10 hp Phase-a-matic Rotary to run 5.5kW (7.4 hp) lathe motor?

I have a Summit 14 lathe on a static phase converter. A a local auction there is a 10 hp Phase-a-matic, auction closing in a couple of days. Trying to understand if it might work with my lathe and my limited power.

Background: Right now I am limited by a 30A breaker at the subpannel (30A is self imposed, 8 ga wire in underground conduit from main panel 40 ft away). The lathe has a 5.5kW (7.4hp) 2-speed motor. With the static converter and the 30A breaker, I am unable to use the highest lathe speed (1800 rpm) with the motor in high speed (breaker trips after about 10 sec, peak inrush of 94A). I could go with a 40A breaker at the subpannel but I want to be certain that I do not trip the breaker at the main panel - rental house, no access if tenant is not home.

Might I be able to use the 10 hp RPC to drive a 7.4 hp lathe? In this recent thread there is discussion of tuning/balancing the load with capacitor(s): Phase converter sizing
Am I close enough to do the same? I have no knowledge on how one might do this, have access to a digital clamp-on Amprobe.

I had a Phase-a-Matic once. It there was no balance capacitor(s) inside. Just a run cap of small value. I returned it.
The idler motor was built by Baldor and the rotor was lightened to spin up easy on start with no start capacitor. Or maybe
what they do is use the same small capacitor for starting and then it stays in circuit for running.

Before that I bought a RPC for $450 from Scott Machinery in Sacramento. I got a little knowledgeable and checked some things.
The RPC consisted of:
A 5HP used Baldor motor. Going on $1 per hp on a used motor.
One small metal box. Maybe $10
One oil filled capacitor. Maybe $15

I returned it to those ripoff artists.

That's when I became a RPC builder. I suggest that you become a little knowledgeable and build your RPC. The thought of doing it is much
worse than learning how to do it.

Scott Machinery is worse than MSC or Grainger. At least with those two you get ripped of but get something that is substantial... (besides the bill)

I have a 20hp phase a matic rotary converter, had to replace caps 10 or so years ago, lots of them, do not recall how they were wired in. According to PAM the largest single motor a 10hp rpc can run is 6hp, and full output will require a 60 amp feed. I also recall having some problems running a 2 speed motor on the lower speed, iirc it would trip the overloads, but cannot be sure, its been some years. Do you really need 7.5hp?
https://phaseconverters.phase-a-matic.com/Asset/RB.pdf

Its possible if you buy a new breaker (40A) it will trip more often. i would search around for the oldest used 40A breaker you can find and put it in the panel.

in my experience, a 3 phase motor will draw 57% of its nameplate full load amps when run at no load on single phase power. a 7.5hp 3p motor is nominally 22 amps full load so i would expect 15 amps no load on single phase.. your breaker should not be tripping. you can try offsetting a lot of this current by adding about 100uf of capacitors across the line, at the input of your static phase converter box.

ideally you would add enough run capacitors to offset the power factor drawn by a rotary phase converter, but most people don't.. they just add the output capacitors across lines 1-3 and 2-3 to boost the output voltage. (line 3 being the generated leg, lines 1-2 are utility)

my guess is you should be able to get the no load amps drawn by a 10 hp rotary phase converter to just 2 amps at 240volts, unity power factor. you'll have to add around 100uF across lines 1-2 (the utility feed) in addition to the 50-150 uF caps already in the box to boost the generated leg up to 240volts.

so anyhow your 94 amps inrush current is about 57% of the inrush current i would expect from a 7.5 hp motor (22 amps * 7 =154 amps).. and this is an interesting coincidence, but i digress.

so anyhow, starting your lathe motor from a 10hp rpc will draw more current from the mains than starting it from a static phase converter, i'm certain of that. but it will start up quicker and run with less copper losses.. which in theory will decrease the total amps drawn by the system.. if the power factor is corrected. the 10hp rpc will add about 500 watts parasitic loss however.


however if you don't need the 7.5 hp (which your 30 amp breaker cant provide anyways)

then go find yourself two 240vac:16/32vac buck boost transformers and connect them to provide 208 volts to your lathe from your 240v rotary converter. the starting currents may be dropped enough to sucessfully start the motor (start it on low speed then switch to high speed) and you'll still be able to safely pull 5hp from the motor on the reduced 208 line voltage.


if you are not already trying to start the lathe in low speed then switch to high speed, doing so may get you by. but the 10 second breaker trip time is curious. you can buy a kva meter on ebay for like 16$, it will tell you the actual watts consumed and the amps and power factor.

who knows, there could be something wrong with your lathe, try warming it up in low speed for half an hour then start it in highspeed.

With limited source current, you may be better off to use a VFD.

