Q1. [20 marks]
A (15+X) HP, 240-V, 18YX RPM shunt motor with draws (55-Y) A when operating at rated conditions. The motor
parameters are Ra=0.28 Ω and Rf=(137+Y) Ω. Determine:
a. The rated torque.
b. Armature current at locked rotor if no starting resistance is used.
c. The external resistance required in the armature circuit that would limit the current and developed 200%
rated torque when starting. Assuming constant flux.
d. Assuming the system voltage drops to (215-X) V, determine the locked-rotor torque using the external
resistance in c.
Q2a. [16 marks]
A (120+XY) V series motor runs at 20Y0 rpm at full load and takes a current of 12.XY A. The armature resistance
is 0.3X Ω and the series field resistance is 0.2 Ω. A diverter resistance of 0.2 Ω is connected across the series field,
and the load on the motor is adjusted. If the motor now takes 2X A, what is the new speed of the motor. Also
determine the torque in each case.
Q2b. [2 marks]
Provide one application where the induction motor is usefully working at regenerative braking.
Q2c. [2 marks]
Sketch the speed torque characteristics of the induction motor if the frequency is increased.
Q3. [20 marks]
A 4X0 V shunt motor draws (30-Y) A while supplying the rated load at a speed of 100 rad/s. The armature
resistance is 1.X Ω and the field winding resistance is 200 Ω.
1. Determine the external resistance that must be inserted in series with the armature circuit that the armature
current does not exceed 1Y0% of its rated value when the motor is plugged.
2. Determine the braking torque (a) at the instant of plugging and (b) when the motor is approaching zero
speed.
Q4. [20 marks]
A 3-phase Y-connected (30+X) HP (rated output), 480-V, 6-pole, 60 Hz, slip ring induction motor has the following
parameters:
R1=0.5Y Ω, , R'2=0.5 Ω,
Assume that the rotational loss is 500W and the core loss is 600W.
a) Calculate the power developed by the motor
b) Calculate the motor speed
c) For a constant torque load, calculate the added resistance to the rotor to reduce the speed by 20%
Q5a. [15 marks]
A 480-V, (600-XY) RPM, (20+Y) HP PM motor is operating at full load. The efficiency of the motor is 8X% and its
armature resistance is 0.4X Ω. Determine:
i. The external resistance that must be added in series with the armature resistance that the armature current
does not exceed twice the full load current when plugged.
ii. The braking torque and the armature current when the motor speed is approaching zero.
Q5b. [5 marks]
Explain the concept of counter current braking. How is it achieved? Explain the methods.
SAMPLE EXAM PAPER