Although the commutating pole is a good solution for commutation, it does not prevent distortion of the main-pole flux by armature reaction. The flux set up across the main-pole face by the armature mmf is shown in Fig. 8-30a.

If the pole face is provided with another winding, as shown in Fig. 8-30b, and connected in series with the load, it can set up an mmf equal and opposite to that of the armature. This would tend to prevent distortion of the air-gap field by armature reaction.

Such windings are called compensating windings and are usually provided on medium-sized and large dc machines to obtain the best possible characteristics. They are also often needed to make machines less susceptible to flashovers.

The use of compensating windings reduces the number of turns required on the commutating pole fields, and this materially reduces the leakage fluxes of the field and, in turn, the pole saturations at high currents. The ampere-turns on the commutating field are reduced by about 50% with the use of a compensating field.

This new winding may be considered to be some of the turns taken off the commutating-pole winding and relocated in slots in the main-pole faces. The number and location of the compensating slots must be carefully chosen to match, as closely as possible, the rotor ampere-turns per inch.

However, the slot spacing must not correspond closely to that of the rotor. This would cause a major change in reluctance to the main-pole useful flux every time the rotor moved from a position where the rotor and stator slots all coincided to where the rotor slots coincided with the stator teeth.

This would occur once for every slot-pitch movement. The resulting rapid changes in useful flux would cause ripples in the output voltage and also serious magnetic noise. If too few slots are used, local flux distortions occur and the compensating winding loses some of its effectiveness.

Compensation of armature reaction effectively reduces the armature circuit inductance. This makes the machine less susceptible to the bad effects of L(di/dt) voltages caused by very fast load current changes.

During manufacture, it is possible to locate the compensating winding nonsymmetrically about the centerline of the main pole. This causes a direct-axis flux, which will give a series field effect. For generator cumulative compounding, the slots must be shifted in the direction of the machine rotation.

This shift gives a motor differential compounding. The effect cannot be adjusted after manufacture. It seldom exceeds 1/2 in, and this does not materially reduce the effectiveness of the compensation.

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