Circuit Protection for VFD driven motors.

The recent thread regarding motor current reminded me of an on-going issue I have with VFDs. Many engineering specs that I see call out a Motor Circuit Protector, a VFD and a motor. I've been making the argument that a Circuit Breaker should be used instead of a Motor Circuit Protector since the power source sees the VFD input circuit which is a big rectifier instead of the inductive load of a motor. I am interested in other people's experience with this setup.

I am sure that DickDV can provide learned insight for you but the proper installation is to protect the entire system properly. Protection coordination is supposed to propagate back toward the feed point. Meaning, if you have a problem or fault, the protection device closest to the fault, but up stream from it, is to trip to protect the rest of the system. If the motor overloads, you would want the motor to be tripped off the line, drive, etc, to stop the fault. If the drive should develop a problem internally then the input protection, either fuses or CBs should take it off the line, etc, etc all the way back to the switch gear. Do not substitute properly designed and engineered installation for good ole boy advice.

I have followed manufacturer's instructions and used UL489 rated circuit breakers instead of fuses ahead of drives.

PC

The AC drives I have dealt with call for semiconductor (very fast acting) fusing if you want to have realistic protection for the front end of the drive. CB's are convenient but are too slow for some fault conditions. It is also commonly held that nothing should be between drive and motor (any type of contacts) due to potential damage to IGBT section. In some applications, a local disconnect for the motor is required for safety during maintenance due to the proximity of drive to motor and other circumstances. The drive should be disabled before this disconnect breaks the circuit. I think I have seen about every combination imaginable in my experience, whatever the recommendations may be. A reputable drive manufacturer will have guidelines and the supporting literature for those guidelines.

I do agree that a motor circuit breaker is not the right way to go. I also agree that semiconductor fuses offer the best protection for the drive. A study of the I2T curves of standard HRC fuses to the semiconductor protection type show these to be far quicker acting. To protect the drive properly, semiconductor fuses should be used.

One of the reasons motor circuit breakers may be being used is to get the fault level up but semiconductor fuses will do the same thing and be far more effective in protecting the drive.

By the way, I once worked for GEC (English Electric) and used to sell a lot of semiconductor fuses for the purpose of protecting drives.

As you say BobB, the semiconductor fuses do provide great protection. One drawback is the added cost. The drive manufacturer recommended the voltage levels to exceed the DC buss, so the ratings were different. These had an RMS rating as well as a DC rating of 700 volts. Most fuses will have an RMS rating such as 600VAC with a peak value. This drive recommended the semiconductor fuses with the 700 volt DC rating. These fuses also used ceramic fuseblocks which added more to the cost. If you common bussed any of the drives, you would use a 2 pole ceramic block with the same fuses. Of course here you are directly fusing the dc buss which can potentially rise to whatever the trip level is set at in the drive.

And there are many applications where class J or CC fuses were used with no problems. The manufacturer even listed these as choices but not the "preferred" method of protection.

The cost of semi conductor fuses is insignificant if they save an expensive drive.

Have you had a look at the British BS88 fuses? they used to use phenolic fuse holders. They used to be English Electric/GEC and are probably now Alsthom.

True. When I say more expensive, it is still a small addition relative to the entire control cabinet. I was speaking of Edison fuses. I believe the ceramic blocks were Gould. I have not priced the fuses you have mentioned but I'm sure there are cheaper ways to go than the ones I have mentioned.

Norml said:

The recent thread regarding motor current reminded me of an on-going issue I have with VFDs. Many engineering specs that I see call out a Motor Circuit Protector, a VFD and a motor. I've been making the argument that a Circuit Breaker should be used instead of a Motor Circuit Protector since the power source sees the VFD input circuit which is a big rectifier instead of the inductive load of a motor. I am interested in other people's experience with this setup.

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I think you may be caught in a semantics issue here. In North America, the term Motor Circuit Protector is used when describing a magnetic-only circuit breaker that is sold STRICTLY for the use of starter manufacturers in making tested and listed combination starters. Contrary to popular belief, THERE IS NO OTHER LEGITIMATE OR ACCEPTABLE USE for that type of circuit breaker (check it out in Article 430 of the NEC if you don't believe me). For this reason, it would be entirely inappropriate for a VFD manufacturer to suggest that you use that device, your equipment would (should) fail inspection every time.

