# VFD or RPC? proper sizing help



## MacCallum Electric (Sep 26, 2016)

Hey all,

im looking at a job for a fella and hes setting up a production facility. The property has an existing 400A single phase service. He has one machine that has three 3 phase Motors 7.5kW, 4.0Kw, and 0.75 kilowatt all @220v. there is a small control panel that has the on/off for the 2 largest motors. These are self starting i believe and all will be started seperately. i realize the .75kw motor will have to be hooked up seperately with a cut off. also there is a 3hp 3PH motor for the refridgeration unit on route however we dont know what V it is at. all of these motors are single speed and customer has no desire to adjust speed.

this property currently cannot get 3ph power however it will be there in 1 year so this is a temporary setup until we change over the service.

coverting those KW to HP we get 10, 5.5, and 1 respectively. and with addition of a 3Hp motor puts the total at 19.5HP. 

never working with vfds or rpcs before im wondering whats the proper size and what unit is best for this setup? 

my inital idea was to take 100A off the 400A service into a 20Hp RPC feeding a 100A 3Ph panel then wire each motor seperately from the panel to cutoff to motor. 

now thats what i thought. worrying about if it should be 22hp or 25hp to be safe ? guy at supplier started going on about vfds and made it seem way more complicated. what would you guys do for sizing and choice between rpc and vfd?

i will add a few pictures of the machines and motors,control panel and plates.

THANKS

Alex


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## MacCallum Electric (Sep 26, 2016)

so heres some of what i got. in the picture of whole machine you see the on off for the 2 biggest motors.(also realize that we need new wiring between motors and control panel). also you see inside control panel as well as the plates of the 3 motors on the machine.


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## John Valdes (May 17, 2007)

Sorry. I did not read your post correctly.
If each motor needs its own control, then you will need a drive for each motor.
You must add up the current and price it both ways.


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## just the cowboy (Sep 4, 2013)

*Phase convertor*

If the motors were under 3 hp you could get single phase suppy VFD's but when you get over that you have to upsize the drive.
Since you don't need the speed control I would go with a LP 3 phase gas generator. If you go with the generator it just feeds all machines that need it, then when 3 phase comes replace feeders to panel and you are done. This way you don't have to mess with each machine. Also if they add more machines you have it ready. 

I would go with bigger may be better, you can then use it as backup power for outages or sell it.


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## telsa (May 22, 2015)

VFDs at your scale go for _*peanuts*_ out of Red China -- a perfect match for your motors, BTW.

Just remember that you have to up size the VFD 'one bump' as you're feeding them single phase power.

Once given the opportunity to dial in 'perfect' RPMs -- the owner will never look back.

It will become a feature that he insists upon ever afterwards.


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## Big John (May 23, 2010)

You could use VFDs for this, but if you use a single big drive for phase conversion you can't have motor contactors downstream or you will cause the drive to fault.

So in your case each motor needs it's own drive, and each drive needs to be sized at 200% of the motor rating. This handles the 173% phase conversion plus inefficiencies in the drive itself.

The advantage of using drives is that you no longer need additional motor controllers like you would with an RCP, and you can also get much better voltage balance.


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## JRaef (Mar 23, 2009)

Big John said:


> You could use VFDs for this, but if you use a single big drive for phase conversion you can't have motor contactors downstream or you will cause the drive to fault.
> 
> So in your case each motor needs it's own drive, and each drive needs to be sized at 200% of the motor rating. This handles the 173% phase conversion plus inefficiencies in the drive itself.
> 
> The advantage of using drives is that you no longer need additional motor controllers like you would with an RCP, and you can also get much better voltage balance.


I don't disagree with anything Big John said here, but to further this though just a little:

You would use separate VFDs to _REPLACE_ the motor starters you have now; it's not a good idea to have the starters in the circuit at all, whether it's 3 starters behind one VFD or one starter behind one VFD. It can be done if done right (and is), but it's not for the novice. There are some potentially costly pitfalls.

Aside from the fact that this is how I and several others in here would do it, in deference to your implied relative inexperience in using VFDs as phase converters, *I'll actually vote for an RPC here*. You can simply install it ahead of a small 3 phase panel and feed all of your 3 phase machines with one unit. An RPC is inefficient, so if this were permanent I might think otherwise. But for a year, I'd say go find a low cost RPC and leave all of the existing controls the same, then sell the RPC when you get your real 3 phase and the difference is your "rental cost". 

Just one thing to think about on that however. If your control panel gets its control power from a transformer, make sure the transformer is tapped off of the REAL 2 phases, not the manufactured phase from the RPC.


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