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Topic: Switching load between two sources. (Read 1 time) previous topic - next topic

tocpcs

Apr 09, 2013, 02:53 pm Last Edit: Apr 09, 2013, 03:01 pm by tocpcs Reason: 1
I need a mains switch that can switch a load between two input sources.

Any thoughts?
I'm thinking something like the on / off / auto switch, but it only needs two positions (off peak, normal), and allow for the load to be switched between either of the two.
It'll want to be a 32A 240V rating (or thereabouts).

Erdin

Search Ebay for ssr-60da or ssr-90da. Two of those. For 9 dollars each.

retrolefty

As standard AC SSRs are in effect SPST switches you can't totally isolate the two voltage sources from each other to one output load, the neutrals would have to be wired together which would only work if the two sources are is perfect phase with each other. So draw up your proposed switching circuitry and post jere before committing to purchasing parts and building it.

So requirement questions:
Does the transfer have to be instantaneous with the load not missing even a single half cycle of power?

Does switching have to by just a one time event or after switching from primary source to secondary source and then primary source returns does the circuit have then switch back to the primary source?

What is the phase relationship between the two voltage sources?

Lefty

Henry_Best


As standard AC SSRs are in effect SPST switches you can't totally isolate the two voltage sources from each other to one output load, the neutrals would have to be wired together which would only work if the two sources are is perfect phase with each other. So draw up your proposed switching circuitry and post jere before committing to purchasing parts and building it.


He could use 4 SPST relays, two for the live side and two for the neutral side. Then there would be no problem with phase differences, assuming that all four relays switch at the same time.

retrolefty

#4
Apr 10, 2013, 05:12 am Last Edit: Apr 10, 2013, 05:16 am by retrolefty Reason: 1


As standard AC SSRs are in effect SPST switches you can't totally isolate the two voltage sources from each other to one output load, the neutrals would have to be wired together which would only work if the two sources are is perfect phase with each other. So draw up your proposed switching circuitry and post jere before committing to purchasing parts and building it.


He could use 4 SPST relays, two for the live side and two for the neutral side. Then there would be no problem with phase differences, assuming that all four relays switch at the same time.



That gets pretty expensive pretty quick, and as two SSRs (for each voltage source) will be operating in series with the load I'm not even sure that would work, it's a pretty unusual arrangement. Again without knowing the phase relationship of the two voltage sources it's hard to propose a good solution. A single normal electromechanical relay would be pretty simple, a DPDT should meet the requirements with the proper sensing circuitry of course to drive the relay coil. That however would not be a 'bumpless' transfer.

Lefty

tocpcs

I think it's a single phase in from the street.

One is just a controlled load.
I realise I posted this on a microcontroller forum, but I'm not fussed with manually switching it.
The idea is that the element in the hot water heater may get power directly when we wish, else it's controlled by the utility controlled load signal.

What I am missing is the name of such a switch (i.e. 3 terminals, load, source 1, source 2).

retrolefty


I think it's a single phase in from the street.

One is just a controlled load.
I realise I posted this on a microcontroller forum, but I'm not fussed with manually switching it.
The idea is that the element in the hot water heater may get power directly when we wish, else it's controlled by the utility controlled load signal.

What I am missing is the name of such a switch (i.e. 3 terminals, load, source 1, source 2).


You really need a six terminal switching device, called double pole double throw (DPDT). This can be either a manual switch or a DPDT electromechanical relay device depending on if you want just manual or automatic switching capabilities. The two common contacts wire to the load, the two normally closed contacts wire to line and neutral of voltage source #1 and the two normally open contacts wire to voltage source #2.

http://www.khazar.com/academics/portal/ucsc/2010fall/art22/images/symbol-dpdt.png

Lefty

jsfetzik

The best thing to use would be what is called a Solid State Contactor. They contain multiple solid state relays along with circuitry to insure they switch at zero crossing. Depending on the specifics, voltage, current, etc., they can quickly get into the multiple hundreds of dollars.

sonnyyu

#8
Apr 11, 2013, 04:39 am Last Edit: Apr 11, 2013, 04:42 am by sonnyyu Reason: 1
I will vote "SSTS" (solid state transfer switch).

The SSTS is an electrical device that allows instantaneous transfer of power sources to the load.
There are SSTS in the market, but all for high voltage and 3 phases. for OP need, OEM or ODM is needed by ask SSR guy.

Henry_Best


I think it's a single phase in from the street.

One is just a controlled load.
I realise I posted this on a microcontroller forum, but I'm not fussed with manually switching it.
The idea is that the element in the hot water heater may get power directly when we wish, else it's controlled by the utility controlled load signal.


Your water heater draws 32 Amps (~8kW)? Are you heating an Olympic swimming pool?
Most UK domestic 'immersion' heaters draw no more than 16 Amps (~4kW), many much less.

sonnyyu

Quote
I realise I posted this on a microcontroller forum, but I'm not fussed with manually switching it.


Reliance Controls 100 Amp Utility / Generator Transfer Switch, from HomeDepot  $139.99
This one is for US voltage, You need to get same type but UK one.

http://arduino.cc/forum/index.php/topic,152330.msg1144293.html#msg1144293

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