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The variable-frequency drive can be operated as a stand-alone unit or it can be controlled by a PLC or other electronic control system that sends a 0-10 volt or a 4-20 mA signal. When the drive is operated as a stand-alone system, a potentiometer may be provided directly on the face of the drive. The operator of the machinery can set the speed for the drive by adjusting the potentiometer from 0-100%. In some applications, the potentiometer is mounted on the machine's front panel instead of on the face of the drive. The reason for this is that in some cases the drive must be mounted in an electrical cabinet, and the potentiometer on the face of the drive is not accessible when the electrical cabinet's door is closed.
If the drive is controlled by a 0-10 volt DC or 4-20 mA signal, the command signal will be changed by the logic of the controller it's connected to. The controller generally will have some type of sensor that indicates speed or position if the system is a closed-loop system. Or the command signal can be generated from a predefined table in the controller that sets the motor to the proper speed for the application. In this type of application, the local speed potentiometer can be used to jog the motor or when the motor must be operated for maintenance.
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