Thermal Overload Device w/ VFD and PLC

[c2alpha]

Good Day,
My guestion is: can you use a thermal overlaod relay without subsequent contactors on the load side of a vfd? Then use the signal from the relay to a plc to shut down the system. The system being multible motors with acme screws in a lift (non-personnel) application.

 

[dpc]

Probably, but I doubt you would need a PLC. Most VFDs can accept discrete contact inputs.

But the overload protection built into the VFD is generally going to be better than an external OL relay that was not designed for the harmonic current and PWM waveform it will be seeing.

There will be some VFD experts along shortly with better opinions.

 

[c2alpha]

I should mention that there is a 20hp vfd controlling (4) 5hp motors with a line reactor on the load side of the vfd feeding into (4) OLR's (one for each motor). Thus "the system".

 

[ozmosis]

c2alpha
You should use external thermal overload devices into a VFD when using multiple motors. The in-built 'electronic' thermal overload in most VFD's will not be functional in this case as the drive will not know which motor is overloaded.
You should be able to connect all therm-overloads into the VFD to shut down the system if one of the motors goes into overload. Presumably you would want the complete system to shut down if just one motor trips (sometimes you wouldn't want this). So contactors on the output should not be necessary, use the logic into the drive to provide a shut down assuming the drive can do this. Some drives will provide a NC circuit that can be esed as an 'external alarm' and register a specific fault so that you know what has caused the drive to shut down.

 

[dpc]

Sorry - I missed the bit about multiple motors. What ozmosis said.

 

[georgeshall]

You don't want to ever disconnect a motor, either with a switch or contactor, while the VFD is running as high voltages can be generated which can severly damage the VFD. Also, the VFD may not be able to handle the high inrush currents when closing a switch or contactor start a motor while the VFD is running and this will likely shut the VFD down. The VFD should start and stop all motors simultaneously.

 

[jraef]

I should mention that there is a 20hp vfd controlling (4) 5hp motors with a line reactor on the load side of the vfd feeding into (4) OLR's (one for each motor).

It bears mentioning that you MUST have separate OL relays in this situation; it is not just a recommendation. Plus, those OL relays should be wired directly to the VFD's external fault input. I like using the Thermistor input if it has one, because it will show up as an Overload fault in the fault display and register, not just a non-descript fault. I don't think running them through a PLC would suffice as "fail safe" overload protection unless the PLC / VFD have been specifically tested and listed for that purpose (assuming you are in the US by virtue of your use of "HP").

 

[DickDV]

With four motors on a single drive, I would recommend separate branch circuit fuses for each motor as well. Once you get beyond three motors on a drive, the drive ampacity becomes so large it could literally roast one motor without ever faulting on overload or overcurrent. Thus the need for fuses.

 

[c2alpha]

Thanks everybody,
I will rap my brain around your comments.

As far as the last few comments, I will incorporate them into the design. Make's good sence.

Thanks again.

 

[Watthour]

The other consideration is that you will be saturating multiple stators simultaneously. Depending upon the motor(s) design(s), this may present a higher saturation current than a single 20HP stator, and potentially more than the drive was designed to provide. Ramp timing of the drive, initial DC boost, and other tuning may be necessary to have a successful installation.