DC Series Motor MCQ || DC Series Motor Questions and Answers

Answer: 2. Interchanging field terminals

Explanation: 

In the case of the DC series motor, the field and armature winding are connected in series. If the polarity of the input voltage or supply terminals are interchanged, then it changed the direction of the field and armature current both get changed so the direction of speed will remain the same.

The direction of rotation of the DC series motor may be reversed by changing the polarity of either field or the armature winding.

Answer: 2. Resistance is connected across its armature terminals

Explanation: 

• In a dc series motor, an increase in armature circuit resistance will cause more voltage drop in the armature circuit, so speed will be reduced.
• The torque which is proportional to the product of the flux and armature current will also be reduced with the increase in armature circuit resistance as the increase in armature circuit resistance will reduce the armature current and the flux which also depends upon the armature current.
• Therefore, in this method of speed control, reduced speeds are obtained by inserting resistance in the armature circuit.

Answer: 1. No-load

Explanation: 

A DC series motor should always be started with load because at no load it will rotate with dangerously high speed

• When the motor is connected across the supply mains without load, it draws a small current from the supply mains
• This current will flow through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude
• The increase in back emf weakens the armature current and hence the field current
• This cause again increases the speed so the back emf
• Thus, the field continues to weaken and speed continues to increase until the armature produced such centrifugal force that it is coming out from its shaft and gets damaged

Answer: 2. Rise in speed and drop in current

Explanation: 

In a DC series motor, the series field winding is connected in series with armature winding i.e., it is carrying high armature current.

Therefore, series field winding will have low resistance value.

In DC series motor, I_a = I_f

∵ we know that, torque τ

τ = K_aϕI_a

Here ϕ ∝ I_a

∴ τ ∝ I_a²

Also, E_b = V_t – I_a(R_a + R_se)

I_a = V_t – E_b(R_a + R_se)

If the supply voltage is reduced then armature current will reduce.

Also, E_b = V_t – I_a(R_a + R_se) ∝ Speed

If armature current is reduced then back emf will increase. Hence, speed will increase.

Answer: 3. No-load speed is zero

Explanation: 

• DC series motor has high starting torque; This is one of the most required characteristics of DC series motor.
• Speed control of DC series motor is easy and various methods are available for this.
• No-load speed is dangerously high
• The size of the DC series motor is small. (i.e. small size and high-power rating).
• This motor can handle overload easily.
• Regenerative braking can be applied to DC series motor, But with some modifications.
• The maintenance cost required is less.

Answer: 4. I_a

Explanation: 

In a DC motor, T ∝ ϕI_a

In a DC series motor, for smaller values of armature current, ϕ ∝ I_a

Now, T ∝ (I²

For higher values of armature current, the flux will be constant because of saturation.

In this case, T ∝ I_a

(torque developed by motor for small values of armature current)/(torque developed by motor for higher values of armature current) = I_a

Answer: 2. Limit the speed

Explanation: 

A DC series motor should always be started with some load because at no load it will rotate with dangerously high speed.

• When the motor is connected across the supply mains without load, it draws small current from the supply mains
• This current will flow through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude
• The increase in back emf weakens the armature current and hence the field current
• This cause again increases the speed so the back emf
• Thus, the field continues to weaken and speed continues to increase until the armature produced such centrifugal force that it is coming out from its shaft and gets damaged

Answer: 2. High starting torque requirements

Explanation: 

DC series motor is best suited for heavy load, as it provides high starting torque and easy speed control which is the primary requirement of DC traction.

Application of different motors:

Answer: 1. Torque is proportional to the square of armature current and speed is inversely proportional to torque

Explanation: 

• DC series motor has high starting torque.
• DC Series motor speed-torque characteristics are such that as torque increases speed decreases.
• DC series motor has develops high torque at low speeds, low torque at high speed, this is the basic requirement of the traction unit.

In DC series motor, the current passing through the field winding is the same as that in the armature.

If the mechanical load on the motor increases, the armature current also increases. Hence, the flux in a series motor increases with the increase in armature current and vice-versa.

In a DC motor, T ∝ ϕI_a

In a DC series motor, for smaller values of armature current, ϕ ∝ I_a

Now, T ∝ (I²

Since in DC series Motor the torque is directly proportional to the flux, therefore, the torque is inversely proportional to the speed. The torque sped curve is, therefore, a rectangular parabola.

Answer: 3. Belt Load

Explanation: 

Belt loads are never recommended for d.c. series motors because,  Since on no load, the armature current and hence flux of d.c. the series motor is very small, speed being inversely proportional to the flux will be dangerously high.

When the motor is loaded, its armature current I, increases, and speed fall. Belt-driven loads are never recommended for d.c. series motor because the slip of the belt may cause the motor to run without load, which may damage the motor.