Control.com - Nerds in Control

Dear Friends
I am developing float Rectifier cum Charger for our Telecom Product. I am supporting two alternate AC utility supply (230V AC 50 Hz) input, out of which only one at a time has to be selected. The load is 6 Amps.

I am concerned about a few aspects :

* What would be the best way of judging the power supply to be selected.
* How to best implement it. ( Contactor etc)
* How about the Earthing issue. ( Vis a vis two different AC Sources)
* MCB in such case should be double Pole or Single Pole (OR Three pole)???

PLease advise.
THANKS
ashokg@cdotp.ernet.in

Ashok Gupta
CDOT
India

If I understand your question right this is what you want:
You want to have two independant power sources, and be able to select one of them, I suppose this is because one might fail and the operation can continue uninterrupted. Did I get it right, you want DC voltage out of it?

If all this is correct:

Two cases, case one: you want to use some kind of power supply to produce the actual output voltage (probably lower than the input voltage). I would do like this: use two independant power supplies and connect them togheter using a pair of diodes.
Depending on how the actual power supplies are made you might not even need the diodes.


Case two: you more or less just want to rectify the AC line voltage and diretly use it for something. Put an insulation transformer on one of the inputs, reactify each AC voltage
separately and then just connect them to each other.


With either of these methods the currently strongest source is the one feeding the load. If they are equal enough the load will be divided between the two sources (not necesarily 50% each).


Grounding: if you use an insulation transformer on one of the sources you just cut that ground and use the other one.


If what you really want is an AC output, forget about this reply.


/Johan Bengtsson

----------------------------------------
P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/
----------------------------------------

Hallo Friend,
Thanx for the response.
Yes, I want to have two independant AC power sources, and be able to select one of them, Yes, this is because one might fail and the operation can continue uninterrupted. I shall be getting AC volts out of it. (230V AC 50 Hz, 5 Amps max.)

PLease suggest me suitably. I would also like to monitor the voltage of each of these before I could actually connect the power sources to the
load side.

ashokg@cdotp.ernet.in
Ashok Gupta
Research Engineer
C-FITL
CDOT
II Floor
#39, Main Pusa Road
N. Delhi 110 005

I would like to add a 2c note on this. i think you may probably might have to use an overlap switchover type relay... i.e., make before break type so that in case your power fails, when the relay drops, it makes the contact before breaking the other contact

thanks

sudarshan.k

I would not take this 2c note without knowing what the AC sources are. You can not connect (short circuit) 2 independent AC sources without
synchronising voltage and frequency first. This can even lead to severe damage of the power generating equipment. So without knowing what's behind not my 2c note

Regards Rolf

Hold that thought a moment. With this type of setup you stand the chance of paralleling two power sources out of phase after the failed power supply has returned. That situation can result in catastrophic failure of your relays and/or the equipment it's supplying. In a 3-phase supply a phase mismatch of 120 degrees would result in currents in excess of a million amps trying to
flow when the contacts are both shut. I've seen the results of that and they aren't pretty.

A better solution, IMHO, if the switchover time is a problem, would be to get the fastest relays available and wire to break before make. I've seen
this work with switchover times of less than 50 ms.

Paul Butchart
Process Control Engineer
Qualitech Steel SBQ, L.L.C.
pbutchart@qualitech-steel.com

I would say, don't use that!
1. You have no reason if the power already have gone
2. If it would make any difference it means you make a switchover when you have power from both sources, since they are probably not the same phase you will short circuit both your sources.


/Johan Bengtsson

----------------------------------------
P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/
----------------------------------------