Is it possible to use a 120v relay as an automatic transfer switch?

[12VoltInstalls]

I’m glad you’ve had good results for your purchase. MSW inverters? I have no use or faith in. The first thing I burned up with one was a computer monitor and I didn’t learn from that. Immediately anyway.
I’m not so convinced that that bestek is a good idea. Do they make a true sine wave model?

 

[HammaMan]

I’m glad you’ve had good results for your purchase. MSW inverters? I have no use or faith in. The first thing I burned up with one was a computer monitor and I didn’t learn from that. Immediately anyway.
I’m not so convinced that that bestek is a good idea. Do they make a true sine wave model?

My shit costs too much to use cheap inverters. I could buy several large AIO chargers / inverters for the price of 1 of my computers.

 

[pollenface]

I configured the load output of an epever mppt to control a couple of relays to switch the mains power on to a 36v psu to top up the battery bank using typical 40a 24v automotive relays. Not exactly a transfer switch but it stops the batteries from dying if it's been raining for a few days. Haven't had any issues thus far, I'm on 230v mains here in Western Australia.

 

[Mike Davison]

My fridge is currently running off my inverter/battery bank and is by far the biggest drain on the system. I was wondering if it would be possible to use a relay to run the fridge on grid power while the grid is on, but on inverter power if the grid goes off. Basically I want to power the relay coil with grid power, and jump grid power to the normally open contact. The inverter would be connected to the normally closed contact, and the common would go to the fridge. All the neutrals would be then tied together. I've currently got my neutrals tied together anyway so I have a neutral-ground bond at my main panel, so I know tying all the neutrals together won't have any ill effects. I can't see anything wrong with this setup. It seems like it would work as a cheap automatic transfer switch. I will post a diagram if I'm not describing my idea clearly enough. Let me know what you think.

Why not something as simple as a dual zone automatic transfer switch.
e.g https://www.ebay.com/itm/GCQ2-63-2P...tch-ATSE-Switch-CB-Class-Switch-/174427269392

 

[Texas-Mark]

Why not something as simple as a dual zone automatic transfer switch.
e.g https://www.ebay.com/itm/GCQ2-63-2P...tch-ATSE-Switch-CB-Class-Switch-/174427269392

I use a transfer switch similar to that, because using a relay blew out an inverter (like someone else mentioned).

 

[Red Squirrel]

I've been thinking about this exact idea myself. I am upgrading my shed's solar setup to a 24v 3kw inverter with 4 golf cart batteries and putting in a small sub panel in the shed and one in the house fed by the shed. I will then have outlets throughout the house that are solar powered. I want a way to transfer certain loads over, either manually or automatically, so I've been thinking of making a device that uses relays to do it and is self contained.

My train of thought is to have a box that has two power cords and an output. The A power cord will be plugged into the grid and the grid ground will be used for the entire system, the B cord will plug into the solar, and it's ground will be ignored. This is to prevent weird grounding issues as lot of devices have chassis connected to ground so since it's inside the house I want every chassis to be on the same ground no matter what. The device will have a microcontroller which will handle the relay switching and have various configurable options such as the wait time to transfer and manual override buttons. I plan to eventually make a home automation system so these will probably tie in to it as well.

Off the shelf transfer switches, even manual ones, are very expensive, so this will be a cheaper way to do it while providing more options too. Once it's tied in with my home automation system I'll also be able to transfer to solar based on the battery voltage, so I can maximize my use of solar. Goal would be to try to run my whole server rack on solar during the day. The solar batteries would also act as an extension of my UPS as I could transfer to them if my UPS batteries run low, if power goes out. From this point on it would also make more sense for me to upgrade the solar batteries and then eventually downsize the UPS.

