Autotransformers

To be conservative you don't want to draw any more than the rated current out of any one terminal, to avoid overheating that set of windings. The 'real' allowable draw is somewhere between that amount and an amount that causes the total winding dissipation to be equal to the transformer used it it's maximum rating -- but without knowing the details of how it's wound and how good it's thermal conductivity is, you can't tell.

John

If it's rated for 60 VA at 240, I'd expect no more than 30VA at

Good Luck! Rich

You should be able to get 60 watts.

An autotransformer is generally rated at twice the VA as the same device used as a transformer (with no common connection). If the center tap is on the primary and it is designed to work at the same VA rating with 120 VAC input, then that half of the winding should be able to handle the current for 60 VA at 120 VAC, or 0.5 A.

If you have 240 VAC on the primary, and a 60 VA load on the center tap, there will be 0.25 A flowing in the top half of the winding, and 0.25 A flowing out of the common connection back to the source. The center tap will provide a total of 0.5 amps into the load, with 0.25 amps in each winding. The vector sum of all currents at each node will be zero.

The only thing that might limit this capability is the size of the wire to the actual center tap of the transformer, but usually this is provided by the two windings being brought out to a terminal or a flexible stranded wire which is probably more than adequate to handle the current.

If you do an LTSpice simulation (as follows), you will see that with V(IN)=240 VAC and R=240 ohms, and a transformer consisting of two coupled 10 H inductors:

I(IN) = 0.25 A @ 240 VAC = 60 VA I(L1) = I(IN) = 0.25 A I(L2) = 0.25 A (180 deg) I(R1) = 0.50 A @ 120 VAC = 60 VA

Paul

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I should clarify that you can get 120 VA if you have a 60 VA transformer with two 120 VAC windings. For a center tap you only get the same VA as rated.

Paul

expect to

Yes, that'll probably work (assuming there IS a center tap). The 60 VA rating, for primary and secondary current flowing in the winding resistance, is lower than what you'd expect from the autotransformer connection. If the primary winding is 4 ohms, 60 VA at 240V creates 0.25W of resistive heating in the primary, and another 0.25W in the secondary.

It would seem that, on heat-generation alone, the autotransformer rating would be higher than the transformer number.

Thanks all, I'd suspected it would work but confirmation is always nice.

The pirmary is two 120V windings designed to be joined to make one

Figure 30VA for each half of the primary, so 60 VA from the centre tap (with no load on the secondary) should be fine.

You can probably do more as there's no heating in the secondary.

What's the VA rating of the motor? and is it designed for the right mains frequency?