Multiple motors on a VFD

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jasonmartin1313

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When installing multiple motors (1HP) behind a single VFD is individual short circuit protection required. It is clearly spelled out that individual over current protection is required. Is line side fusing acceptable or the reliance on the VFD to detect short acceptable and if either or true or not true where does the code address this? I have read several places online, specifically Rockwell literature, where it states individual short circuit AND individual over current protection are required. Thanks in advance.
 
If all motors are under loaded except one how could the common system tell? Need overload relay on each one. Electronic overloads are so cheap these days I don’t understand why it would be questioned. Just run them all in series back to the VFD as an enable input.
 
Correct not questioning the overloads at all, it’s the individual short circuit detection that I am questioning. Does code require it or not. Currently only have fusing ahead of the vfd.
 
Paul - I believe he was asking about short circuit protection. He stated that it is clear that individual overload protection is required...

To the OP, how would individual short circuit protection even be accomplished? An additional breaker for each on the output? It would be news to me if you needed something like this. I’ve done several installs with a single VFD and multiple motors. Just needed overloads for each motor.
 
Right. The common breaker should end up providing short circuit protection equally well for all of the individual moior circuits. If you have long motor runs with small enough wire that a bolted short circuit at the far end will not trigger the instant trip of the breaker, then you have a serious design problem.
 
Most VFD's would trip on short circuit or ground fault to protect the drive from the fault current sooner than most any aux devices you might install after the drive.
 
ok I know that the VFD will detect the fault/short and I know that the line side fusing would also protect, but I couldnt find a place where it said that was acceptable or not. I was treating them more like a branch circuit. I will look at 450.52 C/D once more. Thanks for all the input.
 
Line side protection IMO is to protect the line side conductors. Drive may specify a specific fuse type if available fault current is above a certain level, but really about all that does is limit how spectacular a failure of the input rectifier may be to witness, the input rectifier is toast regardless what kind of overcurrent protection was provided. Load side faults the drive usually will respond well before any external overcurrent device will. All that is needed is individual overload protection when there is multiple motors.
 
So many have argued that a VFD is designed to protect its given motor for equal short circuit protection, ie a 10HP drive will protect a 10hp motor, and the line side fuses are sized accordingly. As soon as i have ten 1hp motors behind that VFD the short circuit protection is thrown into the wind. The available current and energy available for said event is 10x the rating acceptable for that 10hp motor, thus individual short circuit protection, ie MPCB's , IS required. To further that discussion an example of a single strand of wire across L1 L2 on a motor was given, the VFD wont even start the motors, however when all the motors are up and running at 100%, try that same experiment and look out. I thought this was an interesting comparison, necessitating the requirement. It is definitely a debated topic between multiple panel shops, code inspectors, even UL.
 
jasonmartin1313 said:
So many have argued that a VFD is designed to protect its given motor for equal short circuit protection, ie a 10HP drive will protect a 10hp motor, and the line side fuses are sized accordingly. As soon as i have ten 1hp motors behind that VFD the short circuit protection is thrown into the wind. The available current and energy available for said event is 10x the rating acceptable for that 10hp motor, thus individual short circuit protection, ie MPCB's , IS required. To further that discussion an example of a single strand of wire across L1 L2 on a motor was given, the VFD wont even start the motors, however when all the motors are up and running at 100%, try that same experiment and look out. I thought this was an interesting comparison, necessitating the requirement. It is definitely a debated topic between multiple panel shops, code inspectors, even UL.
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If you said ten 100 HP motors you might have a point. On ten 1 HP motors the energy stored in those motors is so low that fault current standards say to ignore them.

The issue really isn’t the loads though. Since the ampacity is for the 10 HP motor we’d be wiring the wire sized for 10 HP to the 1 HP motors. A dead short is a dead short either way. So the example makes no difference at all.

The issue is your 10 HP microdrive has usually a 5 kA short circuit rating. If the line impedance is around 3% at roughly an 8-10 kva rating, the drive can easily survive it. But if it’s connected to a 75 kva or larger transformer (rule of thumb 10x kva rating) we will easily exceed 5 kA and the drive is toast. The solution is to add a 3% line reactor or semiconductor fuses.

This is not debatable. UL says install per manual and this information is in the manual. Drive manufacturers all have goofy ways of stating this information and sometimes it’s written in Chinglish, but the general principal works on any VFD.

There is nothing to debate for panel shops. UL 809A and now even NEC requires SCCR. Local inspectors are looking for it. Too many blown up panels, and too much shrapnel ejected through the covers as transistors explode have forced the industry to change.
 
jasonmartin1313 said:
So many have argued that a VFD is designed to protect its given motor for equal short circuit protection, ie a 10HP drive will protect a 10hp motor, and the line side fuses are sized accordingly. As soon as i have ten 1hp motors behind that VFD the short circuit protection is thrown into the wind. The available current and energy available for said event is 10x the rating acceptable for that 10hp motor, thus individual short circuit protection, ie MPCB's , IS required. To further that discussion an example of a single strand of wire across L1 L2 on a motor was given, the VFD wont even start the motors, however when all the motors are up and running at 100%, try that same experiment and look out. I thought this was an interesting comparison, necessitating the requirement. It is definitely a debated topic between multiple panel shops, code inspectors, even UL.
Click to expand...
Drives protect the motor from overload according to what you set in motor overload parameter settings.

When there is a ground fault on load side any reaction from the drive is going to be to protect the drive more so than the motor.

I do agree a detected ground fault at startup will result in much lower current and immediate trip of the drive but introduce a short circuit or ground fault to the drive output when motor is running closer to full volts/frequency and that fault can be more spectacular to witness, how the drive responds and how well it can take it likely will depend on how the drive is designed to some extent as well as some the factors of the installation. Installation with load side reactor may take it better than same setup without a load side reactor.
 
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