Machine Voltage/Phase Mismatch and Retrofit 3

[MitchusMaximus]

Good day, I am helping a machinist getting some equipment operational that he has purchased with mismatched voltages. I'll lay out what he has:
-240V single phase power
-51kVA 230 - 460 step-up transformer
-12.5HP 460V lathe
-3HP 575V mill
-A tight budget

My plan is to use an oversized 480V drive to feed the lathe, and tap off the output to feed a step-up autotransformer to jump from 460V to 575V, and use the transformer output to run the mill. I would use interlocked contactors for selection, so only one motor can be energized at a time, but my transformer selection raises a question I haven't run into before. Autotransformers by default seem to be Wye-Wye connected, with a neutral terminal for the line and load side. If I am feeding this transformer off a VFD I don't have a neutral to work with, is it possible to make this wye connection work? Or should I opt to try and get a custom delta-delta transformer built for this application?
Thanks for the help, it is shocking how often people will buy auction equipment without considering a means to achieve the power requirements. If you weren't looking for it, it's not a good deal.

 

[jraef]

Don't feed the 460V drive output to another transformer.
Just boost the 480V single phase with a buck-boost transformer to feed a 600V drive.

Just a fair warning; not ALL 480 and 600V drives are capable of running from single phase power, because they are looking at the input and don't provide the ability to disable the Phase Loss protection. So make sure to double check with the suppliers.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[BrianPetersen]

Do the lathe and mill need variable-frequency power? These aren't CNC machines, are they? Just regular old skool manual machine shop equipment?

If it's regular old skool machine shop equipment, then it's going to contain regular old skool induction motors that can be swapped out with other motors that are meant for the voltage that you have ... That type of equipment has been around since long before VFDs were invented.

 

[waross]

I agree with the suggestion to first look for suitable motors.
But for general information on three phase autotransformers.
I did many voltage conversions both ways between 480 V and 600 V. with two autotransformers in open delta with no neutral needed.
Take A to B phase and boost it from 480 V to 600 V single phase, for A-B-B+
Take A to C phase and boost it from 480 V to 600 V single phase, for A-C-C+
You will have usable 600 Volts between B+ and C+.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

I have the following suggestion for your consideration.
1. Based on 240V single phase power. 51kVA 230V - 460V step-up transformer. 12.5HP 460V lathe and 3HP 575V mill. Attention: Check to ensure that 460V lathe and 575V mill are single-phase. If they are three-phase, it would NOT match/work with your single-phase power source.
2. Assuming the step-up auto-transformer is single-phase, I suggest to rewind it to Tap1- Neutral 240V (connect to single-phase power source); Tap2-Neutral 460V (lathe); Tap3-Neutral 575V (mill).
3. Caution: If your power source is single-phase; it is possible, but involves complicated circuitry to run three-phase motors.
Che Kuan Yau (Singapore)

 

[edison123]

How?

@OP


Single phase power to run a 3 phase lathe and a mill?

Cheaper to get 3 phase supply.


Muthu

http://www.edison.co.in

 

[MitchusMaximus]

My customer was looking into getting 3-phase brought in, but it sounds like it was even more cost-prohibitive. It is good to know that the load side auto transformer can work without needing a neutral connection, my transformer guy says that with a little upsizing it will be able to absorb the VFD's output harmonics without causing damage. This is an odd situation where I'm trying to play with three different voltages for sure, and it looks like the motors are custom made for the machinery so replacing them with 230 single phase or even 3 phase motors to use with a phase converter is probably not viable.
Stepping up the single-phase input to 600V and then using a 600V drive would definitely be another way to go about this. The drive I am using has programmable options for two different motors, with a digital input assigned as the selection input, and this would allow me to swap between 600V and 480V depending on selection without causing a potential user error nightmare. That is something I will consider with my customer.
Thank you for your insight, all.

 

[waross]

Try pricing this out:
1. A 240 Volt to 480 Volt transformer.
2. A 240 Volt rotary phase converter.
That will run the lathe.
Pair two dry type, 480:120 Volt dry type transformers connected in open delta to the 575 Volt motor. Switch/control the transformers and motor as a unit.
Sizing the dry type transformers is quite simple. The 120 Volt winding must carry full rated motor current.
Example.
If the motor draws 4 Amps at 600 Volts, a three phase isolation transformer should be rated at 4 Amps x 600 Volts x root 3 = 4.2 KVA
Auto transformers: 4 Amps x 120 Volts = 0.48 KVA
0.48 KVA times two transformers. = 0.96 KVA capacity versus 4.2 KVA capacity for a conventional transformer.
Economy setup;
Manual switch to supply the 240:480 Volt transformer.
Manual motor starter from the transformer to the Rotary Phase Converter.
Manual motor starters from the Rotary Phase converter to both motors.
Electrically it would work to energize the 240:480 Volt transformer with a motor starter and direct connect the 240:480 transformer permanently to the Rotary Phase Converter, but that may not be accepted by the AHJ.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ Mr edison123 (Electrical)7 Jun 24 15:54
".. (Caution: If your power source is single-phase; it is possible, but involves complicated circuitry to run three-phase motors.) How...?"
1. In some remote locations only single-phase is available from the utility. Three-phase is NOT available or cost prohibitive.
2. Google it. There are information on using capacitor to create 3-ph to run the 3-ph motor, on the 1-ph source. It is complicated to a layman, but certainly NOT to your good self a respected guru, and a regular contributor in this forum.
3. Rotary Phase Converter is another possible solution, but cost prohibitive ! If the need is to run some SMALL kW 3-ph motors.
Che Kuan Yau (Singapore)

