Methods of Improving Commutation
There are three methods of improving commutation:
- Resistance Commutation
- EMF commutation
- Compensating winding
- The reversal of current in a coil takes place while the coil is short-circuited by the brush. Therefore, there are two parallel paths for the current as long as the short circuit exists.
- If the contact resistance between the brush and the commutator is made large, then current would divide in the inverse ratio of contact resistances (as for any two resistances in parallel). This is the key point in improving commutation.
- This is achieved by using carbon brushes (instead of Cu brushes) which have high contact resistance. This method of improving commutation is called resistance commutation.
the quick reversal of current will occur in the desired direction. R = ρl / a
- It may be noted that the main cause of sparking during commutation is the production of reactance voltage and carbon brushes cannot prevent it.
- The carbon brushes do help in improving commutation.
- The other minor advantages of carbon brushes are:
- The carbon lubricates and polishes the commutator.
- If sparking occurs, it damages the commutator less than with copper brushes and the damage to the brush itself is of little importance.
2. EMF commutation
- In this method, an arrangement is made to neutralize the reactance voltage by producing a reversing voltage in the coil undergoing commutation.
- The reversing voltage acts in opposition to the reactance voltage and neutralizes it to some extent.
- If the reversing voltage is equal to the reactance voltage, the effect of the latter is completely eliminate and we get sparkless commutation.
- We can produce reversing emf in two ways
- By brush shifting
- improving commutation by the brushes are shifted forward direction for the DC generator and in backward direction for the DC motor for producing the sufficient reversing emf for eliminating the reactance voltage.
2. By using inter poles or commutating poles
- The best way to produce reversing voltage to neutralize the reactance voltage is by using inter poles or commutating poles.
- These are small poles fixed to the yoke and spaced mid-way between the main poles.
3. Compensating winding
- The main function of commutating winding is to produce reversing e.m.f. in order to reduce the reactance voltage.
- In addition to this, the m.m.f. of the commutating winding neutralizes the cross magnetizing ampere-turns in the space between the main poles.
- The compensating winding neutralizes the cross-magnetizing effect of armature reaction under the pole faces.