Electrical question re transformers

[GoCreateParticipant @gocreate]

VA rating of transformers conected in series

[GoCreateParticipant @gocreate]

Hi

At the risk of showing how dumb I an at electrics, I have a question regarding ratings of transformers connected in series.

If I have 2 transformers 12v 1amp they are rated 12 x 1 =12 VA?

If I connect them in series I get 24v. Will the current rating be the same? so I end up with an ac power supply of 24 VA?

I expect the current rating cannot increase because the wire gauge on the winding can't change.

Reference the following diagram.

 

The reason for my question is that I am looking at Ben Fleming's pulse EDM, he uses 3 off 24v 10 amp transformers in series to get 72v. I was wondering if I could replace these with say a single 75v 750VA transformer, or 2off x 36v 350va transformers? I don't at present know if this approach would be OK, available or cheaper, just an idea.

 

Thanks in advance.

 

Nigel

 

Edited By tractionengine42 on 04/06/2015 16:52:56

Edited By tractionengine42 on 04/06/2015 16:54:03

[frank brownParticipant @frankbrown22225]

Spot on, you can connect transformers in series, just check that its the right way round or you get 12V -12V =0V. The maximum current will be determined by the lowest current rating of either transformers. The trouble is when trying to connect transformers in parallel as the smallest miss match in their voltages will cause large current to flow in them.

Frank

[MuzzerParticipant @muzzer]

That should be fine. You can series up secondaries like this. Just make sure you don't exceed the max current rating (= temperature) of any of them. And don't parallel secondaries.

BTW, if you connect one of the secondaries antiphase to the first, you will get something close to zero voltage out of them. In which case, simply swap one of the secondaries (start and finish connections) over and you should then see 24V.

Murray

[MuzzerParticipant @muzzer]

Snap!

[Brian OldfordParticipant @brianoldford70365]

I did a slightly similar thing using PC SMPSUs. The only thing I added was to ensure the frame of the PSUs were isolated from any metallic chassis and add inverse "flywheel" diodes to the OP of each PSU to ensure easy start-up.

[Brian GroomeParticipant @briangroome43093]

Murray, I would be interested to know your reason for being against paralleling transformers.

Provided that the secondaries are identically wound then there will be no circulating currents, and the output current available will be doubled at the same voltage. Brian

[Clive FosterParticipant @clivefoster55965]

Paralleling random transformers is generally frowned upon because its pretty much impossible to know how closely they are matched unless you have some pretty sophisticated test equipment. If you have the test kit odds are you will be in a position to obtain the proper transformer for the job anyway rather than resorting to a "field expedient" solution.

However its quite common to find dual transformers wound on a single core which have two identical primary and secondary windings which can either be run as an independent pair connected in series or parallel give either twice the voltage at the same current as running independently or twice the current at the same voltage. Maximum power being the same in all cases.

Clive.

[Bruce EdneyParticipant @bruceedney59949]

In answer to the OP's question. Yes the VA will be 24VA as the VA is Volt Amps. so 24v and 1 A is 24VA.

[Neil WyattModerator @neilwyatt]

The problem with paralleling transformers is that, unless they are near identical they may have very different output voltages depending on the load they see.

Lots of info HERE.

The simple answer is that you can parallel identical transformers or windings on the same transformer that are of the same voltage (as long as you get the polarity identical). Non-identical transformers are very unlikely to have suffiuciently similar performance.

Putting them in series should cause no problems at all but the current rating is that of the lowest rated transformer, not the average of the all.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I've had no problem paralleling my two identical 110V site transformers to run my 9" angle grinder. I did do some voltage checks between the two before going headlong into fully connecting them and measured zero voltage difference between them with my digital volt meter, which has a resolution of 0.001V AC, I then did a resistive load test between each phase and got zero amps and then on opposing phases and the readings were the same (can't remember what they were) I then wired them in parallel to a 110V socket and allowed them to sit in a no load state for a couple of hours and found no rise in temperature in either of them. I then did an hour or so of general grinding with many starts and stops and there was still no rise in temperature in either of them.

I was then satisfied that all was ok with the setup and the next day went down to London and used them every day for a week cutting stone slabs for a niece of mine, for her patio and they still didn't even get warm even though they were switched on each morning until each evening.

Regards Nick.

[MuzzerParticipant @muzzer]

Paralleled secondaries will tend to share loads due to their fairly soggy load regulation but if you have a different number of turns or winding distribution, the risk is you may get recirculating currents and higher losses. It's quite common to have multifilar primary and secondary windings but they are carefully designed (hopefully) to avoid problems.

If you are prepared and able to go through the palaver of measuring the voltage, checking for recirculating currents and checking for temperature rises then there's no big issue. Similarly if you are confident in the parentage of the transformers and know they are pretty much identical, then there is not likely to be a major problem either.

