Fig shows the connections of the DC series motor.
Torque proportional to flux times armature current
Up to magnetic saturation,
Therefore,
After magnetic saturation, flux is constant, therefore,
- In DC series motor, the same current flows through field winding and armature winding.
- When the mechanical load on the motor shaft increased then armature current also increased hence flux in the DC series motor increases with an increase in armature current and flux in the DC series motor deceased with decreased in armature current.
1. Torque vs Armature (Ta/Ia) Characteristics of DC Series Motor:
We know that:Torque proportional to flux times armature current
Up to magnetic saturation,
Therefore,
After magnetic saturation, flux is constant, therefore,
- Therefore up-to magnetic saturation, the armature torque is directly proportional to the square of armature current. If the armature current is doubled, Torque is almost quadrupled.
- Therefore, Ta/Ia curve up to magnetic saturation is parabolic, then after magnetic saturation, torque is directly proportional to the armature current. So that torque v/s armature current curve is a straight line after magnetic saturation.
- It is observed that in the initial portion of the curve (i.e. up to magnetic saturation), torque is proportional to the square of the current.
2. Speed vs Armature Current (N/Ia) Characteristic of DC Series Motor:
DC series motor speed is given by-
From the characteristics, it is clear that the DC series motor developed high torque at low speed and low torque at high speed because of the increased torque required increased armature current as well as field current.
- With an increase in armature back EMF decreased because of Ia(Ra + Rse) drop cause increased in flux. Under normal conditions, Ia(Ra + Rse) drop is very small and it may be neglected.
- So that speed v/s armature current curve follows a hyperbolic path after magnetic saturation as shown. After saturation, flux is constant therefore speed also.
3. Speed vs Torque (N/Ta) Characteristic of DC Series Motor:
- The N/Ta characteristic of a series motor is shown in Fig below.
From the characteristics, it is clear that the DC series motor developed high torque at low speed and low torque at high speed because of the increased torque required increased armature current as well as field current.
Conclusions from the characteristics of DC Series Motor:
- It has a high starting torque.
- It is a variable speed motor, its speed is changed with the change in load.
MCQs on DC Series Motor Characteristics:
1. What happens when the mechanical load on the shaft of a DC series motor increases?
A) Armature current decreases
B) Flux decreases
C) Armature current increases
D) Flux remains constant
Answer: C) Armature current increases
2. In a DC series motor, what happens to the flux as armature current increases?
A) Flux decreases
B) Flux increases
C) Flux remains constant
D) Flux fluctuates
Answer: B) Flux increases
3. The torque in a DC series motor is directly proportional to which factor up to magnetic saturation?
A) Square of armature current
B) Square of field current
C) Armature resistance
D) Speed of the motor
Answer: A) Square of armature current
4. What is the characteristic of the torque vs. armature current (Ta/Ia) curve up to magnetic saturation in a DC series motor?
A) Parabolic
B) Linear
C) Exponential
D) Hyperbolic
Answer: A) Parabolic
5. After magnetic saturation, the torque vs. armature current curve becomes:
A) Parabolic
B) Linear
C) Constant
D) Exponential
Answer: B) Linear
6. Which of the following is true regarding the speed vs. armature current (N/Ia) characteristic of a DC series motor?
A) The curve follows a hyperbolic path after magnetic saturation
B) The curve is linear at all stages
C) Speed increases with increasing armature current indefinitely
D) Speed decreases after magnetic saturation
Answer: A) The curve follows a hyperbolic path after magnetic saturation
7. What is the effect of increased armature current on the back EMF in a DC series motor?
A) It increases
B) It decreases
C) It remains constant
D) It fluctuates
Answer: B) It decreases
8. In a DC series motor, what causes the speed to remain constant after magnetic saturation?
A) Constant armature current
B) Constant flux
C) Constant load
D) Constant resistance
Answer: B) Constant flux
9. What does the speed vs. torque (N/Ta) characteristic of a DC series motor indicate?
A) High torque at high speeds
B) Low torque at low speeds
C) High torque at low speeds
D) Constant torque at all speeds
Answer: C) High torque at low speeds
10. Which of the following is true about the DC series motor?
A) It has a high starting torque
B) It is a constant speed motor
C) Its speed is not affected by the load
D) It has a low starting torque
Answer: A) It has a high starting torque