Question

Consider the following statements about DC motors and generators
A. Starting torque of series motor is considerably greater than shunt motor
B. Armature is the rotating part of DC generator
C. In series motor, the field is connected in series with armature
D. Series motors are not suitable to drive devices which must run at high speed at light loads.
E. In shunt motor, torque is independent of armature current.
Choose the correct answer from the options given below:

• A. B, C and D only
• B. C, D and E only
• C. A, B and C only
• D. A, D and E only

Hint:

To easily identify incorrect statements:
Torque in any DC motor is given by \(T \propto \Phi I_a\). Since \(I_a\) is the current driving the motor, torque can never be independent of the armature current, which immediately rules out Statement E.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
To evaluate these statements, we must look at the mechanical configurations and electrical characteristics of DC shunt and series motors, as well as the design of DC generators.

Step 2: Detailed Explanation:
Let us analyze each statement:
- Statement A: "Starting torque of series motor is considerably greater than shunt motor."
This is correct.
For a series motor, the starting torque is proportional to the square of the armature current (\(T \propto I_a^2\)). For a shunt motor, the torque is directly proportional to the armature current (\(T \propto I_a\)). This makes the starting torque of a series motor much higher.
- Statement B: "Armature is the rotating part of DC generator."
This is correct.
In standard DC machines (generators and motors), the field magnets are placed on the stationary stator, while the armature conductors rotate on the rotor to facilitate mechanical rectification through the commutator.
- Statement C: "In series motor, the field is connected in series with armature."
This is correct.
This series connection is the defining characteristic of series motors, ensuring that the same current flows through both the field and armature windings.
- Statement D: "Series motors are not suitable to drive devices which must run at high speed at light loads."
This statement is technically misleading. At light loads, the series motor naturally runs at a very high (and potentially dangerous) speed. If a device *must* run at high speed under light loads, the series motor can achieve this, but it is dangerous to operate it without load.
- Statement E: "In shunt motor, torque is independent of armature current."
This is incorrect.
In a shunt motor, the torque is directly proportional to the product of flux and armature current (\(T \propto \Phi I_a\)). Since the flux is constant, the torque is directly proportional to the armature current, not independent of it.
Looking at the options, the combination of statements A, B, and C represents the most accurate set of properties.

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