# powering 3 motors using 3 VFDs through a single CB



## Zylonite (May 15, 2019)

We have 3 2HP motors that need to be synchronized with motion and position. We have installed 3 VFDs that can handle motion and positioning, but the power conductors are protected with a single 3-pole CB and then the power conductors are daisy chained on the VFD inputs. 

Is that a safe practice or does each set of conductors (3 wire) going to the VFD need their own 3-pole CB? We need the whole circuit to shut off if one motor pulls too much current. Also we are installing overloads for each motor, but right now I just want to know if a single CB is enough for the whole circuit. 

FLA for each motor is about 7A, so we are using a 30A CB (240VAC).


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## oliquir (Jan 13, 2011)

check manual of the vfd they will tell you what size breaker or fuse to use, secondly, no overloads on the output of a vfd (only time they are needed is when more than one motor is connected to one vfd)
vfd should not be stopped by removing power (unless it is an emergency), just use the stop function on them for stopping, for e-stop you can use a contactor on the line side of vfd


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## pudge565 (Dec 8, 2007)

Zylonite said:


> We have 3 2HP motors that need to be synchronized with motion and position. We have installed 3 VFDs that can handle motion and positioning, but the power conductors are protected with a single 3-pole CB and then the power conductors are daisy chained on the VFD inputs.
> 
> Is that a safe practice or does each set of conductors (3 wire) going to the VFD need their own 3-pole CB? We need the whole circuit to shut off if one motor pulls too much current. Also we are installing overloads for each motor, but right now I just want to know if a single CB is enough for the whole circuit.
> 
> FLA for each motor is about 7A, so we are using a 30A CB (240VAC).


The breaker should never open on a motor overload as it is there for ground fault and short circuit protection only in a typical motor install. The overload protection is taken care of via the VFD.

You would use an enable circuit for what you are trying to do. You run that enable circuit through NO contacts on each VFD and set that contact to close when the OL is not tripped.

No offense, but from this and other posts you made it sounds like you are in over your head and should be hiring a controls company to set this system up for you, they will have the experience necessary to make this happen with no damage to equipment or personnel.


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## gpop (May 14, 2018)

As pudge and other have said you are in over your head. If a drive shuts down due to one of a hundred different problems the others need to stop. 

As for the breaker that's fine as long as you have fuses that are rated to the manufacturers specs between the breaker and drives (and the wire is correctly sized)


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## frenchelectrican (Mar 15, 2007)

Zylonite said:


> We have 3 2HP motors that need to be synchronized with motion and position. We have installed 3 VFDs that can handle motion and positioning, but the power conductors are protected with a single 3-pole CB and then the power conductors are daisy chained on the VFD inputs.
> 
> Is that a safe practice or does each set of conductors (3 wire) going to the VFD need their own 3-pole CB? We need the whole circuit to shut off if one motor pulls too much current. Also we are installing overloads for each motor, but right now I just want to know if a single CB is enough for the whole circuit.
> 
> FLA for each motor is about 7A, so we are using a 30A CB (240VAC).


Do not install the overload blocks at all due the VSD will take care of it. 

I think you should back this out a little and let someone more experiniced one to deal with this set up so you can watch as you learn along the way.

you will need a comm cable to tie all three VSD so they can work as one unit. and any fault show up it will shut down all of it. again have someone who is well verised with this set up.


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## tates1882 (Sep 3, 2010)

Personally I would have ran the three motors under one Vfd with separate OLs for each motor.


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## Zylonite (May 15, 2019)

tates1882 said:


> Personally I would have ran the three motors under one Vfd with separate OLs for each motor.


The motors need to be sync'ed and the PLC needs feedback control from each motor. So it's impossible to run it through a single VFD.


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## gpop (May 14, 2018)

Zylonite said:


> The motors need to be sync'ed and the PLC needs feedback control from each motor. So it's impossible to run it through a single VFD.


Now we are starting to get some of the details that should have been in the original post. 

So we now have a plc and feedback from the equipment

still put a bet that most of us controls guys could do it with one vfd and 3 starters if we were ever challenged to do it.


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## Zylonite (May 15, 2019)

gpop said:


> Now we are starting to get some of the details that should have been in the original post.
> 
> So we now have a plc and feedback from the equipment


yes we have a PLC and encoders.


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## gpop (May 14, 2018)

Zylonite said:


> yes we have a PLC and encoders.


So keep it simple.

Breaker 30 amp. #10 wire to 3 finger safe fuse blocks. 
fuses sized by drive manufacture
#12 wire from fuses to drive

1 e-stop field mounted button to e-stop relay.
drive enable inputs via e-stop relay

plc input from each drive (hardwired or network)
run signal (not a at speed signal)
fault signal
inputs from encoders
maybe 3 home switch's depending on the encoders

plc outputs (hardwired or network)
run signal
forward/reverse signal
Analog for speed control. 

Programming requires someone with a good understanding of pid loops

Add buttons, switches or a hmi


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## frenchelectrican (Mar 15, 2007)

Zylonite said:


> yes we have a PLC and encoders.


Ahh that clear up that part.,,

Follow Gpop listing above me that is pretty much clear cut on that set up. 

Just prepare to get some nonstandard parts but basically you should able deal with this pretty good once you understand the set up.


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## JRaef (Mar 23, 2009)

The breaker protects the conductors feeding the VFDs, so if you have sized the conductors appropriately, there is no problem. The VFD manual will list the MAXIMUM size OCPD (breaker or fuse) that the drive can have upstream. Anything that size or smaller is fine. Take note however, a lot of small VFDs like this are NOT going to be listed to be used behind breakers at all, they might require fuses. RTFM.

The VFDs are required to protect the motors, not the breaker. If it is a UL / cUL listed drive, each VFD will have _*both *_the Short Circuit and Overload Protection built into it that is programmed based on the motor data that you enter; that became a requirement for listing in 2005. 

If you are using some kind of cheap no-name Asian drive bought off of a website, you are playing with fire anyway and all bets are off as to what protections they do or don't provide.


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