Speed drift

We have a machine that has to comply with critical quality regulations, and one of the regulations is the consistency of the selected speed. The speed is set via a potentiometer while a visual display, shows the calculated speed in meters per minute.

The display uses the 0-10vdc from the pot to calculate the speed, the “actual” speed is not referenced from the drive nor from a motor feedback.

Now the speed is set by the operator to say, 6m/min, he then controls the operation of the machine and by the time the batch has passed through the machine the speed display has increased to 7m/min.

I am tasked with the job to regulate this “drift” and keep the machine constant at the desired speed. I do not know what would cause this fluctuation or how to fix this problem. Any advise is welcomed.



So, you have a VFD drive to control the speed of a motor. You are using a Potentiometer to vary the speed setpoint and your are able to see the measured/set speed in a display.

The VFD calculates the motor speed by measuring the equivalent current to the load i.e. Motor. VFD is configured with the motor speed and other settings, so it calculates the motor speed based on these parameters.

Just check all the VFD drive configurations and settings. You may find some options where we can select the torque settings, control algorithm, speed control types in the VFD drive, depends on the make and model.

Also, check the motor condition.

If you have two sets (VFD+Motor) then interchange the motors connections (VFD 1 to Motor 2 and VFD 2 to Motor 1) and observe again before proceeding to any changes.

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You need a closed loop speed control system.

You can add a stand-alone PID controller to your VFD drive. (or use a PLC/DCS)

Take the Measured Speed signal from the VFD drive i.e. an Analog Output Signal (AO).

Connect this Measured Speed signal to the input of the PID controller.

Connect the PID controller output in place of the existing Potentiometer or in an Analog Input of the VFD drive and do the configuration as required.

You can set the required speed in the PID controller (setpoint).

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This sounds like a viable option. I can remove the pot out of the circuit. I can replace the speed display unit which occupies a 48x48 instrument cutout, and replace it with a PID controller of the same dimensions. I will give it a try. Many thanks!

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