EE1100, Basic Electrical Engineering – HW 4 (part-II)
DC Machines
1. The commutator in a dc generator converts
(a) ac to dc, (b) dc to ac, (c) both (a) and (b), (d) none of the above
2. A ´´p´´ pole (p/2 pole pairs) lap wound dc machine has an armature current of Ia.
The conductor current in the armature winding is
(a) Ia, (b) Ia/p, (c) pIa (d) None of the above
3. The armature resistance of a 6-pole, lap-wound dc machine is 0.05Ω. If the
armature is rewound using a wave winding, then the armature resistance will be
(a) 0.45Ω, (b) 0.3 Ω, (c) 0.15 Ω, (d) 0.10Ω
4. The voltage at the terminals of dc series generator running at rated speed and noload will be
(a) more than rated voltage (b) full rated voltage (c) a very small voltage (d) zero
5. A 200 V dc machine has an armature resistance of 0.5 Ω. If the full load armature
current is 30A, the induced emf when the machine run (i) as generator and (ii)
motor will be respectively ____________V and ________________V.
(a) 230V, 170V (b) 225V, 175V, (c) 185, 215V, (d)215 V, 185V
6. Running a motor at no-load is inadvisable for
(a) dc shunt motor (b) dc series motor (c) induction motor (d) synchronous motor
7. The direction of rotation of a dc shunt motor can be reversed by interchanging
(a) the supply terminals (b) the field terminals, (c) the armature terminals (d)
either field or armature terminals
8. An 8-pole, wave-connected armature has 800 conductors and is driven at
600 rpm. If the flux per pole is 25 mWb, determine the generated emf. For the
same machine, determine the generated voltage if the conductors are lapconnected.
9. A separately excited generator running at 1000 rpm supplies 250A at 125V to a
constant load resistance. What will be the load current, when the speed is dropped
to 800 rpm with the field current unaltered. The armature has a resistance of
0.05 Ω and the total drop at the brushes is 1.5V.
10. The torque required to drive a DC generator at 20 rps is 2 kNm. The sum of core,
friction and windage losses in the machine is 7.0 kW. Calculate the power
generated in the armature winding.
11. A DC motor takes an armature current of 100A at 400V. The resistance of the
armature circuit is 0.15 Ω. The machine has six poles and the armature is lap
wound with 864 conductors. The flux per pole is 0.05 Wb. Calculate the (a) speed
(b) the torque developed by the armature.
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Induction Motor
12. The stator frame is made of
(a) Copper (b) aluminum (c) cast iron (d) stainless steel
13. The rotor of a 3-phase Induction motor has to rotate in the same direction as the
stator field. This is in accordance with
(a) Faradays´s laws of electromagnetic induction (b) Lenz´s law (c) Fleming´s left
hand rule, (d) Fleming´s right hand rule
14. A 6-pole, 50 Hz Induction motor can not run at a speed of
(a) 500 rpm, (b) 750 rpm, (c) 950 rpm (d) 1000 rpm
15. The stator of a 4 pole Induction motor is supplied from a 50 Hz source. The
frequency of induced emf in its rotor is 1.5 Hz. The corresponding slip is
___________ and the speed is __________rpm.
16. The rotor circuit of a squirrel induction motor is
(a) Normally open (b) normally closed (c) always closed (d) any one
17. A 3-phase, 400V, 4-pole induction motor is fed from 50 Hz mains. It runs at a
speed of 1440 rpm. The rotor frequency is___________ Hz
18. The starting torque of a 3-phase Induction motor is __________ supply voltage
(a) Independent of (b) directly proportional (c) directly proportional to the square
of (d) none of the above
19. Two of the power supply terminals of a 3-phase induction motor got interchanged
during reconnection after maintenance of the motor. When put back into service,
the motor will
(a) fail to rotate (b) rotate in the same direction as in was prior to the maintenance
(c) rotate in the reverse direction to that prior to the maintenance (d) none of the
above
20. A 3-phase, 6 pole induction motor is connected to a 60 Hz supply. The voltage
induced in the rotor bar is 4 V when the rotor is at stand still. What will be the
induced voltage in the rotor bar at 300 rpm?
21. The power input to a 3-phse induction motor is 60 kW. The stator loss for this
machine is 1 kW. Find the mechanical power developed and the rotor copper loss
per phase if the motor is running with a slip of 3%.
22. A star connected (stator winding) 3- phase induction motor has 6 poles, and runs
from a 440V, 50 Hz supply. The rotor has a resistance of 0.1 Ω and reactance of
1.2 Ω. The ratio of the stator to the rotor turns is 2.25. The full load slip is 3.5 %.
For this load, calculate (a) the torque (b) maximum torque and (c) speed at
maximum torque.
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