interpole

An interpole is a small auxiliary magnet or pole piece mounted on the stator of a direct current machine, positioned in the magnetic gap between the main field poles. Its purpose is to generate a localized magnetic field that opposes the distortion caused by armature reaction, thereby improving commutation and reducing sparking at the brushes.

In a DC motor or generator, the armature current creates its own magnetic field that opposes and distorts the main field. This distortion, known as armature reaction, shifts the neutral plane and causes the conductors being switched by the commutator to cut through uneven field lines. The result is uneven voltage induction in the switching coils, leading to sparking, arcing, and brush wear. The interpole generates a compensating field that straightens these field lines back through the commutation zone, allowing the brushes to switch coils when the induced voltage in those coils is nearly zero.

Physical construction and polarity

Interpoles are narrower than the main poles and are wound with a few turns of conductor, typically connected in series with the armature circuit. This connection is critical: the interpole winding carries the same current as the armature, so its field strength increases and decreases with load. The polarity of each interpole is opposite to the main pole it is adjacent to. In a four-pole machine with north and south main poles, interpoles between them are magnetized in the opposite sense, creating the opposing field needed for compensation.

The effectiveness of an interpole depends on its magnetic strength, width, and placement. The air gap between the interpole and the armature must be uniform and small enough to provide adequate field penetration. Interpoles are standard in most commercial DC machines rated above fractional horsepower, though their absence or poor design can cause severe commutation problems. Poor commutation manifests as heavy brush sparking, carbon dust accumulation, burned commutator bars, and ultimately shortened brush and commutator life.

Interpoles are sometimes called commutating poles or commutator poles. They are distinct from compensating windings, which are embedded in the main pole faces and provide broader compensation across the entire pole span; many machines use both for optimal performance across variable loads.