Variable Speed Motor Control


Thread Starter


I'm new to automation and have been doing a lot of research on the various topics of automation using a PLC. I understand that in most applications in order to control a variable speed motor it requires a variable speed drive. However, my question is, if the ventilation fan system currently has a variable speed fan with manual controller in the building, can I connect a PLC to the controller to gather the necessary information to control the fan? Any other suggestions that you have would be great. The PLC I'm looking to implement is a Automation Direct Direct Logic 06.
If the VFD has the option of accepting a 4-20 mA signal it will be easy enough to control the speed from PLC

If the VFD only has a potentiometer for setting the speed you may still be able to control from the PLC using a Voltage signal.

If you post the make and model of the VFD someone can probably look it up for you.

The drive seems to be a temperature controller.

The setpoint is in °F, you could probably measure the Voltage across the setpoint pot and substitute that with the PLC output to give cascade control.

Just be aware that input of some cheap devices may not be referenced to ground.
It controls temperature but also has another setting for fan speed. Since it does have temperature I would also monitor that but I'm more interested in controlling the motor speed using the PLC.
Then do the same with the speed pot, measure the voltage across it then inject the same Voltage from the negative terminal to the slider terminal.

You may need to add a signal isolator unless the speed pot is referenced to ground.

Richard Mann

> You may need to add a signal isolator unless the speed pot is referenced to ground.

aren't they referenced to ground most of the time? If i understand correctly they act just as a voltage-divider where the contact position determines the ratio and thereby the output voltage, right?