Commutator

What is Commutator?

A commutator is a rotary electrical switch that periodically reverses the current between the rotor and the external circuit. The commutator was discovered by William Ritchie and Hippolyte Pixii in 1832.

Commutators are used in DC machines (DC motors and DC generators) universal motors.

  • In a motor, a commutator applies an electric current to the windings. A steady rotating torque is produced by reversing the current direction in the rotating windings each half turn.
  • In a generator, the commutator reverses the current direction with each turn serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit.

Definition of Commutator:

A commutator is a rotary electrical switch that periodically reverses the direction of the current between the rotor and the external circuit.

Construction of a Commutator

A commutator is built with a set of contact bars and is set into the revolving shaft of a DC machine allied to the armature winding.

Commutator

The commutator reverses the current flow within a winding when the shaft turns. Once the shaft completes a half-turn, the windings are connected so that current supplies through it in the reverse of the first direction.

In a DC motor, the magnetic field uses a rotating force or a torque over to the winding to rotate. On the other hand, in a DC generator, mechanical torque is applied in the direction of the shaft to maintain the armature winding motion. In both cases, commutators reverse the direction of current flow throughout the winding. The flow of current within the circuit that is external to the machine is in a single direction.

Commutation in DC Machines

In a DC machine, each armature coil contains two commutators attached to its end. The commutator segments and brushes should maintain a continuous moving contact for the better transformation of the current. More than one coil is used in DC machines to avail larger output values.

Now, let us consider a DC motor in which the width of the commutator bars is equal to the width of the brushes. Let the current through the conductor be Ia. Let a,b and c be the commutator segments of the motor. The commutation steps on the coil can be understood by the following steps:

Position 1

Commutator Working

As the armature starts rotating, the brushes move over commutator segments. Let the first position of the brush commutator contact be at b as shown in the figure. As the width of the commutator and the brush is equal, the whole of the commutator segment and the whole of the brush is in contact with each other. The total current conducted by the commutator segment into the brush at this position is equal to 2Ia.

Position 2

Commutator Working

As the armature rotates towards the right, the brush comes in contact with the commutator bar a. At this position, the total current is 2Ia. The current in the coil changes as the current flows through two paths A and B. 3/4th of the 2Ia comes from the coil B and remaining 1/4th comes from coil A. When KCL is applied to the segment a and b, the current through coil B is reduced to Ia/2 and the current drawn through segment a is Ia/2.

Position 3

Commutator Working

At this position, each half of the brush is in contact with segment a and segment b as shown in the figure. Using KCL we observe that the current in coil B will be zero.

Position 4

Commutator Working

In this position, one-fourth of the brush is in contact with segment b and three fourth with segment a as shown in the figure. The current drawn through coil B is – Ia/2. We notice that the current in coil B is reversed.

Position 5

Commutator Working

In this position, the brush is in full contact with segment a and the current from coil B is Ia but in reverse direction to the current direction of position 1.Thus, the commutation process is completed for segment b.

Similar Reading:

Slip Rings and Commutators

Slip rings and commutators are electric devices used to maintain electrical circuit continuity between a rotating and a static system. The most important difference between them is that commutators can convert AC to DC but slip rings cannot.

In contrast to commutator, slip rings have only one operation, that is to enable a connection between a fixed conductor and a rotating conductor.

Frequently Asked Questions on Commutator

When was the commutator invented?

Commutators were invented in 1832 by William Ritchie and Hippolyte Pixii.

Where is the commutator used?

Commutators are used in DC motors and universal motors to ensure that the current in the rotor windings always flows in the same direction.

How do commutators work?

A commutator is a rotatory electrical switch that reverses the direction of current between the rotor and the external circuit periodically. The reversal of the current each half-turn gives rise to a steady rotating torque.

What are commutators made of?

Commutators are made up of a set of copper segments that are fixed around the rotor.

Is the commutator and slip ring the same?

Although both are used to maintain the electric circuit continuity between a rotating and a static system, they are distinct from each other. Slip rings are a continuous ring that provides a constant transfer of signal, power or data. On the other hand, commuatators are used in DC motors to reverse the polarity of the current in the armature windings.

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