# VFD overcurrent Protection



## Wirenuting (Sep 12, 2010)

Welcome to ET

You need to protect the branch circuit going to the drive. 
You can’t tap off the mains.


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## Bird dog (Oct 27, 2015)

It's good to educate yourself, but, it sounds like you need to call an electrician to do the install properly. Your drive documentation should give you the parameters for the branch circuit to feed the drive.


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## oliquir (Jan 13, 2011)

most of the time they tell you in the manual the exact very fast fuse to use (often class T) ive seen a lot that people just use standard fuse or breaker and the vfd blows if there is short-circuit since fuses are not fast enough


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## MDShunk (Jan 7, 2007)

Of course the fuses protect the circuit, but my experience has been that if a fuse feeding a drive ever does blow, 9 times out of 10 that drive has suffered an irrecoverable failure.


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## paulengr (Oct 8, 2017)

The drive electronics (SCR, transistors, diodes) are all semiconductors. To work well they need to be thin. As semiconductors and not conductors they get very hot very fast in fault conditions. So when they fail if you don't have very fast short circuit protection they launch parts out of them. This drives the specs. If the drive front end is the failure there is nothing in the drive itself to stop it. Most of the drive lines I distribute do have optional built in breakers if you don't want to supply your own.

As to the comment about load contactors, these are not terrible and with for instance 3 contactor bypasses, common. The big problem is the precharge circuit. Most drives derive power off the DC bus so the electronics are offline. The drive has to "fill" the DC link. Some drives use just a contactor and a rectifier for this. Usually this circuit is undersized so if you frequently turn the drive on and off like feeding it off a contactor in a starter retrofit it burns up the precharge circuit. For whatever reason medium size drives fire the front end and gradually bring up the DC bus nice and smooth but the larger drives almost always cheap out the precharge circuit. Very irritating that a $100k+ drive fails on a $10 current limiting resistor or rectifier. 


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## Wiresmith (Feb 9, 2013)

welcome, sweet username. if the manufacturer requires it then the NEC requires it because the manufacturer requires it.


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## paulengr (Oct 8, 2017)

MDShunk said:


> Of course the fuses protect the circuit, but my experience has been that if a fuse feeding a drive ever does blow, 9 times out of 10 that drive has suffered an irrecoverable failure.




Older drives could have latchup where high currents prevent transistors from switching off. Like SCRs. On some the feedback is so slow they don’t react in sub cycle speeds. But that’s only in the few left. Dealt with an AB 1336 plus ii last week with this problem, a late 1990s vintage drive.


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## MDShunk (Jan 7, 2007)

paulengr said:


> Older drives could have latchup where high currents prevent transistors from switching off. Like SCRs. On some the feedback is so slow they don’t react in sub cycle speeds. But that’s only in the few left. Dealt with an AB 1336 plus ii last week with this problem, a late 1990s vintage drive.
> 
> 
> Sent from my iPhone using Tapatalk


The exact mechanism of failure is little more than a curiosity to the rank and file electrician. Bottom line- if the fuses feeding the drive blew, expect in your gut you'll likely also need a drive.


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## John Valdes (May 17, 2007)

Welcome to Electrician Talk. 
Thanks for taking the time to fill out your profile.


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## MDShunk (Jan 7, 2007)

John Valdes said:


> Welcome to Electrician Talk.
> Thanks for taking the time to fill out your profile.


You're welcome.


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## paulengr (Oct 8, 2017)

MDShunk said:


> The exact mechanism of failure is little more than a curiosity to the rank and file electrician. Bottom line- if the fuses feeding the drive blew, expect in your gut you'll likely also need a drive.



Not if you don’t fix the actual problem. Just replacing what is broke works about 50% of the time but it is NOT troubleshooting. The rest of the time it takes much longer and costs a lot more to fix it, partly because the “repair” destroys all the evidence.

This attitude is the reason IT morons think that just reboot followed by just reinstall is troubleshooting when both demonstrate they don’t know how to troubleshoot because both of them not only might not fix the problem but more importantly destroy all the evidence! How many times has an IT moron worked on something and not only didn’t fix it but actually made it worse?

