DC Shunt Motor Operating Characteristics MCQ || Operating Characteristics of DC Shunt Motor Questions and Answers

11. The ratio of starting torque to full load torque is least in the case of

  1. Series Motor
  2. Shunt Motor
  3. Differentially Compounded
  4. Cumulative Compounded

Answer: 2. Shunt Motor

Explanation: 

  • In case of the DC shunt motor, the flux per pole is considered to be constant, torque increases with the increase of load current.
  • If the load current increases then the armature current also be increased and the speed slightly falls due to increase in voltage drop in the armature.
  • As the torque is proportional to ϕ (flux) and Ia (armature current).
  • Flux and armature currents are almost constant, so torque is almost constant from no-load to full-load, which can be seen same from the graph also.
  • The ratio of starting torque to full load torque is nearly one and least among all motors. 
  • It is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans, woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.

 

12. A 220 V DC shunt motor takes 3 A at no-load. It draws 25 A when running at full-load at 1500 rpm. The armature and shunt resistances are 0.5 Ω and 220 Ω, respectively. The no-load speed in rpm.

  1. 1580
  2. 1500
  3. 1550
  4. 1600

Answer: 1. 1580

Explanation: 

Voltage (V) = 220 V

No load current (INL) = 3 A

Full load current (IFL) = 25 A

Full load speed (NFL) = 1500 rpm

Armature resistance (R = 0.5 Ω

Shunt resistance (Rsh) = 220 Ω

Shunt current Ish = V/Rsh = 220/200 = 1 A

Armature current (I = IL – Ish

When motor is operating at no load

IanL = INL – Ish = 3 – 1 = 2 A

Eb1 = V – IanL Ra

= 220 – (2) (0.5) = 219 V

When motor is operating at full load

IaFL = IFL – Ish = 25 – 1 = 24 A

Eb2 = V – IaFL Ra

= 220 – (24) (0.5) = 208 V

We know that, in a DC shunt motor

Eb α N

Eb1/Eb2 = NNL/NFL

NNL = 1500 × 219/208 = 1579.33 RPM

 

13. What will be the effect of the opening of the field of a DC shunt motor while motor is running?

  1. The speed of motor will be reduced
  2. The armature current will reduce
  3. The motor will attain dangerously high speed
  4. The motor will continue to constant speed

Answer: 3. The motor will attain dangerously high speed

Explanation: 

If the field circuit is opened accidentally, the field flux will suddenly decrease to its relatively small residual value. If the armature circuit is not opened immediately, the motor speed will increase to dangerously high values and the motor will destroy itself in a few seconds either by the windings being forced from the slots or the commutator segments being thrown out by cen- centrifugal force.

Since the sudden decrease in the field flux reduces the counter voltage to a very small amount, the armature current of the motor will increase to a very high value. This will take place before the motor starts to rotate at a high speed.

The field circuit of a shunt motor must never be opened if the motor is running. Otherwise, the motor will “run away” and will destroy itself in a few seconds

 

14. Speed of DC shunt motor is directly proportional to___________

  1. Flux
  2. Back emf
  3. Terminal voltage
  4. Armature resistance drop

Answer:b. Back emf

Explanation: 

In DC shunt Motor N ∝ Eb/φ.

As the shunt field resistance is constant, the shunt field current is constant. Accordingly, the flux per pole is practically constant. The back emf is also practically constant. Hence, by speed relation, it is seen that the speed of a DC shunt motor is constant.

The speed of a DC shunt motor is dependent on back emf because the flux is practically constant in a DC shunt motor.

 

15. Which motor should be started on no-load?

  1. Series motor
  2. Shunt motor
  3. Cumulatively compounded motor
  4. Differentially compounded motor

Answer:b. Shunt motor

Explanation: 

DC shunt motor should be started with no-load and never start with heavy load because

  • The change in field flux is very less due to the load, so we can assume the flux to be constant. So flux (ɸ) is constant, and the speed is directly proportional to back emf (E.
  • Now Eb is also constant, so eventually, speed is also constant. But in practice, there is a slight decrease in speed from no-load to a full load of DC shunt motor.
  • This is why DC shunt motors are known as constant-speed motors.
  • Since speed is constant and torque (τ) is proportional to armature current (I. So, starting a shunt motor at a heavy load requires a high starting current.
  • In order to avoid this high starting current, shunt motors are not started at heavy loads and they should be started without any load.