 

[edison123]

che

No, you cannot produce 3 phase power from a single phase source 'with complicated circuitry'. The scenario you post of using capacitors is used in 3 phase source where one phase is lost (or cut intentionally by utilities).

And thanks for the compliment, however backhanded it may be.

Muthu

http://www.edison.co.in

 

[waross]

The law of conservation of energy comes into play when developing three phase from single phase.
Example;
A 5 HP motor draws 5 Amps on 480 Volts three phase.
A similar 5 HP motor draws 8.66 Amps on 480 Volts single phase.
If you try to run the three phase motor on single phase with capacitors, at 5 HP output the energized winding will be overloaded with the 8.66 Amps.
So, while it may be possible to run the three phase motor on single phase with capacitors, the safe HP will be reduced.
And there are more complications with a wye connected motor. That is a NEMA 9 lead or 3 lead motor with a buried wye connection, or an IEC motor connected in wye for the higher voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

OP,
There are some excellent suggestions on here. One suggestion I have not seen mentioned is a bridge rectifier. It's been years since I've used this product and I forget the manufacture, but we commonly used it on remote well sites, where only single phase was available. Line to the rectifier, then DC to the DC bus of the drive (not all drive let you do this). This method was actually suggested by the drive manufacture. Even though some drives say they can handle single phase and I've seen plenty of DIY installations over the years, that worked....I have never been a fan of this practice.

I am suspect of your statement that the motors appear to be custom, not saying you are mistaken, I've just never seen it, especially for the HPs you reference. I'd circle back around to BrianPetersen's suggestion to first see if it's possible to swap these out for single phase or like voltage 3 phase. You might even consider taking pictures of them and nameplates and going down to a local motor repair / rebuild shop to see if they can assist you, might even be willing to swap you out, and maybe with the help of an (as Johnny Cash would say) A-dapter kit. This could be much cheaper than purchasing and installing all of the equipment myself, you and others have mentioned.

 

[waross]

A lot of machine tools have "special"motors.
A special motor is normally a standard frame size with the exception of the shaft extension.
Special shafts are not uncommon on face mount or flange mount motors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

Thank you waross, this is good information.

 

[MitchusMaximus]

Thank you for your suggestion, we actually are the local motor repair shop, and we might be able to rewind the motors to get them into 240V 3-Ph, but we couldn't reconfigure them to be single phase, and two rewind jobs would be more expensive than the drive + transformer route. Plus I would still have to set him up with a phase converter/VFD after the fact. One of these motors is Italian, I think the other is custom Chinese, but neither of them have a standard frame size listed which would make finding subs a real bother. I'll attach the nameplates just for kicks.

 

[Heaviside1925]

OP,
HaHa, your original post makes a lot more sense now, thank you for the context. I agree, your labor costs of rewinding would make it prohibitive. I haven't messed with the Chinese custom ones but I have seen the Italian one-offs before, Mamma mia! I will point back the ideas BrianPetersen and waross suggested in their posts. Do you really need VFDs? You said your customer is a machinist. If he will be using this equipment on a daily basis or this is part of his business, I can see where the transformer - VFD route would be the cleanest and could be repaired fairly quickly with off the self-equipment to minimize downtime. It should also pass muster with the AHJ, provided everything else was up to code. If this is for personal use and AHJ is not an issue, do you have a "back-room" inventory that you could build him a couple rotary phase convertors? You have likely considered this based on what I read and your competency level but I'm just throwing it out there because a blind squirrel finds a nut or something like that........

 

[MitchusMaximus]

Honestly, my first suggestion was to just get a phase converter and be done with it. Would have saved me a lot of trouble. We used to build rotary converters but got out of that game, we still provide digital phase converters and a company in my province manufactures rotaries, so there are options. Problem is this guy does not want to spend the money on a proper converter, and the VFD + transformer route is cheaper- although I don't think he fully understands what a VFD can and can't do when used as a phase converter. But I've given him suggestions on what can be done "on the cheap", but if he wants it done right for the easiest use testing or using equipment, he's gonna need to shell out the cash for a proper phase converter.

 

[Heaviside1925]

Good luck OP. Let us know when he comes back after frying two drives.

 

[LionelHutz]

You can run the 575V motor up to 80% speed on the output of the 460V VFD, and possibly even faster depending on its load.

 

[waross]

Good catch, Lionel

You can run the 575V motor up to 80% speed on the output of the 460V VFD, and possibly even faster depending on its load.

And up to 80% load at rated speed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!