The bad thing about the RPC is that it does nothing about the start surge. The VFD will.

Using a VFD can operate a motor on a source that normally would not be possible. And a smaller VFD than the motor rating (in current, not just HP) can operate the motor if you do not need full motor HP. You normally cannot go lower than about 50%, but odds are that a 5 HP VFD would run the 7.4 HP motor so long as you do not need all 7.4 HP.

The bad thing about a VFD is that they tend to cost more, and ones at 5 HP and above may have difficulty operating from single phase without de-rating.

What is the breaker rating back at the main panel?

Thank you all for your responses
dalmationgirl61: Thank you for the link. The model at auction is the R-10, I did not find this page when I was looking to see what the maximum the R-10 would run. No, I do not need 7.4 hp - with the static, I am already giving up 1/3 of the power. I expected the lathe to have 4 or 5 hp - auction purchase, no external tag with electrical power requirements on the lathe other than 230v 3 phase - I was a bit dismayed at finding a 5.5kW motor.
johansen: Why would a new breaker trip more readily? I need to study up about the use of buck boost transformers. With the static phase converter, I need to use 3 of the 4 capacitors supplied to run the motor in low speed, or all 4 to start in high speed. I never even considered switching between speeds with the motor running - I'll have to try that tomorrow. Out of convenience I have been running the motor in low speed as it gives me both the low speeds for threading an the highest top speed that I can achieve with my limited power. I have run the lathe for more than half an hour to warm it up and then took the current inrush measurements.
JST: The lathe has an electrical panel full of "stuff". From memory, there are 7 contactors (relays), along with half a dozen fuses and a transformer. I was guessing that all that would not play nice with a VFD. Breaker at the main panel is 50A. My understanding is that 8 ga wiring is nominally 40A (I used THHN in 1 inch conduit, 26 year old me could not imagine 62 year old me wanting more than 40A or that I should have used an oversize conduit so that I could upsize the wiring).

I have considered replacing the motor with a 5 hp. I was looking for a solution outside of the confines of the lathe motor housing.

Most of the hardware in the electrical box can essentially be bypassed to use the VFD. The function of most of the controls is taken over by the VFD. You do not need to tear it out, but you do not want to use most of it.

If there is a coolant pump, it can be run separately, or possibly re-motored, depending.

If price is right on the rpc grab it, if nothing else you can always flip it. I would try starting lathe with the rpc, it may or may not work, might need to upgrade the incoming power though. I would also think finding a 5hp motor in Detroit would be fairly easy, a 2 speed might make it a bit more leg work. Or just go with a vfd, wire it straight to motor bypassing all the controls.

rons said:

I had a Phase-a-Matic once. It there was no balance capacitor(s) inside. Just a run cap of small value. I returned it.
The idler motor was built by Baldor and the rotor was lightened to spin up easy on start with no start capacitor. Or maybe
what they do is use the same small capacitor for starting and then it stays in circuit for running.
...........................

Click to expand...

Some, such as my Arco, do have a single capacitor that functions as both a start and a "balance" unit. No switch at all.

You returned it just because it "had no balance capacitors"? Seems extreme......... as mentioned the Arco is like that, and works fine.

aribert said:

Thank you all for your responses
johansen: Why would a new breaker trip more readily? I need to study up about the use of buck boost transformers. With the static phase converter, I need to use 3 of the 4 capacitors supplied to run the motor in low speed, or all 4 to start in high speed. I never even considered switching between speeds with the motor running - I'll have to try that tomorrow. Out of convenience I have been running the motor in low speed as it gives me both the low speeds for threading an the highest top speed that I can achieve with my limited power. I have run the lathe for more than half an hour to warm it up and then took the current inrush measurements.

Click to expand...

because breakers are available in different trip curves and what is available at the big box stores is tending towards more sensitive breakers over the past 20 years. also new breakers frequently have a magnetic component to their construction that wasn't built into older breakers.

At my folks house, I accidentally shorted out a pump i was running in a bucket of water, 3 feet from the electrical panel. the 15 or 20 amp breaker on the outlet tripped without me even seeing a spark as the two wires touched each other. same thing at my fiance's house, i was changing the electrical outlets live, shorted one out without even seeing a spark. can't do that on older circuit breakers...

As johansen mentioned, you're running right at the limit to try to run 7.5HP out of 30A. 5.5kw is 25A. Throw in motor inefficiency, and you're pushing 30A.

An RPC is going to add some wasted current, spinning the idler. I'm skeptical that it'll work without going to a 40A breaker. And even then you may need a pony motor to spin up the RPC to limit startup surge, even with good capacitors. I'd try for a 15HP VFD, to get the soft start.

johansen said:

.........................

