Electric Traction and Vehicles [EE 22331]
Assignment – 1
[1.] For a 230-V DC shunt motor, R armature =0.35 Ω and Rfield= 150 Ω. The armature current is 25
A, while driving a constant torque load at 600 r.p.m. Assuming that the magnetic circuit is
unsaturated, calculate the value of resistance to be inserted in the field circuit to increase this
speed to 1,000 r.p.m.
[2.] The speed of a 25-HP, 440-V DC shunt motor is reduced by 35% using a controller. The field
current is 3 A, and Rarmature =0.4 Ω. If the torque remains constant and the efficiency is 70%,
calculate the resistance of the controller.
[3.] For a 230-V, 15-HP DC shunt motor R armature =0.25Ω and Rfield= 150 Ω. If the full-load
efficiency is 85% and the torque varied as the square of the speed , calculate the resistance inserted
in the armature circuit for reducing the speed to 600 r.p.m. from 1,000 r.p.m.,
[4.] A 400-V DC series motor (R armature =0.04 Ω and Rseries-field= 0.02 Ω) takes a line current of 65 A
and runs at a speed of 800 r.p.m. What resistance should be connected in series with the armature
to reduce the speed to 400 r.p.m. The load torque at this new speed is 70% of its previous value.
Assume that flux is proportional to the load.
[5.] For a 3-φ induction motor, the ratio of maximum torque to full-load torque is 3:2; R2 = 0.32 Ω
and X 2 = 3 Ω. For Y − Δ starting, compute the ratio of actual starting torque to full-load torque.
[6.] A 40-kVA, 440-V, 50-Hz, 3-φ squires-cage induction motor, with a standstill impedance of
0.7Ω/phase, has a full-load slip of 5%. It is started using a tapped autotransformer. It's maximum
allowable supply current at the time of starting is 100 A. Calculate (i) the tap position, and (ii)
the ratio of starting torque to full load torque.
[7.] When delivering full-load torque, a six-pole, 50-Hz, 3-φ induction motor runs at 3% slip. It has
a standstill rotor resistance = 0.25 Ω, and a reactance =0.7 Ω/phase. If an additional resistance of
0.25 Ω/phase is inserted in the rotor circuit, and the full-load torque remains constant; calculate
the speed of the induction motor
[8.] A cascaded set of motors A (with six poles) and B (with eight poles) is connected to a 50-Hz
supply. If the losses are neglected, determine (i) the speed of the motor set; and (ii) the electric
power transferred to motor B, when the input to motor A is 50-kW.
[9.] A 15-kW motor, with a heating time constant = 50 minutes, and a cooling time constant =80
minutes, attains a final temperature of 50°C. Calculate the motor's temperature after a full-load
operation of 45 minutes, followed by the switching off duration of 40 minutes.
