Transformer and rectifier math

My transformer and rectifier math for 3-phase supply is a bit rusty.

I'm looking to design a ~70VDC power supply for a large number of servos. They require 75VDC, but with the number of servos I'm looking to run, the factory-supplied power supplies will consume too much room in the control cabinet.

I'm looking at building my own unregulated DC supply using:

(3) 1kva 48VAC secondary transformers
(1) 3-phase bridge rectifier module
(2) 18,000uF filter caps

The transformers are rated at 234VAC primary, 48VAC secondary; if I feed this power supply from 208VAC, that should give me a ~42VAC secondary voltage.

I will rectify the outputs of the transformers, and if my math is right, I should get 42V x 1.65 = ~70VDC. With filtering caps, I should have a fairly solid DC supply. The servos have a 90VDC max rating, so I have plenty of headroom.

Is my math correct to derive my peak DC voltage, using the 1.65 multiplication factor for rectified 3-phase voltage?

I'm also slightly unclear on what my ultimate capacity of the power supply will be; if the transformers' secondaries can each supply 20A, I should have 60A DC available, yes?




Thanks,

rpoet

The DC is 1.414 of the RMS AC voltage so 48 x 1.414 67.5v (approx. with a capacitor .)

parky said:

The DC is 1.414 of the RMS AC voltage so 48 x 1.414 67.5v (approx. with a capacitor .)

Click to expand...

Agreed - the DC voltage is 1.414 the RMS AC voltage on single phase, but I'm looking at using (3) transformers wired as a three-phase delta-connected bank.

I thought the DC voltage is 1.65 the RMS AC voltage with full-wave rectification of a 3-phase source.


I was using the following equation to get the 1.65 value:





Thanks,

rpoet

rpoet
Hi, try this link https://www.electronics-tutorials.ws/diode/diode_6.html
Also Mike Holt has some good information on his forums. Hope this helps.

the 75 VDC is an odd viltage standard ransformers and rectifiers are not going to cut it they will need to be special.
It may be simpler and more reliable to just use a 240V 3Ph DC drive as your source
hang a few caps on the output to filter it. set the output voltage at 75VDC youe done.
With anything you use the output should be regulated at level otherwise as the load changes the DC Buss voltage will also change.

Does your client agree with the solution?

Can you provide certification for your power supply setup? I wouldn't feel comfortable approving a system where I'd be on the hook for damage to a bunch of servos because of cabinet space.

Yes you are correct, I forgot that three phase full bridge, my apologies when you said supply of 234 v I thought this was a bit odd and assumed it was to be single phase, in UK 240v is single phase 415 is three phase I thought US was 110v & 208v