Just replacing a burned up fuse and/or drive is the same thing. If the issue is the fuse is way too slow or the motor is dead shorted and the drive is too slow (or both) just replacing it results in destroying a perfectly good drive right out of the box.



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## Helmut (May 7, 2014)

MDShunk said:


> The exact mechanism of failure is little more than a curiosity to the rank and file electrician. Bottom line- if the fuses feeding the drive blew, expect in your gut you'll likely also need a drive.


And possibly a motor as well.


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## gpop (May 14, 2018)

In the old days like danfoss you were expected to test everything (motor, wiring. disconnect. etc) then field strip the drive and test all the sections of the drive. Replace the bad parts then re-assemble and test.
Today there sealed units with non-serviceable stickers so its more of a test everything and if it points to the drive just replace it. 

There are certain signs that the drive is done. Liquid running out of the drive, black marks on the panel behind the drive, etc. 

Then its a simple case of checking everything else before replacing the drive.


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## LBC Jesse (Apr 26, 2012)

I can't say for certain about NEC codes, but I would highly recommend protection on both the line side of the drive (correct breaker or fuses) and the load side (EOL feeding motor leads for example). 

It's also not as simple as throwing in "X" amp breaker or fuses. The drive's spec book and/ or manual should call out the correct amperage and aic rating required to protect the drive.

I wont warranty any of our drive packages without the correct line side protection. There are SO MANY other factors that need to be considered when installing a drive.. a few examples:
-ambient temperature where the drive is located
-environment where drive is located (dusty/ wet/gas/ corrosive)
-distance form power source to drive (line reactor may be needed)
-distance from drive to motor (output filter may be needed)
-will it be susceptible to power surges and/or lightning (TVSS may be needed)
-is the drive properly sized/ rated for the constant or variable torque loads.

Surprisingly, alot of people just yank out a 100hp "across the line starter" and throw in a 100hp drive thinking they just solved all of life's problems, when in fact, if done incorrectly, just created a living hell for themselves or the next guy that has to work on it.
Drives are too costly to install half-ass... a few extra bucks to do it properly will be worth it.


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## LBC Jesse (Apr 26, 2012)

I'm actually hoping JRaef will opine- if not mistaken, Allen Bradley drives are his wheel house. Id like to hear his thoughts. (I deal more in ABB's ACS and ACH drives)


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## just the cowboy (Sep 4, 2013)

*Motor overloads*



ceilingwalker said:


> Good day,
> 
> First post on this forum. I would like to know if VFD's require an external overcurrent protector? My particular drive is an Allen-Bradley PowerFlex 755 and it does provide internal protection however, I have been searching NEC to see if external is also required but not having much luck. Thank you much.


Are you asking does it need external motor overloads, the answer is no.


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## gpop (May 14, 2018)

As long as the vfd only has one motor and is capable of being programmed to protect the motor then no external protection is required unless recommended by the manufacture.


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## gpop (May 14, 2018)

LBC Jesse said:


> I'm actually hoping JRaef will opine- if not mistaken, Allen Bradley drives are his wheel house. Id like to hear his thoughts. (I deal more in ABB's ACS and ACH drives)


My abb rep doesn't require load side fuses for warrantee.


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## oliquir (Jan 13, 2011)

gpop said:


> My abb rep doesn't require load side fuses for warrantee.


never put fuses on the load side, if one blows, the vfd may blow also !!


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## LBC Jesse (Apr 26, 2012)

oliquir said:


> never put fuses on the load side, if one blows, the vfd may blow also !!


agreed, i use electronic overloads on the load side to the motor. 

I'm also not a huge fan of fusible style on line side, especially on drive & soft start applications. On occasion the specs may require 100Kaic protection, and if the customer chooses cheap fuses instead of a costly 100kaic breaker, I'll let them make that call. so many "cons" to fusible these days with all the sensitive phase imbalance, phase loss equipment, that require higher quality protection. Most VFD's and SS's have built in protection for that sort of thing, but why not try to stop that problem BEFORE it gets to your $30k drive? just my two cents.


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## LBC Jesse (Apr 26, 2012)

gpop said:


> My abb rep doesn't require load side fuses for warrantee.



