Picking Rotary Phase Converter Size - How big do you really?

Kevin101 · Mar 9, 2017

I found myself a nice condition second-hand Milltronics cnc lathe to buy. It has a 15hp spindle, so per the American Rotary site, they'd recommend a 30hp converter. One lathe maker suggested getting a phase converter good for 1.5x the spindle rating, which puts me near 20 hp. Looking at the nameplate, it looks like the machine has a potential for 23hp draw if all the axes max at the same time.

Most of the cnc machines I was shopping were under 10hp spindles. I'm working long skinny aluminum rods to begin with (20:1), so I think I'll be chatter, not power limited. If I'm not pushing for max horsepower, is there a major downside using a 20 hp converter, running it from a 15 hp wall plug (220V 50A Welder type)? Am I right believing the failure mode is a breaker trip with broken cutter and lost part? I think the phase converter might look nice on the floor next to my welder, or perched on a stand above it.

Thanks in advance,
Kevin

Garwood · Mar 9, 2017

I would recommend 30HP RPC on a 100 amp breaker.

My 30 HP lathe is run off a 50 HP RPC fed by 2/0 welding cable off the bus bars in one of my panels. It draws 177 amps single phase @ 240V with the load meter at 100%.

I like the idler bolted to wood and restrained so it can't walk across the floor. Heavy load they really grumble. The wood dampens well.

garyhlucas · Mar 10, 2017

Unlike a mill a lathe starts and stops a heavy rotating mass even when the part itself has little mass. A 3 phase motor started across the line, no VFD, can put out about 6 times rated HP. Just like a Tesla leaving the traffic light! With a VFD the motor might still do twice the Hp on acceleration. So don't skimp on the converter.

CGornet · Mar 14, 2017

When I was sizing mine, American Rotary recommended a 2:1 rating on the phase converter, but said a 1:1 would work fine which is what I went with. So far it runs the VMC without any issues.

Garwood · Mar 15, 2017

CGornet said:
When I was sizing mine, American Rotary recommended a 2:1 rating on the phase converter, but said a 1:1 would work fine which is what I went with. So far it runs the VMC without any issues.

doesn't fly with a CNC lathe.

Parkerbender · Mar 15, 2017

I second that... I got stuck in the stupid location I am at now that only has 200 amps single phase... I have a 35 and a 15 HP converter, wired together in parallel. Way back in the garage days that 15 horse ran my mill and air compressor just fine, both of them together wired straight to the mains won't get my lathe main chuck over 740 rpm. I think it really depends on how smart your servo drives are. My lathe has new Rexroth drives, and I don't think there is actually a power problem that early, it's just that the drives are smart enough to see the one leg pulling down differently than the others on accel/decel. If you were stuck in a situation, you can slow your accel curves to get you a little more top end, but if you are starting from scratch, may as well go bigger. Can never have too much power! (trust me, I know. :-/ )

-Parker

Monarchist · Mar 15, 2017

Kevin101 said:
I found myself a nice condition second-hand Milltronics cnc lathe to buy. It has a 15hp spindle, so per the American Rotary site, they'd recommend a 30hp converter. One lathe maker suggested getting a phase converter good for 1.5x the spindle rating, which puts me near 20 hp. Looking at the nameplate, it looks like the machine has a potential for 23hp draw if all the axes max at the same time.

Most of the cnc machines I was shopping were under 10hp spindles. I'm working long skinny aluminum rods to begin with (20:1), so I think I'll be chatter, not power limited. If I'm not pushing for max horsepower, is there a major downside using a 20 hp converter, running it from a 15 hp wall plug (220V 50A Welder type)? Am I right believing the failure mode is a breaker trip with broken cutter and lost part? I think the phase converter might look nice on the floor next to my welder, or perched on a stand above it.

Thanks in advance,
Kevin

IF.. I was going to do a 20 HP when a 30 HP had been / is being recommended ..I'd have another 10, 15, even another 20 HP I could switch-in as supplementary idler.

That's not really a savings over a 30 HP unless you buy the extra idler cheap, but it is perhaps the safest way to see if you really can sustain a day's work with only a 20 HP.

