Reference no: EM132726008

MBS4105 Electrical Technology - Hong Kong Institute of vocational Education

Question 1: In an experiment, a D.C. series motor is connected to 220 V supply. It takes an armature current of 25 A when running at 1000 rpm. Neglect the field coil resistance and given that the armature resistance=0.4 Ω, the flux is proportional to the field current, E = KNΦ and Torque = k I_aΦ:
i) Calculate the armature voltage of the motor.
ii) Prove that the Torque is proportional to I_a²
iii) If a student wants to reduce the speed to 800 rpm with the torque reduced to 0.8 of its previous value:
(a) Calculate the new value of armature current;
(b) Additional resistor inserted in series with the motor.

Question 2: A 20 kW, 3-phase, 380V (Line volt), 4-pole, 50 Hz, delta-connected induction motor has a full load line current = 40A. The power factor at full load = 0.85.
(a) Calculate the synchronous speed,
(b) Calculate the input power and efficiency of the motor at full load.
(c) If the gross mechanical loss = 652W and the rotor copper loss = 400W at full load,

Find the full load speed. Given the gross mechanical output = 3 I₂² r₂(1-s)/s

Attachment:- Electrical Technology.rar