Resistance commutation DC Macnines (DC motors or DC generators)
Resistance commutation in Direct Current machines is a method to improve commutation (both in DC motors and DC generators). It reduces the occurrence of sparking during the process of commutation. In this method the low resistance copper brushes are replaced by high resistance
carbon brushes. The sparking is reduced due to the high resistance offered by the carbon brushes in the trailing segment of the commutator. As resistance increases with decrease in area of cross-section and vice versa, as commutator segment advances through the carbon brush the resistance for the current path always decreases for leading segment as the contact area increases and increases for the trailing segment as the contact area decreases. Hence it favours the current path to be taken through the leading segment as it cross the commutator segment.
carbon brushes. The sparking is reduced due to the high resistance offered by the carbon brushes in the trailing segment of the commutator. As resistance increases with decrease in area of cross-section and vice versa, as commutator segment advances through the carbon brush the resistance for the current path always decreases for leading segment as the contact area increases and increases for the trailing segment as the contact area decreases. Hence it favours the current path to be taken through the leading segment as it cross the commutator segment.
Working of Resistance Commutation
It is shown in the below figure with three coils 1, 2 and 3 and commutator bars i, ii, and iii. The current I from the coil 3 can take two paths;
1. Coil 3-commutator bar ii-brush or
2. Coil 3-short circuited coil 2 - Commutator bar i - carbon brush.
Let r1 be the resistance between the bar i and brush, r2 is the resistance between the bar ii and brush. If the copper brushes are employed, they take the first path due to their low resistance whereas the carbon brushes prefer the second path, because the resistance r2 increases as the area of contact of brush with bar ii is decreasing and the resistance r1 decreases as the area of contact of brush with bar i is increasing. Hence this reduces the sparking at the brush.
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| Picture Illustrating Resistance Commutation |
The carbon brushes also offer to some extent, self-lubrication capabilities and also even if the sparking occurs the damage caused will be lesser in comparison with Cu brushes.
Note: The carbon brushes alone cannot fully obtain the sparkles commutation because the main cause of the sparking commutation is due to the reactance voltage (self-induced emf in armature).
Limitations
1. Power loss and Voltage drop up to 2 volts due their high resistance.
2. Need of large brush holders due low current density of carbon.
3. Due to the higher losses it increases the size of commutator for effective heat dissipation.
Read also:
commutator
commutator
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