VFD disconnect clarification needed

Q

Questions3000

Member
Location
Chicago
Occupation
Electrician
Hey everybody,

I've got a 10HP motor (fed from 3 phase 208V Y panel) for a bag filter/dust collector that I'm hooking up (Nederman S1000). After reading through all the installation manuals I am now confused.

The Nederman S1000 (dust collector) comes with a VFD (with a start/stop button and programming screen).

According to the dust collector manual I would need an 80A inverse time breaker or a 60A dual element time delay fuse (fused disconnect).

FLA- 26.3A

x 250%= 65.75 (80A breaker inverse time breaker)
x 175%= 46.025 (60A time delay fuse)

*if this was just a basic contactor setup with start/stop there'd be no questions, it's the addition of the VFD that is putting me into analysis paralysis

Now according to the VFD manual for a 10HP motor I need to setup a fused disconnect for the VFD with 40A high speed class J fuses.

The question,

If I provide a fused disconnect with 40A J Class fuses for the VFD, am I still sizing the breaker at 80A? Does the breaker need to be an inverse time breaker? (If I were to guess, the answer would be yes for both)

Between the VFD and the motor do I still need to put in another fused disconnect for the motor with 60A time delay fuses? (Motor, vfd and all disconnects would be within close sight of one another) Or will the single fused disconnect for the VFD (40A class J fuses) suffice?
 
Depends on the situation. We had dozens of AHU'S from 40 to 125 HP that all had a non fused safety switch inside of the air handler. None had a NC micro switch to open before switch contacts but the electricians & mechanics were taught to only open these safety switches after turning off drives & LOTO power to drives before opening load side safety switches. A few months ago somebody posted that some drive manufacturers provide protection if a load side switch us turned off while their drives are running.
 
You are required, by code, to follow the manufacturer's installation instructions, even if you think there is a conflict with some other code article. Instructions trump all.

Besides, the 250 or 175% rules are MAXIMUM, there is nothing wrong with less.

Worth noting here that in addition, the CONDUCTORS feeding the VFD must be rated for 125% of the VFD Maximum Input Current, not the motor current. The MOTOR leads (VFD to motor) must be sized per the motor FLC table. It generally works out that the inpout conductors could be smaller, but you have to double check to make sure.
 
Jraef said:
You are required, by code, to follow the manufacturer's installation instructions, even if you think there is a conflict with some other code article. Instructions trump all.

Besides, the 250 or 175% rules are MAXIMUM, there is nothing wrong with less.

Worth noting here that in addition, the CONDUCTORS feeding the VFD must be rated for 125% of the VFD Maximum Input Current, not the motor current. The MOTOR leads (VFD to motor) must be sized per the motor FLC table. It generally works out that the inpout conductors could be smaller, but you have to double check to make sure.
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Good luck with majority of foreign made equipment. We had foreign made $10 Millon state of the art machine that had no instructions other then power & air requirements. The machine could be run if one of the stop buttons was held in and start button pushed in. German tech said that was not a problem.
 
Jraef said:
You are required, by code, to follow the manufacturer's installation instructions, even if you think there is a conflict with some other code article. Instructions trump all.

Besides, the 250 or 175% rules are MAXIMUM, there is nothing wrong with less.

Worth noting here that in addition, the CONDUCTORS feeding the VFD must be rated for 125% of the VFD Maximum Input Current, not the motor current. The MOTOR leads (VFD to motor) must be sized per the motor FLC table. It generally works out that the input conductors could be smaller, but you have to double check to make sure.
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On several important pieces of equipment my boss liked to upside a VFD to next size. On a 50 HP would go with a 60 HP drive. We would downsize input fuses inside of drive and adjust the overload relay to motor ampere. No way did we ever increase the wires feeding such drives. Thinking was drives would last longer especially if in warm environments. Did the same thing years ago with motor starters especially where motors started & stopped & even reversed frequently.Did notice that on 75 & 100 HP lab exhaust fans that took care of three floors could have a JCI or Siemens BSO system sending a 100% run signal but input ampere was 5 to 10% less then motor nameplate ampere.
 
I'd be inclined to just use the 40A fuses and be done with it. In the real world it won't matter any. The reality is the VFD input will never get anywhere near the motor current in most cases.

I would not be oversizing anything either. Modern drives just do not need it. Decades ago it was kind of common because a lot of drives were marginal as far as heat dissipation went but modern ones tend to be pretty good about such things.
 
Jraef said:
Ok, good point
Click to expand...
Jraef said:
Ok, good point
Click to expand...
Jraef said:
Ok, good point
Click to expand...
Often wonder the education, field experience , degrees etc the person who writes equipment instructions. I know they do not have to follow the NEC. Back in the early 1980's we called out a VFD manufacturer who stated that we must use at least 2/0 copper to feed a 480 volt 100 HP motor. They only used #4 copper inside of the drive which of course added heat inside if the drive.
 
