Jul 20, 2014

Commutation in DC Machines


The voltage generated within the coil (armature), placed in an exceedingly rotating magnetic flux, of a DC generator is alternating in normally. The commutation in D.C. machine or additional explicitly commutation in D.C. generator is that the method during which generated AC armature coil windings of a d.c. machine is regenerate into direct current when looking the commutator and also the stationary brushes.

Again in DC Motor, the input DC is to be regenerate in alternating kind in armature which is additionally done through commutation in DC motor.
This conversion of current from the rotational coil of a d.c. machine to the stationary brushes has to maintain ceaselessly moving contacts amongst the commutator segment and also the brush. once the armature start to rotate, at that moment the coils located below one pole (N pole) rotates between a positive and negative brush consequently and also the electrical current pass through this coil is in an exceedingly direction internal to the commutator segment. Now the coil is short circuited with the assistance of a brush for an awfully short fraction of your time (1/500 sec). It’s known as commutation period. When this short circuit duration the armature coil rotate below S pole and rotates between a negative brush and its succeeding positive brush. Now the direction of becomes reversed that is within the removed from the commutator segments. This development of the reverse of current is named as commutation method. We tend to get electricity from the brush terminals.

The commutation is termed best if the commutation method or the reverse of current is accomplished by the end of the short circuit time duration or the commutation interval. If the reverse of current is completed throughout the tangency time then there's sparking happens at the brushes contacts and also the commutator surface is broken attributable to heating and also the machine is termed poorly commutated.

Commutation in DC Machine

For the reason of commutation method, allow us to take into account a D.C. machine having armature coiled with ring winding. Allow us to additionally take into account that the breadth of the commutated bar is up to the breadth of the brush and current passing over the conductor is IC.
The brush can moves from the right to left when the commutator moves from the left to right.
At the initial position, the brushes connected to the commutator bar B which is shown in the fig-a. Then the entire current lead by the commutator bar B into the brush is 2IC.
When the armature starts to maneuver right, before the brush involves contact of bar A. Then the armature current flows through 2 methods and thru the bars A and B (as shown in fig-b). The entire current (2IC) collected by the brush stay same.

As the contact space of the bar A with the brush will increase and also the contact space of the bar B decreases, this flows through the bar will increase and reduces at the same time. Once the contact space become same for each the commutator bar then same current flows through each the bars (as shown in fig-c).

When the brush contact space with the bar B decreases any, then this flowing through the coil B change its directions and start to flows counter clockwise (shown in fig-d).
When the brush wholly comes below the bar A (shown in fig-e) and cut off within the bar B then current IC flows through the coil B within the counter-clockwise direction and also the short circuit is detached. In this method the reverse of current or the method of commutation is completed.



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