Three Phase AC systems Relays

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neuralc

Joined: 06 Nov 2001
Posts: 274
Helped: 6

25 Jul 2003 19:20 Three Phase AC systems Relays

Hi all,

I'm designing a three phase AC system (380 v), and I should use relays to switch the phases, but normally all relays have 250V, but we put the in series with the lines, what voltage should have the relays contacts?

ThX

NeuralC

jorgito

Joined: 26 Mar 2003
Posts: 198
Helped: 13
Location: Argentina

25 Jul 2003 20:00

Hi Neuralc
Any decent AC contactor has a 400VAC rating.
I would NOT attempt to put several contacts in series to rise the blocking voltage. If you use, lets say, two 200 V contacts in series you may end with arcs in both relays when you attempt to switch off.
Best regards.

venus

Joined: 29 Aug 2003
Posts: 14

11 Sep 2003 14:16

as per my opinion you can keep two 250 v relay in series to increase voltage capacity up to 380 v, you are suggested to keep 1 megaohm with appropriate wattage rating resistor in parellel to each relay contact to equalise voltage between two.

unimx

Joined: 10 Apr 2002
Posts: 18
Location: mars

20 Feb 2004 3:42 Re: Three Phase AC systems Relays

The contactors up to 600 VAC are very cheap these days so there is not any need to use relays (they are used for low power signals mainly).

It is also possible to make a solid state contactor formed by two SCR connected back to back for each phase.

For 380 VAC you will need SCR´s rated for 1200 or 1400 V, the firing board for these is not difficult to build so if you are still interested send pm.

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neuralc

Joined: 06 Nov 2001
Posts: 274
Helped: 6

20 Feb 2004 10:54

Hi all,

I use contactors in many cases and relays for control. But in this case I'm building a standalone system, with everything inside, just connect motor and it's done.

Serie of relays with balancing resistors sound me a good choice....

ThX

NeuralC