Question { DRDO, 4449 }

A separately excited dc motor having 2 ohm armature
resistsance draws a current of 10A and torque developed is
20 Nm under rated field condtion. the speed of the motor
under the rated field condtion , when it is connected to a
100V DC source and delivering 10Nm torque....

Answer

Basic separately excited motor equations are

V=E+Ia*Ra-------------(1)
E=K*W-----------------(2)
T=K*Ia----------------(3)

from the first set of data ie Ra=2, Ia=10 T=20,
and from equation (3)
20=K*10
therefore K=2.

with this knowledge of K
continuing to 2nd set of data ie V=100 and T=10
again from eqn(3) we get Ia as 5A

knowing Ia Ra and V from eqn (1) we get E as 90V

from eqn(2) knowing the values of E and K we obtain w(omega,
ie speed) as 45 rad/sec or 416.67 rpm

Question { Essar, 9359 }

What happens if we give ac supply to dc shunt and series
motors individually

Answer

both the motors will not rotate
when the teminal voltage supply for a dc motor is changed
its direction of rotation changes.
With a ac supply to its terminals there will the change in
the direction of supply every 20ms(in case of 50hz freq)
Due to this very high change in the direction of terminal
voltage and inertia of the motor the rotor does not rotate.