21) The torque developed by a DC motor is directly proportional to ________

  1. Flux per pole ∗ armature current
  2. Armature resistance ∗ armature current
  3. Armature resistance ∗ applied voltage
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
As torque equation of a DC motor is given by:

Torque equation of DC motor

Here, Z,P and A are construction features of the machine, therefore are constant.

T∝ Φ Ia

Hence, for a given DC motor, torque developed in its armature depends on its flux per pole and armature current taken by it.

22) The shaft torque (Tsh) in a DC motor is less than the total armature torque (Ta) because of ___________ in the motor.

  1. Cu losses
  2. Field losses
  3. Iron and friction losses
  4. None of the above
ANSWER
Answer: ( 3 )
EXPLANATION
The total torque developed in the armature is not available at the shaft as some percentage of it is used up in overcoming mechanical losses ( friction and iron losses ) in the motor.

23) Armature reaction in a DC motor is increased _________

  1. When the armature current increases
  2. When the armature current decreases
  3. When the field current increases
  4. By interpoles
ANSWER
Answer: ( 1 )
EXPLANATION
When the load on the DC motor increases the armature current increases. The armature current set up the magnetic flux which opposes the main field flux. Thus the net field flux gets reduced. This phenomenon is called the armature reaction.

The motor performance gets adversely affected due to reduction of the field flux.

  1. The motor attains the speed above its designed rated speed.
  2. The torque delivering capacity of the motor gets reduced.

To nullify the effect of armature reaction, the additional field winding is connected in the series with the armature winding to produce the additional amount of the flux to compensate the armature reaction.

24) In a DC motor, the brushes are shifted from the mechanical neutral plane in a direction opposite to the rotation to _______

  1. Decrease speed
  2. Increase speed
  3. Reduce sparking
  4. Produce flat characteristics
ANSWER
Answer: ( 3 )
EXPLANATION
Under Construction.

25) In very large DC motors with severe heavy duty, armature reaction effects are corrected by ________

  1. Using interpoles only
  2. Using compensatory winding’s in addition to interpoles
  3. Shifting the brush position
  4. None of the above
ANSWER
Answer: ( 2 )
EXPLANATION
Some large DC motors perform a series of heavy duty operations. They accelerate, decelerate, stop, reverse all in matter of seconds. The corresponding armature current increases, decreases, reverses, producing very sudden changes in armature reaction. For such motors, interpoles do not adequately neutralize the armature m.m.f. To eliminate this problem, additional compensating winding are faces of the main field poles. Like interpoles, these winding produce an m.m.f. equal and opposite to the m.m.f. of the armature. 

26) The speed of a ________ motor is practically constant.

  1. Cumulatively compounded
  2. Series
  3. Deferentially compounded
  4. Shunt
ANSWER
Answer: ( 4 )
EXPLANATION
In DC motors 

Speed ∝ Back emf(Eb)/flux(φ)

The field winding in shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage is constant then flux also becomes constant. At the rated speed the back emf also becomes nearly constant if the load is same. Hence the speed of the shunt motor can be assumed almost constant under normal running conditions.

27) Which of the following is not necessarily the advantage of DC motors over AC motors?

  1. High starting torque
  2. Wide speed range
  3. Better speed control
  4. Low cost
ANSWER
Answer: ( 4 )
EXPLANATION
DC motors have the advantage of:

  1. Higher starting torque
  2. Quick starting and stopping
  3. Reversing, variable speeds with voltage input
  4. They are easier and cheaper to control than AC

AC motor advantages include:

  1. Lower power demand on start
  2. Minimal maintenance.

While the market for AC motors is larger than DC, as they are cheaper in cost as compared to DC motors.

28) Which motor will have least percentage increase of input current for the same percentage increase in torque?

  1. Series motor
  2. Shunt motor
  3. Separately excited motor
  4. Cumulatively compound motors
ANSWER
Answer: ( 1 )
EXPLANATION
A series motor has huge starting torque because both field and armature windings carry same current from source as connected in series. Since, torque is directly proportional to product of current in field and armature windings,for a given source current the torque produced is square of this current. So, even there is low current change, the torque change is great. On the other hand, the source current supplied to DC Shunt motor is divided into two parts, field current and armature current. The field current is kept constant for constant flux generation for a given workload. So the torque depends only on armature current where field current remains constant. So, to produce same torque as that in series motor, more amount of current is to be supplied through source compared to series. However by nature changes in torque for DC shunt motor is very low compared to DC series motor

29) The most commonly used method of speed control of a DC motor is by varying ________

  1. Voltage applied to the motor
  2. Field strength
  3. Effective number of conductors in series
  4. Armature circuit resistance
ANSWER
Answer: ( 2 )
EXPLANATION
The fact that the speed of a DC motor varies with field excitation provides a convenient means for controlling the speed of shunt and compound motors. 

30) The running speed of a DC series motor is basically determined by ______

  1. Field excitation
  2. Load
  3. Armature resistance
  4. None of the above
ANSWER
Answer: ( 2 )
EXPLANATION
Under Construction.