The semantics trap you are likely to have fallen into is because of the fact that most VFDs are not made in the US, and the off-shore manufacturers hire translators, often in the UK or India, who use different terminology that we do here. In the IEC world, many people use the term Motor Circuit Protector to mean the integrated Thermal-Magnetic protective switches made by all of the IEC component manufacturers. There really is no direct NEMA equivalent, but the closest would be a Manual Motor Starter, which is how many of them are UL listed and sold here. For this reason, we refer to them under different names such as Motor Protective Switches, because the term Motor Circuit Protectector was already taken. This subtle change in semantics is usually lost on literal translators and some engineers who are unaware of the difference.

Bottom line, don't sweat it, but don't use what we in the US call MCPs, they would be a code violation.

"For this reason, it would be entirely inappropriate for a VFD manufacturer to suggest that you use that device, your equipment would (should) fail inspection every time. "

"Bottom line, don't sweat it, but don't use what we in the US call MCPs, they would be a code violation"

Thanks for your thorough response. That's exactly my point except the use of MCPs is not coming from VFD manufacturers. It is coming from American engineering firms who are writing the specs for wastewater pumping stations. I don't know how this got started but I've seen a lot of specs get copied and used for other jobs using the same terminology and MCP equipment. I do NOT use MCPs in my submittals but it becomes difficult if I get "caught" since I have some convincing to do. Maybe your NEC reference will help. Thanks.

Soon, new arcflash regulations will take care of this problem for everybody. We will be required to use "low peak" fuses everywhere.

Here's a thought on drive power source fusing. Drive manufacturers generally recomment semi-conductor fusing to limit "collatoral" damage if a power circuit component should fail.

This is a noble purpose when the drive is repairable but what about those cases where the drive is simply not built to be repaired or is not economical to repair. This point in current production drives is right around 50hp 460V and 25hp 230V.

For those drives that simply cannot be repaired, why not just use Class J or other inexpensive protection for the branch circuit. If these little drives fail in their power circuit, you are going to dump it anyway so who's concerned about collateral damage?

Ferraz-Shawmut came out with a line of "high-speed Class-J" fuses about 2 years ago, specifically for protection of drives. Has anyone out there used them/have any feedback on them? This might be a good solution. I think they are called HSJ fuses.

Norml said:

"For this reason, it would be entirely inappropriate for a VFD manufacturer to suggest that you use that device, your equipment would (should) fail inspection every time. "

"Bottom line, don't sweat it, but don't use what we in the US call MCPs, they would be a code violation"

Thanks for your thorough response. That's exactly my point except the use of MCPs is not coming from VFD manufacturers. It is coming from American engineering firms who are writing the specs for wastewater pumping stations. I don't know how this got started but I've seen a lot of specs get copied and used for other jobs using the same terminology and MCP equipment. I do NOT use MCPs in my submittals but it becomes difficult if I get "caught" since I have some convincing to do. Maybe your NEC reference will help. Thanks.

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I'll tell you EXACTLY why you see it in American CE firm specifications; because many are too lazy to actually write their own specifications, relying instead on "cut-and-paste" engineering by using the recommended speciications spoon fed to them by manufacturers. There is nothing inherently wrong with starting off using those specs, but the better firms will actually read them and know what they are reading so that they can correct any glaring errors like that. The less-than-professional firms don't bother, so this kind of junk spec gets put out there for manufacturers to scramble to try and meet. Then they argue to defend their reputation even though you may point out their folly. Later when a good inspector (another rarity) catches it and red tags the project, they come back at the supplier saying "you should have known".

...but I'm not bitter sounding, am I?

jraef said:

I'll tell you EXACTLY why you see it in American CE firm specifications; because many are too lazy to actually write their own specifications, relying instead on "cut-and-paste" engineering by using the recommended speciications spoon fed to them by manufacturers. There is nothing inherently wrong with starting off using those specs, but the better firms will actually read them and know what they are reading so that they can correct any glaring errors like that. The less-than-professional firms don't bother, so this kind of junk spec gets put out there for manufacturers to scramble to try and meet. Then they argue to defend their reputation even though you may point out their folly. Later when a good inspector (another rarity) catches it and red tags the project, they come back at the supplier saying "you should have known".

...but I'm not bitter sounding, am I?

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Hallelujah! I'm glad I'm not the only person out there who's sick of this. Most of the engineers we deal with get ROYALLY ticked off if you point out inconsistencies in their specs, but you sure better not quote it their way then expect a change order approval to make it work later on. It's a lose-lose situation a lot of the time for contractors and equipment suppliers. But, they are the "engineers", so they know best, don't they?