Question

Question (1)

Explain with relevant diagrams, resistance commutation and how it helps in improving commutation.

 

Question (4)

A 240 V, 4 – pole shunt motor running at 1000 r.p.m. gives 15 h.p. with an armature current of 50 A and field

current of 1 A. The armature winding is wave connected and has 540 conductors. The armature resistance is 0.1

Ω and the drop at each brush is 1 V. Calculate (i) the useful torque (ii) the total torque (iii) useful flux per pole

and (iv) iron and friction losses.

 

Question (5)

A 1100 V, 50 Hz, delta-connected induction motor has a star-connected slip-ring rotor with a phase transformation

ratio of 3.8. The rotor resistance and standstill leakage reactance are 0.012 Ω and 0.25 Ω per phase respectively.

Neglecting stator impedance and magnetizing current, determine (i) the rotor current at start with slip-rings

shorted (ii) the rotor power factor at start with slip-rings shorted (iii) the rotor current at 4 % slip with slip-rings

shorted (iv) the rotor power factor at 4 % slip with slip rings shorted (v) the external resistance per phase

required to obtain a starting current of 100 A in the stator supply lines.