# Rotary converter size question... and shop help



## timcki (Sep 4, 2017)

Trying to understand rotary phase converters,
  I have a 7.5 HP Clausing lathe knock off. It runs off of three phase ,do I understand it correctly that I will need  a converter greater than 15 HP to correctly run this off of 220v single phase?

My "shop" is in a storage area with no electricity... I have successfully run my 2HP 220v single phase mill with the generator. I understand 7.5 HP is a big jump from 2 HP...

My only options are:
a bigger generator...
a bigger 3 phase generator.......
 Any suggestions??


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## Karl_T (Sep 4, 2017)

I'd suggest a VFD if your power generation is that limited.

Yes, the guide line for an RPC is twice the size of the motor you are starting. You can do this with two or more motors. for example, I have a Mazak M4 20 hp lathe. My RPC first starts a 10hp idler, then a 25 hp idle five seconds later. this really reduces starting loads.


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## Doubleeboy (Sep 4, 2017)

If your motor is in good shape and does not present too great a starting load you can frequently get by with smaller RPC especially if you are not spinning a huge chuck.  I know a gunsmith who has same 5 hp lathe as mine, he runs it successfully with 8 hp RPC and has had no problems in several years of use.  If you were to want to run other motors besides your lathe a larger one would be right call.


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## Ulma Doctor (Sep 4, 2017)

you can easily start a 7.5hp from a 10 hp RPC, i have done it too many times to count.

that being said, 
you will need large single phase input wiring to the RPC due to the high start up current of the 10 hp motor on single phase.
after the idler motor is running, it will produce 3 phases of power.
the 3 phase output of the idler motor can then be used to power your lathe (or any single other 3 phase operated machine tool {within reason})

now that i said that,
your lathe should be started unloaded, in the neutral position or with the belt drive disengaged (if equipped).
the 7.5 hp lathe motor will not start well if the motor is loaded, minimum friction is the idea.
short of those 2 points i can only offer the suggestion that you run the 10 hp idler on a 40 amp circuit for longevity.
a 30 amp circuit may facilitate the operation, but you may/will use up all 30 amps doing it on start up.


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## Ulma Doctor (Sep 4, 2017)

7.5 hp single phase to 3 phase VFD's are available...
http://www.ebay.com/itm/10HP-7-5KW-...283387?hash=item283df6e87b:g:0NgAAOSw8thZoM6T
or
http://www.ebay.com/itm/HIGH-QUALIT...650269?hash=item3f56f2285d:g:zjgAAOSwHnFVnhP9

i suggest the 10hp (7.5Kw) drive to overrate the drive for longevity
a 5.5Kw VFD would provide enough power to drive your lathe like this

http://www.ebay.com/itm/7-5HP-5-5KW...406179?hash=item2edffd2a23:g:L18AAOSwx6pYorad

(i have no connection to any of the vendors listed)


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## Keith Foor (Sep 5, 2017)

If this is the only thing that you expect to have in the shop that is going to be 3phase, then go with a VFD.  They are less expensive both to buy and to power over time.  
If not, I would look closely at a RPC. 
Now there are some truths to an RPC that many don't know or forgot, that I will let you in on.  
First is starting power. 
The starting power of an RPC is directly related to the STARTING load of the equipment that you are running.  
Meaning that if you are trying to start an air compressor, fan, winch (with load on) or other motorized device where the motor you are starting is expected to immediately see the full rated mechanical load of the motor you are trying to start then yes, you need 1.5 to 2 times the size RPC of the rating of that motor.  

SO, you have to ask yourself, are you going to be starting the lathe in the highest spindle speed it has, with a cutting tool buried in the material being cut?
Of course the answer is NO.  
So does the lathe have a clutch?  If so, it gets started with little or no load.
We already decided that you will not be starting the motor with a tool against the material being cut, so again, not full load.
Now you will need to consider that if there is no clutch, and it has the clausing belt variable drive, you CAN'T run the speed down with the lathe not running.  SO, there is some loading to consider that will differ with the spindle RPM setting at startup  so we are still looking at a minimum of a 1 to 1 ratio.  

Personal experience.
I have started 20 HP motors with 10 HP RPC's with the 20 HP motor sitting loose with nothing on the shaft to test the motor.  It was never an issue.
I have started 40 hp motors with a 15 HP RPC.  Again, no load, no issue.  
All RPC's I have or have had are fully balanced.  Meaning that I had run caps on both the L1 to L3 and L2 to L3.  This is what is referred to as a "CNC READY" RPC by the manufactures.  Basically it gives a leg to leg voltage a balance within 10%.  Running caps on L1 to L3 or L2 to L3 and not both will decrease the starting and running ability of the RPC.  Or decrease it's rating.  But of course it's also cheaper to build.  

Now, mind you I am a pro RPC kind of guy.  But I run welders, plasma cutters and other 3 phase stuff that require 3 phase and the normal VFD stuff will not cut it for that equipment.  I also build them to fuel my home machinist addiction.
All that being said, I have also run my Clausing with a clutch and  variable speed with a 7.5 motor off a 5 HP RPC I built to see if it would run it.  I was in back gear, and I was turning about 110 RPM on the spindle, but I was taking a .125 DOC (depth of cut) off a mild steel bar.  So it was loaded, and it didn't complain, but it did chatter a bit thanks to a loose gib.


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## timcki (Sep 9, 2017)

I would like to thank everybody that replied...
I have decided to go with a  Rotary phase converter... After doing the research  and watching  ShadonHKW's videos it seems like the most logical thing to do...
I know running a shop with a generator is a bad idea but it is my only option.....


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