 

[JohnAR]

you can build the circuit but with the cost of PLC or Automatic Transfer switches being so cheap I do not understand why you would

 

[Steve_switch]

I had a similar idea except with a little twist. I like the idea of having a Transfer Switch but I want to use the energy that I've collected from solar panels and stored into batteries. I don't want to just sit there waiting for the grid to go down, because mostly, it's dependable and only goes ofline when there is a very terrible storm or a tornado. So I thought why not tap into the data coming from my battery monitor and pick a variable that I want to control on, Battery %, Amp Hours removed from 100%, Battery Voltage or Average Battery Voltage, then pick a turn-on point and a turn-off point to let the Automatic Transfer Switch run on the grid normally running on the Normally Closed contacts of a 30 Amp Double Pole Double Throw Relay. When my set point reaches the turn-on point, I have an Arduino Mega send an output to a small relay that completes the circuit to turn the inverter on. I have a 5 vdc wall wort plugged into one of the inverters outlets and when the inverter turns on, that wall wort sends that 5 vdc to another input pin on the Arduino Mega. When the Mega gets that signal, it sends another output pin high energizing another mini relay that completes the 120 vac 30 Amp relay coil. I've use this system for 5 years, running the deep freeze in the garage and the refrigerator in the kitchen. I can select the controlling variable, and the set-points. I usually only let it operate in a narrow window of operation so that the batteries are never under stress. So if/when the grid does go down, (and it has happened a few times in 5 years), I've got power to keep my stuff. The batteries should last forever.

 

[Bossrox]

My fridge is currently running off my inverter/battery bank and is by far the biggest drain on the system. I was wondering if it would be possible to use a relay to run the fridge on grid power while the grid is on, but on inverter power if the grid goes off. Basically I want to power the relay coil with grid power, and jump grid power to the normally open contact. The inverter would be connected to the normally closed contact, and the common would go to the fridge. All the neutrals would be then tied together. I've currently got my neutrals tied together anyway so I have a neutral-ground bond at my main panel, so I know tying all the neutrals together won't have any ill effects. I can't see anything wrong with this setup. It seems like it would work as a cheap automatic transfer switch. I will post a diagram if I'm not describing my idea clearly enough. Let me know what you think.

I use a dpdt relay on my system to switch between grid & solar so yeah, you sure can. I have my grid & inverter neutrals tied together & you can use just a spdt on the hot side. I use the other pole for a signal light to tell me at a glance if I'm on grid or solar. A word of caution tho, flipping between grid & inverter can stall a compressor if the sine wave is opposite to what it's switched from & that will shorten it's lifespan so I'd add a power interrupter that kills power to the fridge for 3 or 4 minutes.

 

[Goboatingnow]

I personally HATE Manufactured Item Dependency Syndrome.

However, in this case, $115ish for an auto solar transfer switch makes sense. In fact, the more panels you have (to a point) the more it makes sense.

Wow how do you get out of bed then

 

[MaximusAnonus]

How about something like this? (excuse the caveman like drawing I couldn't be arsed to make a proper one)

This won't have fast transfer times (and really that's not the point here) but I need something to switch my EV charger (only 230VAC/10A) from solar to grid (winter use, when grid electricity is essentially free during the night) and back again.
The idea for the safest switchover from IN1 to IN2 I can think off:
OUT relays are switched
IN relay are switched
SWITCHOVER relays are switched
OUT relays are switched
reverse the order for switching back to IN1

(The reason I use 2x SPDT instead of 1x DPDT etc. is simply I'm working with what I have left over from other projects)
Whole thing would be automated with ESP and HA.
(The reason for a mix of NC/NO relays is to make it efficient. Magnetized relays consume power. This way IN1 (solar) and OUT is connected with 0 power draw.)

 

[Hedges]

Just because you could (use a relay between inverter and grid) doesn't mean you should.

The following shows industrial devices, but same basic principle of being able to disconnect under load, and providing safe isolation from utility source (relay contacts likely aren't far enough apart for complete safety.)

https://electrical-engineering-portal.com/disconnectors-load-switches-switch-disconnectors-cbs

I've had a relay arc across. Load was just a heating element, but due to my circuit, the arc resulted in a short that tripped breaker. Electronics it could have blown up.

I'm not terribly worried about connecting a spinning refrigerator motor out of phase. I think it'll just tug a tiny bit and settle in to the new condition. Would be different if you had a generator you were supposed to synchronize with the grid; I've read of those ripping anchor bolts out of concrete.

