Multi motors on VFD
- Thread starter davidb123
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petersonra
Senior Member
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- Northern illinois
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- engineer
If the OCPD on the input to the VFD is adequate for the motor, then you don't have to provide it separately, just OL protection.
For instance, suppose you had a VFD with a 15 A CB on the input. Any motor of any size on the output of the VFD would be protected from SC by the 15 A CB.
However, if you had a 20 A CB on the VFD input, and that was inadequate to provide SC protection for the motor on the VFD output, than additional protection would be required on the output.
Incidentally, there are manual motor protectors you could use in lieu of fuses and overload blocks.
I'd provide an overload and a contactor for every motor but I don't know what the NEC mandates.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
It mandates OL protection specifically, but as Bob pointed out, the OCPD is less directly addressed in this situation. Generally, this is done with small motors and his 15A rule applies anyway because that's the smallest branch breaker you can get. But as per his example once the breaker feeding the VFD is larger, it usually no longer fits the rules for the OCPD of the individual motor circuits. A more graphic example: 7-1/2HP VFD feeding 6 x 1HP motors, each with an FLA of 1.4A; the VFD is protected by a 15A CB and since that's the smallest breaker you can get anyway, the NEC allows that for the 1HP motors. But if you have a 20HP drive feeding 12 of these motors, the 20HP drive needs a 40A CB, but the max OCPD for the motors is 250% of 1.4A, so that's only 3.5A and that CB is now too big for the motors and too big for the 15A exception.
That's where those little IEC style motor protective switches come in, mounted down stream of the VFD feeding each individual motor circuit. They provide the OCPD, OL and disconnecting device all in one, sized and listed exactly for that motor. Some are not rated for use in anything but 50/60Hz however, so you must be careful when choosing brands.
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
why would you add a contactor?
To disconnect the motor that had an overcurrent fault whilst letting the others continue running. We've done this for steel mill roller tables where many motors are connected to one VFD.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
The little MPS devices will do that too, and preclude the ability to easily remotely re-engage them while the drive is running. Using the MPS devices requires (if properly designed) that the cabinet be opened to reset them, which ostensibly means the power to the drive was killed first in order to open the door. Of course any of that can be defeated, but anyone working that hard at it would (hopefully) understand the risks of re-energizing a motor when the VFD is already running. You can do that with contactors as well by controlling the coil circuitry, and I imagine you (Besoeker) have the experience to make sure that happens correctly, but in general I don't trust end users that much and I do what I can to make sure that risk is mitigated in a more passive way.
And for the novices, the risk is; blowing up the transistors on the output of the drive, or hopefully, at least making the drive trip to prevent that, which means you should have just turned it off first anyway.
Opening a set of contacts on the load side of a VFD that is powering loads used to be risky, because in opening a set of contacts under load, the arc that forms is something that could cause the transistors to misfire and be damaged. But most newer designs have now mitigated that risk. The remaining risk in a multi-motor setup is that the high inrush current from CLOSING a set of contacts into an induction motor will damage the transistors directly. The VFD output must be Off before re-closing those contacts.
MPS is a term I'm famaliar with. A link might be helpful.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/140m-pp004_-en-p.pdf
In your world you likely call them something else, such as "circuit breakers", but over here that term has a specific definition and those devices don't meet that, so almost every mfr has a different term for them. Motor Protective Switch (MPS); Motor Starter Protector (MSP); Motor Circuit Protector (MCP, although that also has another meaning and is a trade name); Motor Protection Circuit Breaker (MPCB); Manual Motor Starter (MMS); etc.
The contactor and thermal overload is usually a cheaper solution.
kwired
Electron manager
- Location
- NE Nebraska
If no additional control circuit is needed maybe not.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
these days you can get a 1/4 HP drive for not much more than $100. Might be less expensive overall to go that direction rather than screw around with extra overloads, extra contactors, and extra wiring.
kwired
Electron manager
- Location
- NE Nebraska
I agree that may be worth consideration, but you still have multiple units to wire either way, the manual motor starter or whatever you decide to call it only needs line in and line out - no controls unless you want to signal the drive or other device that it has tripped.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I think you would want some kind of feedback that something has tripped. last time I used some of the manual motor protectors or whatever the heck they are, they were like $60 each. They were ABs so they might have been more expensive than some other brand but they are not cheap.
We almost always had 110Vac or 24Vdc controls. And sometimes both.
kwired
Electron manager
- Location
- NE Nebraska
The application would dictate how important such feedback is necessary. I have one place I put multiple cooling fans for a process on a single VFD. Product is cooled on an open table by wall mounted fans - if one should quit it isn't too detrimental to the process and the operators will notice it isn't running, they may just increased the speed of the others a little to compensate until the problem is resolved.
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