Post By Jraef
A few transformer questions 3 phase 480 to 240 to 380
I would like to hook up a machine to power in order to test it before buying. The machine is 3 phase 380 volt (22 amp?) . I bought a properly sized 3 phase 240 to 380 transformer thinking there would be 240 near by and also to use if I buy it. I was wrong and there is only 480. I can't find a cheap 480 to 380 transformer but I have found a local auction with a 480 to 240 that might go cheap. The transformer at the auction is much larger KVA wise than I need. Two questions
1 Any negatives or problems using the two transformers to get the right voltage (both long and short term)?
2 Any negatives or prooblems using to big a KVA for the 480 to 240 again both long and short term?
The only down side to using a much larger transformer then strictly required is that the magnetising current is higher. This a current that sets up the magnetic field, in small (50W) transformers it may be be up to 10-15% of the running (useful) current. Above the 3 KW level it is down to a loss of 1-3%. So if you have a very overrated transformer, the losses seem higher. 2% loss at rated 50KW (full rating) = 20% loss @ 5KW loading.
This is a "I am testing the machine" scenario, right?
On edit: Yes, this isn't the correct scenario, you have only a higher voltage, but I'm leaving this post up, as generic advice.
Are you taking any cuts, any heavy cuts? If No, then a lesser voltage can be applied, since you aren't requiring maximum force, you aren't requiring continuous duty. You will get less torque at the expected RPMs (so, less HP).
Just bring along a helper (with a wooden dowel) to assist in holding the contactor coil (if the coil can't hold at the lesser voltage.
If you "know what you're doing", you could quickly switch the input taps on the machine tool transformer, and avoid the issue with coil voltage; just remember to switch it back.
I say all this, because:
You aren't expecting to stress the motor, so the overload heaters aren't an issue.
You aren't expecting to stress the machine, so you aren't requiring maximum motor torque.
You aren't running production, so the machine has little chance of overheating.
You could also start a "Test a machine at a lesser voltage?" thread, and have others vet what I am proposing.
Milacron once spoke of a 220/440 wood planer that ran at the lesser voltage, though "it seems weak" was the seller's opinion.
In some situations, a 400 volt class VFD could be configured to produce 380 volts from 480 (3 phase) input, but then you run up against "no switches between the motor and VFD" best practice. A clever guy would just jumper the contactor (be careful with a mechanical interlock reversing contactor), and control (only) the main machine motor by the VFD panel.
If the seller is ignorant of the possible work-arounds of "wrong voltage, can't use it", I suggest putting the VFD in a "black box" , or not revealing how/why you are comfortable with testing the machine at a lesser voltage. Otherwise, you run the risk of losing your negotiating advantage.
If the machine is some exotic with magnetic clutches, or multi-speed motor you are just screwed...sorry.
Alright, Too Much Information...
Not entirely so, at a lesser voltage, the motor speeds will be less. How much less depends on the motor characteristics which we don't have.
380V 50Hz motors will run at 480V 60Hz without problem. They will however run 20% faster (60/50). This is because motors run best when the ratio of voltage and frequency remains constant, or within 10% of the design. So the V/Hz ratio of a 380V 50Hz motor is 380/50 = 7.6:1 and the V/Hz ratio of a 480/60 supply is... drum roll please... 8:1, 95%. No problem.
If there are control circuits inside of the machine that are tapped off of the line supply, that may be a problem. In IEC countries that use 380V 50Hz, the power supply is always Y, meaning it is 380/230V, with 230V being the phase to neutral voltage. So they tend to use that 230V as the control power a lot. If you feed 480/277V that we have here, you are over-exciting all of your control devices, such as starter coils etc. But check into it, often times, and especially if the machine is newer, Europeans are using 24VDC controls and it will have a separate 24VDC power supply. Most of those power supplies will accept a really wide input voltage range.
Thanks for the help. This is an old german CNC mill with lots of other electronics besides just the motors. Because of the electronics and the fact that it isn't mine I would rather not try things that might work. The mill had at least a few things changed for the US market to deal with the 50Hz to 60 Hz from the factory. I think it has 9 motors in all and all of the controls so I would rather not mess with VFD or try to run it on 480. I mainly wanted to see if using the two transformers will work and if there are any problems with that set up? The transformer I might be able to buy is a Square D 112.5 KVA HV480 LV240 I think it will go for less than scrape. Will it work and what are the problems with using it? At less than scrape price I really can't lose buying it. The 2% loss dosen't seem to be a problem. If there is a problem using either of these then I will try and find a 480 to 380 transformer of the right size.
only issue that jumps out at me may be a show stopper: 115kva is BIG - it will have a big inrush current at startup. you may not be able to turn it on without always popping the cb feeding it.
115kva/230v/1.73= about 290amps rating so inrush of 4-6x typical? maybe too much to turn on at home? the inrush actual level depends on stuff we dont need to go into here but can be anywhere fro almost nothing to this 6 even 10x rated current level. if you read the particula xfmr spec close you probably can find the inrush rating for sure but i suspect it wont be less than 4x, more likelly 10x.
if you are ok with popping cb 9 out of time turn-on atempts it prob would work. yes, magnetizing current is way higher than you need but you dont pay for that so the standby current you pay for is not horendous, but expect say 10% of rating so maybe 20-30 amps just to be powered up coming and going up and down your wires... sounds way to big - even for a 'goog' price. but this is all just my opinion.
That is just the kind of help I was looking for. The mill is in an old warehouse south of Pittsburgh and I'm not sure what size service is there other than the 480 3 phase. I think I'll just keep looking for the right transformer at the right price. That one might have worked for the test but seems to big for anything else. Physically it is big enough that it would be a pain to move the transformer 3or4 times for just the test.
I suggest asking to speak to the building's maintenance man, the electrician, etc. You could always look at the service entrances, see what the meters indicate. An "old warehouse" may still have a service of 240 volts. Is the seller under any pressure of a deadline? The seller has a vested interest in assisting your inspection of the machine.
south of Pittsburgh and I'm not sure what size service is there other than the 480 3 phase.
If the correct power was "around the corner", "down the hall", etc. perhaps a reel of SO rubber cord would be an attractive option?
I suspect you need to start beating the bushes, asking around, see if you can buy, or borrow, or rent a suitable, more portable temporary power option. Not sure if I would suggest renting a genset. Some electrical contractor or machinery rigger may have a transformer they don't care to sell, but would provide its use with the purchase of the firm's services.
One more option...
I'm not batting 1000 with my electrical comprehensions, so Mike Kilroy or others should sign off on this proposal:
What's the chance there are "derived systems" in that warehouse? Perhaps for a specific machine, a specific panel, etc.?
If that 480 primary was actually a 240/480 primary, you could use the primary as an auto-transformer to derive your 240 volts.
This idea may be a "little knowledge is dangerous"; don't try it unless our more knowledgeable members agree to it.
your thinking man! i agree; if seller wants to sell it they should be willing to power it up for you. i cannot imagine a 460v 3ph shop with out 230 somewhere..... i dont think it would be worth taking chance of wiring wrong and breaking something to do the 240v input tap but i do think it would be in the sellers interest to wire to a 240v secondary somewhere on another machine and run so wire over...
The seller is just really the owner of the building. She is in NJ and the warehouse is just south of Pittsburgh and untill I told her about it she didn't know the mill was even there (left buy a tenant along with a bunch of other stuff). She has already made a deal with a scrap man to remove everything not bolted down. There is no one on site, and this is just a warehouse not a shop. She could care less about powering it up all she wants is new good paying tenants to move in to an empty clean space. I'll see what I can do.