# Rotary Phase Converter Size overkill



## Scra99tch (Jun 19, 2017)

I am getting a free 30 HP American rotary Phase converter CNC model.  I need one to power my Van Norman 2HP main motor and 1.5HP Feed motor and eventually <5HP Lathe.  I have read to double the required HP of your RPC, but what harm is running this 30 HP motor. 

Just how much power is required to keep the motor turning vs a 10HP motor? 

I may just try to sell and get something a bit more my size, or even an RPC for the Lathe and Main motor, and a VFD for the mill feed motor.


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## British Steel (Jun 19, 2017)

That's going to drink electricity... Probably 5 - 10 kw (so 20 - 40 Amps of 220v) at idle to keep it spinning, so downsizing might be a good idea, or at least making sure it's switched off when you don't need it...

I'm building a 7.5 HP / 5.5 kw for my milling machine (and the rest of the kit as I buy it), which has 1 HP for feeds, 3? HP spindle so the RPC will be a little oversized, but I plan on putting a contactor, run from the Garage of Danger's lighting circuit, to switch it off when I'm not around (someone else's bright idea, shamelessly stolen!)

Dave H. (the other one)


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## Keith Foor (Jun 19, 2017)

Couple things here.  One is SUPER important, so I will start with that.  If you decide to run the equipment off the 30HP RPC, you NEED a 3 phase panel to distribute the load out to the machines.  Reason is that a 30HP RPC is gonna need a 80 or 100 amp breaker if memory serves me.  Now a 2 or 3 or even a 10 HP machine is not gonna need that.  So more importantly it's not gonna be WIRED for that.  SO if you just stick a twistlock outlet on the ouput of the RPC, the available current at that outlet is gonna be what ever the breaker is.  If you loose a winding of have a short in a machine that should be wired to a 20 amp breaker, it very well may not trip the 100 amp main breaker and instead start a hell of an electrical fire inside your machine.  

The other consideration for a RPC that size is start current.  I have no idea what power feed you have to your current shop, but if it's not up to snuff, getting it there may be pricy depending on how much wire you need to make the run.


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## Scra99tch (Jun 20, 2017)

The run is < 5' for the mill and yet to be lathe.  I looked at their American Rotary's installation guide and they do recommend the 3PH panel for that purpose.  I would probably hard wire to each breaker the recommended amount for the motors being powered, and add a twist lock for mobile equipment like a welder.  

Would anyone give me a hypothetical on the running cost of just the idle motor when not put under load of a 10HP vs a 30HP?   The starting power draw of course would be much different.

I opened the panel and at least one lead to a capacitor got to hot and melted, wondering if there were frequent start and stops going on prior to my ownership.


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## projectnut (Jun 20, 2017)

I have a 10/15 hp panel and a 10 hp idler/phase generator motor.  I'm in the process of designing a whole new 3 phase system for the shop to replace the multitude of  static converters now in use.  The 10 hp phase generator data plate says the maximum current draw at full load is 28 amps.  To begin with I'll have to add another 100 amp single phase sub panel since all the circuits in the current 200 amp panel are already spoken for.  Then I'll have to add a few breakers with one being at least 40 amps to power the phase converter panel and generator.  Down stream will be a 3 phase load distribution panel with at least 4 20 amp circuits to power the machines.  Add in the wiring, outlets, and 3 phase plugs, etc., etc. 

If I purchase all new panels, breakers, wiring, outlets and plugs, etc., etc., the rough cost estimate will be close to $1,000.00 for materials alone.  I'm thinking of letting some bids for the project.  If I have a contractor do the job I expect the cost to at least double.  I would like to do the job myself.  I did most of the wiring in our previous house and installed the central air. 

The work wasn't hard, but dealing with the inspectors was a nightmare.  They don't like to deal with residents doing their own plumbing and electrical work.  The minute they "think" they find a problem the inspection comes to a halt.  They flag the item and schedule and other inspection 2 weeks down the road.  When that item is passed they move to the next.  I had a running battle with the electrical inspector over installing a certain brand of disconnect for the AC compressor.  Although there was no code stating explicitly a certain brand was to be used he wouldn't pass the inspection until I installed that brand.  The worst part was when I toured several other new houses under construction in the same area none of the contractors were using the brand the inspector required me to use.   So now I have to decide whether it's more cost effective to hire a contractor, or do battle with the inspectors one more time.


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## Ulma Doctor (Jun 21, 2017)

gotta love inspectors 
some are waaay too self-important.
if they had a bad day, you are soon to have a worse day.

i had my share of contact with Welding inspectors, Health inspectors, OSHA inspectors, USDA inspectors, Electrical inspectors, and probably a couple others that i'm not aware of.
the only advise i can offer you is to avoid contact with them, if you value your happiness or bank account.


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## Keith Foor (Jun 21, 2017)

ok.  is the wire melted on the start caps or the run caps?   a lot of times I see these things wired with inadequate wiring for the current involved.  for a 30 hp RPC the wire gauge should be a number 6. now of course this gets a bit difficult to find number 6 1/4 blade connectors. and a blade will not carry that current anyway.  so each cap should be wired independently to a common point with a number 10 wire.  then run to the contactor with a number 6.   start current will be a good bit higher than the run current on the run caps because you are trying to start a 30 hp 3 phase motor with single phase power and capacitors forcing it to start.  this is not efficient and therefore draws lots of power.   if you are looking a using number 6 wire for the run caps.  the start caps should be wired with number 4 wire.  and there are zero number 4 blade connectors  so you have to wire them individually tk a common point and run them from there 

Sent from my SPH-L720 using Tapatalk


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## Cheeseking (Jun 21, 2017)

Scratch I think the cost to run will depend on the $/kwh your utility charges.    Oversizing by 2x seems prudent but a 30hp? Wow not sure on that.   If its been toasted I'd pass but thats just me.  If you do run that beast, fortunately most don't bother charge residential customers for negative power factors on inductive equipmet!  I forget all the calculations I used but my 7-1/2 hp rpc rating plate shows 47.8 FLA input and 27.6 FLA output @240V.  I fed it with #6 conductors on 60A breaker 10 ft run to a 60A fused safety disconnect.   That feeds a motor starter with overloads to protect idler.  3Ph output of rpc goes to 30A fused disconnect which acts as the main breaker feeding a 3Ph panel.  10 ga conductors /30A fuses there.
4 dedicated circuits each on 20A breakers, 12ga conductors w/20A twist lock recepticals.   
Bought most of the stuff new on ebay from surplus outfits pennies on the dollar.  Retail cost on the gear would be ridiculous.


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## markba633csi (Jun 22, 2017)

In my area you can do "owner-builder" and I did my remodel and electrical myself, no inspections, much simpler.
You didn't hear that.
M
ps nice setup you got there, Cheese


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## Cheeseking (Jun 24, 2017)

New construction aside, ultimately I think the way it works is you can avoid inspections/ do anything you want inside your 4 walls after the fact and they won't know until the day theres a fire and they investigate and discover electrical work not typically found in a residential home.   Thats when it pays to have everything done to code.  Even if it's done properly, the lack of a permit, Insurance co will latch onto anything if they think it absolves them of liability.  I didn't get any permits but made damn sure nothing I added would cause a fire.


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## kenscabs (Jul 12, 2017)

Real numbers:  my American Rotary 20 hp uses 12.4 amps idling.  That's about 3kwh, at my local rates .25 kwh, so about 75 cents an hour idling.


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