An electric motor is plugged into a standard wall socket and is running at norma
ID: 1698380 • Letter: A
Question
An electric motor is plugged into a standard wall socket and is running at normal speed. Suddenly, some dirt prevents the shaft of the motor from turning quite so rapidly. What happens to the back emf of the motor, and what happens to the current that the motor draws from the wall socket?a.) The back emf decreases, and the current drawn from the socket decreases.
b.) The back emf increases, and the current drawn from the socket decreases.
c.) The back emf decreases, and the current drawn from the socket increases.
d.) The back emf increases, and the current drawn from the socket increases.
Explanation / Answer
SOL:
acc . to the standered expressoin of back e.m.f produced in the coil ,
e.m.f(e) =NABW-------------(1)
where N is the number of thre turns,
A is the area of the coil ,
B is the magnetic field ,
W is the rate moving of the coil,
if suddenely , some drift prevents the shaft of the motor from turning quite so rapidlly ,
which means the rate of moving is decreases (W) then ,from the above formula
the e.m.f induced also decreased as it is directly propotional toit.
thus " e" propotional to "W"
but, acc. to self inductance formule we have
the induced e.m.f is equal to e.m.f (e) = - L di /dt ,-------------(2)
where L is the self inductance of the coil ,
di/dt is the rate of change of current ,
from this formula,
e.m.f is directly propotional to the - ve rate of griwth of current in the coil,
thus, when W is decreases e.m.f will decreases ( from eqn:1)
if em.f decreases then current growth will increases (from eqn:2)
on combining these two results
option (c) is correct : as back e.m.f decreases the current drawn from the socket will increases
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