The frequency, the field flux and the number of turns in the winding have influence on the power factor.
Ques. How does the frequency affect the power factor?
Ans. Lowering the frequency tends to improve the power factor.
The use of very low frequencies has the disadvantage of departing from standard frequencies, and the probability that the greater cost of transformers and alternators would offset the gain.
Fig. 1,886.—General Electric 5 H.P., 6 pole adjustable speed single phase compensated repulsion motor. This type is suitable for service requirements demanding the use of a motor whose speed can be adjusted over a considerable range, this speed at a fixed controller setting remaining practically unaffected by any load within the motor's rated capacity. With the controller on the high speed points, the motor possesses an inherent speed regulation between no load and full load of approximately 6 per cent. At the low speed points, under similar load conditions, the speed variation will be approximately 20 per cent. To secure adjustable speed control, the armature circuits employ transformers, whose primaries are excited by the line circuit. The secondaries of these transformers are divided into two sections; the first or "regulating" circuit is placed across the energy brushes; the other section, since it is connected in series with the compensating winding, maintains the high power factor and speed regulation obtained in the constant speed type. The speed range is 2:1, approximately one-half of this range being below and one-half above synchronous speed.
Series Motors.—This class of commutator motor is about the simplest of the several types belonging to this division. In general design the series motor is identical with the series direct current motor, but all the iron of the magnetic circuit must be laminated and a neutralizing winding is often employed.
It will be readily understood that the torque is produced in the same way as in the direct current machine, when it is remembered that the direction of rotation of the direct current series motor is independent of the direction of the voltage applied.
At any moment the torque will be proportional to the product of the current and the flux which it is at that moment producing in the magnetic system, and the average torque will be the product of the average current and the average flux it produces, so that if the iron parts be unsaturated, as they must be if the iron losses are not to be too high, the torque will be proportional simply to the square of the current, there being no question of power factor entering into the consideration.