is it posible to use a 3 phase transformer on single phase 1

[RickHeid]

okay here is my question, I bought a bridgeport cnc mill and it has a 3 phase 240 to 60 volt transformer in it. The transformer looks like 3 seperate single phase transformers with primary h1-h2...h1-h2...h1-h2. Any rate the 60 volts coming out of each secondary supplys the power for one of the drive dc power supplies. I could replace the transformer with a single phase one and tie the 3 power supplies to it but I would prefer to use the transformer already there if possible. I wondered if the transformer can be wired as 3 seperate transformers so I can run off single phase. I would still have to supply the spindle motor with three phase but would use a vfd for that. Any help would sure be appreciated. Thanks in advance.

 

[mc5w]

Are your drive power supplies 60 volts single phase input? If so, you could get away with wiring the 240 volt primaries in parallel and running them off of 240 volts songle phase.

One product that you might want to try is Mirus International's 1Q3 LINEATOR(R) that does single phase to 3 phase conversion for VFDs and servo drives. DO NOT try to operate a VFD off of a rotary phase converter as the 5th and 7th harmonics from the drive will burn up the phase converter.

Mike Cole

 

[skiier]

When you say "Any rate the 60 volts coming out of each secondary supplys the power for one of the drive dc power supplies" do you mean you are supplying the Drive with a 3 phase, 3 wire, 60vac input or is it a 3 phase, 4 wire, 60vac input? Some clarification here on the type of 3 phase transformer connection would help; ie: wye-wye, delta-delta, delta-wye, you get the idea.
If the drive is recieving a 3 phase voltage then your single phase plan is pretty much shot. The drive bases it's DC rectification on a 3 phase supply. Connecting equal in-phase voltages sources across the terminals of your drive will accomplish nothing. It may even damage it.

Sorry, but if you still want to use that drive you are getting to have get a 3phase supply of some sort. Be it a single phase to three phase converter or your local utility - you take your pick.

The transformer not only steps down the voltage for rectification purposes but it also offers some measure of isolation form the supply it is connected to or vice versa.
Drives are largely silicone beasts and as such offer little in the way of protection for the utility or the electrical system which they are connected to should a motor or device connected to a drive either faults or short circuits somehow. The small impedance introduced by the transformer has a great effect in reducing the level of ground faults downstream of the transformer as well protecting the internal components of the system itself.

If you want your drive to run trouble free then you should use it as it was designed. Feed it the proper voltage.

 

[RickHeid]

Mike, yes the drives are single phase. You say to wire the tranformer primaries in parallel. I tried that removing the connections from the secondaries. I first wired one phase in and checked the current draw into the transformer, then added the second and checked again and it was drawing way to much current. I guess they affect each other because of the way they are physically tied together. Any other ideas. Oh and I would feed single phase into an oversized vfd. I have a 3 phase 25 hp vfd running the 10 hp lathe motor being supplied with single phase. Works fantastic, I set it up to ramp up slow for safty. It also stops on a dime with the vfd's electrical breaking and the motors mechanical break. Thanks for the response, Rick

 

[RickHeid]

skiier, The mill has 3 axis control. Each phase of the transformer supplies 60 volts single phase to each axis'es power supply. If I put single phase only into the 3 phase transformer only one of the drives runs. If I had a 240 volt to 60 volt transformer of suficient size, I would put that in and be all set. Then I'd just run a vfd on single phase to run my spindle motor. Thanks, Rick

 

[skiier]

Ok I have re-read the question and I understand what you have.
Yes, you can some times connect a 3 phase VFD to 1 phase supply. Most manufacturers provide this connection feature nowadays for the drive. We are talking about a t/f though, not a VFD.
Ok let me be more specific. Have you "rung out" the transformer with a meter so we can know exactly what type of connection you have? Can you physically see the connections? I'll bet you a million bucks you have a standard 3 phase delta-wye shell type transformer. Probably looks like a miniature power t/f doesn't it? Europeans love to use this wee beastie. Sorry, you can't use the "parallel- single phase" connection method you tried. You will just burn out the t/f. However you can probably use the 3 phase t/f to feed one axis drive- maybe two. You already know this stuff though.
The core of a 3 phase shell t/f is shaped like a "double-d" with 3 center core sections joined by 2 end sections. Over the center cores are wound each primary and secondary winding. The flux produced by each winding "sees" 2 paths over which to make it's journey inside the core. This flux splits thru the other core sections and then rejoins and returns to the coil from which that flux was produced - just like a parallel electric circuit.
When you connect the t/f to a single phase source it will act like a single phase t/f. The secondary winding that is on the same core as the connected primary winding will see the required flux density in the core and produce a near normal output voltage. However when you try your "single phase parallel method" you will have problems because you drastically change the flux density in the core. It hurts my head trying to think of the relationships involved with the flux density and crap.
You may be able to get 2 single phase sources out of this t/f though. If the t/f is slightly oversized for the application you may be in luck. Here's my theory.
The "additive sum" of the voltages of the other 2 secondary windings should be close to 60V. Since the other 2 secondary windings see the flux from the connected primary winding they will have induced voltages on them. Since the flux splits in 2 paths thru the core I am guessing each of the other secondary windings will have an induced voltage somewhere near 30V. 30V+30V=60V.
Give it a try. Got nothing to lose. Put a fan on the thing. A good cooling fan could increase the t/f capacity 20 to 30%. Maybe just enough to run 2 drives. Monitor the current draw and watch the temperature of the t/f. Maybe it could work.
The other alternative is to get a standard 480-120/240 t/f and connect it for the other 2 axis drives. Ensure that you tie the common points of both t/f's to each other and to gnd.

skiier

 

[RickHeid]

Thanks Skiier, after many attempts to get the transformer to work I decided to strip the bridgeport of the old controls and ordered 3 Larken stepper drives and will upgrade to pentium computer control. I have everthing orderd except an ac drive and I am looking for one to run the 2 hp spindle. Thanks again for everyones help. Rick