Load side? not an issue really- customer's call. 
Line side? I'll still warranty a "brown box drive" from manufacture default, but if it blows up and customer didn't have it protected by a breaker or fuses, (or any other environmental protection) he will be buying another drive. I highly doubt ANY manufacture would warranty a drive not installed/ protected/ sized correctly. 
I try to avoid this in advance by asking ALOT of questions prior to the sale. As previously stated, so many DIYers can wreck havoc on their equipment (as well as their employers check book). 
Heck, I build/ sell ICP's all day, but even I call a licensed electrician to install/ repair electrical in my own house and shop


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## pudge565 (Dec 8, 2007)

paulengr said:


> Older drives could have latchup where high currents prevent transistors from switching off. Like SCRs. On some the feedback is so slow they don’t react in sub cycle speeds. But that’s only in the few left. Dealt with an AB 1336 plus ii last week with this problem, a late 1990s vintage drive.
> 
> 
> Sent from my iPhone using Tapatalk


Sounds familiar lol, see my profile pic. I installed that as my senior project at vo-tech in 2010. The thing was collecting dust and I had to convince my instructor to let me do it. I was the ONLY one he trusted to do so and I had to padlock the enclosure so no one could mess with it.

He tried to tell me it wouldn't work on the 208 wye system the school had as it was a 240v drive. Well I dug up the decades old manual and showed him it can run on 208v but, IIRC, had to be derated.

I really miss control work :crying:


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## pudge565 (Dec 8, 2007)

LBC Jesse said:


> Load side? not an issue really- customer's call.
> Line side? I'll still warranty a "brown box drive" from manufacture default, but if it blows up and customer didn't have it protected by a breaker or fuses, (or any other environmental protection) he will be buying another drive. I highly doubt ANY manufacture would warranty a drive not installed/ protected/ sized correctly.
> I try to avoid this in advance by asking ALOT of questions prior to the sale. As previously stated, so many DIYers can wreck havoc on their equipment (as well as their employers check book).
> Heck, I build/ sell ICP's all day, but even I call a licensed electrician to install/ repair electrical in my own house and shop


I worked for a control shop that did an install of 2 75 HP Powerflex I think they were 755s. We were told the motors were 60 HP but could not verify as the nameplates were trashed. So these 2 motors drove exhaust fans at a foundry. One fan fed the second fan, this was part of their scrubbers which used a venturi system.

Well at commissioning the drive for the first fan kept faulting out on current limit. I suggested that the first fan was pushing more air than the second fan and therefore causing a back pressure and higher load. Well maintenance did not look into it. Instead my boss just had me set the max frequency to about half of the second drive. A few weeks later the facility had a burnt out drive. It kept going out on current limit and instead of investigating they just kept resetting it... Once it smoked they wanted us to send it back for warranty which my boss told them no way they will open it up and know exactly what happened and deny it.

Turns out, someone replaced the belts on the first fan and used the wrong sheaves of the multi sheave pulley so I was in fact exactly correct with what was happening. Prior to us doing this install they were running on across the line starters which occasionally blew fuses but once again just replace no need to investigate...


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## gpop (May 14, 2018)

oliquir said:


> never put fuses on the load side, if one blows, the vfd may blow also !!



lol So I guess you would be surprised when you come across a drive with 13 starters hooked to the load side each powering 10 hp motors. We don't stop the drive just open the starters and leave the drive running. On starting we just time delay between motor and close the starters. 


It may also surprise you that we have been doing for this over 25 years.


