Electrical Engineering Practice questions (Mcq): DC Motor (Part 1)

1.    No-load speed of which of the following motor will be highest ?

(a)          Shunt motor

(b)         Series motor

(c)          Cumulative compound motor

(d)         Differentiate compound motor

Ans: b

2.    The direction of rotation of a D.C. series motor can be changed by

(a)          interchanging supply terminals

(b)         interchanging field terminals

(c)          either of (a) and (b) above

(d)         None of the above 

Ans: b

3. Which of the following application requires high starting torque ?

(a) Lathe machine

(b) Centrifugal pump

(c) Locomotive

(d) Air blower

Ans: c

4.    If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?

(a)          Series motor

(b)         Shunt motor

(c)          Differentially compound motor

(d)         Cumulative compound motor Ans: a

5.    Which D.C. motor will be preferred for machine tools ?

(a) Series motor

(b) Shunt motor

(c) Cumulative compound motor

(d) Differential compound motor
Ans: b

6.    Differentially compound D.C. motors can find applications requiring

(a)         high starting torque

(b)        low starting torque

(c)         variable speed

(d)        frequent on-off cycles

Ans: b

7.    Which D.C. motor is preferred for elevators ?

(a)        Shunt motor

(b)       Series motor

(c)        Differential compound motor

(d)       Cumulative compound motor

 Ans: d

8.         According to Fleming's left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of

(a)     current in the conductor aovtaat of conductor

(c)        resultant force on conductor

(d)       none of the above

Ans: a

9.    If the field of a D.C. shunt motor gets opened while motor is running

(a)   the speed of motor will be reduced %

(b)   the armature current will reduce

(c)    the motor will attain dangerously high speed 1

(d)   the motor will continue to nuvat constant speedAns: c

10.    Starters are used with D.C. motors because

(a)        these motors have high starting torque

(b)       these motors are not self-starting

(c)        back e.m.f. of these motors is zero initially

(d)       to restrict armature current as there is no back e.m.f. while starting Ans: d

11.       In D.C. shunt motors as load is reduced the speed will increase abruptly

(a) the speed will increase in proportion to reduction in load 

(b) the speed will remain almost/constant

(c) the speed will reduce  

Ans: c

11.       A D.C. series motor is that which

(a)        has its field winding consisting of thick wire and less turns

(b)       has a poor torque

(c)        can be started easily without load

(d)       has almost constant speed

Ans: a

13.    For starting a D.C. motor a starter is required because

(a)        it limits the speed of the motor

(b)       it limits the starting current to a safe value

(c)        it starts the motor

(d)       none of the above

Ans: b

14.    The type of D.C. motor used for shears and punches is

(a)        shunt motor

(b)       series motor

(c)        differential compoutid D.C. motor

(d)       cumulative compound D.C. motor Ans: d

15.    If a D.C. motor is connected across the A.C. supply it will

(a)          run at normal speed

(b)         not run

(c)          run at lower speed

(d)         burn due to heat produced in the field winding by .eddy currents Ans: d

16. To get the speed of D.C, motor below the normal without wastage of electrical energy is used.

(a)        Ward Leonard control

(b)       rheostatic control

(c)        any of the above method

(d)        none of the above method

Ans: a

17.       When two D.C. series motors are connected in parallel, the resultant speed is 

(a) more than the normal speed

(b) loss than the normal speed

(c) normal speed

(d) zero 

Ans: c 

18.       The speed of a D.C. shunt motor more than its full-load speed can be obtained by

(a)        decreasing the field current

(b)       increasing the field current

(c)        decreasing the armature current

(d)       increasing the armature current 

Ans: a

19.    In a D.C. shunt motor, speed is

(a)        independent of armature current

(b)       directly proportional to the armature current

(c)        proportional to the square of the current

(d)       inversely proportional to the armature current 

Ans: a

20. A direct on line starter is used: for starting motors

 (a)           iip to 5 H.P.

(b)       up to 10 H.P. (c)         up to 15 H.P.

(d)       up to 20 H.P. 

Ans: a

21.    What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

(a)        The motor will stop

(b)       The motor will continue to run

(c)        The armature may burn

(d)       The motor will run noisy

Ans: c

22.    In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

(a)        back e.m.f. is equal to armature drop

(b)       armature drop is negligible

(c)        flux is proportional to armature current

(d)       flux is practically constant in D:C. shunt motors

 Ans: d

23. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be 

(a) almost negligible 

(b) rated full-load current 

 (c) less than full-load current

(d) more than full-load current 

Ans: d

23.       These days D.C. motors are widely used in

(a)        pumping sets

(b)       air compressors

(c)        electric traction

(d)       machine shops

Ans: c

25.       By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?

(a)        Frame

(b)       Shaft

(c)        Commutator

(d)       Stator

Ans: c

26.    In which of the following applications D.C. series motor is invariably tried?

(a)       Starter for a car

(b)       Drive for a water pump 

(c)      Fan motor

          (d) Motor operation in A.C. or D.C. 

Ans: a

27.    In D.C. machines fractional pitch winding is used

(a)          to improve cooling

(b)         to reduce copper losses

(c)          to increase the generated e.m.f.

(d)         to reduce the sparking

Ans: d

28.    A three point starter is considered suitable for

(a)  shunt motors

(b)  shunt as well as compound motors

(c)   shunt, compound and series motors

(d)  all D.C. motors

Ans: b

29.       In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be

(a)          100%

(b)         around 90%

(c)          anywhere between 75% and 90%

(d)         less than 50%

Ans: d

30.    The ratio of starting torque to full-load torque is least in case of

(a)        series motors

(b)       shunt motors

(c)        compound motors

(d)       none of the above Ans: b

32.    In D.C. motor which of the following can sustain the maximum temperature rise?

(a)        Slip rings

(b)       Commutator

(c)        Field winding

(d)       Armature winding

Ans: c

33.       Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?

(a)        Lenz's law

(b)       Faraday's law

(c)        Coloumb's law

(d)       Fleming's left-hand rule

Ans: d

34.       Which of the following load normally needs starting torque more than the rated torque?

(a)        Blowers

(b)       Conveyors

(c)        Air compressors

(d)       Centrifugal pumps

Ans: b

35.    The starting resistance of a D.C. motor is generally

(a)        low

(b)       around 500 Q

(c)        1000 Q

(d)       infinitely large

Ans: a