- Commutation Definition: Commutation in a DC machine is defined as the process of converting the alternating current generated in the armature winding into direct current using a commutator and stationary brushes.
- Continuous Contact: The process requires continuous contact between the commutator segments and the brushes to maintain current transformation.
- Ideal Commutation: Ideal commutation means the current reversal is completed within the commutation period, avoiding sparking and damage.
- Current Reversal: During commutation, the current flowing through the armature coil reverses direction, essential for the DC machine’s operation.
- Improving Commutation: Techniques like resistance commutation, voltage commutation, and compensating windings help improve the commutation process and reduce sparking.
The voltage induced in a rotating armature winding of a DC generator reverses as each coil passes under alternating stator poles. The armature rotates through a stationary field; this differs from the rotating magnetic field used in many AC machines. Commutation in a DC machine switches each coil’s connection at the correct rotor position, so alternating coil emf appears as unidirectional terminal current at the brushes.
In a DC Motor, commutation reverses current in each armature coil as it crosses the neutral region, keeping torque in the required direction.
Stationary brushes maintain sliding electrical contact with the rotating commutator segments. As a coil approaches the magnetic neutral region, a brush spans the two segments connected to that coil and briefly short-circuits it.
Brush overlap defines the commutation period. Its duration varies with brush width, commutator geometry and rotor speed, so a universal value such as 1/500 second does not apply. During this interval, coil current must fall from its original value, pass through zero and rise in the opposite direction before the trailing segment leaves the brush.
Ideal commutation completes this reversal by the end of the commutation period, with the correct current in the coil and no damaging arc at contact separation.
If current has not reached its required reversed value when contact breaks, the coil inductance drives an arc between the commutator segment and brush. Repeated sparking heats and wears the brush and commutator surface. This is poor commutation.
Physical Concept of Commutation in DC Machine
Consider a simplified DC machine with an armature wound with ring winding. Assume that brush width equals one commutator-bar width and each conductor initially carries IC.
In the developed diagram, the commutator moves from left to right relative to the stationary brush.
At the first position, the brush contacts bar b. Bar b carries a total brush current of 2IC.
As the armature moves, the brush begins to contact bar a while it still contacts bar b. The brush current divides between the two contact areas, but the total remains 2IC.
Contact area with bar a increases as contact area with bar b decreases. The currents through the two bars therefore change during brush overlap. Equal contact areas give equal contact resistances in the simplified linear-commutation model.
As overlap shifts further to bar a, the current in coil B passes through zero and begins flowing counter-clockwise.
When the brush leaves bar b and fully contacts bar a, coil B carries IC counter-clockwise and is no longer short-circuited.
This completed current reversal is commutation.
Methods of Improving Commutation
Three design measures can reduce commutator sparking:
- Resistance commutation uses carbon brushes with enough contact resistance to help current transfer from the leaving segment to the entering segment.
- Voltage or emf commutation uses interpoles in the neutral region to induce a reversing emf in the short-circuited coil. Interpole strength follows armature current.
- Compensating windings oppose armature-reaction flux under the main poles. They support stable neutral conditions in heavily loaded machines, while interpoles directly assist current reversal.
