In the DC machine, the interpole is a very essential part that contributes to making the machine smaller and cheaper than a non-interpole machine. Interpoles are small poles placed in between the main poles. The interpoles are also called commutation poles.
What is an interpole in a DC machine?
➥ Between the main poles, there are little poles that are attached to the yoke.
➥ To lessen the excess air gap flux under the trailing pole tip, the interpoles are tapering in shape, i.e., they have a broad base and a smaller pole shoe area.
➥ To prevent saturation, the air gap under the interpoles is larger than the air gap under the main field poles.
➥ Interpoles are wrapped and arranged so that, in the direction of rotation, each interpole has the same magnetic polarity as the main pole in front of it. The interpoles' field has the same result that the compensatory winding does.
➥ They are connected in series with the armature and wound with very few turns of heavy gauge copper wire, carrying the entire armature current.
➥ In the case of a generator, their polarity is the same as the main pole.
➥ To provide the commutating pole strength that yields the optimal commutation, a resistor connected in parallel with the commutating pole (interpole) windings is modified and permanently set at the factory.
Important terms.
There are two different types of magnetic fluxes present in a DC machine: armature flux and main field flux. Armature reaction refers to the impact of armature flux on the main field flux.
The effect of armature reaction can be reduced.1. Using compensating winding
2. Using commutation poles or interpoles.
3. By reducing the cross section of pole pieces.
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| Fig: Compensating winding The image is used courtesy of studyelectrical |
What is the function of an interpole in a DC machine?
➥ Their function is to assist commutation by producing auxiliary or commutating flux. With the interpoles, sparkles commutation is possible up to 20–30% overload with a fixed brush position. That means brush spark is practically eliminated.
➥ Another function of the interpoles is to neutralize the cross-magnetizing effect of armature reaction.
How are the windings connected?
♦ Interpole windings are connected in series with the armature windings.
♦ In the above picture, we can see the winding connections.
WWhere are the compensating windings and interpoles used?
♦ By physically moving the brushes, the effects of armature reaction are minimized in small generators.
♦ Except in smaller generators, it is not standard procedure to change the brush position in response to changes in the current. In larger generators, armature reaction is eliminated using alternative techniques. This is accomplished via compensating windings or interpoles.
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