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

doctorcupcakes

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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.
 

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doctorcupcakes

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And even if tying all the neutrals together could cause problems, some relays have 2 NO and 2 NC contacts in them anyway, so you could break both the neutrals and hots.
 

Bill Taylor

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You're on the right track. I've installed dozens of those. Use a 20a DPDT relay and switch both hot and neutral. You can just switch the hot with some inverters, (I've done it in a pinch) but some units will see a fault and trip out. It's best to get the correct relay and it will work fine.
 

efficientPV

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You can do anything, but I wouldn't use a relay with 120V rated contacts. Due to possible out of phase from two sources that could cause arcing they should be rated for at least 277V
 

Bri from IA

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I know others have said they have done it repeatedly, but I would be at least a little concerned about the affect on your fridge's compressor motor by fast-switching out-of-phase.

The transfer from a dead (or nearly dead) utility to solar inverter isn't much of a concern. Switching from inverter to a live utiltiy is the trouble-area, but only if the fridge happens to be running while the transfer occurs. Fridges cycle on an off, so the concern is limited to those times that a transfer occurs while the compressor is actually running.

While running, statistically, about a 1/4 of the time, the wave forms will match up reaonsably close... not really any concerns. About 1/4 of the time, you'll get a nearly 180-degree out-of-phase transfer, i.e. 240v volts (2x120V)... concerning. About half the time, you'll get this gray zone of 45-135 degree leading/lagging switch which could also be a concern. When switched out-of-phase or unsynchronized, motors will quickly adjust to line up with the new magnetic fields via some pretty large motor surge currents, similar to motor starting from rest. This stresses the motor windings and all the mechanical equipment connected to the motor shaft.

When transfers occur, sometimes you'll be lucky. Sometimes the waves will be 180-degrees out of phase and you'll have higher surge currents. You may experience occasional breaker tripping over the course of time. As more transfers happen, shortened life of your fridge motor may also come into play. But since fridges don't run all the time, its more of a roll of the dice on how lucky or unlucky you are. Odds, however, are in your favor, but bad things can happen.

If you're already handy with schematic wiring diagrams and custom control circuits, you might consider going the route of two power relays (normally-open contacts) with minimum of 125VAC contact ratings and a control timer with N.O. and N.C contacts that drive each power relay to make sure there's a time delay when switching from solar to utitily. There might be a way to do something with a current-switch and only one power relay, but I haven't thought through that completly. Yes there would be extra cost, but also reduced worry.

Some may say that's overkill, but that's how I would do it.
 

Samueru

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I know others have said they have done it repeatedly, but I would be at least a little concerned about the affect on your fridge's compressor motor by fast-switching out-of-phase.

The transfer from a dead (or nearly dead) utility to solar inverter isn't much of a concern. Switching from inverter to a live utiltiy is the trouble-area, but only if the fridge happens to be running while the transfer occurs. Fridges cycle on an off, so the concern is limited to those times that a transfer occurs while the compressor is actually running.

While running, statistically, about a 1/4 of the time, the wave forms will match up reaonsably close... not really any concerns. About 1/4 of the time, you'll get a nearly 180-degree out-of-phase transfer, i.e. 240v volts (2x120V)... concerning. About half the time, you'll get this gray zone of 45-135 degree leading/lagging switch which could also be a concern. When switched out-of-phase or unsynchronized, motors will quickly adjust to line up with the new magnetic fields via some pretty large motor surge currents, similar to motor starting from rest. This stresses the motor windings and all the mechanical equipment connected to the motor shaft.

When transfers occur, sometimes you'll be lucky. Sometimes the waves will be 180-degrees out of phase and you'll have higher surge currents. You may experience occasional breaker tripping over the course of time. As more transfers happen, shortened life of your fridge motor may also come into play. But since fridges don't run all the time, its more of a roll of the dice on how lucky or unlucky you are. Odds, however, are in your favor, but bad things can happen.

If you're already handy with schematic wiring diagrams and custom control circuits, you might consider going the route of two power relays (normally-open contacts) with minimum of 125VAC contact ratings and a control timer with N.O. and N.C contacts that drive each power relay to make sure there's a time delay when switching from solar to utitily. There might be a way to do something with a current-switch and only one power relay, but I haven't thought through that completly. Yes there would be extra cost, but also reduced worry.

Some may say that's overkill, but that's how I would do it.

I tried using a DPDT relay as transfer switch, it killed the inverter when running inductive loads as it would arc across while switching. I tried adding some snubbers but that didnt help. (The inverter is surprisingly easy to repair, the only thing damaged are AC switching transistors). It has 8 IRF640 in a H bridge configuration (2 fets in parallel for each switch in the H bridge).