Otherwise, the safest practice is not to mix and match different types of transformers, particularly by paralleling secondaries. This is pretty much the failsafe approach, as there is no possibility of recirculation. Generally, if you have the choice, you are better off connecting several low voltage windings in series than paralleling several higher voltage windings – particularly if you do not consider yourself expert in these matters, which is what the OP stated.

Murray

[GoCreateParticipant @gocreate]

Thanks everyone for your reassuring and informative responses.

Nigel

[Gordon WParticipant @gordonw]

Nick Farr, off topic question -Why do you need to use 2 transformers to run an angle grinder? My 9" 110V grinder runs no bother off one transformer, a cheap ally wound 1.8kva. continuous 3 kva intermittent. I'm only asking for interest, and to check I'm not doing something wrong.

[JAParticipant @ja]

Nigel

A question, have you found a source of sensible priced transformers?

I looked at Ben's EDM design and was put off by the cost of transformers. Also it seems that they were expensive for Ben.

JA

[GoCreateParticipant @gocreate]

Hi JA

I have only made a tentative look so far and found equivalent toroidal transformers @ 104 GBP from Maplin for 3 off. I thought that I could perhaps get away with a single transformer at a lower cost something like a 36v 0v 36v with would give 72vac but then it would have to be around 700va.

I was wondering if 60v ac would be OK as there seems to be a better choice of transformers, but the cost doesn't appear to work out any cheaper for a single large transformer. Ben does say 50vac should be workable i.e. 2 of his transformers instead of 3.

Anyway, I will be looking further and will let you know.

Nigel

 

Edited By tractionengine42 on 05/06/2015 13:50:14

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Gordon W on 05/06/2015 11:45:46:

Nick Farr, off topic question -Why do you need to use 2 transformers to run an angle grinder? My 9" 110V grinder runs no bother off one transformer, a cheap ally wound 1.8kva. continuous 3 kva intermittent. I'm only asking for interest, and to check I'm not doing something wrong.

Hi Gordon, when in my garage (but outside actually) I have a 110V transformer rated in the range of 4KVA continuous use which is no problem for my 110V power tools, but is not very portable and it is fixed in my garage anyway. My Bosch 9" grinders have a full load of 2000 Watts, my two site transformers are 2KVA max each. So OK you may well believe that one 2KVA transformer should run my 9" grinder, and it will for short periods without any bother. Transformer outputs are always rated as a KVA rather than in watts as various things affect the actual wattage that you may get out from them. Using cutting discs will very often make an angle grinder pull its full load and even a little bit more if you get a bit to heavy handed with them. With this in mine and constant starting and stopping and in continuous use will put my 2KVA transformer up to its maximum and the trip will be triggered within a short space of time. I did not have the time to wait around for the transformer to cool down enough to reset the trip, nor did I wish to subject my transformer to constant overloading.

Your transformer should be able to run your grinder for a fair while before tripping, if it ever does, but normal grinding operations don't normally pull at the full load rating. Your grinder may be rated lower anyway.

Regards Nick.

P.S. sorry for going a bit off topic Nigel.

[Ian S CParticipant @iansc]

It wouldn't be the first time I;v rewound a transformer for a different voltage, big old valve type TV sets used to supply good transformers for doing that sort of thing, The transformers were about the same physical size as the one in a microwave oven, only problem with those ones, the laminations are welded.

Ian S C

[AjohnwParticipant @ajohnw51620]

Transformers are often rated for an insanely high temperature rise at full load. Worth looking at the specs if some one intends to do that. There are sometimes industrial rated ones about – much lower temperature rise but they will be a lot larger. The regulation with load is also worth looking at – that is determined by the winding resistance to a certain extent so poor regulation will often mean a high temperature rise. The 2 tend to go together,

Personally if for something serious I tend to buy transformers of RS Components or Farnel as specs will be available or Ebay at times if they give a brand and specs can be found or there is sufficient info in the listing.

Given Nicks apparent problem I might use 2 well under rated when in parallel too even though it's frowned upon. Sounds like the regulation will be crap anyway and that will help them share.

It's sometimes possible to wind a few extra turn on a normal transformer using insulated wire to get a bit more volts but the coupling is likely to be poor. RS Components did do 100VA transformer kits. They may still do. I have built a couple in the past using these. They specify the turns per volt and it isn't too difficult to lay a neat winding by hand. Only thing is they may gave uprated them now so some bigger number but the specs should give a clue to how hot they will get and that can be more or less be guestimated pro rata with load.

John

–

[Johnboy25Participant @johnboy25]

Neil….

"The problem with paralleling transformers is that, unless they are near identical they may have very different output voltages depending on the load they see.

Lots of info HERE."

Arr… Reminders me of my college days! Almost a distance memory!

John

[Ian S CParticipant @iansc]

When I rewind a transformer, I strip off the secondry, and wind on a new one to suit. As my transformers are for low voltages, there are not a great number of turns.

Ian S C

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