graph (3-6) of the torque versus power factor
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Week 7
Practical Work
Graph 3-6 : the relationship (T VS p.f.)
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