A 240v dc shunt motor with an armature resistance of 0.5 has a full load current of 40A. Find out the ratio of the stalling torque to the full load torque when resistance of 1 ohm is connected in series with the armature
A 240v dc shunt motor with an armature resistance of 0.5 has a full load current of 40A. Find out the ratio of the stalling torque to the full load torque when resistance of 1 ohm is connected in series with the armature
Right Answer is:
4
SOLUTION
Stalling torque is the torque at which speed would fall to zero.
With N = 0 , Eb = 0
Hence the value of armature current
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 = 240 − Ia(0.5 + 1)
Ia = 220 ⁄ 1.5
Ia = 160 A
The torque developed by a d.c motor is directly proportional to Flux per pole × Armature Resistance i.e
T ∝ ΦIa (Now flux is constant hence )
T ∝ Ia
Hence Ratio of stalling torque to full load torque will be
T2 ⁄ T1 = I2 ⁄ I1
T2 ⁄ T1 = 160/40 = 4
T2 ⁄ T1 = 4