Unstable VFD on lathe?

[MushCreek]

A lathe I use a lot at work runs off of a VFD (built in). Lately, it has started hunting rather than settling in a set RPM. It varies about 20 RPM at 400 RPM spindle speed, as an example. It goes up and down continuously. It used to just stay where you set it. Any ideas? Motor? Circuitry? Power supply from the building? My main concern is if it craps out altogether- it's the only accurate lathe we have.

 

[Fallon]

Some people want that random factor...

In your case it sounds like electronics in the VFD getting ready to **** the bed. A VFD converts AC to DC then back to AC again. The last AC stage is variable at frequencies & voltages you define in the VFD. If it can't output frequencies as defined, something on the control bus is likely on it's way out.

 

[RPH]

Start by checking for loose connections, that includes any mechanical couplers. Does the unit have any feedback devices? Is the display showing this change? Have you confirmed the velocity change with an external instrument?

 

[MushCreek]

It shows on the display; I don't know if the speed is actually changing. It happens fast- almost like a flicker at times.

 

[RPH]

If you can get a contact or non contact tachometer this will resolve where the problems.
I’ve seen plenty of display drivers go flakey.

 

[rsanter]

How old is it? VFDs have capacitors in them, that is often one of the first components that fail from age (verses some external screwup damage)

 

[LXCam]

Check to see if it has external tach pick up. You say it does this at 400rpm but have you confirmed it doesn't or does happen at other settings?

 

[MushCreek]

I tried it at different speeds, and the problem went away- for now. It doesn't vary at 400 RPM now, either. I wonder if it could have been a building supply problem? Would a VFD do that if the voltage in the building were unstable?

 

[American Locomotive]

VFDs can operate in a number of different modes - V/Hz mode and Vector mode are very common. V/Hz is a "dumb" mode, where the VFD just outputs a certain frequency and voltage, and the motor is free to do whatever it wants. In vector mode, the drive is constantly looking at the current motor speed and makes corrections to try and keep it at the target speed.

Vector mode can work in both "open loop" sensorless configuration, and "closed loop" configuration with a speed sensor/shaft encoder. In open loop mode, the VFD relies on feedback from the motor's electrical connections, while in a sensor configuration, it relies on the shaft sensor for all its information.

If it's a closed loop drive, you may have a problem with the shaft encoder/speed sensor. If its open loop, the drive it self may be failing, or you may have something introducing a lot of electrical noise into the circuit.

Also, are you sure it's a VFD, and not a DC motor drive?