University of Jaffna, Faculty of Engineering
EC5040 Electric Machines
Assignment 03
1. (a) Calculate the synchronous speed of an eight–pole induction motor when supplied with
power from a 60–cycle source.
(b) Calculate the synchronous speed of 60–cycle induction motors having four poles.
(c) The rotor speed of a six–pole 50 cycle induction motor is 960 rpm.
Calculate the percent slip.
(d) Calculate the speed of a 60cycle,14 pole motor if the slip s is 0.05.
2. A 220–volt, 60–hertz, 10–hp, single-phase induction motor operates at an efficiency of
86% and a power factor of 90%.What capacity should be placed in parallel with the motor so
that the feeder supplying the motor will operate at unity power factor?
3. A 3–phase Y–connected 220–volt (line to line) 10–hp, 60–cps 6–pole induction motor has
the following constants in ohms per phase referred to the stator:
r1 = 0.294
r2 = 0.144 x1 = 0.503
x2 = 0.209
xɸ = 13.25
The total friction, windage, and core losses may be assumed to be constant at 403 watts,
independent of load. For a slip of 2.00 percent, compute the speed, output torque and power,
stator current, power factor, and efficiency when the motor is operated at rated voltage and
frequency.
4. (i) An induction motor draws 25 A from a 460–V, three-phase line at a power factor of
0.85, lagging. The stator copper loss is 1000 W, and the rotor copper loss is 500 W.
The "rotational" losses are friction and windage = 250 W, coreloss = 800 W, and stray load
loss = 200 W.
Calculate
(a) the air-gap power, Pg,
(b) the developed mechanical power, DMP,
(c) the output horsepower, and
(d) the efficiency.
(ii) If the frequency of the source in part (i) is 60 Hz, and the machine has four poles, find
(a) the slip,
(b) the operating speed,
(c) the developed torque, and
(d) the output torque.
5. The equivalent circuit of 5 hp squirrel cage induction motor is shown in figure 01. The
motor rated speed is 1800 rpm and 400 V, 50 Hz. The full load current and locked rotor
current is 7 A and 39 A respectively.
a. Calculate the per phase current at breakdown
b. Calculate the total breakdown torque for the motor.
c. By considering various values for the slip(0.0125, 0.025, 0.026, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8,
1.0), tabulate the slip(s), rotor current (I), power delivered to the rotor (Pr), induced torque
(T) and rotor speed (ω) in rpm. d. Draw the speed torque characteristic curve for the given
induction motor.