Read Out “Electrical DC Motors” topic to better understand all those MCQs

1) Brushes of DC Motors are made of

  1. Carbon
  2. Soft carbon
  3. Hard carbon
  4. Aluminium
ANSWER
Answer: ( 1 )
EXPLANATION

The brushes in a DC motor have two purposes.

  1. They carry current to the armature (the rotating part).
  2. The brushes work with the commutator to switch the current to the proper winding of the armature as it rotates. This creates the correct magnet fields to make the motor run. Essentially, a brush is one contact of a switch; the commutator is the other.

DC motors brushes are usually made of carbon, both to lubricate the sliding contact made with the commutator and to withstand the arcing that occurs when the switching happens at speeds of a hundred times per second or more in small motors. Because the electrical resistivity of the carbon brush is considerably higher than the copper commutator, most of the wear caused by the arcing happens to the brush, not the commutator.

2) Armature reaction of an unsaturated DC machine is

  1. Cross-magnetizing
  2. Demagnetizing
  3. Magnetizing
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
Initially at unsaturated condition in a DC machine armature reaction lies along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

3) Eddy current are induced in the pole shoes of a DC machine due to

  1. Oscillating magnetic field
  2. Pulsating magnetic flux
  3. Relative rotation between field and armature
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
In addition to the voltages induced in the armature conductors, there are also voltages induced in the armature core. These voltages produce circulating currents in the armature core . These are called eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency.

In pole shoe, only eddy current losses are present which occures due to the ‘to and fro sweeping effect of air gap flux’ (i.e. due to rotation of armature on which slots are present). Hence you find that in DC machine only the pole shoes are laminated to reduce this eddy current.

4) In a DC machine, short circuited field coil will result in

  1. Odour of baring insulation
  2. Unbalanced magnetic pull producing vibrations
  3. Reduction of generated voltage for which excitation has to be increased to maintain the voltage
  4. All of the above
ANSWER
Answer: ( 4 )
EXPLANATION
The field winding is supposed to generate a magnetic field, which is used to make the machine work. A shorted field winding won’t generate any field. There might be some permanent magnetism in the field magnetic circuit and the machine might be able to use it, but the permanent magnet field will be weak compared to the one the field windings is supposed to produce. So a feeble motor or generator, probably useless for the intended service.
5) In a DC machine, fractional pitch winding is used

  1. To increase the generated voltage
  2. To reduce sparking
  3. To save copper because of short end connection
  4. Due to both 2 and 3
ANSWER
Answer: ( 4 )
EXPLANATION
Fractional pitched windings are purposely used to effect substantial saving in copper of the end connection and for improving commutation.

6) A DC motor is used in industrial application because it ________

  1. Is cheap
  2. Is simple in construction
  3. Provide fine speed control
  4. None of the above
ANSWER
Answer: ( 3 )
EXPLANATION
One potential reason for the wide spread use of DC motors in industry is that their speed can be changed over a wide range by simple methods. Such a fine speed control is not possible with AC motors.

7) The field poles and armature of a DC machine are laminated to _________

  1. Reduce the weight of the machine
  2. Decrease the speed
  3. Reduce eddy currents
  4. Reduce armature reaction
ANSWER
Answer: ( 3 )
EXPLANATION
Stator core is stationary and field exciation is DC hence we need not laminate it since there is no flux cutting action. But if we use DC supply from a rectifier then the DC will be pulsating DC and hence it induces emf in the core and causes eddy currents to flow. So lamination of the core is done only when your power electronic converter feed the field excitation to the DC machine. Similarly, armature core is laminated to reduce eddy current losses. By using thin laminations we increase resistance of eddy current path thereby reducing the eddy currents and eddy current losses.

8) The back emf in a DC motor _______

  1. Opposes the applied voltage
  2. Aids the applied voltage
  3. Aids the armature current
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
When the motor armature rotates, its conductors cut the magnetic field. therefore, the e.m.f. of rotation is induced in them. In the case of a motor, the e.m.f. of rotation is known as back e.m.f. or counter e.m.f. opposes the applied voltage and always less than the applied voltage, although this difference is small when the motor is running under normal conditions. The back emf opposes the current which causes it.

9) The value of back e.m.f in a DC motor is maximum at _______

  1. No load
  2. Full load
  3. Half load
  4. None of the above
ANSWER
Answer: ( 1 )
EXPLANATION
The back EMF is directly related to the speed of the motor. With no load on the shaft (free-running), the motor runs at the no-load speed (NLS), the fastest possible speed for that voltage. When the shaft is fully loaded and not allowed to move, the speed is zero and the motor is producing its stall torque (ST), the maximum possible torque.

