I'm somewhat new to electronics, so please pardon the possibly stupid question.

I am building a variable speed pottery wheel for the wife for Christmas. I have the wheel assembly and 2HP 1725 RPM 3-phase motor. I also have a 3 phase motor controller, that needs 220V-240V input.

So the question is this, we just got a 5KVA step-down toroid transformer at work (I work at a scrap yard), from a server backup power supply. I was wondering if I could use this as a step-up transformer. The reason I ask is that I know toroid primary winding has to take frequency into account as well as everything else. I don't know if the secondary takes this into account as well (or just does the simple 2:1 winding).

I will be making a soft start circuit for this (being that toroids can have a nasty inrush current problem). I'm looking at this transformer because I can get it CHEAP ($21 is a decent deal).

  • 1
    \$\begingroup\$ Good grief, a 2 HP motor for pottery? Are you making a one ton pot? . . Now the important stuff : do you really have 3 phase electrical utility at your house ? I wish I did. Rest assured that if secondary is rated at frequency, so is primary. Sounds like you love your lady very much. Don't worry about frequency. Seriously, what are the transformer specifications ? \$\endgroup\$
    – Marla
    Commented Jul 3, 2014 at 2:04
  • \$\begingroup\$ The thought of 3-phase residential service makes me giggle. \$\endgroup\$ Commented Jul 3, 2014 at 2:05
  • \$\begingroup\$ I need the bigger motor for huge amounts of low speed torque. Plus it came in at work, and I pay $.45/lb. So might as well (and I can't really be choosy about 3 phase motors that people scrap. The motor controller requires single phase 220-240v input and drives a 3 phase motor. \$\endgroup\$
    – user46694
    Commented Jul 3, 2014 at 3:43
  • \$\begingroup\$ aha, very good user46694, now things can be seen, knowing that you have a single phase input , to 3 phase output motor drive. So I take it that you need a step up from 120 or some lower voltage , to step up to 240 v ? \$\endgroup\$
    – Marla
    Commented Jul 3, 2014 at 4:11
  • \$\begingroup\$ Split-phase supplies are already capable of 240V, but require a different socket/plug for it. \$\endgroup\$ Commented Jul 3, 2014 at 4:30

1 Answer 1


Assumed facts : Single phase low voltage electrical utility.

Needs : Has single phase input, 3 phase AC motor controller and 3 phase motor on hand. Can transformer be used to step up voltage.

If you have a single phase transformer (5 KVA) which is rated as step up (or step down) rated for 120 VAC to 240 VAC (or vica versa), can you use said transformer?

Yes, and not much concern for inrush. Part of the fun and learning is just going ahead and trying it. Just be careful. The KVA size could be a problem with inrush current in a home environment (circuit breakers). Use prudence and stand back. You should be ok. Doesn't matter if it is toroidal or not in your application.

  • \$\begingroup\$ Thanks for the answer! I remember reading in a book in the library that toroid transformers "remember" the last state of power cycle, and if you reconnect power on the opposite side of that cycle it pulls a huge surge. But that was a while ago. \$\endgroup\$
    – user46694
    Commented Jul 3, 2014 at 5:25
  • \$\begingroup\$ Residual magnetism in the core of a transformer is real and does influence the inrush current. \$\endgroup\$
    – Marla
    Commented Jul 3, 2014 at 12:35

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