1. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What is the steady state angular velocity at no-load?
a) 480 rad/s
b) 960 rad/s
c) 320 rad/s
d) 490 rad/s
Explanation: v = V/(2*r*l*B)
= 120/(2*0.0005*1*0.25)
= 480 rad/s.
2. If the torque is in the direction of rotation, the DC machine acts as ________
a) generator
b) motor
c) amplidyne
d) any of the mentioned
Explanation: In a DC motor, the torque produced by the interaction of the armature current and the magnetic field is in the direction of rotation.
3. The cogging torque is absent in the permanent magnet dc machine is due to ________
a) non-magnetic nature of rotor
b) magnetic nature of rotor
c) absence of ac supply
d) any of the mentioned
Explanation: It is due to the fact that no field poles are induced in PMDC to create magnetic flux leading to harmonics.
4. DC Motor torque depends on ________
a) geometry
b) magnetic properties
c) any of the mentioned
d) both geometry and magnetic properties of the structure
Explanation: The torque will indeed depend on the geometry as well the magnetic materials used in the construction of the dc machine. Though this remains constant with the machine
5. For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a current of 5A is applied, what will be generated torque vary to?
a) 25 N-m
b) 40 N-m
c) 50 N-m
d) 100 N-m
Explanation: The torque is proportional to the square of the current.
T1/T2 = (I1)2/(I2)2.
100/T2 = 100/25
T2 = 25
So the new torque will be 25 N-m.
6. Pulsating torque frequency in universal motor is ________
a) 2f
b) f
c) f/2
d) 4f
Explanation: The torque reverses its direction in the negative cycle of the ac supply and thus it always unidirectional.
7. If the terminal voltage of 220-V dc generator having armature resistance of 1 ohms. The induced emf produced is 200-V. The armature current for the above machine is?
a) 20 A
b) -20 A
c) -10 A
d) 10 A
Explanation: I = V-E/r
= 220-200/1
= 20 A.
8. The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.
a) True
b) False
Explanation: The voltage drop at brush-commutator contact is variable (1-2V) and independent of armature current.
9. What is the shaft power at the DC Generator?
a) sum of mechanical power and rotational losses
b) only mechanical losses
c) no-load losses and mechanical power
d) any of the mentioned
Explanation: The shaft power is sum of mechanical power and rotational losses
10. If the electromagnetic torque in a DC shunt-generator is opposite, what can be further concluded?
a) Mechanical power is absorbed by the machine
b) Mechanical power is delivered by the machine
c) Electromagnetic torque is in same direction of prime mover
d) None of the mentioned
Explanation: When the electromagnetic torque is in opposite direction, it is of motoring nature.