In a shunt-wound dc motor with the field coils and rotor connected in parallel,
ID: 1421338 • Letter: I
Question
In a shunt-wound dc motor with the field coils and rotor connected in parallel, the resistance Rf of the field coils is 101 , and the resistance Rr of the rotor is 5.9 . When a potential of 120 V is applied to the brushes and the motor is running at full speed delivering mechanical power, the current supplied to it is 4.74 A.
(a) What is the current in the field coils?
A
(b) What is the current in the rotor?
A
(c) What is the induced emf developed by the motor?
V
(d) How much mechanical power is developed by this motor?
W
Explanation / Answer
(a) The field current is: V/R = 120/101 = 1.19A
(b) The only other current flowing is the rotor current: Ir = Itot - Ifld
Ir = 4.74- 1.19 = 3.55A
(c) The emf: The rotor current multiplied by the rotor resistance is the difference between the line voltage and the counter emf:
emf = Vline - Irotor*Rrotor = 120 - 3.55*5.9 = 99.1 V
(d) P = V*I =99.1* 3.55 = 351.81 W