Changing Voltages or Direction of Rotation

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Since the PSC motor is rather specialized and inexpensive, it generally is not made to have its direction of rotation changed or be reconnected to operate on a different voltage. Instead, it's common practice to stock a clockwise and counterclockwise motor for each voltage application that one has in the facility. Since these motors are generally used for blade fan applications, they are commonly used for small air-conditioning units used to cool small offices erected on the factory floor or the main offices of the facility. They are also used in the air-conditioning units to cool electrical control panels on larger equipment that have numerous electronic boards or motor drives mounted directly in the cabinet.

Typical starting torques of PSC motors are low -- from 30 to 150% of rated load -- so these motors are not for hard-to-start applications. However, unlike split-phase motors, PSC motors have low starting currents, usually less than 200% of rated load current, making them excellent for applications with high cycle rates. Breakdown torque varies depending on the design type and application, though it's typically somewhat lower than with a cap start motors.

PSC motors have several advantages. They need no starting mechanism and so they can be reversed easily. Designs can be easily altered for use with speed controllers. They can also be designed for optimum efficiency and high power factor at rated load. And they're considered to be the most reliable of the single phase motors, mostly because no starting switch is needed.

One problem that the PSC motor has is that it may run in the wrong direction when it's used to drive a condenser fan on an outdoor air-conditioning unit. This condition is caused when it's exceedingly windy and the wind blows across the fan blade when the motor is de-energized, causing the motor rotor to spin in the opposite direction than it would normally operate. When voltage is applied to the motor, the rotor will continue to rotate in the direction the wind is blowing when power is applied. If the rotor is stationary when voltage is applied, the rotor will spin in the proper direction. If the wind is blowing the fan blade in the wrong direction, it will continue to spin in that direction when voltage is applied. If the fan is running in the wrong direction, it will cause insufficient air movement across the coils. This will cause the air conditioner to overheat and cause high pressure in the refrigerant coils.

A ratchet mechanism is mounted on the motor shaft of condenser fans when this is a problem. The ratchet allows the motor to spin in the correct direction and prevents the motor shaft from spinning in the wrong direction when the wind is blowing.

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