31) After a shunt motor is up to speed, the speed may be increased considerably by ________

  1. Increasing field circuit resistance
  2. Decreasing field circuit resistance
  3. Increasing armature circuit resistance
  4. Reducing the load
ANSWER
Answer: ( 2 )
EXPLANATION
In shunt motor the field current is practically constant. Speed can be increased by reduced the field flux hence by field current.

32) ________ has the high speed regulation.

  1. Series
  2. Cumulatively compounded
  3. Shunt
  4. Deferentially compounded
ANSWER
Answer: ( 3 )
EXPLANATION
If the load is applied to the DC motor or machine, the speed of the motor decreases automatically. Thus to maintain the constant speed, the difference between no load and full load speed known as speed regulation should be maintained very less and a motor which has nearly constant speed at all load below full rated load, have good speed regulation.In case of DC, DC Shunt Motor and in case of AC, squirrel cage induction motor has good speed regulation.

33) The deciding factor in the selection of a DC motor for a particular application is its ________ characteristics.

  1. Speed – torque
  2. Speed –  armature current
  3. Torque – armature current
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
The performance of a DC motor can be judged from its characteristic curves known as motor characteristics curve. There are various types of characteristic curve of motor but one of the important properties of DC motor is its speed-torque characteristic. Ideally, when the voltage is connected to the motor, it must provide a constant torque to accelerate the rotor and when the required speed is reached the torque must become zero.These are also called mechanical characteristics. The nature of these characteristics can easily be obtained by using speed and torque equations. These characteristics also show how much torque can be developed at a given speed to stay within the rating of the drive. Hence speed-torque characteristics play a very important role in selecting a type of motor for a particular application.

34) The demand of a large increase in torque of a DC shunt motor is met by a _____________

  1. Large decrease in speed
  2. Large increase in speed
  3. Large increase in current
  4. Small increase in current
ANSWER
Answer: ( 3 )
EXPLANATION
For a DC motor, Ta is directly proportional to ΦIa. But in a shunt motor, Φ is practically constant so that Ta is directly proportional to Ia. If torque is doubled, armature current is also doubled.

35) The demand for a large increase in torque of a DC series motor is met by a _________

  1. Large decrease in current
  2. Large decrease in speed
  3. Large increase in speed
  4. Small decrease in speed
ANSWER
Answer: ( 2 )
EXPLANATION
DC series motors exert largest torque at low speeds, i.e. when starting. As the load decreases, they automatically raise the speed.

36) A series motor will over speed when _________

  1. The load is increased
  2. The field is opened
  3. The armature circuit is opened
  4. The load is removed
ANSWER
Answer: ( 4 )
EXPLANATION
In a DC series motor, flux per pole depends upon the armature current. If the load is decreased, armature current and flux per pole are decreased. Consequently, the speed of the motor increases. If the load is small, the speed may rise to dangerously high values. For this reason, we never permit a DC series motor to operate on no load.

37) The mechanical power developed by a DC motor is maximum when the ratio of back e.m.f. / applied voltage is

  1. 1.0
  2. 3.0
  3. 0.5
  4. 0.7
ANSWER
Answer: ( 3 )
EXPLANATION
The maximum power in a DC motor is the maximum power transferred by the motor. The power is given as P = (torque x speed). During the maximum power condition the core loss should be equals to the copper loss and the induced EMF should be half of the supply voltage. The value (torque x speed) should be very high at maximum power condition.

38) The speed of a series wound DC motor

  1. Can be controlled by shunt field regulator
  2. Cannot be controlled by diverter
  3. Increases as flux decreases
  4. Increases as armature circuit resistance increases
ANSWER
Answer: ( 3 )
EXPLANATION
A series wound DC motor like in the case of shunt wound DC motor or compound wound DC motor falls under the category of self-excited DC motors. In series wound DC motor  torque is directly proportional to electric current. For a DC motor, magnitude of the back emf is given by the same emf equation of a DC generator i.e. Eb= PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ. 

39) As the load increases, a _________ motor will speed up

  1. Series
  2. Shunt
  3. Cumulatively compounded
  4. Differentially compounded
ANSWER
Answer: ( 4 )
EXPLANATION
A differential-compound DC motor, particularly if over-compensated will tend to increase speed if the load is increased. If a sudden heavy load is thrown on the motor, the series field winding will draw a heavy current from the supply and thus cause the total flux to reduce to a very small value. The motor will now attain a dangerously high speed like a series motor. Therefore, this motor is not used at places where there is a possibility of overloading the motor.

40) The speed regulation of a DC motors is

  1. Highest speed/lowest speed
  2. No load speed/full load speed
  3. Full load speed/(no load speed-full load speed)
  4. (No load speed-full load speed)/full load speed
ANSWER
Answer: ( 4 )
EXPLANATION
The speed regulation is defined as the change in speed from no load to full load. Therefore, as per definition per unit (p.u) speed regulation of DC motor is given as,

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