Commutator Maintenance
- Basically, commutation is a process whereby, with the aid of the brushes and the commutator, current is reversed in a coil each time that coil passes under a pole face of opposite polarity in order to maintain a torque in the machine in the same direction.
Besides the normal current load carried by a brush, a circulating reactive current is induced in the brush the moment it shorts two commutator segments which have a potential difference between them, the voltage causing these circulating currents is proportional to the speed and load current. Thus, as the rating and speed of modern motor/generators increase this voltage becomes a serious problem. This reactive voltage is under certain circumstances sufficiently large to cause sparking and consequently from a design aspect it is essential to introduce commutating or inter poles to reduce this effect in the rotor circuit. It is thus important to ensure a good and continuous contact between brushes and commutator surface.
Good commutation does obviously not depend solely upon carbon brushes, the commutator or the electric circuitry, but is achieved by the best combination of the electrical and mechanical properties of the motor.
Commutator segments have always to be tight. When loose, they cause brush bounce and thus arcing and bad commutation. Commutator and slip rings should have smooth surface which are concentric about their axis of rotation.
When vibrations occur it is essential to determine their origin immediately. Worn bearings produce overheating, non uniform operation and finally abrasion of the armature and field poles.
Commutation can be such a complex problem that it is essential to ensure that all variants are strictly to specification. Good commutation is sometimes not attainable due to the working environment. For example: dust, gas, high room temperatures, humidity, oil, oily vapours, corrosive acid vapours and excessive vibrations.
The maintenance electrician must reduce these problems to a minimum with correct cleaning and a planned maintenance programme. Problems are reduced to a minimum when equipment is kept clean and regular electrical and mechanical maintenance adopted.
Whenever commutation is bad or a change of brush grade is deemed essential, refer to your brush supplier.
Assuming the design and manufacture of a commutator is correct, the only reasons for the deterioration of the commutator surface can be the environment within which the machine is working. Special maintenance is required on machines having more that one brush per brush arm. If the voltage drop across one brush is lower than that across the others on the same brush arm due to excessive spring pressure or a commutator film break in the carbon brush track, this brush will carry a larger load current than the other brushes of the same polarity.
On large machines it is also possible that a similar situation exists as above, with the exception that the brushes in one brush arm are conducting more current than the brushes in the other brush arm of the same polarity. The above conditions result in poor commutation.