 

16. A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved then:

  1. The speed becomes 2 pu but armature current remains at 1 pu
  2. The speed remains at 1 pu but armature current becomes 2 pu
  3. Both speed and armature current become 2 pu
  4. Both speed and armature current remain at 1 pu

Answer:b. Shunt motor

Explanation: 

Given

The terminal voltage and the field current of the machine are halved

V2 = V1 / 2  and If2 = If1 / 2

1) Armature current

VIa1 = V2 Ia2

Ia2 = VIa1 /V= VIa1 /V1/2

Ia2 = 2Ia1 = 2 pu

2) Speed of motor

T1 N1 = T2 N2  {T ∝ ϕ Ia}

${{\rm{N}}_2} = \dfrac{{{{\rm{T}}_1}{{\rm{N}}_1}}}{{{{\rm{T}}_2}}} = \dfrac{{{{\rm{I}}_{{\rm{f}}1}}{{\rm{I}}_{{\rm{a}}1}}{{\rm{N}}_1}}}{{{{\rm{I}}_{{\rm{f}}2}}{{\rm{I}}_{{\rm{a}}2}}}}$

 

${{\rm{N}}_2} = \dfrac{{{{\rm{I}}_{{\rm{f}}1}}{{\rm{I}}_{{\rm{a}}1}}{{\rm{N}}_1}}}{{\left( {\dfrac{{{{\rm{I}}_{{\rm{f}}1}}}}{2}} \right)\left( {2{{\rm{I}}_{{\rm{a}}1}}} \right)}} = {{\rm{N}}_1}$

Speed N2 = 1 pu

 

17. . What will be the effect of reducing load on DC shunt motor?

  1. Speed will increase abruptly
  2. Speed will increase in proportion to the reduction in load
  3. Speed will remain almost constant
  4. Speed will reduce

Answer: 2. Inversely proportional to the armature current

Explanation: 

  • When an external load is applied to the shunt motor it tends to slow down slightly. The slight decrease in speed causes a corresponding decrease in counter emf. If the armature resistance is low, the resulting increase in armature current and torque will be relatively large. Therefore, the torque is increased until it matches the resisting torque of the load. The speed of the motor will then remain constant at the new value as long as the load is constant.
  • Conversely, if the load on the shunt motor is reduced, the motor tends to speed up slightly, The increased speed causes a corresponding increase in counter emf and a relatively large decrease in armature current and torque.
  • The change is so negligible that in many cases it is assumed that the speed of the DC motor remains constant. Hence, characteristic is also called as shunt characteristic.

 

18. Speed-torque characteristic of DC shunt motor is____________

  1. Almost Linear
  2. Non- Linear
  3. Either 1 and 2
  4. None of the above

Answer: 1. Almost Linear

Explanation: 

From Speed Torque characteristics of DC Shunt Motor When load torque increases,  armature current Ia, increases but speed decreases slightly. Thus with the increase in load or torque, the speed decreases slightly as shown in Fig. speed-torque characteristic of DC shunt motor is almost Linear.

Speed-torque characteristics DC shunt Motor

The Equation of speed-torque characteristic of DC shunt motor is

 

19. The DC shunt motor is running with a certain load. The effect of adding external resistance in the field circuit is to:

  1. Increase the speed
  2. Stop the motor
  3. Reduce the motor speed
  4. Reduce the armature current of the motor

Answer: 1. Increase the speed

Explanation: 

Field control method:

By adding external resistance in the field circuit, we can control the speed of DC shunt motor above the normal speed.

  • In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field.
  • An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed.
  • This method of speed control is independent of load on the motor. Power wasted in controlling resistance is very less as field current is a small value.

We know that,

N ∝ Eb

N ∝ (V − Ia R/φ

ϕ ↓  → N ↑ 

By varying flux, we can increase the speed more than its base speed. This method is a constant power and variable torque drive.

 

20. The armature torque of the D. C. shunt motor is proportional to ___________

  1. Field flux only
  2. Armature current only
  3. Both 1 and 2
  4. None of the above

Answer:2. Armature current only

Explanation: 

For DC Machine, torque is directly proportional to armature current and flux.

T ∝ ϕ Ia

In dc shunt motors, flux is constant (not dependent on armature current). Hence torque is proportional to armature current.

T ∝ Ia

In dc series motors, flux is directly proportional to armature current. Hence torque is directly proportional to square of the armature current.

Ta ∝ Ia2

In dc series motor at saturation condition, flux is constant. Hence torque is proportional to armature current.

T ∝ Ia

Where, T = torque, Ia = armature current, ϕ = field flux

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