Before reading this article, please read previous one, Why a Single Phase Induction Motor is not Self Staring?
As discussed in the previous article single phase induction motors are not self starting. It is because a single phase supply cannot produce a rotating magnetic field. But we can create a rotating magnetic field by a two phase construction. So simply we can say in order make a single phase induction motor self starting, we have to temporarily convert it into a two phase motor during its starting period.
winding known as Starting or Auxillary Winding in addition to the Main or Running Winding.The two winding are 90 degree electrically displaced and are connected in parallel across the single phase supply.
The phase difference between the currents in main winding and running winding can be obtained through different methods. The phase shift can be achieved by connecting a resistance, inductance or capacitance in series with the starting winding.
Creating Phase difference between two windings
Split Phase Machine
Hence, as shown in Fig (b), the current Is drawn by the starting winding lags behind the applied voltage by a small angle whereas current Im taken by the main winding lags behind V by a very large angle. Phase angle between Is and Im is made as large as possible because the starting torque of a split-phase motor is proportional to sin α.
A centrifugal switch S is connected in series with the starting winding and is located inside the motor. Its function is to automatically disconnect the starting winding from the supply when the motor has reached 70 to 80 per cent of its full load speed. The centrifugal switch is necessary because the auxiliary winding cannot support high currents for more than a few seconds without being damaged because it is made of fine wire. In case of a capacitor start motor its is necessary because most motors use a cheap electrolytic capacitor that can only carry a.c current for a short period of time.
- In the case of split-phase motors that are hermetically sealed in refrigeration units, instead of internally mounted centrifugal switch, an electromagnetic type of relay is used. As shown in Fig. 31-6, the relay coil is connected in series with main winding and the pair of contacts which are normally open, is included in the starting winding. During starting period when Im is large, relay contacts close thereby allowing Is to flow and the motor starts as usual. After motor speeds up to 75 per cent of full-load speed, Im drops to value that is low enough to cause the contacts to open.
- These motors are often used in preference to the costlier capacitor-start motors. Typical applications are : fans and blowers, centrifugal pumps and separators, washing machines, small machine tools, duplicating machines and domestic refrigerators and oil burners etc. Commonly available sizes range from 1/20 to 1/3 h.p. (40 to 250 W) with speeds ranging from 3,450 to 865 r.p.m.
Capacitor-start Induction run motors