Help! Trying to wrap my head around the transformer... But my head is too hard.
 

So, I think I have finally figured this out, if so, please just confirm that I'm on the right track here.

I have Motionking's 9801 motors, which are rated at 4.0A, .98ohms, and 4.1mH. It is an 8 wire motor. I am planning on wiring it half-coil, and I kept asking myself if that means that it needs half the Amps, as the ongoing discussion about wiring in different methods either scales things up or down by 1.4, depending on what is done.

BUT, I just realized that people seem to be talking about rating per phase.

Are the numbers I posted above correct for a half-coil winding? And a half coil winding really means that I'm using only 1 of the 2 phases of the stepper motor? So all the numbers given by the motor are the rating PER PHASE (per half motor?). Does that make sense?

I think a lot of my confusion stems from the size of the transformer and power supplies people are using on here. Most everyone seems to have a 300-400VA power supply, whereas my motors appear to require a 600-700VA power supply.

So when I break this down, I come out with the following:
2/3*(4 motors x 4.0A) = 10.7 A
Per Mariss' suggestion, I'm looking at a max of 65V DC power supply, so a 600-700VA supply seems to be what keeps coming up.

Is mine so much higher just because the motors run off of 4.0A instead of 2ish that I see others running off of on their spec sheets?

Sorry, I know that is a lot of questions, just trying to make sure I'm clear on this, and not missing something obvious (to everyone else).

Hi Eric,
Your 600~700VA is based on the theoretical assumption that the motors are drawing max current continuously. Which never happens in reality.
Mine is wired half-coil, & the driver is set to lowest 3Amp setting, without any loose step. I wish my drivers can set lower current but that will do.

65Vdc is correct. everything else is correct too.

You really don't want/need your motor running hot to brink of melt down...

If your motors are rated at 4A when wired parallel, they will pull up to 2.8A when wired half-coil.

The secret number is 1/SQRT(2) or 0.7071. Half-coil requires 1 / SQRT(2) X Parallel Amps, so if the motor takes up to 4A when wired parallel, 1 / SQRT(2) X 4 = 2.8.

2.8A X 4 motors = 11.2A and then 66% of 11.2 = 7.3A.

The motors are rated 4.1mH so using Mariss's formula, 32 X SQRT(4.1) = 64VDC maximum. To get 64VDC, you need a transformer that is rated at 45VAC which is found by the forumla, 1 / SQRT(2) X DC voltage, so 1 / SQRT(2) X 64 = 45.

To find the VA needed, multiply AC voltage X Amps, so 45VAC X 7.3A = 328VA.

You would be safe using a transformer rated at 300VA or higher.

As Ken pointed out, the motors will not be drawing full current all the time. That is why a 300VA transformer is able to handle the job. On my test bench, I use a 500VA transformer. Its temperature never rises above room temperature.

I think I found a winner...
 

So, I'd just about given up on Antek, and then I noticed, on their description on the side, it says "Two outputs." So all the ones that I couldn't add up the numbers properly, actually have 2 outputs. -.- I feel really dumb now.

http://www.antekinc.com/details.php?p=105

So I think this is a winner. 2x 45V, giving me 63VDC, which is close enough to peak draw for me to be comfortable. And it has 2 12V and 18V taps for other applications (fans, etc.).

Am I on the right track?

Yes Eric looks good for the motion king. I run mine at 45 Vac and less VA on the transformer than the 500VA you have there.
Now check what other windings you need for control voltages, fans etc, I ended up with three plug packs as well. Would be neater if the one transformer supplied all the AC requirements.
David

Unfortunately, looks like Antek is limited to the 12 and the 18v options. Hopefully that will cover what I need, if not, I'll add in as necessary... :-/ I think it'll do the job pretty well, though.

Eric,
Just call Antek and speak with John. They custom build everything to order - just ask for what you want...it won't cost you any more.
Good luck
Sean

Wow, I did not know that. Thanks. I saw their stock numbers and I guess I assumed that it would be a couple of weeks leadtime to get something built vs. something off the shelf.

So what might actually use an 18V line? I'm thinking about asking for all 12V secondaries instead of the 12 and 18.

Eric,
You don't have to exact with 63Vdc. it is only a reference voltage, you can get 66~68Vdc using with 48Vac transformer. The motor will never notice a few V difference & it is below your motor driver rating of 80V so no melt down. My 9801's are happy with this arrangement for 2~3 years already.