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.
For this purpose, stator of the single phase induction motor is provided with an extra
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.
It is so arranged that the phase difference between the currents in two stator windings ( main and running windings ) is very large ( ideal value is 90 degree ). Hence the motor behaves like a two phase motor. These two currents produce a revolving flux and hence make the motor self starting.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
In split-phase machine, the main-winding has low resistance but high reactance whereas the starting winding has a high resistance but low reactance. The resistance of the starting winding may be increased either by connecting a high resistance R in series with it or by choosing a high-resistance fine copper wire for winding purposes. 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.
As shown in Fig. 31-8, the direction of rotation of such motors can be reversed by reversing the connections of ono of the two stator windings (riot both). For this purpose, the four leads are brought outside the frame.
As seen from Fig. 31-9, the connections of the starting winding have been reversed.
The speed regulation of standard split-phase motors is nearly the same as of the 3-phase motors. Their speed varies about 2 to 5% between no load and full- load’ For this reason such motors are usually regarded as practically constant-speed motors.
Capacitor-start Induction run motors
The capacitor is designed for extremely short-duty service and is guaranteed for not more than 20 periods of operation per hour, each period not to exceed 3 seconds. When the motor reaches about 75 per cent of fulls peed, the centrifugal switch S opens and cuts out both the starting winding and the capacitor from the supply, thus leaving only the running winding across the
lines.
As shown in Fig, current Im drawn by the main winding lags the supply voltage V by a large angle whereas Is leads V by a certain angle. The two currents are out of phase with each other by about 80° (for a 200-W 50-Hz motor) as compared to nearly 30 degrees for a Split phase motor. Their resultant current I is small and is almost in phase with V as shown in Fig. 31-12.
Since the torque developed by a split-phase motor is proportional to the sine of the angle between Is and Im, it is obvious that the increase in the angle ( from 30 to 80 ) alone increases the starting torque to nearly twice the value developed by a standard split phase induction motor. Other improvements in motor design have made it possible to increase the starting torque to a value as high as 350 to 450 per cent.
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