We all know that dc machine is an electromechanical device. It acts as two types of electric machines either dc generator or dc motor. In case of generator, the output electric power is taken out from the machine.
The dc generator converts mechanical motion into electrical power by electromagnetic induction. The induced emf produced alternative current in nature. but, the need is direct current connect to Electrical Appliances! how to convert alternating current into unidirectional pulsating current.
The answer to the complex questions is 'commutator'.
What is commutator?
A commutator is a rotary electrical switch divided by segments and fixed with armature conductors. The function of commutator in dc generator is to collect current from the armature conductors and fed into load.
Commutation in dc machine
The commutation is nothing but the process of reversal of electric current in a coil with the help of carbon brushes and commutator segments.
The update of slip ring commutator is split ring commutator. The slip ring commutator is a device used to take output current as bidirectional. Where as split ring commutator is fixed with armature conductors. so, every half cycle the direction of electric current reversed. consider the dc machine with 2 poles double layer lap wound armature conductors and rotating in anti clock wise direction.
The armature coil is connected with split ring commutator segments the number of armature coil is equal to the number of commutator segments. The mica insulation provided in between the commutator segments.
Each coil having two ends is connected to commutator. Let's see what happens when its running as dc generator. connected with load from the dc generator diagram. consider coil a connected with commutator segment 1 and 2 in simple ways.
case1
The brush is in exact position of commutator segment 1. The width of the brush is equal to the commutator segment.
The coil between 4 and 1 is Ic and other coils carries Ic. By kcl, the current 2I enter into the commutator segment 1.
case 2
The commutator is rotated such a way that 3/4th and 1/4th of commutator segment 1 and 2. 3/4th of current is entering into segment 1 and 1/4th of current is entering into segment 2. By KCL, the resultant current is 2I.
case 3
In this case, brush is equally distributed in the commutator segment 1 and 2. At this instant, no emf is induced in the coil a. Because of armature conductors is out off main field flux. so, the current is 2I.
case 4
The commutator is rotated such a way that 3/4th and 1/4th of commutator segment of 2 and 1. 3/4th of current is entering into segment 2 and 1/4th of current is enter into segment 1. By KCL, the resultant current is 2I.
case 5
The brush is exactly is in commutator segment 2 and current from the coil between 2 and 3 is Ic. another coils current from 4 to 1 is Ic. The resultant current is 2Ic
Effects of Commutation
The commutation discussed above process is called as Linear commutation. The speed of the dc machine is high such that RPM is high. The time required to shift the commutator segment is very less in terms of milliseconds. so, the contact of brushes produces motor sparks in brushes when the current reversal is occurs. It might damage the commutator segments and brushes. There are some methods to improve commutation in dc machine.
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