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Tuesday, 24 December 2019

Process of Commutation in DC Machine

Commutation: The current induced in the armature of a d.c. generator is alternating and a commutator is used to make the alternating current into unidirectional current by reversing the negative part of alternating current. The reversal of current takes place in the magnetic neutral axis. The brush short circuits the particular coil undergoing reversal. The method by which current in the short circuited coil is reversed as it crosses the magnetic neutral axis is referred to as commutation.


Process of Commutation in DC Machine

If the reversal of current is gradual, then it is called smooth commutation. Smooth commutation means no spark at the brush and the surface between brush and the segment is unaffected. Consider a portion of the segments with armature coil between them touching a brush at any instant of rotation of armature coil as shown in Fig. In the figure, the coils have been labeled as A, B, C, D, E and corresponding segments have been numbered as 20 to 25. In Fig (a), the brush is in contact with segment no. 21 to which coils A and B feed the current of magnitude lc. The direction of current in the coil A is clockwise while in coil b is anticlockwise. As the armature moves in the direction of rotation the brush slides and makes contact with commutator segments 21 and 22 as shown in Fig. (b). At this position, the coil B is getting short circuited thereby current in coil B decreases. Note that as the brush slides and touches the commutator segment No. 22 which is in contact with segment No. 21, the current in coil B which was an it clockwise drops down. If i1 is the current which enters the brush from coil C through segment No. 22 (Ic — i1) is the current which enters brush from coil B through segment No. 21. The area of the copper carbon contact decides the distribution of current. As the area of copper carbon contact in segments 21 and 22 become equal, i1 would have become Ic. Then the current in coil B becomes zero as shown in Fig. (c).
Process of Commutation in DC Machine
After another brief period, the area of contact of brush in segment No. 22 will be more than in segment No. 21. If at this position the current entering the brush from coil A through segment No. 21 is i2 the remaining current (lc - i2) will try to pass through B and segment No. 22 to reach the brush. In this process the current in coil B rises from zero to a value i, in the reverse direction to that shown in Fig. 2.59 (a). As the brush slides further the value of i2 increases and when the brush is in contact with segment No. 22, the current i2 would have risen to a value equal to lc. From the above, it can be seen that for every shift of position of brush from one segment (say 21) to its adjacent segment, say (22).


a. The current in the coil connected in between segment Nos. 21 and 22 decreases, becomes zero and increases in the reverse direction.

b. Due to reversal of the current in the coil in question, a static e.m.f. is induced in the coil, which opposes the flow of current, the least is the time for reversal, higher is the magnitude of induced e.m.f. The time required for the coil current to change by + Ic to - Ic is called the commutation period time and is given by

Tc = Brush width /Commutator peripheral speed

CAUSES FOR SPARKING AT COMMUTATOR

The nature of current flowing in the local circuit of the coil being commuted depends on the following factors.

a. Resistance between the surfaces of brush material and segment material.
b. Resistance of the coil under commutation.
c. E.m.fs induced in the short circuited coil due to

i. Self inductance
ii. Mutual inductance with other coils undergoing commutation simultaneously. This is possible only when brush is more than one commutator segment width as in the case of duplex winding.

d. E.m.f. induced in the coil due to its rotation in the armature cross magnetising flux.

1. Sparking at Commutator :

The armature coil possesses an appreciable amount of reluctance because it is embedded in the armature core which has high magnetic permeability. Due to the current reversal in commutation, self induced e.m.f. is produced in the coil. The quicker is the time of reversal, higher is the magnitude of self induced e.m.f. This voltage although of little magnitude, makes a large current through the coil whose resistance is low due to short circuit. When the coil undergoes short circuit in the magnetic neutral axis, no e.m.f. is induced due to rotation of the armature and the self induced e.m.f. which is present during this time causes severe sparking at brushes. Higher is the rate of rotation of armature, higher is the rate of reversal of current in the short circuited coil and higher is the magnitude of self induced emf resulting in large spark at brush-contact.

2. Effect of Sparking

Sparking at the brush contacts has the following effects :

a. The surface of the commutator segments gets carbonised. Consequently it short circuits all the coils of the armature.

b. Sparking can damage commutator surface decreasing the contact surface. Due to this the brush jumps resulting in more sparking.

c. Sparking can result in excess heating of the commutator segments raising its temperature which can unsolder the armature coil leads.

3. Reactance Voltage :

Let Tc = be the time of commutation
= (brush width - width of mica) /Peripheral velocity

Wb = Width of brush in metres
Wm = Width of mica in metres

v = Peripheral velocity of commutator segment in cm/sec

Then T = [Wb — Wm]/V seconds

If I is the current through the armature conductor, then the total change in commutation current is equal to

I - (- I) = 2I Amperes

Reactance Voltage = L (2I /Tc), for linear commutation
Reactance Voltage = 1.11 x L (2I /Tc) for sinusoidal commutation

Note: If the current varies at a uniform rate, it is called linear commutation.

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