Commutator in DC Machine

Commutator in DC Machine

A commutator is an essential part of a DC machine and behaves as a reversing switch.

In the case of a DC generator, the commutator is used to convert the alternating current (AC) into direct current (DC).

In the case of a DC motor, it is used to reverse the current available from the DC supply and helps to maintain unidirectional torque.

The commutator is made up of wedge-shaped hard drawn copper wires. (Hard drawn wire means the wire is drawn in such a way that they provide more strength to the machine.)

It is used in DC machines (DC motor, DC generator, Dynamo) and universal motors.

Generally, it is used along with the brushes. And the brushes are stationary parts, and the commutator is a rotating part.

Role of Commutator

The commutator connects the rotating armature circuit to the stationary circuit.

As we know, the armature is a rotating part. And the load or source connected with the DC machine must be connected with the stationary terminals.

Hence, the commutator and the brushes help to connect the rotating armature conductors to the stationary terminals.

A DC generator converts the mechanical input into the direct current (DC) electrical output. If a coil rotates in a magnetic field, it will generate an alternating current. Therefore, the commutator converts the alternating current into a direct current.

A DC source supplies a DC motor. The DC supply enters the machine via brush and commutator. And the commutator applies electric current to the armature winding. At each half turn, it reverses the current direction in rotating winding. And it helps to produce a steady rotating force.

How Does Commutator Work in DC Generator?

To understand the working of a commutator, we take an example of a single loop.

First, we consider the operation without a commutator in a single loop. The circuit diagram of a single loop without a commutator is shown in the figure below.

Single-loop Generator Without Commutator
Single-loop Generator Without Commutator

Here a single loop (ABCD) is placed between the magnetic field generated by the permanent magnets. The terminals of coils are connected with the slip ring and brush assembly.

The direction of a magnetic field is always N-pole to S-pole. So let’s consider this arrangement for a generator action. And by external means, the loop is rotating in a clockwise direction.

In this condition, the EMF is induced in the loop conductors. And due to EMF, the current starts flowing to the conductors.

The direction of current in conductor-1 is from A to B. Similarly, the current direction in conductor-2 is from C to D. Therefore, the current direction through the load is from F to H.

After the half rotation, the above arrangement looks like the below figure.

Without Commutator Single-loop Generator
Without Commutator Single-loop Generator

Now, in this condition, the direction of a magnetic field is not changed. But the position of conductors is changed.

Hence, the current flowing through the conductor-1 is from B to A. And current flowing through conductor-2 is from D to C.

So, we can see that the current direction is changed to the previous condition.

The output waveform of this arrangement is shown in the figure below.

Single-loop Generator Without Commutator Output Waveform
Single-loop Generator Without Commutator Output Waveform

The output waveform changes the polarity (positive to negative) in this arrangement. Hence, this arrangement produces alternating current. But we need direct current.

To do so, we need to replace the slip-rings with the commutator. And the arrangement is shown in the figure below.

Single-loop Generator With Commutator
Single-loop Generator With Commutator

The terminals of a coil (conductors) are connected with the commutators. And the commutator rotates with the coil.

The commutators are connected with brushes. And the brushes are stationary. The load is connected across the brushes.

Now, the direction of a magnetic field is from N-pole to S-pole. And the current that passes through the conductor-1 is from A to B. The current that passes through the conductor-2 is from C to D.

Therefore, the current passes through the load are from F to G.

After half rotation, the arrangement looks like the figure below.

With Commutator Single-loop Generator
With Commutator Single-loop Generator

In this condition, the current passing through the conductor-1 is from B to A. And the direction of current through conductor-2 is from D to C. Therefore, the current direction through the load is from F to G.

Hence, the current direction through the load does not change after the half rotation. Even further, the coil rotates in a clockwise direction. But the current passes through the load remain in one direction.

The graph of this arrangement is shown in the figure below.

Single-loop Generator With Commutator Output Waveform
Single-loop Generator With Commutator Output Waveform

Here, we can see the current flows in only one direction. Hence, we can get the pulsating DC output from this arrangement with the help of a commutator.

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