At my folks house, I accidentally shorted out a pump i was running in a bucket of water, 3 feet from the electrical panel. the 15 or 20 amp breaker on the outlet tripped without me even seeing a spark as the two wires touched each other. same thing at my fiance's house, i was changing the electrical outlets live, shorted one out without even seeing a spark. can't do that on older circuit breakers...

Click to expand...

That really should happen with any wiring that is adequate for the load, maybe not "instantly", but fast.

I once was called in to look at a problem in a home shop..... there was a long run of 14 ga (on a 15A circuit) to a shop area that was at the far end of the basement in a large ranch style home. I paced it off, and I do not recall the total anymore, but quite significant.

It had enough resistance (it did not even run straight) in all that wire, that a 1/3 HP motor took almost 5 seconds to start up. I have no idea what actually would have happened if there had been a short at the far end in the shop, but my gut feeling is that there would have been at least several seconds before the trip. I am pretty sure it would have tripped...pretty sure.

I think people are getting a little in the weeds on loads and capacity
Back in the day I ran 5 3 phase machines, 2 CNC mills, 1 3hp manual, 2 hp tumbler, 5hp saw, 12ck with I think 5hp motor on a pair of 5 hp home built converters with no caps, at the end of 150 feet of 6 g wire

NO problems. OK, if two guys hit the run button on the 2 CNC at precisely the same moment the might trip,sometimes

Since I still own and run 4 of those 5 machines 25 years later I think there was no long term harm

IF the lathe starts unloaded I think it will be fine, unlike a 7 horse air compressor

JST said:

That really should happen with any wiring that is adequate for the load, maybe not "instantly", but fast.

I once was called in to look at a problem in a home shop..... there was a long run of 14 ga (on a 15A circuit) to a shop area that was at the far end of the basement in a large ranch style home. I paced it off, and I do not recall the total anymore, but quite significant.

It had enough resistance (it did not even run straight) in all that wire, that a 1/3 HP motor took almost 5 seconds to start up. I have no idea what actually would have happened if there had been a short at the far end in the shop, but my gut feeling is that there would have been at least several seconds before the trip. I am pretty sure it would have tripped...pretty sure.

Click to expand...

It's a requirement in NZ, Aus, the UK, and others to test that the fault current at every new socket is adequate to trip the protection within 0.4s, unless supplied by an RCD.

We have tables to indicate how far you can expect to run a given size cable on a given breaker and maintain adequate fault current.

johansen said:

because breakers are available in different trip curves and what is available at the big box stores is tending towards more sensitive breakers over the past 20 years. also new breakers frequently have a magnetic component to their construction that wasn't built into older breakers....

Click to expand...

Thank you for responding - good to know.

dalmatiangirl61 said:

If price is right on the rpc grab it, if nothing else you can always flip it. I would try starting lathe with the rpc, it may or may not work, might need to upgrade the incoming power though. I would also think finding a 5hp motor in Detroit would be fairly easy, a 2 speed might make it a bit more leg work. Or just go with a vfd, wire it straight to motor bypassing all the controls.

Click to expand...

I dropped out of the bidding at about $175, since I was not confidant it would work any better than the current set up - sold at $210. There was a Baldor 5.5 hp in the same auction but is was a 3000+ rmp motor. Mine is 1800 rpm in high speed. Initially after buying the lathe, and before I sold off my Clausing, I considered replacing the motor but its not straight forward - the motor is face mounted on the endbell. Not insurmountable but more fiddly on just a mill if ones only lathe is dismantled.
If/once I start having major issues with the controls, I think a VFD will be the way forward - I had not considered that as an option until JST mentioned that in an earlier response. I just found out a couple of days ago that while the foot brake disconnects the power it is not engaging the actual brake (or the friction surface is worn away or out of adjustment).

The VFD has pluses:

It does NOT draw added idle current as an RPC does.

Of course you get variable speed.

It does soft start, every time. The RPC requires a hard across-the-line start of the machine, adding the surge current to whatever the RPC draws by itself.

When the VFD itself is turned on, it normally has a soft start to charge the capacitors slowly, so that it does not contribute a large surge.

And minuses:

It does not change the 1.73x current issue going from 1 to 3 phase, and can add some to that. But probably not as much as the RPC does.

You need to have controls, which can be new, or can be the originals, but using the originals requires disconnecting them from the old relays if you wanted to keep those in place for possible later use.

It has to go somewhere, and needs a box.

SomeoneSomewhere said:

It's a requirement in NZ, Aus, the UK, and others to test that the fault current at every new socket is adequate to trip the protection within 0.4s, unless supplied by an RCD.

We have tables to indicate how far you can expect to run a given size cable on a given breaker and maintain adequate fault current.

Click to expand...

You assume the wiring was done right... in any way.... Not quite true, obviously.