Difficulty understanding transformer rating numbers

[MK_CATANZARO]

I'm gathering parts and pieces to start my build, and the transformer specs are throwing me off. I'm using the motors, drivers, and bob from this thread. If i were to buy the antek AS-4215, would that make sense for this use? It's a 15v/15v/13.3amp 400va transformer. The top of the thread recommends 38vdc @ 10amps and I believe that this transformer should give me 42vdc @ 9.5 amps. Am I doing the math right on this?

[MetalHead]

Give us your motors part numbers. Have you bought them already?

[MK_CATANZARO]

I'm using the following setup that seems so popular.
4-PMDX-122
4-G203V
4-PK296B2A-SG7.2

My issue isn't so much about understanding the power supply circuit, but about the transformers themselves. Most of the threads on here refer to the transformers by a single output number and an overall VA rating. Most of the transformers I look at however have multiple duplicate outputs.

If for example, I found an Antek transformer with dual 115V inputs and dual 28VAC outputs and an overall rating of 400VA, will that make 39.2VDC (28*1.4)at 10A when rectified if both inputs and outputs are wired in parallel? Or can the outputs be wired in series for 78.4VDC?

[domino11]

Mike,
The dual primary windings are there so you can hook them up in parallel for 120V mains input. or in series for 240V mains input. Now the VA rating is for the whole transformer. Then you have two identical secondaries. If you wire them in parallel, you get double the current (V x A = VA) but still only up to the total VA rating of the transformer. If you wire the secondaries in series (carefull here with the phasing when wiring in series) you will get double the voltage but only half the current of the parallel combination. Again you only get the rated VA from the transformer.

[bradm]

Heath has pretty much nailed it; you get 200VA worth of power per winding. Use both inputs (series for 240, parallel for 12), you have 400va worth of input. Use both outputs (series for 56vac at 5a, or parallel for 28vac at 10a) for 400va worth of output.

However, the real reason I'm posting is to point out that you MUST BE CAREFUL WITH PHASING REGARDLESS OF SERIAL OR PARALLEL. Get it wrong, and you WILL be creating a short circuit, which can happen on either the primary or secondary side.

[domino11]

Thanks Brad,
My earlier post did not properly address all phasing requirements. :o

[MK_CATANZARO]

That's exactly what I needed to know. Thank you both.

As to the "phasing" concern, I take it you mean to make sure that the correct wires are connected to each other.

For example, on the input end to connect in parallel.
Connect both lines together and both neutrals together. Then connect the lines to the line voltage from the wall and the neutrals to the neutral from the wall.

To connect the outputs in series,
Connect one 28v and one 0v to each other. Then connect the loose 28v and the loose 0v to the rectifier.

In my instance, I'll be using both in parallel, but I'm sure it would be of benefit to be clear.

[domino11]

Mike,
You got it down now! :) Use a fuse as well when you are playing with transformers for some added protection and safety! Just remember your fuse might need to be a little higher than the rated input for "inrush" current. This is a slightly higher current that is present for a short time on power up only.

[MK_CATANZARO]

Good! Now I feel comfortable ordering the transformer for my power supply. Thanks again.

[smreish]

...and this public service transformer announcement has been provided by your affiliated MM community.

LOL

[Robert M]

Sean.... :D