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## gpop (May 14, 2018)

pudge565 said:


> I worked for a control shop that did an install of 2 75 HP Powerflex I think they were 755s. We were told the motors were 60 HP but could not verify as the nameplates were trashed. So these 2 motors drove exhaust fans at a foundry. One fan fed the second fan, this was part of their scrubbers which used a venturi system.
> 
> Well at commissioning the drive for the first fan kept faulting out on current limit. I suggested that the first fan was pushing more air than the second fan and therefore causing a back pressure and higher load. Well maintenance did not look into it. Instead my boss just had me set the max frequency to about half of the second drive. A few weeks later the facility had a burnt out drive. It kept going out on current limit and instead of investigating they just kept resetting it... Once it smoked they wanted us to send it back for warranty which my boss told them no way they will open it up and know exactly what happened and deny it.
> 
> Turns out, someone replaced the belts on the first fan and used the wrong sheaves of the multi sheave pulley so I was in fact exactly correct with what was happening. Prior to us doing this install they were running on across the line starters which occasionally blew fuses but once again just replace no need to investigate...




isn't current limit on ab normally short for hardware current limit which is real bad (press reset and something is going to go bang) compared to motor overload which is a basic overload. 


Always cheaper the first time to have the factory tech come out and program the drive. It might cost 1000 dollars but it extends the warrantee and 99% of the time they cover it with no questions asked. (unless the alarm was hardware current limit and there was enough left of the drive to recover the fault log)


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## paulengr (Oct 8, 2017)

Overload for the motor is a simulation and just a trip like the real one. You can set the torque/current limit to motor rating and the motor can go all the way to stall (with enough cooling and an encoder) without tripping. This behavior is easy enough to program especially with variable torque loads if you set up stall tripping and occasionally I set this up in general purpose drives but it is all but automatic in servo drives, elevators, cranes, and excavators where stopping under load is normal.

The inverter (DC to AC) part of the drive switches the DC link (source) on and off in one of seven possible arrangements in a typical 6 pulse drive. Switching is all voltage control. The drive monitors current and can adjust voltage to control it but current is not technically “controlled”, voltage is. In pure V/Hz you can short out the drive and burn it up but it will literally throw shrapnel. Also it is necessary to fire a transistor before the one in the opposite polarity fully opens so technically for a few microseconds drives short themselves out. Transistors are not perfectly matched and their characteristics change with age and temperature so actual drives monitor gate voltage which indirectly gives current for basic normal drive performance reasons even in cheap HVAC drives. Pure V/Hz died with analog PWM. Along with that comes automatic detection and tripping of shorted components.

At this point I have to backtrack a little. The DC link can be a capacitor, inductor, or both. A pure inductor design is current sourcing. Only AB still makes one. You have to have at least output contactors on those to stop a dead short.

Another possible output problem is load reactors where I’ve seen times where one goes bad and the extra reactance is enough to do in the drive.

In multilevel drives often the number of components in the power stack makes it either necessary or prudent to fuse against individual module failure like in a big locomotive drive but usually in the smaller ones tripping the front end is all that is needed and the drive is destroyed anyways.

Occasionally DC links fail especially capacitors with age. Fusing used to be a thing but I haven’t seen it since the 1990s. Large drives distribute the DC link components so the thyrstor fusing does it.

At this point the load by the way is isolated so shorts in loads stop at the DC link at worst.

The front end handles maintaining voltage in the DC link. It can be a passive rectifier; SCR bank; or transistor for the main system with optional precharge and brake chopper. There is nothing “upstream” so an internal fault or self commutation induced by a surge can’t be stopped by the drive. This is where external fuses/breaker/MCP are used along with surge protection. Good protection doesn’t stop drive damage that is already done. It keeps the smoke and parts inside.




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## paulengr (Oct 8, 2017)

There is no doubt that a bad motor puts more stress on a drive. A bad motor can cause an old weak drive part to finally fail but by design that’s not the case:. Most drives have a design 100,000 hour life. Motors if treated right and go to insulation failure last 20-30 years (200,090+ hours), neither of which usually happens but motors can easily outlive drives.


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## tmessner (Apr 1, 2013)

read the manual. I think you always need ocp on the line side. The drive internally provides motor protection on the load side unless you are driving more than one motor.


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## pudge565 (Dec 8, 2007)

gpop said:


> isn't current limit on ab normally short for hardware current limit which is real bad (press reset and something is going to go bang) compared to motor overload which is a basic overload.
> 
> 
> Always cheaper the first time to have the factory tech come out and program the drive. It might cost 1000 dollars but it extends the warrantee and 99% of the time they cover it with no questions asked. (unless the alarm was hardware current limit and there was enough left of the drive to recover the fault log)


Yes, that is exactly what that fault means, you are trying to pull more current than the output of the drive is rated for. We are pretty sure the motors were 75 HP at least but the facility told us they were 60 HP because that is what their records said. The nameplates were covered in metal particulate from the thermite burns they use to melt the metals. I offered to clean them off to see if we could verify and was told not to.


