Auto Isloating Transformer

There are a number of questions that you put below, that suggest that you are unsure of what you are doing. Electricity kills!

415 Volts is the UK between-phases voltage, so there is an implication that you are running on a 3-phase supply.

I am unaware (and I stand to be corrected, light curent background) but I have not heard of an autotransformer being used on a 3-phase supply

(Reply cross-posted to alt.engineering.electrical for a better response?)

That's right he owns an Engineering company. Fully paid up card carrying member of the human race as opposed to .......................

What more can I say?????????

******* ** ** *** ** **** * ** ********** *****

Lionel if you have the odd 5 or 6 hours could you give Harricot Bean a quick resume of your credentials??

-- Regards,

John Stevenson Nottingham, England.

Lionel,

Your Lathe has a transformer fitted which takes the incoming supply has, I hope, several tappings i.e. 380 - 400 - 415 and I assume from your comments that the 3 phases are set on 400, if you change the wires to 415 this will stop the over voltage. Your general running voltage may read slightly lower during the day but this should not effect your machine. I would also think that the over voltage is your actual problem, you do not describe the fault but I wonder if you have a sensitivity problem with the CNC because most transformer voltage supplies allow for a + or _ error in the supply and your machine should cope with this.

Martin P

Martin,

In the day I am getting 420 volts. Checked one evening and got over 430 volts, I think it was 436 volts. Never been around to check immediately after a shutdown to see if we are getting over the 440 volts.

Specs say machine needs 400 volt plus/minus 10% (360 - 440 volts) Just had it serviced yesterday and the engineer has said that he has seen this before and I need an isolating transformer, and set at 380 volts should stop the over voltage in the evening.

I suspect I am getting evening power surges that go over the 440 volts causing the machine to fault out. Error Message reading OVERLOAD BRAKE RESISTOR is caused by over voltage in the supply.

There is a transformer in the machine, but I only see 240 volt tappings, and the 400 volts in. If there was adjustment for output the engineer would have changed any links for this.

Very nice machine, Emco with driven tooling run by PC in the back running Windows 98 and Siemens 840D control.

Just need to find out where to get a transformer from. Engineer will get his company to quote me one, thought I would ask on group to see if anyone has used these before.

Thanks

Lionel

Lionel, As a service engineer myself I would not assume that he would change the voltage because he has to make money for his company :-)) Sorry for being cynical that just comes with middle age, I would also like to add that I dont work that way :-))

Martin P

That would be two (open delta) or three single phase transformers in a single assembly. In the industry, such an assembly is referred to as 'a transformer' rather than 'transformers' if it is nameplated as a single unit intended for three phase operation.

If someone told you to get an 'isolating' transformer, make sure you get someone else to do the actual design and installation.

An autotransformer is what you need.

Lionel

If your machine is rated to operate from 400V +/-10% then it should operate day in, day out, on 440V which is after all within its specification.

You don't state the precise fault but in any event the it is only likely to be the electronics of the lathe affected and that probably only operates of only one off the phases. If a transformer were needed it would only need to be rated at a few watts just to feed the electronics.

Can you tried to contact the manufacturer?

Ian Phillips

---------------- Three phase autotransformers are quite common. Possibly not in this size range. Many 3 phase motor starters use an autotransformer but such units would not be suitable as the ratio would be fixed and they would be disigned for intermittent use. High voltage transmission systems often use autotransformers -e.g. 380/240KV and would likely be star-star with a tertiary delta winding to take care of triplen harmonics.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

>

How is the power supplied to your work place?

If you are in the UK for instance you may well have 11000 volt feed coming into a distribution transformer to supply 433v (unloaded) on the secondary.

Such transformers commonly have tapped HV windings to accommodate variations in the supply voltage; these taps are generally in five 2.5% steps i.e. Tap position 1 = 11550/433, tap position 2 = 11275/433, tap position 3 =

11000/433, etc - check the rating plate of the transformer for the exact details.

So for example if your transformer is currently on tap position 3 and you changed it to tap position 2 you would reduce the LV voltage by 2.5%, from

420 to 409.5 in the day and from 430 to 419.25 in the evening which would seem acceptable for your needs. It would also give you an acceptable phase voltage for any single-phase loads that it may be powering, (assuming that you are in the UK).

If you can change the tap position in this way please be aware that the tap switches are generally for use OFF CIRCUIT and NOT OFF LOAD which means that you MUST ISOLATE THE HV SUPPLY BEFORE CHANGING THE TAP POSITION.

IF THERE IS ANY POSSIBILITY OF THERE BEING A BACK FEED TO THE TRANSFORMER YOU MUST ISOLATE THE LV SIDE ALSO.

I've seen several failed transformers and heard of one engineer being killed in the UK this year through failure to properly isolate the supplies to transformers before changing the tapping position, it's not worth saving a few minutes switching time for.

Hope this is of some help.

Best regards John

If I can put my 5 penath in here!!!

There is no reason why an auto transformer cant be used on a 3 phase supply.......... are you cahps aware of the difference between an auto transformer and an isolating transformer???? They are very different beasts!

Also, 430V is prob. not too high for the input to the lathe.....if it is causing trouble then it will only be with the CNC section.... which prob runs off single phase anyway...so a much smaller (and cheaper :-) transformer could be used to run this section only if that really is the problem.

Adrian

the small trans for the electonics sounds right, still i don't know a lot about it... if you do need a trans try morite windings, i found them through a google search, they custom wound a transforem at a very good price for me..

-- richard

Try Power Capacitors - the people who make the Transwave 3 phase converters - they sold me a 5 Kva 235v to 415v autotransformer at quite a reasonable price as it was one of their standard sizes from the converters.

Regards,

Andrew Mawson Bromley, Kent

er it's you again, landy paint removal! by the way did you see the post about having wheels blasted with a polymer bead? I checked and it appears places are stripping cars and leaving primer and surface coatings but removing the paint!! sounds rather promising.. I should have said I gave up commercial blasting in 1989.

so andrew what sort of dosh was the trans? I got a single phase 7kva 240-415[40kg] and it cost 120 quid.. I would like a cheap trans then i could have another convertor, one tuned especialy to the compressor.. I have yet to sort out proper switch gear to the compressor which switches more capacitance in and turns on the convertor.. I am manually switching it on when i hear the switch shut at the moment!

-- richard

Richard,

Yes we pop up all over the place don't we !

The transformer was circa £100 iirc and they delivered it to their stand at the Wembley exhibition for me - (in fact even took it to my car as I was suffering with back problems at the time).

Mine was to provide 415 input to an inverter drive running a three phase hydrovane compressor - I had the inverter so it was cheaper to go that route than change the motor, and it has the benefit of controlled acceleration which keeps the noise down.

Andrew Mawson

thats sounds good, i was fairly fit but years of watching the telly and surfing have taken their toll. when i first cobbled my converter together i didn't have a box to put it in. I decided to put it on a shelf, this must have looked like something from the strongest man competition. I had to pick it up and get over two motorbikes and then lift the trans up onto a 5' high shelf..... needless to say i didn't rush to move it..

My converter runs my compressor ok but i have never seen a motor start so slowly, well i have but not on a compressor. I was tempted by a very cheap 3 phase in vfd, it would fill the gaps in my mills speed range nicely.

I am suprized at the price, i asked the phase convertor man' for prices and most of them wanted a lot more than that, thats why i had morite wind me one..

-- richard