Questions3000 said:
According to the dust collector manual I would need an 80A inverse time breaker or a 60A dual element time delay fuse (fused disconnect).
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This sounds about right if you would happen to have across the line contactor for a control.

A VFD controller will never need such high overcurrent protection as it will never have such high starting current, a VFD would never "start" with 60 hertz at full voltage being a major reason.

Bad instructions IMO.
 
kwired said:
This sounds about right if you would happen to have across the line contactor for a control.

A VFD controller will never need such high overcurrent protection as it will never have such high starting current, a VFD would never "start" with 60 hertz at full voltage being a major reason.

Bad instructions IMO.
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Years ago while we installed two 100 HP 480 volt drives to replace starters only had 150 amp time delay fuses in bench stock so installed them thinking we would replace them when 200 amp fuses came in. Motor nameplates for these four pole plain Jane NEMA frame motors was 124 amps. Set ramp up time to 20 seconds and ten years later the 150 amp fuses were still in service. Was fun to install a Amprobe onto one of the line wires feeding a VFD and watch the ampere slowly increasing. Old school and liked to use the smallest quality time delay 200KIAC fuses. Most of the time only went 125 to 140% over nameplate ampere. On air compressors that restarted at 140#'s & Centrifuges that took over a minute to reach full speed would need fuses at 175%.
 
NEC requirements are based upon art 430 and pertain to protecting the building's wiring. not the VFD. (i'm going to calculate assuming this application is not continuous duty)

10hp @ 208v = 30.8a x 2.5 = 77a max inverse time breaker
10hp @ 208v = 30.8a x 1.75 = 54.5a max time delay fuse
30.8a = #10thhn

the NEC requires you to install the minimum wire size of #10 thhn CU for a 10hp 208v 3phase motor, with MOCP device to protect the wire. this is so we ensure the building's permenantly installed conductors are protected.

now, the VFD is requiring you to provide EXTRA protection for the VFD itself, by installing more stringent fuses that are no higher than 40a and are fast-acting. you can put these fuses right next to the VFD, or at the beginning of the branch circuit. up to you. personally, i'd put them right before the VFD, either in a disconnect, or inside of the control cabinet or VFD enclosure. that way, if the VFD needs swapped out, you can change these fuses to whatever the next one requires.

this of this sort of like overload protection for a specific motor, when it gets changed out. the NEC-required wiring for the building stays the same as long as the horsepower stays the same. but the service factor on the new motor, may be different, so the overload protection needs to change. in this case, you're providing fault protection for the VFD. and the VFD itself will protect the motor with overload proctection (as long as you have it programmed correctly.)

does this help?
 
Master Nater said:
NEC requirements are based upon art 430 and pertain to protecting the building's wiring. not the VFD. (i'm going to calculate assuming this application is not continuous duty)

10hp @ 208v = 30.8a x 2.5 = 77a max inverse time breaker
10hp @ 208v = 30.8a x 1.75 = 54.5a max time delay fuse
30.8a = #10thhn

the NEC requires you to install the minimum wire size of #10 thhn CU for a 10hp 208v 3phase motor, with MOCP device to protect the wire. this is so we ensure the building's permenantly installed conductors are protected.
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Motors are treated as continuous duty unless they have a marked duty cycle, then you can follow allowances for whatever that duty cycle is when it comes to conductor size selection, overcurrent and overload protection.

Max OCPD you mentioned is allowed to apply next size up standard overcurrent device from the calculation result. Even then there are allowances to possibly increase beyond those settings if the motor will not start.

Conductor protection: yes we are protecting the conductor with the branch circuit device but most cases we are only providing short circuit/ground fault protection. The required motor overload protection is selected to protect the motor, but inherently also protects the conductors from overload.

Master Nater said:
now, the VFD is requiring you to provide EXTRA protection for the VFD itself, by installing more stringent fuses that are no higher than 40a and are fast-acting. you can put these fuses right next to the VFD, or at the beginning of the branch circuit. up to you. personally, i'd put them right before the VFD, either in a disconnect, or inside of the control cabinet or VFD enclosure. that way, if the VFD needs swapped out, you can change these fuses to whatever the next one requires.

this of this sort of like overload protection for a specific motor, when it gets changed out. the NEC-required wiring for the building stays the same as long as the horsepower stays the same. but the service factor on the new motor, may be different, so the overload protection needs to change. in this case, you're providing fault protection for the VFD. and the VFD itself will protect the motor with overload proctection (as long as you have it programmed correctly.)
Click to expand...
The fast acting fuses are so if the front end of the drive has a catastrophic failure, it will interrupt the high level current quicker than the typical time delay fuse or inverse time breaker. Many drives I have dealt with in recent years tell you in the instructions that such fuses are needed when the available fault current is above a certain level. Basically if the front end of drive has such failure, the drive is toast, all that high speed fuse does is help keep the resulting incident energy down if the available fault current is high and the resulting destruction may be less of a fireworks show than if you did not use those high speed fuses.
 
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