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## gpop (May 14, 2018)

pudge565 said:


> Yes, that is exactly what that fault means, you are trying to pull more current than the output of the drive is rated for. We are pretty sure the motors were 75 HP at least but the facility told us they were 60 HP because that is what their records said. The nameplates were covered in metal particulate from the thermite burns they use to melt the metals. I offered to clean them off to see if we could verify and was told not to.



please don't take this as bitchy but you need to read about the fault in the manual.


You can not program a drive to hurt itself. (well abb you carnt and I have tried, never been challenged to try it on a ab). Hardware over current means something has shorted and the drive could not react in time to protect itself so it faulted.


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## JRaef (Mar 23, 2009)

gpop said:


> ...
> You can not program a drive to hurt itself. (well abb you carnt and I have tried, never been challenged to try it on a ab). Hardware over current means something has shorted and the drive could not react in time to protect itself so it faulted.


Well, sort of. See below*.


But first, in the PoweFlex 7xx drives, there is no "Current Limit Trip" per se. There is "Current Limit", and "HARDWARE Over Current _trip"_, and "Fault, Current Limit Stop", which is not a trip ON Current Limit, it is some OTHER fault that EMPLOYS a Current Limit function and shows up on the display as "Flt CL Stop". They are all separate and different functions. 

"Current Limit" is a programmable function you can enable that means the drive will (try to) NOT allow excess current to be delivered to the load.* When enabled, and the _load attempts to pull more current_ than you want it to, the drive will override your commanded speed value and "fold back" the output frequency in an attempt to keep the current below that setting. This is not a "fault", it is a programmable function and the drive default is disabled, so YOU have to say you want it.

HARDWARE Over Current (HW OC) is a non-programmable trip based on a current sensing system in hardware that protects the transistors from delivering current that will damage them, whether that is a maximum peak value over a period of time or a maximum rate of change value (di/dt) approaching a peak that will cause damage. This can actually be faster than the Short Circuit trip function if it is based on that di/dt function. So if you get a Hardware Current Limit trip, it might be a short in the load, but if you rule that out, then it is a severe overload in a relatively quick step-change. But the fault that is displayed doesn't say anything about "Current Limit", it says "HW OC".


If your drive said "Flt CL Stop" which is the closest thing to what you may have interpreted as a "Current Limit Fault", that is a PURPOSEFUL setting somewhere in one or more of several OTHER protection parameters. When the drive trips from one of THOSE issues, it doesn't just coast to a stop, it will brake OR decelerate the motor AND ALSO apply a current limit to that braking or decel function. It's a very specialized option that someone must purposely select, it is never a default setting.

*In PowerFlex 7xxx series drives, even if you set the Current Limit value to try to not allow the drive to deliver too much current, it does so by folding back the output speed. But if there is a short in the load circuit or even a high step change in load where the motor slip increases too fast for the CL function to react, the HW OC trip or Short Circuit trip will attempt to stop the damage. But even that sometimes can't react fast enough to prevent damage if the issue is a short on the load side. Most high-end VFDs are designed this way (some cheap ones just immediately trip on almost anything). That's the value of a load reactor on a drive, it slows down the rise time of a sharp increase in current on the output so that the drive has time to react to it properly without damage.


Back to the LINE SIDE OC protection. Since 2005, UL has required mfrs to include the MOTOR Over Load _*AND Motor Short Circuit Protection*_ to be part of the VFD UL listing. OL has been required for decades, but not SC. The change came about because the SCPD ahead of the VFD may be much larger than the motor since it has to be the Branch Protective Device for the VFD itself, yet the VFD can be programmed for a much smaller motor, i.e. a 20HP VFD running a 10HP motor. So UL changed their requirements to ensure that the motor is protected based on what you program into the VFD, regardless of the SCPD in front of the drive. What that means is that from UL's standpoint the UPSTREAM device need only be sized for the VFD itself, it's not necessary for it to be based on the motor size. HOWEVER, it's not yet worded that way in the NEC, so if you get an AHJ who is picky, they have on occasion bounced people on this issue.


