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.
 
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