Method of Improving Commuatation

There are three methods of obtaining sparkless commutation.

1. Resistance commutation
2. Voltage Commutation
3. Compensating windings

Resistance commutation
In this method use the carbon brushes to improving the commutation. This make the contact resistance between commutator segments and brushes high. This high contact resistance has the tendency to force the current in the short-circuited coil to change according to the commutation requirements., Namely to reverse and then build up in the reverse direction

Voltage Commutation

In this method, arrangement is made to induce a voltage in the coil undergoing the commutation, which will neutralize the reactance voltage. This injected voltage is in opposition to the reactance voltage. If the value of the injected voltage is made equal to the reactance voltage, quick reversal of current in the short-circuited coil will take place and there will be sparkles Commutation.

Two method may be used to produce the injected voltage in the opposition to the reactance voltage.

Brush Shift

The effect of armature reaction is to shift the magnetic neutral axis (MNA) in the direction of rotation for the generator and against the direction of rotation for the motor. Armature reaction establishes a flux in the neutral zone. A small voltage is induced in the commutating coil since it is cutting the flux.

Commutating Poles or Interpoles

Interpoles are narrow poles Attached to the stator yoke ,and placed exactly midway between the main poles. Interpoles are also called commutating poles or Campoles. The interpoles windings are connected in series with the armature because the interpoles must produce fluxes that are directly proportional to the armature current.The armature and the interpoles mmfs are affected simultaneously by the same armature current. Consequently, the armature flux in the commutating zone, which tends to shift the magnetic neutral axis, is neutralized by an appropriate component of interpole flux.

The interpoles must induce a voltage in the conductors undergoing commutation that is opposite to the voltage caused by the neutral plane shift and reactance voltage.

For Generator,The neutral plane shifts are in the direction of rotation. Thus, the conductor undergoing the commutation have the same polarity of the voltage as the pole they just left. To oppose this voltage, the interpoles must have the opposite flux, which is the flux of the main pole ahead according to the direction of rotation.

For a motor, the neutral plane shifts opposite to the direction of rotation, and the conductors undergoing commutation have the same flux as the main pole. then are approaching.For opposing this voltage, the interpole must have the same polarity as the previous main pole.the polarity of interpole is opposite to that of the next main pole in the direction of rotation.

The polarity of interpoles is shown in figure.

It is to be noted that The Interpoles serve only to provide sufficient flux to assure good commutation. They do not overcome the distortion of the flux resulting from cross-magnetizing mmf of the armature.

The use of interpoles is very common to nearly all dc machines of more than one 1hp.

During severe overloads or rapidly changing loads, the voltage between adjacent commutator segments may become very high. This ionizes the air around the commutator to the extent that it becomes sufficiently conductive. An arc is established from brush to brush. This phenomenon is known as Flashover.This arc is sufficiently hot to melt the commutator segments. It should be extinguished quickly. To prevent flashover, compensating windings are used.

Compensating Windings 

Compansanting winding are The most efficient method of eliminating the problem of armature reaction and flashover by balancing the armature mmf is the compensating windings. The windings are placed in the slots provided in pole faces parallel to the rotor conductors. These windings are connected in series with the armature windings. The direction of currents in the compensating winding must be opposite to that in the armature winding just below the pole faces. Thus, compensating winding produces mmf that is equal and opposite to the armature MMF. The compensating winding demagnetizes or neutralizes the armature flux produced by the armature conductors. The flux per pole is then undisturbed by the armature flux regardless of the load conditions.

The major drawback with the compensating windings is that they are very costly. Their use can only be justified in the following special cases.

1. In large machine subject to heavy overloads or plugging.
2. In small motors subject to sudden reversal and high acceleration.