Recommendations for an overload relay approved for the use on a VFD output?

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MTW

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When using a larger VFD to power a few motors, what are some Over;load Relays, brands and models that are listed for use on a VFD output?

I have read that many types are not suited for use on pulse width modulated outputs, that most types cannot sense accurately on this type output.
Preferably models that are separate mount that don't need to be mounted to a starter contactor. anyone have some pointers on where to look?
 
Most VFDs have had listed overload protection as part of the VFD itself for a long time. I has been at least 25 years since I installed a separate overload device for a VFD.
 
These used to be common,
as long as you are using a bi-metal or melting alloy style. NEMA style were available with lugs in and out, while IEC style had a separate lug kit used with standard relays.
 
Worked on hundreds of older model Danfoss VFD'S that appeared to use a plain Jane adjustable trip overload block The Danfoss VLT drives with bypasses had them in the bottom right side of the oversized panel that contained the three contactors. We had some that were almost twenty years old and over 100,000 running hours and never had to rep!ace a Danfoss overload block. Only once or twice one an overload out while running thru drive because the motor ampere you set would trip out drive first. We had an air handled that had four believe they were 7.5 HP motors each feed thru thier own overload block. Bad thing when one motor grounded out remains fans shut down. We ran VFD'S to a temporary conveyor line where we used Allen Bradley bi metal overload block for each motor.
 
Standard bi-metal OL relays are NOT typically good for use behind VFDs. You can always double check by finding the detailed specifications page for any particular model and look at the frequency spec, most will say 50-60Hz only. Those are the ones you shouldn’t use. But to be clear, the reason may be different than you think. They WILL trip on an overload, but the high harmonic content of the VFD output may cause them to NUISANCE trip at values far less than an actual motor overload. Then the subsequent problem is that the human response to this is usually to turn up the dial on the OL relay, to the point where it NO LONGER provides the proper protection.

Yes, Rockwell/A-B does make a special version of their 140MT bi-metal MPCB (Motor Protection Circuit Breaker, also known as an IEC Manual Motor Starter) and that is a good choice for use on multi-motor VFD applications, because it ALSO provides the necessary Short Circuit protection for the motor, as well as a way to disconnect one motor without shutting them all down (if necessary). I don’t know if anyone else is offering that as of yet.

All eutectic melting alloy type OL relays are inherently frequency insensitive up to 400Hz, which will cover pretty much any application where you will have multiple motors. It’s just that they do NOT provide any SC protection, so if the size ratio of the drive to the motors puts the SCPD of the drive outside of the requirements for the motor circuit, you have to add something anyway, so it might as well be an MPCB (fuses on the output of a VFD are a bad idea because 1 fuse blows and the VFD doesn’t know, so the motor single phases and burns up). As an example, if you have a 10HP drive running two 5HP motors, the SC trip of the breaker ahead of that drive is likely within the maximum required SCPD rating of those motors. But if you have a 10HP drive running ten 1HP motors, it is not, so each 1HP motor needs a lower rating of SCPD.
 
Thank you @Jraef for the detailed reply, you had covered this in the past, but I could not find it in my searches. Back then I think you mentioned some IEC (German) models that were specifically approved for this use. but an AB model that is suitable is more readily available to me. I still have lots of Heater Elements for eutectic models, but the stand alone OL bases are getting harder to find.

One of my reasons for inquiry is shops using a VFD for phase conversion to a multi-motor machine with one larger main motor and a few smaller ancillary motors that are not easily replaceable. Trying to properly protect those motors, while running them all from the same VFD. In this multi-motor configuration the drive should be st to the V/H mode.
Thanks again for sharing your knowledge.

Cat. No. 140MT-D9V devices can also be applied at the output of a variable frequency drive (VFD) in multi-motor applications.

