Most electrical motors used in home appliances and industrial motion-control systems are AC induction motors. The motors consist of a stationary housing known as the stator and a rotating interior part known as a rotor. The AC single-phase induction motor is not self-starting. The types of starting mechanisms can be used to divide these motors into categories.
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Most motors used today are based upon induction. This means that the stator has an electric supply going to electromagnets that create a magnetic field. The rotor does not have a separate power supply and receives the necessary magnetic field from the stator. The stator has electromagnets that cause the shaft to turn part of the way around due to the repelling opposite poles where the attraction of like poles causes the shaft to turn the rest of the way. In order to keep rotating, the current must be reversed. In AC, the current flow reverses every 120 times per seconds.
Insufficient torque is present in the single-phase AC induction motor to start-up. When the current is applied, the rotor will vibrate, but not rotate. Most of these motors will need a separate starting mechanism to take the rotor up to 3/4 of its normal rotating speed. Above 3/4 of the speed, the motor can generally maintain the necessary torque to continue operating.
The split-phase AC induction motor has two windings. The starter winding is made with fewer turns and lower gauge wire. This creates higher resistance and produces torques of 100 per cent to 175 per cent of the necessary torque for operation. The main winding uses heavier wire and is able to maintain operational torque. This design is used for lighter applications such as electric fans, small grinders and blowers that require lower torques.
Capacitor Start Motors
The capacitor start AC induction motor is a modified split-phase design. A capacitor is wired in series with the starter winding. This gives a surge to help start the motor. This motor has a switch that will disconnect the starter circuit when the motor reaches 75 per cent of the rated speed. This motor reaches approximately 200 per cent to 400 per cent of the rated torque. When requiring the starting torque to be less, the capacitor can be replaced by a resistor to lower the cost.
The shaded-pole AC induction motor uses continuous loops of copper wire on a small part of the motor poles. This shaded area causes the magnetic field to lag behind. The difference between the two fields causes the shaft to move. The simple inexpensive designs makes this motor popular for high-production applications. In general, it is cheaper to replace this motor than repair it.
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