HMI shows motor running even though disconnect is on.

[TheColt46]

Here's what happened,
Operator turned on the disconnect for an auger while it was still running to cut power. Auger DID in fact turn off, but the running FDBK indication showed that it was still running. Also, the auger is on a Schneider ATV71 drive and was still showing 100% speed from HMI. No faults were tripped as well which would be expected since a STOP command was never given from HMI. Would this be a drive configuration issue or a lost input back to the drive?

 

[Mandel314159]

I'm new to the world of PLCs and controls, but I know where I'm working the more seasoned guys have redundancies set up on power disconnect switches so when disconnected, there's signals sent to indicate it was shut off. Was that something that happened here and the HMI just didn't receive the signal, or is it something else entirely?

 

[Old Man]

What communication protocol is being used to communicate back to HMI? If Modbus is used once power is lost Modbus can't update.

 

[brstilson]

By default, Schneider ATVs will fault out if it tries to run without a motor connected (as this can be bad for the drive). However, there's a parameter than can change this behavior and allow it to run even without a motor connected. If that's the case, then what you're seeing is unsurprising. The operator may have opened the disconnect, but the drive has no way of knowing that, so it will continue to show that it's running, because it IS running. Your feedback input back to the PLC is only telling you the state of the drive, not the motor. Most motor feedback circuits are done with the feedback from the motor control device rather than the motor or what the motor is coupled to, usually to cut costs. So most are simply assumptions that if the motor control device is "on", then the motor must be spinning. Ideally, a motor control feedback signal should come from what's actually being moved; so if it's a fan, it should be coming from an air flow switch. If it's a pump, an output pressure or flow switch. Etcetera and so on. That way you know if there's, say a fan and there's air flow, you know in addition to air flowing, that the fan is actually spinning, the motor is still coupled to it and spinning, the disconnect is closed, no fuses are blown, etc.

Rant aside, you have what you have. Look in the drive manual and find the parameter that faults the drive when you lose motor connection. Ideally, you should also have an additional early-break contact on that disconnect that is tied into the enable circuit on the drive. That way, the drive automatically stops slightly before the disconnect opens.

 

[TheColt46]

By default, Schneider ATVs will fault out if it tries to run without a motor connected (as this can be bad for the drive). However, there's a parameter than can change this behavior and allow it to run even without a motor connected. If that's the case, then what you're seeing is unsurprising. The operator may have opened the disconnect, but the drive has no way of knowing that, so it will continue to show that it's running, because it IS running. Your feedback input back to the PLC is only telling you the state of the drive, not the motor. Most motor feedback circuits are done with the feedback from the motor control device rather than the motor or what the motor is coupled to, usually to cut costs. So most are simply assumptions that if the motor control device is "on", then the motor must be spinning. Ideally, a motor control feedback signal should come from what's actually being moved; so if it's a fan, it should be coming from an air flow switch. If it's a pump, an output pressure or flow switch. Etcetera and so on. That way you know if there's, say a fan and there's air flow, you know in addition to air flowing, that the fan is actually spinning, the motor is still coupled to it and spinning, the disconnect is closed, no fuses are blown, etc.

Rant aside, you have what you have. Look in the drive manual and find the parameter that faults the drive when you lose motor connection. Ideally, you should also have an additional early-break contact on that disconnect that is tied into the enable circuit on the drive. That way, the drive automatically stops slightly before the disconnect opens.

Thank you. This cleared up a lot. Our disconnect only contains the wiring for the 3-phase to the motor so it's a no-go on the early-break contact. As you guessed, I confirmed that the running signal comes from the drive and not the motor itself. I will see if I can find the correct parameter to change and report back. Thanks again.

 

[bill4807]

Brstilson beat me to it.

This also happens when someone designs a system even with across the line starter and uses an aux contact for "running" then cuts off the motor power with a safety contactor or external disconnect and forgets to cut off the contactor as well. Still having the running contact on.

 

[I_Automation]

Put one of these current trip switches on one of the motor leads coming off the VFD or servo.

Run the RUNNING signal off the VFD through it. If you don't have a hardwired RUNNING coming from your VFD then wire it from DC to a PLC input and monitor that.

I had to add about 32 of these to a plating line that each rack had vibrator motors in it and the PLC had no feedback if they weren't working. I routed the VFD RUNNING signals through them in this application

 

[nicherbert]

I think having the VFD in place, by default, the drive would trip (and you should get status back on that if it's setup correctly), but as people mentioned, this can be disabled. Assuming you have some communication to the drive (not hardwired), then you can, in addition to the running status output, also look at the motor running current and set something reasonable here to also indicate running status. Almost all loads will have some current draw, even if it's just the motor free spinning with nothing attached, so find that magic number and set a threshold to help with "running" status. I've seen this done to detect a decoupling from the load (i.e. belt/chain driven from motor and the belt breaks). This way, you can detect other faults.

 

[TheColt46]

I found the problem. Fault caused by OUTPUT PHASE LOSE was disabled.