A circular armature coil is used in a motor with a uniform magnetic field of 3.1

Question

A circular armature coil is used in a motor with a uniform magnetic field of 3.10 T. The coil has a diameter 95 cm, a resistance of 50 , and generates a back emf of 101VRMS when running a full speed under no load at the angular frequency A.
(A) At what angular frequency, B relative to A does the coil be need to be rotated to generate a back emf of 81 VRMS?

B = _________ A

(B) If this motor is operating on 120 VRMS now with a load (e.g running a fan) and at the angular frequency A, how much current is flowing throught the motor?

___________ A

Explanation / Answer

Here,

B = 3.1 T

radius , r = 95/2 = 47.5 cm

r = 0.475 m

R = 50 Ohm

EA = 101 V

a)

EA/EB = wA/wB

101/81 = wA/wB

wB/wA = 0.801

B relative to A is 0.801

b)

for the motor

back emf =120 Vrms

let the current is I

current = 120/(50)

Current = 2.4 A

the current flowing through the motor is 2.4 A

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