Question

In a DC Machine, the function of the commutator is to:

• A. Convert AC to DC in a generator
• B. Convert DC to AC in a motor
• C. Convert AC to DC in a generator and DC to AC in a motor
• D. Increase the speed of the machine

Hint:

The commutator is essentially a mechanical frequency converter.
It acts as a rectifier in generator mode and as an inverter in motor mode.
In both cases, the current inside the armature winding is always AC, while the current in the external circuit is always DC.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question addresses the fundamental operating principle of DC machines and the critical role played by the commutator.
The commutator is a cylindrical arrangement of copper segments insulated from each other, which rotates along with the armature shaft.
Its main function is to facilitate the transfer of current between the rotating armature conductors and the stationary external circuit.

Step 2: Key Formula or Approach:
The question is qualitative, so no mathematical equations are necessary.
The solution relies on understanding the nature of induced EMF in rotating machine windings and how mechanical switching is used to alter its waveform.

Step 3: Detailed Explanation:

• Generator Action: When the armature of a DC generator rotates within the stator magnetic field, the magnetic flux linkage changes continuously.

• According to Faraday's law of electromagnetic induction, this change induces an alternating electromotive force (AC EMF) in the armature conductors.

• To deliver a unidirectional direct current (DC) to the external load, a mechanical rectifier is needed.

• The commutator, operating in conjunction with stationary carbon brushes, switches the connections of the coils as they move under opposite magnetic poles, thereby converting AC to DC.

• Motor Action: In a DC motor, the electrical input supplied from the external source is DC.

• To maintain continuous rotation and unidirectional electromagnetic torque, the direction of current in each armature conductor must change direction every time they pass from the influence of one pole to the next.

• The commutator acts as a mechanical inverter, reversing the direction of the DC supply current entering the coils, thereby producing an alternating current (AC) in the armature.

Step 4: Final Answer
Thus, the commutator performs the dual function of converting AC to DC in a generator and DC to AC in a motor.