Methods of Improving Commutation in DC Generator

There are two methods of improving commutation i.e., for making current reversal in the short-circuited coil as sparkless as possible. They are,

1. Resistance commutation.
2. EMF commutation (it is done with the help of brush-lead or inter-poles).

Resistance Commutation :

We can improve the commutation by increasing the resistance of the brushes. i.e., placing highly resistive brushes in touch with the commutator to carry the current.

The current I in coil A has two paths to reach the brush. The first path is straight from segment 'b' to the brush and the other path is via short-circuited coil B to segment 'c' then to brush.

If the copper brushes (low resistance) are used, the current follows the first path but not the second longer path. If high resistance carbon brushes are used, then the current I coming from coil A will prefer the second longest path because,

1. The first path resistance will increase due to the diminishing area of contact of brush with segment 'b', and
2. The resistance of the second path will decrease due to the rapidly increasing contact area of segment 'c' with the brush.

The main cause of commutation is the self-induced emf (reactance voltage), so high resistance carbon brushes alone do not give a sparkless commutation, but they help in obtaining it. Another advantage of carbon brushes is that they lubricate and polish the rotating commutator. The carbon brushes cause voltage drop due to high contact resistance (approximately 2 V).

Advantages of Carbon Brushes :

• Improves commutation.
• Lubricate polish the commutator.
• Easily available in the market.
• Economical.

Disadvantages of Carbon Brushes :

• Causes voltage drop due to high contact resistance.
• Needs to be replaced due to wear.

E.M.F. Commutation :

In this method, a reversing e.m.f is produced to neutralize the reactance voltage in the short-circuited coil. If reversing e.m.f. is made to be exactly equal and opposite to reactance voltage, the current in the short-circuited coil will reverse quickly, which results in sparkless commutation. The reversing e.m.f. may be produced in two ways,

1. Either by giving the brush a forward lead sufficient enough to bring the short-circuited coil under the influence of the next pole of opposite polarity.
2. By using inter-poles.

The first method was used in early machines but now it has been ruled out due to many other difficulties it brings along with it.

By Using Inter-poles :

These are small poles fixed to the yoke, having a few heavy gauges turns connected in series with the armature so that they carry full armature current. Their polarity in the case of a generator is the same as that of the main pole ahead in the direction of rotation.

The function of inter-poles is two-fold :

The inter-poles induce an e.m.f. in the short-circuited coil, which neutralizes the reactance emf thereby making commutation sparkless. The e.m.f. induced by the inter-poles is known as commutating or reversing e.m.f. and this e.m.f. is proportional to armature current. This ensures automatic neutralization of reactance voltage because both emf's are due to the same armature current.

Another function of inter-poles is to neutralize the cross-magnetizing effect of armature reaction. OF_m and OF_c represent the main flux and cross-magnetizing flux (m.m.f) respectively. OF_i represents the flux (m.m.f.) due to inter-poles, which is opposite to OF_c, hence they cancel each other.

This cancellation of cross-magnetizing is automatic and for all loads, because both are produced by the same armature current.