This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “DC Motor Starting”.
1. A starting resistance is inserted at the starting in an induction motor as well as dc motor.
a) Induction motor has to control starting torque whereas in dc motor, it is done to avoid large current
b) To limit starting current in both the machines
c) To limit starting speed
d) All of the mentioned
Explanation: For both the machines the purpose is different.
2. Considering a human handed control system for the dc motor speed control, if the resistance wire cut out too slowly, then the _____________
a) starting resistance would burn
b) field winding would burn
c) speed will rise steeply
d) any of the mentioned
Explanation: Due to the slow cut off the resistance the current will be high for a large amount of time and it will cause the machine to burn.
3. A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?
Explanation: n = log(Ra/Rt)/log(Imin/Imax); Rt = Vt/Imax = 250/700 = 0.357 ohms
= 3 (approx).
Explanation: This is due to the resistance steps used in the speed control mechanism.
5. Following are the observations due to large starting current in dc motor.
(i) Sparking at brushes (ii) Mechanical shock at shaft (iii) Source current fluctuations (iv) Burning of the field winding
a) (i), (ii), (iii), (iv)
b) (i), (iii)
c) (ii), (iii), (iv)
d) (i), (ii)
Explanation: All the observations can be seen due to large starting current in dc motor.
6. The shunt motor starters that can be used is/are ___________
a) 3-point and 4-point starter
b) 5-point starter
c) 4-point starter
d) 5-point and 3-point starter
Explanation: Both 3-point and 4-point starters can be used.
Explanation: Due to subsequent addition of the step resistances in the dc motor, the speed build also occurs like steps.
8. The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.
Explanation: The direct-on-line starter can not limit the speed by limiting the current.
9. Thyristor controlled starter is preferred over DOL starter due to _________
a) lesser losses
b) controlled direction
c) least resistance offered
d) all of the mentioned
Explanation: A thyristor controlled method is more efficient as it has lesser resistance and losses while operating.
10. For a 7.46 kW, 200 V dc shunt motor with full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current.
Explanation: Full load current = 7460/(200*0.85) = 43.88 A
Starting current = 1.5*43.88 = 65.883 A
R = V/I = 200/65.883
= 3.038 ohms
Starting resistance = 3.038 – 0.25 = 2.788 ohms.
11. The effect of fringing increases as we _________
a) increase air gap
b) decrease air gap
c) increase in flux density
d) introduce more ferric core material
Explanation: Fringing is the effect in which the magnetic flux lines bulge out of the flux. and it is introduced at the air gaps.
12. The post effects of the armature reaction is _________
a) main field distortion
b) shift in MNA
c) reduction in main field
d) none of the mentioned
Explanation: There is no such post effect of armature reaction. The armature reaction phenomena affect the working condition only.
13. If the students give a forward shift of 10° to the dc generator, then it _________
a) reduces flux per pole
b) improves flux per pole
c) increases the flux density in core
d) none of the mentioned
Explanation: A generator given a forward brush shift will get its flux density reduced at the pole as the flux will be in the opposite direction.
14. A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then it leads to the conclusion of _________
a) incorrect brush placement
b) incorrect pole and core alignment
c) incorrect field supply
d) all of the mentioned
Explanation: Given at the same speed of operation, if the speeds are different then the brushes are placed in not aligned manner.
15. For a dc machine, its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be _________
a) 0.382 ms
b) 0.456 ms
c) 0.573 ms
d) 0.312 ms
Explanation: Time of commutation = brush width/peripheral velocity
= 0.382 ms.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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