10) The motor equation is given by

  1. V = Eb – IaRa
  2. V = Eb + IaRa
  3. Eb = IaRa – V
  4. none of the above
ANSWER
Answer: ( 2 )
EXPLANATION

Let,

                V = applied voltage
                Eb = back e.m.f.
                Ia = armature current
               Ra = armature resistance
Meanwhile back e.m.f. Eb acts in reverse to the applied voltage V, the net voltage through the armature circuit is V-Eb. Now the armature current Ia is given by;
              Ia = V-Eb/Ra
               V = Eb + Ia Ra      
This is known as voltage equation of the DC motor.

11) The fully loaded current of a 20 HP 500 V DC motor will be closer to

  1. 100 A
  2. 60 A
  3. 35 A
  4. 15 A
ANSWER
Answer: ( 3 )
EXPLANATION
Apply

P = V X I

Result will be closer to 35 A.

12) The speed of a DC motor can be varied by varying

  1. Field current
  2. Applied voltage
  3. Resistance in series with armature
  4. Any of the above
ANSWER
Answer: ( 4 )
EXPLANATION
Speed of the motor is directly proportional to supply voltage, inversely proportional to armature voltage drop and to the flux due to the field findings.

Thus, the speed of a DC motor can be controlled in three ways:

  • By varying the supply voltage
  • By varying the flux, and by varying the current through field winding
  • By varying the armature voltage, and by varying the armature resistance

13) In a DC motors, the conditions for maximum power is

  1. Supply voltage = 1/2 × back e.m.f
  2. Supply voltage = √2 × back e.m.f
  3. Back e.m.f = 2 × supply voltage
  4. Back e.m.f = 1/2 × supply voltage
ANSWER
Answer: ( 4 )
EXPLANATION
The gross mechanical power developed by a motor is

Differentiating both sides with respect to Ia and equating the result to zero, we get


Thus maximum efficiency of a dc motor occurs when back EMF is equal to half the applied voltage..

14) If conditions for maximum power for a DC motor are established, the efficiency of the motor will be

  1. 100%
  2. 90 to 95%
  3. 81%
  4. Less than 50%
ANSWER
Answer: ( 4 )
EXPLANATION
The gross mechanical power developed by a motor is maximum when back EMF is equal to half the applied voltage. This condition is, however, not realized in practice, because in that case current would be much beyond the normal current of the motor. Moreover, half the input would be wasted in the form of heat and taking other losses (mechanical and magnetic) into consideration, the motor efficiency will be well below 50 percent.

15) When the speed of a DC motor increases, its armature current _______

  1. Increases
  2. Decreases
  3. Remains constant
  4. None of the above
ANSWER
Answer: ( 2 )
EXPLANATION
Ia = (V – Eb)/Ra

If the speed of the DC motor increases, then the back e.m.f Eb = (PΦZN/60 A) also increases. It is clear from the above equation that the armature current (Ia) will decrease.

16) The amount of back e.m.f of a shunt motor will increase when _________

  1. The load is increased
  2. The field is weakened
  3. The field is strengthened
  4. None of the above
ANSWER
Answer: ( 3 )
EXPLANATION
Under Construction.

17) The speed of a DC motor is ________

  1. Directly proportional to flux per pole
  2. Inversely proportional to flux per pole
  3. Inversely proportional to applied voltage
  4. None of the above
ANSWER
Answer: ( 2 )
EXPLANATION
N is directly proportional to (V – IaRa)/Φ

It is clear that speed (N) of a DC motor is inversely proportional to flux per pole (Φ). 

18) A 200 V DC machine has an armature resistance of 0.5 Ω. If the full armature current is 30 A, the induced e.m.f when the machine acts as (i) generator (ii) motor, will be

  1. 170 V, 230 V
  2. 175 V, 225 V
  3. 185 V, 215 V
  4. 215 V, 185 V
ANSWER
Answer: ( 4 )
EXPLANATION
Given parameters Ra = 0.5 ohm

                              Va = 200 V

                              Ia = 30 A

Inducer EMF when machine acting as a generator

Eg = Va + ia Ra = 200 + 30 × 0.5 

     Eg = 215 V

Induced EMF when machines acting as a motor (Induce) EMF also called back EMF )

Eb = Va – ia Ra = 200 – 30 × 0.5

                        = 200 – 15

Eb = 185 V.

19) The ratio of starting torque to fully load torque is least in case of

  1. Shunt motors
  2. Series motors
  3. Compound motors
  4. Differential compound motor
ANSWER
Answer: ( 1 )
EXPLANATION
In differential compound motor, series field opposes the shunt field. Hence ratio of starting torque to full load torque is minimum.

20) The speed of DC motor is

  1. Always constant
  2. Directly proportional to back e.m.f
  3. Directly proportional to flux
  4. Inversely proportional to the product of back e.m.f and flux
ANSWER
Answer: ( 2 )
EXPLANATION
The emf equation of DC motor is given by

Here,

N = Speed of rotation in rpm.

P = Number of poles.

A = Number of parallel paths.

Z = Total no. conductors in armature.

Hence, speed of a DC motor is directly proportional to emf of rotation (E) and inversely proportional to flux per pole (φ).


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