Whether that SCPD ahead of the VFD is a set of fuses or a CB, that has to do with how the VFD mfr has listed their drive(s). All of them have their drives listed to be protected by fuses, and their manual is required to tell you exactly which fuses are required for their UL file. SOME of the VFD mfrs have also listed their VFDs behind circuit breakers, but because that process is expensive, they will often only do it for THEIR OWN breakers. That too is supposed to be described in their installation manuals. So if the manual does not specifically SAY that you can use a Circuit Breaker, you can't assume it is OK and you will have to use fuses.


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## ceilingwalker (May 25, 2018)

oliquir said:


> most of the time they tell you in the manual the exact very fast fuse to use (often class T) ive seen a lot that people just use standard fuse or breaker and the vfd blows if there is short-circuit since fuses are not fast enough


After reading the PF525 documentation, it recommended a B-curve breaker and overload combo......like a manual starter. I thought it should have quick-blow fuses but went with what AB recommended. Thank you for the reply.


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## ceilingwalker (May 25, 2018)

paulengr said:


> Not if you don’t fix the actual problem. Just replacing what is broke works about 50% of the time but it is NOT troubleshooting. The rest of the time it takes much longer and costs a lot more to fix it, partly because the “repair” destroys all the evidence.
> 
> This attitude is the reason IT morons think that just reboot followed by just reinstall is troubleshooting when both demonstrate they don’t know how to troubleshoot because both of them not only might not fix the problem but more importantly destroy all the evidence! How many times has an IT moron worked on something and not only didn’t fix it but actually made it worse?
> 
> ...


Thank you for the reply. I am talking about circuit protection to the VFD, not the load. I wasn't sure if I was clear enough about that. :smile:


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## ceilingwalker (May 25, 2018)

LBC Jesse said:


> I can't say for certain about NEC codes, but I would highly recommend protection on both the line side of the drive (correct breaker or fuses) and the load side (EOL feeding motor leads for example).
> 
> It's also not as simple as throwing in "X" amp breaker or fuses. The drive's spec book and/ or manual should call out the correct amperage and aic rating required to protect the drive.
> 
> ...


So, are you saying I should provide current protection on the load-side as well, even if it is just one motor connected to it? Thank you


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## ceilingwalker (May 25, 2018)

oliquir said:


> never put fuses on the load side, if one blows, the vfd may blow also !!


Point well taken. Thank you:smile:


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## paulengr (Oct 8, 2017)

ceilingwalker said:


> So, are you saying I should provide current protection on the load-side as well, even if it is just one motor connected to it? Thank you




No. The VFD does that.



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## FF301 (Jan 12, 2014)

Fused disconnect on the power / line side of the VFD. Non fused disconnect at motor for safety ( lock out ) while servicing. ( 50’ and within sight )


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## oliquir (Jan 13, 2011)

FF301 said:


> Fused disconnect on the power / line side of the VFD. Non fused disconnect at motor for safety ( lock out ) while servicing. ( 50’ and within sight )


be sure that the motor disconnect have an early break aux contact so it shutdowns the vfd before breaking power to the motor


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## gpop (May 14, 2018)

oliquir said:


> be sure that the motor disconnect have an early break aux contact so it shutdowns the vfd before breaking power to the motor



Has anyone actually seen a vfd blow due to a sudden loss of load. I have always understood that the aux in the disconnect was less about opening a disconnect on a running drive and more about avoiding shutting the disconnect on a running drive.
I have seen the disconnect opened with no aux thousands of times and on some set ups we have starters on the load side of the vfd for multiply motors which all drop out out the same time so over 15 years plus ive never seen one that i can say failed due to a loss of load. 

The company i work for has over 100 vfds and the main vfd killer is a phase to phase short inside the motor or cap failure due to age (some of our drives are over 25 years old)

Im not saying it doesn't happen especially as there a are a lot of different brands. Im just wondering if certain brands are more likely to fail that others.


