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Author Topic: Transfer switches  (Read 771 times)

Lyle Williams

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Transfer switches
« on: December 23, 2018, 01:24:06 am »

Ok, transfer switches for redundant power switchover.  Single phase.

The simplest are DPDT relays with the coil magentised by the normal power supply.  When the normal supply cuts out, the relay flicks over to the alternate supply.

More sophisticated products have silicon relays for faster switchover, and better cutover logic that deals with sagging or pulsing voltages.

How much difference does it make in the real world?
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Rob Spence

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Transfer switches
« Reply #1 on: December 24, 2018, 12:44:47 am »

Actually, even the simple (automatic) ones are a little bit more complicated.

I have one. It happens to be a 100a model for a Generac unit.

The contactor connects PoCo power when not energized.

When PoCo power drops, the standby genset notices and starts. After about 10 seconds, when speed and voltage are stable, the genset controller supplies voltage to the transfer switch. A bit of relay logic says “no PoCo power & genset ready” and then activates the transfer.


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Chris Hindle

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Re: Transfer switches
« Reply #2 on: December 24, 2018, 08:10:23 am »

Actually, even the simple (automatic) ones are a little bit more complicated.

I have one. It happens to be a 100a model for a Generac unit.

The contactor connects PoCo power when not energized.

When PoCo power drops, the standby genset notices and starts. After about 10 seconds, when speed and voltage are stable, the genset controller supplies voltage to the transfer switch. A bit of relay logic says “no PoCo power & genset ready” and then activates the transfer.


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Much better (and safer) to have no power for a bit, that switch to a Genny while it's spinning up........
In an "establishment" I worked at in the late 70's, we went from POCO to a battery room, and after 4 or 5 minutes to the rooftop natural gas Genny.
Chris.
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Ya, Whatever. Just throw a '57 on it, and get off my stage.

TJ (Tom) Cornish

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Re: Transfer switches
« Reply #3 on: December 24, 2018, 08:42:13 am »

Ok, transfer switches for redundant power switchover.  Single phase.

The simplest are DPDT relays with the coil magentised by the normal power supply.  When the normal supply cuts out, the relay flicks over to the alternate supply.

More sophisticated products have silicon relays for faster switchover, and better cutover logic that deals with sagging or pulsing voltages.

How much difference does it make in the real world?
What gear are you concerned about?  Most equipment powered by an AC->DC power supply have a couple cycles' worth of stored energy, meaning that as long as switchover happens in less than 1/30th of a second or so, there isn't any issue. 

If you have critical loads, a double-conversion UPS that always powers the load from the battery/inverter with the transfer switch (if present) located on the rectifier side is the way to go as there is no interruption to the load at all.  This is also by far the best way to isolate your load from powerline junk you are worried about - sags, spikes, harmonics, etc. 
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Lyle Williams

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Re: Transfer switches
« Reply #4 on: December 24, 2018, 02:47:21 pm »

Right now I am thinking about water pumps.  But the curiosity is generic.  How to cope with one load and two supplies.

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Re: Transfer switches
« Reply #4 on: December 24, 2018, 02:47:21 pm »


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