| 1.Which of the following method of speed control of D.C. machine will offer minimum efficiency ? |
| Voltage control method |
| Field control method |
| Armature control method |
| All above methods |
|
ANSWER : : Armature control method
|
| 2.Sparking, is discouraged in a D.C. motor because |
| it increases the input power con-sumption |
| commutator gets damaged |
| both (a) and (b) |
| none of the above |
|
ANSWER : : commutator gets damaged
|
| 3.The speed of a D.C. motor can be varied by varying |
| field current |
| applied voltage |
| resistance in series with armature |
| any of the above |
|
ANSWER : : any of the above
|
| 4.If I be the armature current, then speed of a D.C. shunt motor is |
| independent of Ia |
| proportional to la |
| varies as (Ia) |
| none of these |
|
ANSWER : : independent of Ia
|
| 5.Which D.C. motor is generally preferred for cranes and hoists ? |
| Series motor |
| Shunt motor |
| Cumulatively compounded motor |
| Differentially compounded motor |
ANSWER : : Series motor
| 6.Speed control by Ward Leonard method gives uniform speed variation |
| in one direction |
| in both directions |
| below normal speed only |
| above normal speed only. |
|
ANSWER : : in both directions
|
| 7.Which of the following motor has the poorest speed regulation ? |
| Shunt motor |
| Series motor |
| Differential compound motor |
| Cumulative compound motor |
|
ANSWER : : Series motor
|
| 8.Three point starter can be used for |
| series motor only |
| shunt motor only |
| compound motor only |
| both shunt and compound motor |
|
ANSWER : : both shunt and compound motor
|
9.Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have
to be |
| level compounded |
| under compounded |
| cumulatively compounded |
| differentially compounded |
|
ANSWER : : cumulatively compounded
|
| 10.Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ? |
| Low cost |
| Wide speed range |
| Stability |
| High starting torque. |
ANSWER : : Low cost
| 11.Following motor is used where high starting torque and wide speed range control is required. |
| Single phase capacitor start |
| Induction motor |
| Synchronous motor |
| D.C. motor |
|
ANSWER : : D.C. motor
|
| 12.At the instant of starting when a D.C. motor is put on supply, it behaves like |
| a highly resistive circuit |
| a low resistance circuit |
| a capacitive circuit |
| none of the above |
|
ANSWER : : a low resistance circuit
|
| 13.As -the load is increased the speed of D.C. shunt motor will |
| reduce slightly |
| increase slightly |
| increase proportionately |
| remains unchanged |
|
ANSWER : : reduce slightly
|
| 14.In D.C. machines the residual magnetism is of the order of |
| 2 to 3 per cent |
| 10 to 15 per cent |
| 20 to 25 per cent |
| 50 to 75 per cent |
|
ANSWER : : 2 to 3 per cent
|
| 15.The armature voltage control of D.C. motor provides |
| constant torque drive |
| constant voltage drive |
| constant current drive |
| none of the above |
ANSWER : : constant torque drive
| 16.For a D.C. shunt motor if the excitation is changed |
| torque will remain constant |
| torque will change but power will remain constant |
| torque and power both will change |
| torque, power and speed, all will change |
|
ANSWER : : torque will change but power will remain constant
|
| 17.Usually wide and sensitive speed control is desired in case of |
| centrifugal pumps |
| elevators |
| steel rolling mills |
| colliery winders |
|
ANSWER : : colliery winders
|
18.As there is no back e.m.f. at the instant of starting a
D.C. motor, in order to prevent a heavy current from flowing though the
armature circuit |
| a resistance is connected in series with armature |
| a resistance is connected parallel to the armature |
| armature is temporarily open circuited |
| a high value resistor is connected across the field winding |
|
ANSWER : : a resistance is connected in series with armature
|
| 19.In a differentially compounded D.C. motor, if shunt field suddenly opens |
| the motor will first stop and then run in opposite direction as series motor |
| the motor will work as series motor and run at slow speed in the same direction |
| the motor will work as series motor and run at high speed in the same direction |
| the motor will not work and come to stop |
|
ANSWER : : the motor will first stop and then run in opposite direction as series motor
|
| 20.The speed of a D.C. shunt motor can be increased by |
| increasing the resistance in armature circuit |
| increasing the resistance in field circuit |
| reducing the resistance in the field circuit |
| reducing the resistance in the armature circuit |
|
ANSWER : : increasing the resistance in field circuit
|
| 21.The armature torque of the D.C. shunt motor is proportional to |
| field flux only |
| armature current only |
| both (a) and (b) |
| none of the above |
|
ANSWER : : armature current only
|
| 22.In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will |
| remain unchanged |
| reduce to one-fourth value |
| increase four folds |
| be doubled |
|
ANSWER : : remain unchanged
|
| 23.The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is |
| 2.36% |
| 4.76% |
| 6.77% |
| 8.84% |
|
ANSWER : : 4.76%
|
| 24.Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of |
| D.C. series motor |
| D.C. shunt motor |
| induction motor |
| all of above motors |
|
ANSWER : : D.C. series motor
|
|
|
|
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