Motor Size For Phase Converter

[Train]

I've watched a few videos on making a rotary phase converter using a 3 phase motor. Different guys used different sizes of motors, but it sounded like the choice was just based on what they had. What would be the smallest size that would work to power say, 5 hp motors on some smaller shop machinery? Or does it not matter as long as it would put out 3 phase?

 

[larry_g]

http://metalwebnews.com/howto/ph-conv/ph-conv.html

read the above link to understand a bit of basic information on rpc's.

Another thread, https://www.garagejournal.com/forum/showthread.php?t=367420&highlight=hanrahan

lg
no neat sig line

Answer from another thread.

lg
no neat sig line

 

[bimmer1980]

It also depends on the load on the motor upon start up....

For example, I can run my 7.5 hp 3-ph iron worker off a 5-hp phase converter, but the start up load is relatively low. I also know that I will not be able to take a full 50 ton load on the iron worker. (Piranha P-3, 50 ton capacity)

You probably will not be able to start an equal size lathe motor on a phase converter. Generally recommend 1.5 to 2 times motor size to phase converter to high load starts....

Check out the American Phase converter website for more details...

 

[kazlx]

It really depends on what you want to power. A 5hp compressor is way different than a 5hp mill. Ideally, your idler should be at least 2x your intended motor size. If you want to run 5hp things, I'd plan on building at least a 10hp. If you're going through the hassle, I'd probably just do a 15-20 if you have the power. They don't really draw much more at idle. I use a 15hp to power my 15hp Fadal CNC.

 

[Train]

Well this helps. Thank you. I don't know how many auctions I've been to and nobody even bids on the 3 phase stuff. Should be able to put one together pretty cheap other than the capacitors.

 

[cvairwerks]

Just remember to watch hp ratings when you start tinkering with 3p stuff. Lots of POCO’s have max hp limits on residential services. In my case, 7.5 hp is the max limit for a single motor, no matter voltage or phase requirements.

 

[rsanter]

Typically you need 50% more motor for the RPC than for,the largest motor that will be running off if it.
I have not tried it but I have heard that if you have multiple pieces of three phase equipment, if you start the RPC, and then start all let idle a couple more of the other motors, that it will add to the "capacity"

 

[matt_i]

I always used the rule of sizing your RPC motor to your max machine motor.

As mentioned a metal-cutting bandsaw and an air compressor are very different electrical loads as far as the startup current needed.

 

[930dreamer]

My 10 hp rpc will start my 5 hp Champion air compressor.

 

[vpd66]

Well this helps. Thank you. I don't know how many auctions I've been to and nobody even bids on the 3 phase stuff. Should be able to put one together pretty cheap other than the capacitors.

To save money on capacitors go to your local salvage yard. These places junk out all kinds of electrical items and they have to remove all the capacitors and pay to dispose of them. I told them what iI was after and they gave me a 5 gallon pail full for free and said "don't bring any back!" I also bought all my motors, switches, and power strips from them for junk prices. I built a 15 hp (10 hp and 5 hp motor) for less then $100. The key was the salvage yard I was dealing with was junking old industrial roof top HVAC units. They had all the right components at scrap price.

 

[930dreamer]

To save money on capacitors go to your local salvage yard. These places junk out all kinds of electrical items and they have to remove all the capacitors and pay to dispose of them. I told them what iI was after and they gave me a 5 gallon pail full for free and said "don't bring any back!" I also bought all my motors, switches, and power strips from them for junk prices. I built a 15 hp (10 hp and 5 hp motor) for less then $100. The key was the salvage yard I was dealing with was junking old industrial roof top HVAC units. They had all the right components at scrap price.

A good place for electrical enclosures too.

 

[MushCreek]

When I got my American Rotary panel, they recommended 2X the size of the biggest motor. You can RUN several motors totaling more than that, but it's the starting that matters. I picked up a 10 hp 3 ph motor at auction (untested0 for $25, and it works great.

 

[slodat]

I have a 20HP American Rotary converter with their Baldor idler.



It is starting my 15HP lathe motor with no problem.

 

[kwb]

Everything I have seen recently is that the way to go is a VFD. There are a number of them out there rated for plenty of capacity and get real 3 phase power, a RPC does okay but the "created" leg isn't usually as clean as the incoming legs.

 

[slodat]

It really depends on the load. On my cnc milling machine I used a modern vfd to power a very nice Yaskawa spindle motor from three phase power. Of course this application needs the speed control and other features a vfd provides. On my 15hp lathe, there’s nothing gained from a vfd. And, I can also run the lube pump. Same for my variable speed (Reeves drive) drill press and 20” disc sander. Once a RPC is installed correctly and powering a three phase panel, you have three phase available when you want it.

 

[MushCreek]

I bought a VFD for my mill, but ended up going RPC. Just for the instant reversing capability alone, I prefer the RPC. My VFD will reverse, but it's slow. When you're power tapping, you want it instant. Plus, I created a three phase circuit for the three 3 ph machines I have now, and can run any or all of them. The VFD should be optimized for a particular motor.

 

[kazlx]

I bought a VFD for my mill, but ended up going RPC. Just for the instant reversing capability alone, I prefer the RPC. My VFD will reverse, but it's slow. When you're power tapping, you want it instant. Plus, I created a three phase circuit for the three 3 ph machines I have now, and can run any or all of them. The VFD should be optimized for a particular motor.

On a side note, you should be able to program most VFDs. They have accelerations and braking time settings. I like the TECO drives, but most are the same. I have one on a drill press and I think I currently have it set to full brake in 2 seconds.

I do think a good RPC is more versatile though. I have a 15hp that powers anything in my shop, including my Fadal. It's nice to just start it up and have 3ph instead of having to wire a VFD to each machine. I do have VFDs on a few tools just for the VFD benefits.

 

[Jim Caudill]

For the actual panel, you can pic one up on ebay from some company in New York. 3-phase motors are relatively cheap if you scrounge around long enough. I have used the RPC and VFD for different equipment, but if you are going to power a variety of 3-phase stuff with different horsepower motors, then the RPC with a 3-phase sub-panel is what I would suggest. I have my lathe and mill hard-wired and have two twist-lock receptacles that allow me to plug in my bandsaw, buffer, or my second lathe.