Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: DC series motors are suitable for buses, trolleys and trains.
Reason R: DC series motors are variable speed motors.
In light of the above statements, choose the correct answer from the options given below

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Always remember:
- DC Series Motor \(\rightarrow\) Used in Traction (high starting torque, speed drops as load increases).
- DC Shunt Motor \(\rightarrow\) Used in Lathes and Blowers (constant speed applications).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DC series motors are characterized by having the field winding connected in series with the armature winding.
This configuration gives them unique speed-torque characteristics that make them highly suited for heavy-duty electric traction applications.

Step 2: Detailed Explanation:
Let us evaluate the statements:
- Assertion A: "DC series motors are suitable for buses, trolleys and trains."
This is correct.
Electric traction vehicles like trains, trolleys, and buses require very high torque at startup to overcome static friction and inertia.
Since the starting torque of a DC series motor is proportional to the square of the armature current (\(T \propto I_a^2\)), it can develop exceptionally high starting torque, making it ideal for these applications.
- Reason R: "DC series motors are variable speed motors."
This is correct.
The speed of a DC series motor is inversely proportional to the armature current/flux (\(N \propto \frac{1}{\Phi}\)).
Under heavy load (high current), the motor runs at a safe, low speed, while under light load, it runs at a high speed. This variable speed behavior is a self-regulating characteristic:
- It prevents overloading by slowing down when carrying heavy loads.
- It speeds up as the load decreases.
This direct link between load and speed is the key reason why DC series motors are selected for traction applications (trains and trolleys).
Therefore, both A and R are true, and R is the correct explanation of A.

Step 3: Final Answer:
The correct option is (A).