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## MDShunk (Jan 7, 2007)

gpop said:


> Has anyone actually seen a vfd blow due to a sudden loss of load. I have always understood that the aux in the disconnect was less about opening a disconnect on a running drive and more about avoiding shutting the disconnect on a running drive.
> I have seen the disconnect opened with no aux thousands of times and on some set ups we have starters on the load side of the vfd for multiply motors which all drop out out the same time so over 15 years plus ive never seen one that i can say failed due to a loss of load.
> 
> The company i work for has over 100 vfds and the main vfd killer is a phase to phase short inside the motor or cap failure due to age (some of our drives are over 25 years old)
> ...


I've had them blow when started into a missing load with some regularity over the years, but I've never had one fail from the load suddenly being missing. Not really sure what the difference is, but there must be one.

I want that local disconnect aux contact going back to the VFD at least as much for monitoring the VFD healthy bit in the PLC so that I can indicate the motor faulted or not on the HMI. Without the aux contact in the local disconnect, the operator would have really no idea that the motor was not running if the VFD was otherwise happy with no other faults.


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## pudge565 (Dec 8, 2007)

MDShunk said:


> I've had them blow when started into a missing load with some regularity over the years, but I've never had one fail from the load suddenly being missing. Not really sure what the difference is, but there must be one.
> 
> I want that local disconnect aux contact going back to the VFD at least as much for monitoring the VFD healthy bit in the PLC so that I can indicate the motor faulted or not on the HMI. Without the aux contact in the local disconnect, the operator would have really no idea that the motor was not running if the VFD was otherwise happy with no other faults.


Some places are hesitant about placing a control voltage aux contact in a disconnect with the line voltage for the motor. The food plant I worked in the programming department had an old PLC5 input card that was blown out because water got into the disconnect and sent 480v to the 120v input card...

Don't get me started on the water in disconnects issue, I had a simple fix of placing a drain in the disconnect (as you'll never keep water out of everything when you give the plant workers 400 PSI water) but my maintenance manager wanted me to find the cause and eliminate it...


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## gpop (May 14, 2018)

MDShunk said:


> I've had them blow when started into a missing load with some regularity over the years, but I've never had one fail from the load suddenly being missing. Not really sure what the difference is, but there must be one.
> 
> I want that local disconnect aux contact going back to the VFD at least as much for monitoring the VFD healthy bit in the PLC so that I can indicate the motor faulted or not on the HMI. Without the aux contact in the local disconnect, the operator would have really no idea that the motor was not running if the VFD was otherwise happy with no other faults.



We use alot of rotation sensors, flow proven devices and load loss output from the vfd. This wasn't a alternative to a aux contact it just works that way.


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## gpop (May 14, 2018)

pudge565 said:


> Some places are hesitant about placing a control voltage aux contact in a disconnect with the line voltage for the motor. The food plant I worked in the programming department had an old PLC5 input card that was blown out because water got into the disconnect and sent 480v to the 120v input card...
> 
> Don't get me started on the water in disconnects issue, I had a simple fix of placing a drain in the disconnect (as you'll never keep water out of everything when you give the plant workers 400 PSI water) but my maintenance manager wanted me to find the cause and eliminate it...



We must be working together. 

We install panel drains or accidentally put a little cut in the bottom seal.

"If you can not make it water proof make it leak"


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## MDShunk (Jan 7, 2007)

pudge565 said:


> Some places are hesitant about placing a control voltage aux contact in a disconnect with the line voltage for the motor. The food plant I worked in the programming department had an old PLC5 input card that was blown out because water got into the disconnect and sent 480v to the 120v input card...
> 
> Don't get me started on the water in disconnects issue, I had a simple fix of placing a drain in the disconnect (as you'll never keep water out of everything when you give the plant workers 400 PSI water) but my maintenance manager wanted me to find the cause and eliminate it...


That is a perfect description of what I deal with every single working day now. I'm a huge fan of water getting inside of things. It makes my personal finances more secure.


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## pudge565 (Dec 8, 2007)

MDShunk said:


> That is a perfect description of what I deal with every single working day now. I'm a huge fan of water getting inside of things. It makes my personal finances more secure.


