DC Shunt Motor MCQ || DC Shunt Motor Questions and Answers

Answer: 1. Decreasing the field current

Explanation:

• In the field control method of the shunt motor, a variable resistance is connected in series with the shunt field and is adjusted to change the field current as required.
• The speed is inversely proportional to the flux when the applied voltage is not changed and the back emf can be considered constant. Thus, increasing the resistance in the shunt field decreases the shunt field current, decreases the flux, and increases the speed.
• In the same way. decreasing the resistance in the shunt field circuit increases the shunt field current, increases the flux and decreases the speed.

Answer: 2. Shunt motor

Explanation:

• The source current supplied to the 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.
• Hence, to produce the same torque as that in a series motor, more amount of current is to be supplied through the source compared to the series.
• Therefore, the shunt motor will have the highest percentage increase in input current for a given percentage increase in torque.

Answer: 2. Shunt motor

Explanation:

• In the series motor, both field and armature winding carry the same current from the source as connected in the series.
• Now, torque is directly proportional to the product of current in field and armature windings, for a given source current the torque produced is square of this current.
• Hence, for a great change in torque, the current change is less.

Answer:1. Will be doubled

Explanation:

Speed of DC shunt motor is given by,

$N \propto \frac{{{E_b}}}{\phi } \propto \frac{{V – {I_a}{R_a}}}{\phi }$

But generally, Ra term is small, hence this term is neglected.

⇒ E ∝ V

Now the speed is,

$\Rightarrow N \propto \frac{{{E_b}}}{\phi } \propto \frac{V}{\phi }$

From data, as V is half, ϕ becomes half, such that speed becomes constant. Also given that,

T ∝ N₂, T ∝ φI_a

Since speed is not changed even V is half, the torque also remains unchanged.

Hence to get constant torque with field flux fallen to half, armature current has to be doubled.

Answer: 1. Milling machines

Explanation:

• DC Shunt Motor is an almost constant speed motor and its torque varies nearly as the current.
• Hence, 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.

Answer: 4. Flux is practically constant in DC shunt motors

Explanation:

In DC shunt Motor the field winding is connected in parallel to the armature winding and the supply.

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.

Answer:3. Not suitable for rapidly changing loads

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 E_b 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.

In DC shunt motor, due to shunt characteristics, speed variation depends not only upon the controlling resistance but on the load current also. This double dependence makes it impossible to keep the speed sensibly constant on a rapidly changing load.

Answer:2. Shunt Motor

Explanation:

DC shunt motor is an almost constant speed motor. Hence it is used for driving constant speed line shafts, lathes, centrifugal pumps, small printing presses, paper making machines, etc.

Answer:4. 1500 rpm

Explanation:

Let applied voltage =  V

Reduced applied voltage =  V / 2

No-load speed = 1500 rpm

As the speed is constant irrespective of voltage variation, the speed will be 1500 rpm at reduced voltage.

Answer:2. 237.5 V

Explanation:

Given that, voltage (V) = 250 V

Armature resistance (R = 0.5 Ω

Shunt current is neglected.

Armature current (I = (I_L) = 25 A

Back emf (E = V – IaRa = 250 – 25 × 0.5 = 237.5 V

Answer:1. Electric locomotive

Explanation:

• A shunt-wound motor is an almost constant speed motor and its torque varies nearly as the current.
• Hence, 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.

From the given option electric locomotive is not an application of shunt motor, so option 1 is the correct answer.

Answer:1. 4

Explanation:

Stalling torque is the torque at which speed would fall to zero.

With N = 0

E_b = V − I_aR_a

240 − I_a(1.5) = 0

I_a = 160 A

T₂/T₁ = I₂/I₁

T₂/T₁ = 160/40 = 4

Answer:4. 476 rpm

Explanation:

Performance eqⁿ of DC shunt motors are

E_b = V_t – I_aR_a

or, E_b = K_aϕω       _ _ _ _ _ (1)

and τ = K_aϕI_a       _ _ _ _ _ (2)

E_b1 = 220 – 50 × 0.2

= 220 – 10 = 210 V

Since Torque is double i.e., armature current is doubled

i.e., I_a2 = 2 × I_a1 = 2 × 50 = 100 A

∴ E_b2 = 220 – 100 × 0.2

= 200 V

E_b2/E_b1 = N₂/N₁

N₂ = 500 × 200/210 = 476 RPM

Answer:2. Armature Resistance control

Explanation:

An armature resistance control method is very simple and usually employed when speeds below normal are required. But this method suffers from the drawback that the motor speed changes with every change in load on the machine.

It is because the speed variation does not depend only upon the controlling resistance but it also depends upon load current. Because of this reason constant speed cannot be obtained in case of rapidly changing loads.

Answer:1. 2:1

Explanation:

DC shunt motor is a constant speed motor

• It is used in applications where constant speed is required.
• It is used in applications where adjustable speed in the range of 2:1, along with a medium starting torque is required.
• It is used in lathe machines, centrifugal pumps, fans, and blowers, machine tools like wood working machines, reciprocating pumps, spinning and weaving machines etc.