AC Motors: Duty Cycle

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The duty cycle of a motor is the amount of time the motor can be operated out of every hour. If the motor’s duty cycle is listed as continuous, it means the motor can be run 24 hours a day and does not need to be turned off to cool down. If the duty cycle is rated for 20 minutes, it means the motor can be safely operated for 20 minutes before it must be shut down to be allowed to cool. The motor with this rating should be shut down for 40 minutes of every hour of operation to be allowed to cool.

Another way to specify the duty cycle of a motor is called the motor rating. The motor rating on the data plate refers to the type of duty the motor is rated for. The types of duty include continuous duty, intermittent duty, and heavy duty. which includes jogging and plugging duty. Continuous duty includes applications where the motor is started and allowed to operate for hours at a time. The intermittent duty includes operations where the motor is started and stopped frequently. This type of application allows the motor to heat up because it will draw LRA more often than will a motor rated for continuous duty.

Motors that are rated for jogging and plugging are built to withstand very large amounts of heat that will build up when the motor will draw large LRA during starting and stopping. Since the motor can be reversed when it's running in the forward direction for plugging applications, it will build up excessive amounts of heat. Motors with this rating must be able to get rid of heat as much as possible to withstand the heavy-duty applications.

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