JM2CW

moonlight machine · Mar 15, 2017

Do what A R says, 30 hp. I use an A R converter on my Miltronics ML20, for sure go with the 30 hp You can't compare a mill with a lathe as far as starting load goes. I have my converter on the wall with the dummy motor on the floor sitting on 1/2" thick rubber pads. A R is a good company to deal with and a top notch product. I have 2 friends that bought from A R as well and their converters have been flawless as well.

Bobw · Mar 15, 2017

My lathe/rotary story..

20hp lathe, continuous, 25hp for 30 minutes.. So like 140 HaasPower. 10" chuck, 2 speed gear box.

It was a 20 or 25hp rotary (it was a while ago).. Had to drop the spindle acceleration down to
5 seconds to get the spindle to start.. AND it would only work in low gear 0-1200rpms.. Even at
a 10 second ramp, it wouldn't get the spindle spinning in high gear.

Never had a problem cutting, it was just the juice needed to get that big ass spindle/chuck assembly
to start spinning.

I don't know a thing about Milltronics, but I would make sure you know how to slow down the spindle ramp.

"failure" wasn't a breaker trip, probably because the breaker was oversized.. The spindle would stutter,
stutter, stutter and then the machine would shut down.

Pete Deal · Mar 15, 2017

I run my Mazak QT15, which I think has about the same size motor as your machine, on a 30hp american rotary converter and it seems to work well.

beckerkumm · Mar 15, 2017

Kay and Arco size their RPC differently by the largest HP motor it can start- hard starting. When you compare prices or look for used, a 15 hp Kay is similar to a 30 hp American Rotary. If you connect two smaller units, you need to use similar designs. My understanding is that Kay and Arco use only run capacitors while most others also use start. Don't want to connect one type to the other. Now you everything I know. Went to a Phase Perfect which is much better but also more expensive. Dave

Milacron · Mar 16, 2017

I wouldn't screw around with a rotary converter in the first place, especially for CNC...buy a 20 HP Phase Perfect and be done with it.

Ox · Mar 16, 2017

Kevin:

You are the one that had the 100hp convertor?

Doo you still have it?

Just curious. I think about that from time to time.

------------------------------

Think Snow Eh!
Ox

Monarchist · Mar 16, 2017

Milacron said:
I wouldn't screw around with a rotary converter in the first place, especially for CNC...buy a 20 HP Phase Perfect and be done with it.

+1

.. and... 'flywheel effect' still applies. If there is a hard-starting, but not hard-running situation? Bring a 5 HP or 10 HP 3-P unloaded motor online first as if it were a supplementary idler on an RPC, shut 'er down once up, run off the P-P unaided rest of the shift.

beckerkumm · Mar 16, 2017

If you have the cash, the PP is the way to go. A 10 hp starts a motor eeassier than my 10 hp Kay. They come up used once in a while. Stay away from the Blue ones or price accordingly. they are older with a main board that is no longer supported. The newer white ones use several replaceable boards that are still available. Dave

I learned the hard way when the used 30 hp I bought crapped out almost immediately and the seller ran for cover. PP has always been great to deal with though.

JST · Mar 17, 2017

When it comes to RPCs, there is really no way to give a rule of thumb that is 100% accurate. The idler motors vary.

What you really MEAN when trying to size an RPC to a load, is selecting a unit where the difference in voltage drop between the input legs and the generated leg is not too much to get it started and run well at full output. That obviously depends on the way the motor is made.

When you find a motor to use in an RPC, as opposed to buying a complete RPC, you may not know what "NEMA design" the motor is made to. Of the three common types, one is sort of average, one is definitely higher impedance, and one is definitely lower impedance.

So "a motor" of any given HP is somewhat of an unknown. There are clues. Look at the loaded rpm. There are 1725 rpm motors (4 pole) and there are 1750 or 1760 rpm motors. Typically, the motor with the higher speed under load will also have less voltage drop when used as an RPC idler.

