In case the two windings in an autotransformer are directly activated one after the other, they can be properly controlled by means of series winding or by a parallel winding. The reduced voltage winding is basically a parallel winding, which can be found parallel to the load in order to decrease the voltage, and in order to increase the voltage, it is found in parallel with the grid. The highest voltage winding is the serial connection of the series and parallel windings. Because the autotransformer is considered to be a real transformer, all principles applied to the normal multi-winding transformer can be applied.
It can be stated that the basic principles of application of the autotransformer are exactly the same as the principles of the multi-winding transformer air vents bell. However, it is important to note that one winding is partially contained in the other. This is another factor that contributes to arouse so much interest in the economic autotransformer. The terminals in all of the autotransformer windings are indicated by a number or by a combination of numbers and letters in the same way as in multi-winding transformers.
In cases where considerably higher speeds can be achieved, which may vary between 80% and 90% of the synchronous speed on average, the torque curve with the combined windings must cross it so that, for speeds above this limit, the electric motor will develop a smaller torque. Because the curves do not always intersect at the same point, and the centrifugal circuit breaker does not always open at the same speed, it is very common for the opening to occur immediately before the curves intersect. The starter capacitor motor basically consists of a split phase comma-like machine.
Among the differences, the main one is based on the inclusion of a series electrolytic capacitor, with auxiliary starting winding. This capacitor has the ability to promote a larger angle of lag between the main and auxiliary winding currents, thereby providing high starting torques. Also in the split-phase motor, the auxiliary circuit is deactivated as soon as the motor reaches approximately 75% to 80% of the synchronous speed. In this pause of speeds, the main winding alone can develop the same torque as the combined windings.