Rotary phase converter versus trying to run a VMC straight from single phase

There are so many ways that VMCs can be wired that you should take any advice only as a suggestion to investigate.

Its unlikely that you'll make the machine run without some sort of three phase converter.

The spindle, of course, uses nearly all the power. What sort of spindle motor and drive do you have? If its a three phase motor and VFD you might be able to rewire it for 1 phase input. A servo drive is very common. AFAIK, these units need three phase input. A home made RPC would suite your needs in that case.

If you use an RPC, be VERY CAREFUL that the "wild leg" is not connected to your control computer. The control will most likely use two legs and a transformer to provide the different voltages needed.

Karl

If your getting the one listed by on craigslist at Performance I'd ask them if they have a phase converter laying around.

I am pretty sure you will find that the 3-phase is only for the spindle motor and maybe the coolant pump. The CNC machines we have here, all have separate 115v line cords to the controller.

It would be a major job to replace the spindle motor with single phase so I would look at the phase converter since you already have the motor. There are several circuits in the drop box and the parts will run to around 50-bucks and a couple of hours work. That's the way I'd go if the seller doesn't have a converter they can toss in.

I have remote push button on/off switches to the converters mounted on the control box.

Dave

If you get a piece of 6/3 SO cable and run it directly to the breaker, you won't have to worry about the inside wiring. I was in the same boat when I started my shop in the garage of a house I was renting. There was a 220 SP outlet in the garage, but it only had #10 wire running to it. I went directly to the panel with 2 pieces of 6/3 cable, hooked up to a single 100 amp 220v SP breaker and ran 2 converters. A 15hp & a 20hp. Started out with one, and added the other about a year later.

Was there for 4 years, both converters would sometimes run 24/7 for

3-4 weeks before giving them a rest, and never had a problem.

Matt

Something else........

I first heard about a rotary phase converter from a guy who had his

3ph motor hooked up to a 110v motor with a washing machine coupling. He would turn on the 110v motor and get the 3ph motor spinning, then hit the 3ph motor with 220v SP power, then turn off the 110v motor. Guess you could call that "soft start":-)=20

The only problem is that with no run caps, you have no way of controlling voltage. He was only running manual equipment, so he had no problems. Don't think I'd try to run a CNC that way, but you could at least get it running and check to see what you get for voltage. You never know, it might just work.

Matt

In principle, you need an rpc only to *start* a 3 ph motor; after that, you can disconnect the rpc, and the 3 ph motor will operate fine, albeit at 2/3 power. fwiu.

This means, ultimately, that any 3 ph motor "on the grid" will act as an rpc for any other 3 ph motor that needs to be started/run.

Which then means that you can have just one oh-ficial small rpc, with a whole bunch of small-ish motors (idlers) "staged". I have one official rpc, and 3 additional idlers, for a total of about 25 hp, that I can kick in as necessary.

You may need to screw around with caps, as gunner suggested, and as is oft discussed on rcm, but I haven't gotten that far yet, beyond the caps on my official rpc.

I seem to recall one type of rpc that once it gets up to speed, a relay on the wild (generated) leg actually *switches out* the caps!

There is also a kind of buffer they make for the wild leg. Forgot what it's called, my rpc mfr (whose name I also forgot) makes it, altho the fellow did say something to the effect that it's not without its own vicissytudes. If inyone is interested, I'll get the name/number. Phase-a-matic proly makes them, as well.

The servo power supplies on most of the cnc's I've repaired have 3ph bridge rectifiers in them.

Wes

Well, I talked to somebody about inrush current on non-soft start motors, and for a 20HP motor, he told me it is 150 amps and could be up to 275 amps, which would pop the main breaker for the house (125 amps) Sooo... if that is true, my 20HP is headed for the scrap bin at whatever the motor/generators go for these days.

Just trying to find the best deal on a soft starting 20HP rotary phase converter.

Anybody here taken a Mitsubishi M320 controlled machine and run the whole thing off of single phase?

