Motor not turning, no drive fault,

[ptonsparky]

Customer called last night with the problem that the VFD was ON, but the motor was not turning. No faults apparent or showing up on VFD or the control HMI.

There are 2 contactors on the load side of the VFD to run either of two motors. Logic to the PLC and the Enable contact on the VFD require that the selected contactor must be pulled in 2? seconds before the VFD starts.

Would the VFD show a fault if no load was connected? IDR what the minimum HZ setting is,but there is one.

Everything works this morning of course. They discovered that before I headed out.

 

[petersonra]

usually when I see this it is a PLC programming error where the drive is getting a speed setpoint of zero. so the drive is on but telling the motor to run at 0 hz.

it's a pretty common program bug as such things go.

 

[ptonsparky]

usually when I see this it is a PLC programming error where the drive is getting a speed setpoint of zero. so the drive is on but telling the motor to run at 0 hz.

it's a pretty common program bug as such things go.

That was one of my questions, if they had it set low. Set point was 68 Hz which is common for that system.

 

[Jraef]

Did they notice if the drive display was SHOWING a Hz output, but the motor was not turning? If the VFD is in V/Hz Mode, there is no feedback mechanism for the drive to know if the motor is there or not (unless you program an Under Current Trip feature if there is one). So if the contactor failed to close, the drive may not have known. If the drive is in SVC Mode, it would know that the motor wasn’t turning because it uses a current feedback to determine the flux vector.

If the display didn’t show anything, then I’d suspect that the Run or Enable signal wasn’t getting to the drive. You say you have a delay for the run command (or enable?) to the VFD. If that is dependent on an aux contact on the contactor, and it’s a Logic level circuit, like the Enable input, sometimes because there is virtually no energy on that circuit, a film builds up on the Aux contacts of the contactor and with no arc, it eventually interferes with the signal. I’ve had to switch to gold flashed contacts to get reliability because of that. Sometimes even that isn’t good enough and I’ve had to go to parallel gold flashed contacts to make sure it gets through reliably.