A d.c. shunt motor runs at 500 r.p.m. at 220 V. A resistance of 4.5 Ω is added in series with the armature for speed control. The armature resistance is 0.5 Ω. The current to stall the motor is
A d.c. shunt motor runs at 500 r.p.m. at 220 V. A resistance of 4.5 Ω is added in series with the armature for speed control. The armature resistance is 0.5 Ω. The current to stall the motor is
Right Answer is:
44 A
SOLUTION
As we know that back EMF of DC motor is directly proportional to the flux and speed.
Eb ∝ Nφ
Eb ∝ N (φ is constant)
We can stall the motor (i.e., reduce its speed to zero) by putting more and more load on the shaft. When the motor is stalled since N = 0 therefore Eb = 0
From the voltage equation, the back EMF of DC motor at no-load
Eb = V − IaRa
Since external resistance, Rext is added in series with the motor. Hence
Eb = V − Ia(Ra + Rext)
0 = 220 − Ia(4.5 + 0.5)
Ia = 220 ⁄ 5
Ia = 44 A