I think bigger problem for compressor is power being disconnected long enough that it stops, then reapplied before "unloader" relief of backpressure. So it tries to start up and is stalled, burning out the motor.

Maximus' concept of opening a circuit and doing switchover cold solves the arcing issue. The 2x SPDT may not provide sufficient airgap to be a completely safe isolator. Maybe acceptable if only equipment not people at risk, e.g. not used with a generator inlet plug having accessible contacts. But, the delayed switching might be just long enough to cause a compressor to stop and get stalled.

 

[MaximusAnonus]

Maximus' concept of opening a circuit and doing switchover cold solves the arcing issue. The 2x SPDT may not provide sufficient airgap to be a completely safe isolator. Maybe acceptable if only equipment not people at risk, e.g. not used with a generator inlet plug having accessible contacts. But, the delayed switching might be just long enough to cause a compressor to stop and get stalled.

My concept is specifically designed for like 1-2 sec transfer times.
It's not really for a fridge or anything. I've had in mind my EV charger, the battery bank chargers and the water heaters I have.
All devices that aren't very transfer time sensitive.

 

[MaximusAnonus]

BTW this is how Victron does it. It's just a 2NO+2NC contractor and some stuff around it in a box.

 

[DarrylG]

I recently saw the new life in this old thread and thought it would be good to get such a device for my fridge as I only have Manual Transfer Switches at my house and there is another outlet nearby to tap off the grid. I was a bit afraid of the gap being too small in some relays, so I bought what I thought was a non relay version:

AC120V 15Amp Automatic Transfer Switch, ATS Auto Transfer Switch ​

Brand: OONO

Unfortunately, It was a well built relay box with little gap between the two sides of the relay. I ruined my inverter by using this while powering a moderate load and switching. I even had a fridge surge protector on the load side to provide a delay when the power was switched. However this OONO switched so fast the protector didn't even notice.

So I will be checking out one of the other options.

 

[Red Squirrel]

You definitely want something that has a bit of intelligence, such as a delay before switching. A fast transfer switch could work for non motor equipment such as computers if you're trying to use the transfer system as a UPS, but for general purpose where you're going to plug anything in it, it's best to have a short delay. I also do wonder what the effects of back EMF are when you quickly connect a load from one AC source to another when they're not in phase. Could be why it blew the inverter perhaps?

Me personally I'd do like a 1 minute delay. That should give enough time for any motorized loads to fully stop, and for refrigeration systems to depressurize enough so they can startup again. Build it with the assumption that important loads already have their own UPS protection, and accept that there will be a short outage. Things like fridges etc are basically their own battery, so if they go out for a minute or more it's not a huge deal.

Actual UPSes that are designed to transfer fast sync their inverter with the grid I believe, so this is why they can get away with switching in under 16ms which is needed to avoid computers shutting down. The really good ones actually run on inverter all the time, but those are rare as a consumer unit and usually take up an entire room.

 

[DarrylG]

Yes. I agree. I had followed robby 's advice and installed a Fridge Guard which should shut off the power for three minutes. However the Fridge Guard I bought did not have any issues with the electricity switching at the 15 milliseconds from OONO Transfer switch and did not turn off the power. At the time the inverter blew the ATS was tripping between utility power and the inverter while powering a 600watt heater with a small fan running.

I have ordered a transfer switch like mikekhi suggested. It has a big gap between the contacts and a slower 2 second switching time.

 

[Hedges]

Some people say a fridge compressor will lock up if switched out of phase, but I'm not sure I believe it. More like, if turned off long enough to stop spinning and then turned back on before pressure bleeds down, it will stall. Out of phase, so what? Starting from a standstill is about as out of phase as you can get. This is not a massive generator being paralleled with the grid.

I blew up some electronics last week (as part of my job.) We were doing EMI/EMC testing, a system with PDU (power distribution unit, fancy power strip) and instruments. During conducted immunity, when RF of 10V amplitude and several MHz was sent down the power cord, the PDU chattered its relays, switching AC power on and off repeatedly. That blew up the power supply of an instrument that per spec would have handled the conducted immunity on its own. But full AC voltage toggled on and off is hard on electronics. They can take the surge once at power-on, but not repeatedly.