I Have an issue with an electric gate motor, it originally had a Toroidal 250va 13.5V. Naturally this is an unusual sized transformer and the space allocated makes a replacement difficult.

The Unit is marked as having a max capacity of 200W.

I could get a 225VA/15V TX that will not fit and will require me to put it in a custom enclosure which is just a pain or i could potentially get two Toroidal transformers and put them in // 100VA/15V.

I Understand its not 13.5V but with the fluctuation in grid supply i do not believe 1.5V will make much of a difference, it also appears they rectify this voltage so i am assuming there is a regulator somewhere. TAU K125M https://www.automatismes.net/doc/Notice%20TAU%20K125M.pdf

Schematic VTX-146-100-215 (2x primary 2 x secondary) in // = 4x primary 4x secondary Schematic VTX-146-100-215

The only concern I have with connecting them in // is checking for circulating currents, as it's been a long time ago since I conducted this test and just wanted to double check. My Primary voltage here is 230v

Primary would be Blue = Neutral, (Grey + Violet), Brown Active

Secondary would be join Yellow + Red together and measure voltage across Black and Orange if i get a reading of 0 that would indicate i am in phase and no circulating current.

Now the question that i have coming around to the second transformer what would that test look like?

I also find some conflicting information on the unit itself. The manual says 50W but the name plate reads 200W. looking at the motor it is a MP56M. So i am going to connect my Variac (500VA) to the unit and test it under load to see what it actually pulls, as i am wondering if this transformer was just oversized to begin with.

enter image description here

  • \$\begingroup\$ a simple test is to run them with no load for a bit and see if they warm up. \$\endgroup\$
    – dandavis
    Commented Sep 25, 2023 at 23:34
  • \$\begingroup\$ you can drop 15v AC to ~13.5v AC with a chain of ~3 10A diodes in series, then repeat that, reverse, and parallel with the first chain to form a short "magic wire" that drops 1.5v both directions. Note: you might need two of those paralleled in order to handle the current, didn't realize the motor was so beefy. A block of a dozen diodes would look cool. \$\endgroup\$
    – dandavis
    Commented Sep 25, 2023 at 23:49
  • \$\begingroup\$ Its funny you mention that, i had a thought, if you look at the pcb secureentry.com.au/product/tau-k125m-control-board looks like they have 2x 10A diode in series, after the filter capacitors, that would take their 13.5 to 12.01v ac i wonder if this transformer was just used as they had them. i might be able to use a 12v TX and just remove the diodes. \$\endgroup\$ Commented Sep 26, 2023 at 1:27
  • \$\begingroup\$ @epicfatigue - Hi, You used the text box below labeled "Your Answer" to post an "answer", but it was asking a question. This isn't allowed on Stack Exchange, as it breaks the whole "Q&A are in different places" approach. It is also too late to extend the question, as you have already received an answer to your original question - extending the question would make that answer look incomplete (see discussions about "chameleon questions" on Meta.SE). \$\endgroup\$
    – SamGibson
    Commented Sep 26, 2023 at 8:04
  • \$\begingroup\$ (continued) My least bad option is to treat the question which you posted as an answer, as a clarification question to the answer you got. Clarification questions get posted as comments on Stack Exchange. Therefore I moved your post to be that. FYI If you want to add image links in a comment, you can use this technique. Note that the draft answer (or question) must not be submitted. It is used only to upload the image & is then discarded. Thanks. \$\endgroup\$
    – SamGibson
    Commented Sep 26, 2023 at 8:04

1 Answer 1


Your plan to test the motor to determine exactly what current it pulls is a good one. From there you can select a suitable 50Hz/60Hz mains transformer. If you cannot find a single transformer to supply the power required under all operating conditions, then using multiple transformers is an option provided that you follow a simple rule:

At least one set of windings must be connected in series, rather than parallel.

For example, you could use 2 x 230V/6.3V transformers, each with its primary winding connected across the 230V AC supply, and with the 6.3V secondary windings connected in series. This arrangement forces the current in the LV windings to be equal, which avoids the issue you raised (circulating currents, leading to unequal sharing of the load current which could cause problems).

  • \$\begingroup\$ Thanks heaps for your reply, i am unable to obtain a lower voltage transformer at a cost effective rate. I might have to suck it up and just buy the 200va model that will not fit and make an external housing for it. This would mean i would only have 1 transformer with two primary and two secondary, one side would be in series and the other side would be in // which should not give me circulating current correct ? \$\endgroup\$ Commented Sep 26, 2023 at 1:20
  • \$\begingroup\$ If i do end up going down the route of two transformers (logically 4) would i have issues wiring them like this? Wiring diagram Noticed i missed a line on transformer 1 but that would have been to blue \$\endgroup\$ Commented Sep 26, 2023 at 6:56
  • \$\begingroup\$ @epicfatigue Hi, OK, I need to clarify the rule I gave: it applies to separate transformers with separate magnetic cores. The drawing you provided shows two Txs (on separate cores) but all windings are in parallel. You may not suffer circulating currents with your proposal, but that depends on how well-matched they are - and if they start off being "well-matched" does that remain the case over the many years you expect this to be in service. \$\endgroup\$ Commented Sep 27, 2023 at 8:36
  • \$\begingroup\$ Connecting the LV windings in series increases the voltage applied to the load, and I am guessing the problem is that you cannot source a transformer with suitably low LV winding to allow series connection. It may be OK to cross-couple the primaries, but I need to think about that before recommending it. \$\endgroup\$ Commented Sep 27, 2023 at 8:37

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