A capacitor is a passive with two-terminal electrical component used to store energy in an electrical field. A capacitor consists of at least two electrical conductors separated by a dielectric (insulator). Electric current flows through the conductor to the dielectric creating an electric field across the dielectric and this energy used to provide extra to start a motor. An ideal capacitor is characterized by it’s capacitance.
This is the ratio of the electric charge on each conductor to the voltage set on between them. The SI unit off capacitance is farad (equal to 1Cb/V)
I. Capacitor Induction Motor
This capacitor-start motor is identical to a split-phase motor except that the starting winding has as many turns as the main winding. The picture of capacitor start induction motor is shows in Figure.H1. Moreover, a capacitor C (3-20µF) is connected in series with the starting winding,the capacitor is generally of the electrolytic type and is usually mounted on the outside of the motor as a separate unit. as shown in Figure.H2. The value of capacitor is so chosen that ${I_s}$ leads ${I_m}$ by about 80° which is considerably greater than 25° found in split-phase motor (see Figure.H3). Figure.H4 shows typical torque speed characteristic.
II. Operation
When the two stator winding are energized from a signle phase supply, the main winding carries current ${I_m}$ while the starting winding carries ${I_s}$. Due to cap acitance the currents ${I_m}$ and ${I_s}$ have a suitable phase angle a (about 80° for a 200W,50hz motor) between them. When starting torque is much more than that of a split-phase motor, the starting winding is opened by the centrifugal switch when the motor attains about 75% of synchronous speed. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed.
Their resultant current I is small and is almost in phase with V (see Fig.H3)
Since the torque developed by a split-phase motor is proportional to the sine of the angle between ${I_s}$ and ${I_m}$, it is obvious that the increase in the angle 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 percent.
III. Characteristics
Although starting characteristics of a capacitor-start motor are better than those of a split-phase motor, both machines possess the same running characteristics because the main winding are identical. The phase angle between the two currents is about 80° compared to about 25° in a split-phase motor. Consequently, for the same starting torque, the current in the starting winding is only about half that in a split-phase motor. Therefore, the starting winding of a capacitor start motor heats up less quickly and is well suited to applications involving either frequent or prolonged starting periods (see Fig.H4)
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