Abstract :
The single-phase capacitor motor is an important type of electrical machine which requires lengthy design calculations to predetermine its performance. The paper describes a method of using a transformer analogue analyser to determine rapidly the currents and torque of the motor from its design constants, by direct reading on the analyser with very little auxiliary manual computation. The basic method assumes no saturation and gives the same results as are obtained by the slide-rule calculations in common use. With the analyser, however, the effect of changing any of the constants can be determined very quickly once the analyser is set up. In the second part of the paper, a method of allowing for saturation is described. Saturation is important because of the increase of voltage on the auxiliary winding caused by the capacitor under certain conditions. In order to develop a theory, the flux is resolved along two fixed axes and a different value of magnetizing reactance is used on each axis. When saturation occurs, the value of reactance for either axis varies not only with the flux in its own axis but also with the flux in the other axis. Theoretical and experimental methods of determing the appropriate reactances are discussed and the values are plotted on families of curves. With the aid of the analyser it is then possible to use a quick trial-and-error method to determine the performance of the capacitor motor with any degree of saturation. Test results on a capacitor motor are compared with calculations made both with the analyser and by manual computation.
