Capacitor Start Induction Motor
A capacitor start induction motor is improved from a split-phase induction motor (resistance start induction motor).
Fundamentally, the capacitor start induction motor is quite similar to the split-phase induction motor, except that the starting winding has a few more turns and is made up of heavier wire than the starting winding.
A large electrolytic capacitor is used in a series of starting winding instead of the resistor in a capacitor start induction motor.
By adequately selecting the capacitor value, the starting current Is at a standstill, leading the main winding current Im by 90˚.
The connection diagram and phasor diagram of the capacitor start induction motor is shown in the figure below.
Generally, the capacitor is mounted in a metal casing on the top of the motor. It also can be placed in a convenient place on a motor frame. It can be mounted inside the motor housing.
In the capacitor start motor, an electrolytic capacitor is used. And this capacitor is designed for concise duty service (20 periods of operation per hour, each period not exceeding 3 seconds).
The capacitor size depends on the motor rating, and it has a value around 50μF to a few hundred micro-farads.
Due to the development of inexpensive and reliable capacitors, the capacitor starts motor has largely replaced all other single-phase motors in high starting torque applications.
Like a split-phase induction motor, the capacitor start induction motor has a centrifugal switch to disconnect starting winding when the motor picks up speed.
Here, the proper operation of the centrifugal switch is essential. If the switch does not disconnect, starting winding at an appropriate time, the capacitor will discharge and shorten its life.
The torque-speed characteristics of the capacitor start induction motor are shown in the figure below.
The torque developed by the motor is proportional to the sine of phase displacement angle θ. The phase displacement angle is between the starting and main winding.
The starting torque increases twice the value developed by the resistance to start the motor by increasing the phase displacement angle.
An increase in the number of turns in starting winding causes a decrease in starting current and produce large rotating flux that results in high starting torque.
In a capacitor start motor, the line current IL is less than a split-phase induction motor for the same stator winding due to a large angle.
The line current of a split-phase induction motor is 2/3 times higher than the line current in a capacitor start induction motor. And the starting torque of the capacitor start motor is twice the resistance start the motor.
Here, it is noted that the capacitor start motor differs from the split-phase induction motor during the starting period. (It develops 3.5-4.5 times full load torque with lower inrush current.) After that, the machine behaves the same in both motors.
The capacitor start motors are manufactured in rating between 1/10 kW to 3/4 kW. Larger sizes are also used for heavy-duty general-purpose applications.
This motor is widely used in the application of;
- Jet pumps
- Centrifugal pumps
- Blowers, etc.
Reversal of Direction of Rotation
The direction of rotation of a capacitor start induction motors may be reversed by interchanging the connection to the main or starting winding supply.
The disadvantages of capacitor start induction motors are listed below.
- This motor cannot be used for applications having starting periods of longer duration. Because the electrolytic capacitor is used for short duty service only.
- An electrolytic capacitor has a maximum voltage rating, and if subjected to a voltage exceeding 25% greater than such rating, it will get damaged.
- The capacitor start motor should not be started more than 20 times an hour. However, if the motor begins frequently, the dielectric of a capacitor will get damaged by the frequent current surges.
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