# Motor protection circuit breaker and VFDs



## Flyingsod (Jul 11, 2013)

I'm not real familiar with that beast. Aside from the fractional horse power starters i can't say I've ever really seen one in the wild. I looked then up though and judging by specs they don't do anything more than what a drive will do.

I can tell you that they are not standard in drive installations in industry. Is say as guesstimate about 80 percent of drives are powered directly from fuses or regular branch circuit breakers. An MPCB seems superfluous in a drive install, at least on first inspection.

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## tates1882 (Sep 3, 2010)

cmdr_suds said:


> Can a MPCB ahead of a VFD provide any additional protection for the motor beyond what the drive will provide?
> 
> Sent from my LG-AS330 using Tapatalk


NO, I've used a mpcb to provide line side protection for drive.


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## sparkiez (Aug 1, 2015)

There should not be a need for load side protection on a vfd. Most drives provide short circuit and overload protection. You do want to provide line side short circuit protection via breaker or time delay fuses. That being said, you do generally want a local disconnect at the motor you are feeding.


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## frenchelectrican (Mar 15, 2007)

sparkiez said:


> There should not be a need for load side protection on a vfd. Most drives provide short circuit and overload protection. You do want to provide line side short circuit protection via breaker or time delay fuses. That being said,  you do generally want a local disconnect at the motor you are feeding .


I wrote a line below that part.,, 

for local disconnection means .,, you will have to add a aux switch concants so that way the VSD can shut down or deengerized the load side before the disconnecting switch kick open .,, 

I know most VSD manufacters will remind that part and make sure you do that if you do have local motor disconnecting. 

the reason why It need a aux switch on the switch due when the VSD is running and suddely running unloaded quick the voltage on dc buss will spike up pretty hard so that why it will quickly shut down the dc bus before the switch kick open.


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## John Valdes (May 17, 2007)

Protect the line and let the drive protect the motor. Thats really all there is to it.
Every drive has the specs for line side OCP listed in the manual.
Be it fuses or breaker.


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## cmdr_suds (Jul 29, 2016)

When I saw the installation with the mpcb ahead of the drives and asked why they were there instead just breakers or fuses, I was given the dubious answer they were to protect the motor. My belief is the drives have far superior ability to protect the motor then a dumb MPCB.

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## frenchelectrican (Mar 15, 2007)

cmdr_suds said:


> When I saw the installation with the mpcb ahead of the drives and asked why they were there instead just breakers or fuses, I was given the dubious answer they were to protect the motor. My belief is the drives have far superior ability to protect the motor then a dumb MPCB.
> 
> Sent from my LG-AS330 using Tapatalk


The MPCB or standard breaker or fuse will take care of the line side of the VSD unit as long you follow the specs what they listed.,, then ya are fine with it. 

Sure the drive do protect the motor but ya need something ahead of it like line side in case the VSD take a dump which I have see it happend once a while.


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## JRaef (Mar 23, 2009)

cmdr_suds said:


> Can a MPCB *ahead of a VFD* provide any additional protection for the motor beyond what the drive will provide?
> 
> Sent from my LG-AS330 using Tapatalk


No. The device on the* line side *only sees the input current of the VFD, which is lower than the output current, so it means nothing with regards to the motor. The VFD will be providing all protection for the motor.

The only reason some mfrs list their VFDs to be protected by MPCBs instead of MCCBs is when the VFD mfr does not manufacture their own MCCBs. So rather that list them with a product that must be bought from a competitor, they list them with something that they themselves make.

MCCBs underwent a radical design change recently because of some newer more draconian RoHS regulations (Reduction of Hazardous Substances) in the EU and elsewhere. Although not applicable to North America yet, it's applicable to exports so the effect is global. The latest change means that all components must be 100% recyclable and in older MCCBs, the molded cases were often made with Fiberglass Reinforced Plastic (FRP) which is not recyclable. So all older designs can no longer be used in anything that ends up in the EU or many other countries, forcing mfrs to redesign. What happened was that only a few of them spend the money to do it, others simply made arrangements to brand-label those that did. But when it comes to listing something like their own VFDs, the MCCBs are still "buy-out" products with less profit built-in, hence using something they themselves manufacture.


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## DesignerMan (Jun 13, 2008)

I've used them on VFDs for various reasons- anything from a smaller foot print to lower cost or a higher SCCR rating.
It all depends on the project...


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## cmdr_suds (Jul 29, 2016)

JRaef said:


> No. The device on the* line side *only sees the input current of the VFD, which is lower than the output current, so it means nothing with regards to the motor. The VFD will be providing all protection for the motor.
> 
> The only reason some mfrs list their VFDs to be protected by MPCBs instead of MCCBs is when the VFD mfr does not manufacture their own MCCBs. So rather that list them with a product that must be bought from a competitor, they list them with something that they themselves make.
> 
> MCCBs underwent a radical design change recently because of some newer more draconian RoHS regulations (Reduction of Hazardous Substances) in the EU and elsewhere. Although not applicable to North America yet, it's applicable to exports so the effect is global. The latest change means that all components must be 100% recyclable and in older MCCBs, the molded cases were often made with Fiberglass Reinforced Plastic (FRP) which is not recyclable. So all older designs can no longer be used in anything that ends up in the EU or many other countries, forcing mfrs to redesign. What happened was that only a few of them spend the money to do it, others simply made arrangements to brand-label those that did. But when it comes to listing something like their own VFDs, the MCCBs are still "buy-out" products with less profit built-in, hence using something they themselves manufacture.


Thanks! That was pretty much what I thought. I felt the person telling me it was for motor protection didn't know what he was talking about. Interesting information about the use listing the drives with mpcb in lieu of mccb. 

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## sparkiez (Aug 1, 2015)

frenchelectrican said:


> I wrote a line below that part.,,
> 
> for local disconnection means .,, you will have to add a aux switch concants so that way the VSD can shut down or deengerized the load side before the disconnecting switch kick open .,,
> 
> ...


This is true and not a bad idea, however in the US LOTO procedure is to shut down the system normally (press the stop button) and THEN switch off the disconnect. This is, in part, to prevent a sudden mechanical disconnect of energy that can cause an Arc Flash. In a smaller motor with a quality disconnect this isn't such a big deal. Now imagine that same scenario on, say, a 250A disconnect. Much greater potential for a flash.

So, while the Aux contacts are great, if proper procedure is followed they aren't necessary.


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## JRaef (Mar 23, 2009)

While that's true, in my experience proper procedures being followed is not the norm when non-electricians are involved. Most operators and millwrights are lazy when it comes to electrical procedures. They SHOULDN'T be, but they are. I can't count the number of times I've seen where there is a Start-stop button on a panel for a machine, and an E-Stop near it, the lazy people will ALWAYS hit the E-Stop rather than move 5 ft to hit the normal Stop button. Same with local disconnects.


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## DesignerMan (Jun 13, 2008)

JRaef said:


> the lazy people will ALWAYS hit the E-Stop rather than move 5 ft to hit the normal Stop button. Same with local disconnects.


So True!!!


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