Methods of Improving Commutation in DC Generator

     There are two methods of improving commutation i.e. of making current reversal in the short-circuited coil as spark-less as possible. They are :

(i) Resistance commutation.     

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

Resistance Commutation :

     This method of improving commutation consists of replacing low resistance copper brushes by comparatively high resistance carbon brushes.

     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 follow 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 second loner path because 1. the first path resistance will increase due to diminishing area of contact of brush with segment ' b ' and because 2. resistance of second path will decrease due to rapid increasing contact area of segment ' c ' with the brush.

Methods of Improving Commutation

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

Advantages of Carbon Brushes :

(1) Improves the commutation.

(2) Lubricate polish the commutator.

(3) Easily available in the market.

(4) Economical.

Dis-advantages of Carbon Brushes :

(1) Causes voltage drop due to high contact resistance.

(2) Needs to be replaced due to wear.

E.M.F. Commutation :

     In this method, an 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 short-circuit coil will reverse quickly, which results in spark-less 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 next pole of opposite polarity or 

2. By using inter-poles.

Methods of Improving Commutation

      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 few heavy gauge turns connected in series with the armature so that they carry full armature current. Their polarity in 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 e.m.f. thereby making commutation spark-less. 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 same armature current.
Methods of Improving Commutation

      Another function of inter-poles is to neutralize the cross-magnetizing effect of armature reaction. OFm and OFc represents main flux and cross-magnetising flux ( m.m.f. ) respectively. OFrepresents the flux ( m.m.f. ) due  to inter-poles, is opposite to OFc, hence they cancel each other. This cancellation of cross-magnetising is automatic and for all loads because, both are produced by the same armature current.

Reference -

       Electrical Machines-I by U.A.Bakshi, M.V.Bakshi

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