If you buy a purpose made RPC from a known builder, such as Kay, ARCO, etc, you will generally be getting a more suitable device than if you find a junkyard motor and set it up. They have selected the best type motor, and have had the motor made to their specs for best performance as an RPC.

if you buy a control box and mate it to your own motor, you may do pretty well, or you may be disappointed. Unless you have (by accident) selected one of the more suitable types of motor, you may not get the performance you want. With the worst type of motor, 30 HP might be hard pressed to equal what a 15 or 20 HP of a more suitable NEMA design will do. It's nothing to do with the box maker, it is going to be mostly the motor selection.

For a general shop RPC, it may not matter. Those are normally sized to run many motors, and are plenty big enough for any individual unit.

If you need good performance with a single large load, as with CNC applications, you should buy a known commercial RPC and consult the makers for advice. That turns a crap-shoot into a pretty much known deal.

Ox · Mar 17, 2017

With that said - I've found that Lincoln seems to be one of the best choices for RPC's.

My 60hp Phasemaster is a Lincoln, and I have another 60 and a 50 that are "home made" RPC's. (those 2 are not currently in use)
I had a GE 30hp, but I think that I may have sold it recently? (<3 yrs?)

--------------------------

Think Snow Eh!
Ox

Monarchist · Mar 17, 2017

beckerkumm said:
If you have the cash, the PP is the way to go. A 10 hp starts a motor eeassier than my 10 hp Kay. They come up used once in a while. Stay away from the Blue ones or price accordingly. they are older with a main board that is no longer supported. The newer white ones use several replaceable boards that are still available. Dave

I learned the hard way when the used 30 hp I bought crapped out almost immediately and the seller ran for cover. PP has always been great to deal with though.

Oh well..

My used 10 HP is a Blue one. Should be OK for the max 5 HP motor it will serve, though.

I'll just put the 7.5 HP Alzmetall AB5 DP on the 10 HP RPC, then.

Thanks for that heads-up!

moonlight machine · Mar 17, 2017

JST said:
When it comes to RPCs, there is really no way to give a rule of thumb that is 100% accurate. The idler motors vary.

What you really MEAN when trying to size an RPC to a load, is selecting a unit where the difference in voltage drop between the input legs and the generated leg is not too much to get it started and run well at full output. That obviously depends on the way the motor is made.

When you find a motor to use in an RPC, as opposed to buying a complete RPC, you may not know what "NEMA design" the motor is made to. Of the three common types, one is sort of average, one is definitely higher impedance, and one is definitely lower impedance.

So "a motor" of any given HP is somewhat of an unknown. There are clues. Look at the loaded rpm. There are 1725 rpm motors (4 pole) and there are 1750 or 1760 rpm motors. Typically, the motor with the higher speed under load will also have less voltage drop when used as an RPC idler.

If you buy a purpose made RPC from a known builder, such as Kay, ARCO, etc, you will generally be getting a more suitable device than if you find a junkyard motor and set it up. They have selected the best type motor, and have had the motor made to their specs for best performance as an RPC.

if you buy a control box and mate it to your own motor, you may do pretty well, or you may be disappointed. Unless you have (by accident) selected one of the more suitable types of motor, you may not get the performance you want. With the worst type of motor, 30 HP might be hard pressed to equal what a 15 or 20 HP of a more suitable NEMA design will do. It's nothing to do with the box maker, it is going to be mostly the motor selection.

For a general shop RPC, it may not matter. Those are normally sized to run many motors, and are plenty big enough for any individual unit.

If you need good performance with a single large load, as with CNC applications, you should buy a known commercial RPC and consult the makers for advice. That turns a crap-shoot into a pretty much known deal.

If you get a RPC from American Rotary it will come with a dummy motor specially designed for rpc work, it will have no shaft, and closed end bells.

rons · Mar 17, 2017

moonlight machine said:
If you get a RPC from American Rotary it will come with a dummy motor specially designed for rpc work, it will have no shaft, and closed end bells.

That is what you get from Phase-a-matic. Dummy motor? I would call it a DUMMY RPC. Specially designed? No. It is built with a light-weight rotor so that it can spin up to speed without a circuit to switch in/out start capacitors. I have seen one opened up.

As prices go, the RPC's (or Dummy RPS's) get as expensive as a Phase Perfect. Buy a Phase Pefect.

Picking Rotary Phase Converter Size - How big do you really?

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