Page 23 in this Product Specification https://literature.rockwellautomation.com/idc/groups/literature/documents/td/140-td005_-en-p.pdf
 
Jraef said:
As an example, if you have a 10HP drive running two 5HP motors, the SC trip of the breaker ahead of that drive is likely within the maximum required SCPD rating of those motors. But if you have a 10HP drive running ten 1HP motors, it is not, so each 1HP motor needs a lower rating of SCPD.
Click to expand...
Something tells me if you have the ten 1 HP motors on a 10 HP drive and have a ground fault on one motor circuit that there is reasonably good chance the drive detects this and shuts down before enough fault current flows to destroy the individual thermal overload element. It almost definitely will shut down without any damages if it detects it when starting.

I could see there being higher risk of damage if it is running at or near full speed then a fault suddenly occurs. I can see more risk of damage to the drive itself in this scenario as well.
 
Jraef said:
Wow, that’s one of the broadest “CYA” statements I’ve seen…
Click to expand...
Definitely broad!

One of the Lenze reps actually just called me, I had submitted this question through their online contact form to verify directly.

He says as long as you're only running one motor at a time, there's no additional overload protection needed, and to make sure the P108 motor draw setting is correct so if the VFD is oversized it knows when the motor is pulling too much and shuts itself off.

He said multiple motors running at the same time off a single VFD should each have overload protection. I gather that the motor which didn't overload would help absorb the feedback inrush from the overloaded motor.

Additionally, he suggested that if you do add a protective device, instead of letting it just cut power to the motor, it should also tell the VFD to stop.
 
Jraef said:
Standard bi-metal OL relays are NOT typically good for use behind VFDs. You can always double check by finding the detailed specifications page for any particular model and look at the frequency spec, most will say 50-60Hz only. Those are the ones you shouldn’t use. But to be clear, the reason may be different than you think. They WILL trip on an overload, but the high harmonic content of the VFD output may cause them to NUISANCE trip at values far less than an actual motor overload. Then the subsequent problem is that the human response to this is usually to turn up the dial on the OL relay, to the point where it NO LONGER provides the proper protection.

Yes, Rockwell/A-B does make a special version of their 140MT bi-metal MPCB (Motor Protection Circuit Breaker, also known as an IEC Manual Motor Starter) and that is a good choice for use on multi-motor VFD applications, because it ALSO provides the necessary Short Circuit protection for the motor, as well as a way to disconnect one motor without shutting them all down (if necessary). I don’t know if anyone else is offering that as of yet.

All eutectic melting alloy type OL relays are inherently frequency insensitive up to 400Hz, which will cover pretty much any application where you will have multiple motors. It’s just that they do NOT provide any SC protection, so if the size ratio of the drive to the motors puts the SCPD of the drive outside of the requirements for the motor circuit, you have to add something anyway, so it might as well be an MPCB (fuses on the output of a VFD are a bad idea because 1 fuse blows and the VFD doesn’t know, so the motor single phases and burns up). As an example, if you have a 10HP drive running two 5HP motors, the SC trip of the breaker ahead of that drive is likely within the maximum required SCPD rating of those motors. But if you have a 10HP drive running ten 1HP motors, it is not, so each 1HP motor needs a lower rating of SCPD.
Click to expand...
Interesting you say that; I've never heard that bimetallic O/Ls are the wrong thing to use with a VFD. I've used Fuji bimetallic overloads several times to protect multiple motors running from one VFD, and once when I had to run a motor so small I couldn't turn the VFD's protection settings down far enough.


The data sheets make no mention of frequency limitations, and I even checked with AutomationDirect, and they said that bimetallic O/L's were appropriate for use on the output side of a VFD.

It's the electronic O/L relays that you have to be careful of. Those definitely DO NOT like VFD power.


SceneryDriver
 
I am doing a machine right now that has two 75HP motors on a single VFD, the individual motors are protected from OL using AB 193-T1DC97OP Overload relays.
Just read the data sheet and be sure the OL relay is rated for a VFD output. The electronic OL's do not like the harmonics from the drive and will create nuisance tripping events.
 
TnDrew said:
Additionally, he suggested that if you do add a protective device, instead of letting it just cut power to the motor, it should also tell the VFD to stop.
Click to expand...
VFD's don't like to have switching on load side of the drive output. It is best to use any overload device you may use to open the run signal to the drive if it "trips" and let the drive shut down in it's own fashion than to suddenly open part of the output load.
 
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