The CNC tech guy at the dealer was saying that they often will try to distribute the loading/legs. He also said that it will shorten the lifespan of the control and drives running it from single phase.

So, I guess a big chunk of change is going to get spent on a soft starting RPC.

I'm simultaneously excited and terrified about finally getting a VMC. Excited because I'll have a toolchanger, it is a good condition, solid machine and chips will be contained. Terrified by the cost, size, weight... having to move it again :-)

I ran a 30 horsepower motor as a RPC, and it was on a 100 amp breaker. The breaker will not trip on an instantaneous overcurrent. They have a certain amount of "inertia". I had two decent VMC's running off of that RPC. Just make sure that the electronics is not powered from the "Wild Leg". A little examination of the schematics will reveal all.

Apparently you weren't listening (reading?) when someone else suggested spinning the 20 horse RPC motor up to near synchronous speed with a 1/3 horse motor ... thus eliminating that inrush of current . -- Snag - but I'd still at least try to balance the legs with some caps .

Hi Snag

Perhaps I havent read all the posts in this thread. But, it has been my experience that the pony motor for spinning a 20 HP idler will need to be much bigger than 1/3 HP. Also, I respect the fact that you prefer to balance the rotary converter with capacitors. But, the balance caps have very little effect on the amount of power the "tool" motor can deliver when the idler is as big or preferably bigger than the tool motor. I mention the minimal benefit from the balance caps because I made some dyno tests of the HP a 3 phase motor can deliver when running on a single phase circuit with various idlers. The data indicated that balancing caps were noticably beneficial for starting lightly loaded 3 phase motors from the rotary converter. It is my opinion that balance caps dont hurt but they have limited benefit related to the amount of tool motor power deliverable.

Jerry

The Cedarburg converters are like this, they are essentially just a starting capacitor and a relay to switch it in when needed. They are horrible pieces of junk, barely adequate for a water pump, I'd hate to see you use one on a CNC machine. Also, some motors run well on these (Bridgeport J-head motors, for instance) and some run hot, have little torque or otherwise misbehave.

Jon

Well the real deal here is that a fairly small amount of voltage imbalance causes a rather significant amount of motor heating--fine if it's cold in the shop but in the summertime it's probly something you don't really want.

Hi Jeffrey

Could you tell me more about the motor heating ? How much heat would we consider to be "rather significant"?

Jerry

THe term you want to google is "negative sequence Voltage" and heating Negative sequence is a mathematical construct, which is used to analyze power systems.

Positive sequence is normal balanced 3 phase rotating one direction Negative sequence is the same thing, but rotating the other way and zero sequence is the same on all3 phases.

You can add these 3 together in the right amplitudes and get any unbalanced condition.

Excess heat generation fairly closely follows the negative sequence number.

In essence (very roughly) the positive sequence voltage is trying to rotate your motor forward, the negative sequence is trying to rotate it backwards, the fight between the two ends up as heat.

jk

And in a ps, there are also positive negative and 0 sequence harmonics as well. When balanced, on all3 phases

1 (fundamental) tries to rotate forward 2 (2nd) tries to rotate backward 3 is zero sequence, because all 3 phase are in sync 4 is positive 5 is negative and so on.

jk wrote:

jk

My point , Jerry , was that he could use the motor he has and lower the start current by using a smaller motor to bring it up to speed . -- Snag

Hi Snag

I sure agree with you. That "good old" pony method is so reliable and uncomplicated that it would really fit well into a low budget system for a

20 HP idler. I got some unwanted surprises when I tried to use a too small pony. I wouldnt be surprised to learn that a 20 HP idler needs a several HP pony.

Jerry

I friggin give up....

WHY, WHY, WHY do you need a pony motor for a 20 horsepower idler????

A few thousand Mfd worth of starter caps, and it will SNAP to full RPM in a Fraction of a second.. WHY make it so extra complicated?