# Electrical Machines Questions and Answers – Operating Characteristics of DC Motors

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Operating Characteristics of DC Motors”.

1. The speed torque characteristics of the dc motor is best described by following the equation.

`w=(Vt/k*flux)-Ra*T/((R*flux)2)`
Explanation: The speed reduced slightly due to armature drops practically.

2. Mark the correct option which affects the terminal voltage of a dc shunt motor.
a) Armature reaction
b) Source voltage variations
c) Compensating winding
d) Any of the mentioned

Explanation: If the dc motor has compensating winding, flux will remain constant regardless of load.

3. For a dc shunt motor of 5 kW, running at 1000 rpm, the induced torque will be ____________
a) 47.76 N
b) 57.76 N
c) 35.76 N
d) 37.76 N

Explanation: Torque = Power/(speed in rev/s)
= 5000/(2*pi*1000/60)
= 47.76 N.

4. The flux and the internally generated voltage of a dc machine is a ________ function of its magneto-motive force.
a) non-linear
b) linear
c) constant
d) inverse

Explanation: The flux and induced emf are non linear function of its mmf.
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5. It is advised not to run dc series motor with no load. Why?
a) Because zero torque at no load will make speed infinite
b) Because zero torque as no load will not let machine start
c) Because infinite torque will be produced
d) None of the mentioned

Explanation: If the torque is zero in dc series motor then the speed will be infinite causing the damage of the machine permanently.
Explanation: It is inverse in nature. At start it is infinite due to zero flux.

7. Choose the best option which identifies about the below characteristics. a) (1)series motor (2) Cumulative compound motor (3) Shunt motor
b) (1)Cumulative compound motor (2) series motor (3) Shunt motor
c) (1)Cumulative compound motor (2) Shunt motor (3) series motor
d) (1)Shunt motor (2) series motor (3) Cumulative compound motor

Explanation: Shunt machine has almost constant speed while the series varies as inverse relation with torque.
Explanation: The shunt motor has speed characteristic of linearly falling characteristics as the load increases the speed also falls.

9. The speed torque of the differential compound dc motor is shown below. What conclusions can be made? a) This is an unstable machine
b) There is regenerative increment in the speed
c) This is impractical to be used
d) All of the mentioned

Explanation: This is basically an unstable scenario which makes the machine unable to be used.

10. It is impossible to start a differential compounded dc motor.
a) True
b) False

Explanation: It is so because of the unstable speed and torque characteristics. At starting, armature current and series field current are high. Since the series flux subtracts from shunt flux, series flux actually reverses the magnetic polarity. This motor will typically be still or turn slowly in wrong direction while burning up because of excessive current.

11. A student is given a differential compound motor and he has been asked to make it start. How will he try?
a) By shorting series field at start
b) To run as shunt motor at start
c) By making rated current at start
d) All of the mentioned

Explanation: At starting, the flux should be rated so that there is no abnormal situation arise.

12. For a 100 hp 250 V, compound dc motor with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?
a) 5 A
b) 5.6 A
c) 4 A
d) 0 A

Explanation: At no load, armature current is zero, so the internal generated voltage is 250 V. So 5 A will be the shunt field current at no load.
Explanation: After saturation, flux becomes constant. After that we observe a linear region.
Explanation: Initially it will act as dc shunt motor but as the speed increases it starts behaving like dc series motor.

15. A dc shunt motor is connected to the source through 3-point starter. Suddenly if we starter handle is moved fastly from off to on position, then the _________
a) motor will draw large current
b) motor will not start
c) motor will burn
d) all of the mentioned

Explanation: As the starter is placed very rapidly from off to on the resistance seen by the machine will be very less and it will draw a large starting current.

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