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What size Rotary Phase Converter do I need?

AmericanMaker

Aluminum
Joined
Nov 23, 2020
Location
Franklin, TN
The new VMC I am getting has a 15 HP spindle. I need an RPC to run the machine in my garage. The seller/mfg of the RPC I 'm interested in buying says I need an RPC that is double the spindle HP? That seems like overkill to me. Do I really need a 30 hp RPC for a 15 hp spindle?

The VMC is this 2011 Sharp SV-2412 SX .
sharp.png
 
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My 2 cents.. Not necessary on a mill.. On a lathe, yes...

Why? On a mill, you can control the horsepower you need, if you've ever
run a small knee mill using modern cutting tools, you do have to back figure
for HP, 2 or 3 horses.. You can do the same on a VMC,, keep it under 10 or
something, and honestly a 10hp isn't super common, though its not way out there
either.

On a lathe, you have a Huge Giant chuck, that weighs a lot, a long big heavy
spindle.. That sucker takes a TON of power to get spinning....

My RPC experience, I've run a 15hp VMC on a 5hp RPC and it was fine, I just
had to keep the cuts to a minimum (the 20hp was out getting fixed).. I've
also run a 25hp(20 continuous) lathe on a 25hp RPC, and I couldn't run it in
high gear, even after stretching the spindle ramp out to 10 seconds..

On a VMC you don't have that huge power spike trying to spin the spindle up.
Yeah, there is a spike, but a VMC spindle is absolutely tiny and light compared
to a lathe spindle and chuck.

Of COURSE the guy trying to sell you the RPC is going to up sale you..
 
The new VMC I am getting has a 15 HP spindle.
The seller of the RPC I 'm interested in buying says I need an RPC that is double the spindle HP?
That seems like overkill to me. Do I really need a 30 hp RPC?
The VMC is this 2011 Sharp SV-2412 SX .

View attachment 372915
What size doo you propose getting ?
And if it proves undersized ? Who's fault will it be ?

Maybe you could spend some time reading ?
 
If the rpc you have is too small add another unloaded motor or two into the circuit. A disk sander or dust collector fan is a common add on for increased hp. Or just buy a used 3 phase motor and tuck it away somewhere and switch it in as needed.
Bill D
 
I have a 15HP RPC for my 7.5 HP Lathe, wouldnt want any less.

 
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What does your power supply look like? How many KVA is your transformer? What size/material is the wire feeding your breaker panel? What is the largest breaker ampacity your panel will accept?

If anything before your RPC is a choke point then there's no point going bigger than what the smallest component will support. Like if you have a tiny transformer you won't be running any machines regardless of whether your power company says you have a 200 amp service.

30HP RPC is a fair match for your standard 200 amp single phase breaker panel. 30HP RPC is good for pulling 100 amps single phase, maybe a bit more. 100 amps is usually the largest breaker you can put in a 200amp panel. That's good for 50+ amps 240 3 phase.

When sizing an RPC you gotta realize all it's doing is making one leg of power and the bigger the RPC, the more resilient that generated leg will be.
 
well the long and the short of it, you want double. minimum.
15 hp on a 30 is good due to the inductive loads and trying to keep the phases "clean" as you also have to think about if the motor powers down, it acts like a generator pushing power back where it came from which can spike the voltage
so yes you need 30HP which also needs a 125A breaker which are ususally around $500 by them selves to drive it.
its expensive to hook these things up. its even more expensive to hook them up wrong.
 
So if you are getting this Vertical Machining Center,
think about what is going on here for a second.
You have 3 phase AC going in to the spindle drive,
which is a VFD. All VFDs convert the incoming AC
to DC trough full wave rectifiers, then filter the DC
ripple through electrolytic caps. Then they make
pulsed AC 3 phase using transistors or IGBTs, what
ever, doesn't matter.
Anyhow, if you only feed the VFD 1 phase and it
rectifies the current to DC, what you get is one
bridge rectifier taking all the load, and lots of ripple
in your DC. You can use a large HP VFD on 1 phase
AC, but you need a really large rectifier and some
really large capacitors for smoothing.
So if you are using an RPC to make 3 phase for your
VMC, then you are feeding the 2nd and 3rd rectifiers
your manufactured phases. All good. BUT if your
manufactured phases are a little weak, current wise,
from a conservatively sized RPC, the 1st power company
supplied stage in the rectifier takes a bit more load.
The gamble is hopefully, the extra current is not enough
to kill it (magic smoke), but if you are supplying the other
2 phases, be it maybe a but weak, the 1st stage in the
rectifier can usually handle some moderate overload.
There is a safety factor after all. But the tolerance to
handle a slightly weaker RPC and still maintain a reasonably
small DC rectified ripple, and not overload the input
rectifier, is actually pretty good. My opinion based on what
I have seen. If you already have a smaller RPC I would say
try it. If ripple is too much, most VFDs will fault out.
The advice to double the RPC is good advice.
If your VMC is 15hp, a 30 hp RPC is best.
But I really think if you had a 20hp RPC, I would try it
without fear of hurting something. If you have to spend
money anyways, get the larger one, to make sure it works.

-Doozer
 
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In my millwork shop, i have a 400 hp generator wired for 480, and a 30 hp Phase-a-Matic RPC providing a 240V 3 phase output.

A transfer switch is fed by the RPC and a step down transformer, allowing me to run my 240 3 phase equipment on either the RPC or the generator (via the step down transformer). All of my 480V equipment requires the generator.

One machine, a 15 hp two blade edger, will edge 2" pine ok being fed from the RPC but if we try to run 2-1/4" oak it will frequently stall out. So when we need to run thick oak thru the edger we supply it from the generator. When placed under heavy loads, there is a noticeable difference between motor power on my higher HP 240 3 phase equipment when being fed by the generator versus the RPC. I ran a 22.5 hp wide belt sander for years off of the RPC, and have a 10hp air compressor.

My RPC was fully rebuilt by a reputable electric engine shop last year and works as should. I've been using it for 12 + years. Wire size feeding the RPC and the panels is oversized too so as to minimize any additional resistance during high current draw.

My point in all of this is to share some real world experience when operating a motor at maximum load off of an RPC.

If there are times that you will be loading that 15hp spindle fully, then I'd recommend at least a 2X ratio of RPC hp to load HP. If not, then you can probably get by with something smaller.

One other note - RPM on the RPC makes difference. My 30 hp Phase-a-matic is 3450 rpm and it will start a larger load than a 1025 rpm 40 hp RPC.

Best of success to you.
 
...
The advice to double the RPC is good advice. If your VMC is 15hp, a 30 hp RPC is best.
If you have to spend money anyways, get the larger one, to make sure it works.
Thanks @Doozer, I have to spend the money.
I was going to try and save $400 by going with the 25 HP RPC.
But based on all the responses the wiser thing to do is drop the extra coin on the 30 HP RPC.
 
My point in all of this is to share some real world experience when operating a motor at maximum load off of an RPC.

If there are times that you will be loading that 15hp spindle fully, then I'd recommend at least a 2X ratio of RPC hp to load HP. If not, then you can probably get by with something smaller.
@scsmith42, thanks for sharing your experiences. I'm new to all this and it does help.
I hope I never fully load the 15hp spindle. But I suppose I should be ready for the possibility.
 








 
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