Updated: Jun 7
Resistance Commutation, Voltage Commutation and Compensating Winding are three methods of improving commutation. The proper commutation means there is no sparking at the brushes.
The voltage commutation has two methods commutating poles or interpoles and Brush Shift.
Now we will discuss the commutation methods in detail in the subsequent sections.
The carbon brushes make contact between commutator segments and coils. The high resistance of carbon brush forces the current in the short-circuit coil as per the commutation requirement. Thus the high resistance of the carbon brush cause good commutation. Therefore, the method is called resistance commutation.
In this method, the voltage induces in the coil undergoing the commutation. The brush shift and the commutating pole are the two methods of inducing voltage in a coil. The induced voltage neutralize the reactance voltage. The injected voltage is made equal to the reactance voltage, which cause quick current reversal in the short-circuited coil.
This injected voltage is just in opposition to the reactance voltage. Thus, a spark less commutation is possible.
The two methods used to produce the voltage in opposition to the reactance voltage are as follows.
The armature reaction shifts the magnetic neutral axis (MNA) in the direction of rotation in the case of the generator, and in the case of the motor, the shifts the magnetic neutral axis (MNA) is against the direction of rotation for the motor. Thus the armature reaction sets up flux in the neutral zone. This flux induces a small voltage in the commutating coil because the armature cut this flux.
Commutating Poles or Interpoles
Interpoles are narrow poles placed in between the main poles. These are attached to the stator Interpoles. The interpoles are also called commutating poles. The interpoles windings connections are in the series with the armature. The reason for this is that the interpoles must produce fluxes that are directly proportional to the armature current.
The armature and the interpoles mmfs interacts by the same armature current. As a result, the armature flux in the commutating zone 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.
In case of a generator:
The neutral plane shifts are in the direction of rotation. Thus, for the conductor undergoing the commutation, the polarity of the Interpole must be the same i.e. similar to the next main pole in the direction of rotation. 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.
In case of a motor:
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. For opposing this voltage, the interpole must have the same polarity as the previous main pole. The polarity of an interpole and main pole is opposite in the direction of rotation.
The polarity of interpoles is shown in the figure below:
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.
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.
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. The primary use of the compensating winding is in particular cases, as given below:
In large machine subject to heavy overloads or plugging.
In small motors subject to sudden reversal and high acceleration.
This is all about Methods of Improving Commutation.