Yea, that manager ended up canning me... I don't tolerate stupid very well and that place had plenty of that in management. Trying to actually do my job was so damn difficult...


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## tmessner (Apr 1, 2013)

Definition of water tight in the electrical dictionary: Water in, not out.


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## JRaef (Mar 23, 2009)

ceilingwalker said:


> So, are you saying I should provide current protection on the load-side as well, even if it is just one motor connected to it? Thank you


The only time you need to add OL protection on the load side of a VFD is if you are feeding more than one motor from the VFD, because the VFD can only see the total current, it has no idea what's happening in any individual motor if there is more than one. 20+ years ago some of the cheaper Asian VFDs used to be sold without motor OL protection, so you had to add it in the field. But starting in 2001 UL began requiring the VFD to have OL protection and then in 2005 they added the requirement for the VFD to provide motor short circuit protection as well. 

So all you need in the upstream Branch Short Circuit Protection Device, fuses of CB. You need to folloow what the VFD mfr says in the installation manual, because that will be tied to the "SCCR" listing of the drive, in that it will be tested and listed with SPECIFIC devices as outline in the instructions. Nothing else matters.



gpop said:


> Has anyone actually seen a vfd blow due to a sudden loss of load. I have always understood that the aux in the disconnect was less about opening a disconnect on a running drive and more about avoiding shutting the disconnect on a running drive.
> I have seen the disconnect opened with no aux thousands of times and on some set ups we have starters on the load side of the vfd for multiply motors which all drop out out the same time so over 15 years plus ive never seen one that i can say failed due to a loss of load.
> 
> The company i work for has over 100 vfds and the main vfd killer is a phase to phase short inside the motor or cap failure due to age (some of our drives are over 25 years old)
> ...


It used to be more common when drives were made with separate Darlington transistors.The issue is that when you open a set of contacts under load, the arc that forms begins to act like a capacitor and as the PWM firing continues to take place as the gap widens, that capacitor looks like a tank circuit that pumps up the charge to where you get a very high voltage across the contacts. That then becomes a "dv/dt" problem for the transistors; too high a a rate of change in voltage, and the silicon can fuse. So 25 years ago yes, you could blow the transistors the first time someone opened that disconnect while the drive was running. 

But about 15 years ago virtually everyone switched to using the smaller-cheaper-faster IPMs (Intelligent Power Modules) that have all of the power components and their firing circuits integrated into a single unit. The mfrs of the IPMs started adding flyback diodes to the transistor circuits to be able to handle the voltage kickback that might happen when you open a circuit under load on the output side. That's not however a perfect "cure", it just lessens the chances of a failure the first few times it happens. If you keep doing it repeatedly, you will eventually fry _those _devices, then the transistors follow shortly thereafter. Sometimes people only do it in emergencies and that's probably fine, but the problem stems from the real world fact that once an operator gets used to just using the knife switch at the motor to shut it down, they tend to keep doing it that way until something fails. If you use the aux. contacts, it never matters. 

Then of course having the aux contacts ALSO prevents someone from closing the switch AFTER the drive has already been told to run. Again, in the earlier days, simple V/Hz drives would run along just fine, not even knowing that the motor was not connected. The latest generation of VFD however now use "SVC", which uses a feedback signal into the drive based on motor current, so no current means the motor is not running and the VFD will trip off line. So you can't really close that local switch into a running drive any more, because the drive wont be running as soon as it notices there is no current. That however may take a few seconds, so it's still POSSIBLE, given a "'better idiot"* situation, to get around those safety measures.


_* Stemming from the joke that says "If you build something to be "idiot proof", the universe responds by creating better idiots!"_


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## gpop (May 14, 2018)

That makes more sense thankfully i haven't seen to many of the older drives. 
Thanks JRaef.


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## paulengr (Oct 8, 2017)

The anti parallel diodes aren’t protection or optional. MOSFETs to some degree and especially SCRs have reasonable and good reverse bias blocking but IGBT blocking is almost nonexistent. Not just an IPM thing. They’re required on IGBT designs.


Sent from my iPhone using Tapatalk


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