This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Circuit Model of DC Machines”.
Explanation: This is a motor performance equation.
Explanation: This is a generator performance equation.
3. 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
= 20 A.
4. The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.
Explanation: The voltage drop at brush-commutator contact is variable (1-2V) and independent of armature current.
5. 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.
6. 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.
7. The conductor EMF and current are in _____ direction and developed torque is in _____ for generating mode.
a) same, opposite
b) same, same
c) opposite, same
d) opposite, opposite
Explanation: The conductor emf and current will be in same direction and the developed torque is in opposite direction for a generator.
8. If the armature terminal voltage is more that its induced EMF, the DC machine given is ________
a) motoring mode
b) generating mode
c) regenerative mode
d) none of the mentioned
Explanation: As the terminal voltage is lesser than armature voltage, the supply is fed to the machine and so it will be acting like a motor.
9. Consider a 200V, 25kW, 30A DC machine lap connected with armature resistance of 0.4 ohms. If the machine is later wave wound, then the developed power is?
a) 25 kW
b) 12.5 kW
c) 20 kW
d) 50 kW
Explanation: The power of the machine remains unaltered by the type of connections.
10. If the DC machine is held constant at 3000 rpm. The DC voltage is 250V. If the field is held constant with 250V. Is this machine generator or motor?
c) None of the mentioned
d) Any of the mentioned
Explanation: From the speed and emf relation, E = 250*2950/3000
= 245.8 V
This is less than the terminal voltage. Hence it is a motor.
11. A shunt generator has an induced voltage on open circuit of 127 V. When the machine is on load the terminal voltage is 120 V. The load current if the field resistance be 15 ohm and armature resistance be 15 ohm.
a) 342 A
b) 350 A
c) 358 A
d) 8 A
Explanation: Ia*Ra = E-V
= 127 – 120
= 7 V
Ia = 350 A
Ish = 120/15
= 8 A
I = 350 – 8
= 342 A
Sanfoundry Global Education & Learning Series – Electrical Machines.
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