Then I tried using a 2NC 2NO contactor, that one had double the clearance between contacts and I could run inductive loads (which were fans) with the inverter without issues. It worked and for almost a year I had it as my ATS, and it pretty much switched every day as where I'm from the grid is not stable.

however one day we were using the hair dryer with the fans running as well, there was a power fluctuation that made the contactor turn off and it switched to the inverter, it arced again and killed the inverter, 8 IRF640s later replaced I tested again running the hair dryer with some fans and it once again, killed the transistors, another 8 transistors later (good thing I have bag full of them xd) I tried adding a NTC thermistor in series with the inverter to see if that way the inverter surivives and well, it killed the inverter once again and damaged the NTC thermistor as well.

And if anyone wonders, yes the inverter can run all those loads together but if the contactor switches it dies, it is arcing.



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.

Get an ATS, on aliexpress sell some ATS that are as cheap as DPDT relays and I checked their clearance distance and it is much much bigger.

You can see it here:

Geya has a 110V version as well. https://www.aliexpress.com/item/4001080619986.html
 
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12VoltInstalls

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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.
 

mikekhi

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i have been using them for over a year now. same scenario as yours, if i switch my inverter off my home is still powered, the ATS will switch my home to grid, and its cheap as
 

efficientPV

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Last year I purchased a 3 phase drum switch ON/OFF/ON for switching panels from parallel to series. Removal of some jumpers makes this three independent switches. Theoretically the center OFF should have guaranteed no arcing. Except someone in China decided to save some plastic and it momentarily makes contact just before OFF shorting the panel with a massive arc. How someone gets this to work on three phase is beyond me.

Anyway, this thread inspired me to order a manual version of these transfer switches. I had been using Anderson Power Poles but pulling on connectors is getting old.
 

HammaMan

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I tried using a DPDT relay as transfer switch, it killed the inverter when running inductive loads as it would arc across while switching. I tried adding some snubbers but that didnt help. (The inverter is surprisingly easy to repair, the only thing damaged are AC switching transistors). It has 8 IRF640 in a H bridge configuration (2 fets in parallel for each switch in the H bridge).

Then I tried using a 2NC 2NO contactor, that one had double the clearance between contacts and I could run inductive loads (which were fans) with the inverter without issues. It worked and for almost a year I had it as my ATS, and it pretty much switched every day as where I'm from the grid is not stable.
You didn't specify what exactly you were using. I use contactors for quite heavy loads and don't experience any issue with them if they're sized appropriately in both voltage and amperage. The ones I use for 120/240v are rated for 500v.

As for your issue, it is quite possibly that the inverter itself is unable to adjust to suddenly having zero load and the voltage on it spikes creating the arc, basically overshooting its regulation capability and over-volting itself. If you want instant failover capability, it's best to use an inverter with such capability built in. It's often referred to as "UPS" mode. Typically the fast switching mode that a UPS functionality provides limits its output power to about 1/2 of its rated for such fast switching. 40ms is typically more than adequate to experience no loss of functionality.

There's quite a few power generation devices (generators / inverters / etc...) that state quite specifically that a device should be powered off and then unplugged. As to what happens if one fails to follow such procedure varies, but even generators will overshoot their frequency target if a sizeable load 1/3 or more of its rated capacity is suddenly removed from it, voltage as well if the AVR isn't much to speak of.
 

Samueru

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You didn't specify what exactly you were using. I use contactors for quite heavy loads and don't experience any issue with them if they're sized appropriately in both voltage and amperage. The ones I use for 120/240v are rated for 500v.

As for your issue, it is quite possibly that the inverter itself is unable to adjust to suddenly having zero load and the voltage on it spikes creating the arc, basically overshooting its regulation capability and over-volting itself. If you want instant failover capability, it's best to use an inverter with such capability built in. It's often referred to as "UPS" mode. Typically the fast switching mode that a UPS functionality provides limits its output power to about 1/2 of its rated for such fast switching. 40ms is typically more than adequate to experience no loss of functionality.

There's quite a few power generation devices (generators / inverters / etc...) that state quite specifically that a device should be powered off and then unplugged. As to what happens if one fails to follow such procedure varies, but even generators will overshoot their frequency target if a sizeable load 1/3 or more of its rated capacity is suddenly removed from it, voltage as well if the AVR isn't much to speak of.

I used a TOCT1-63 2NC 2NO contactor, rated for 400V. It is just one contactor, are you using just one contactor? Or you have two electrically interlocked contactors? The later of course will have no problems.

The Inverter is a Bestek 1000W MSW, it is powered by a 3S lithium battery (10.95V nominal) and as result the peak output voltage of the inverter is 140V (and it turns into a squarewave when underload). It has no regulation between primary and secondary, meaning that it is impossible for the thing to overvolt itself to begin with.

The inverter can take the load no problems, even suddenly, in the testing that I did to make sure that it was not arcing I turned the subpanel breaker off, that also turns the contactors coil off and does the switching as well, that way I am 100% sure there cannot be arcing since there will be no voltage at the contacts one they start moving, and I could do this several times with no issues, then after 3 tests with the hair dryer turning the subpanel breaker off, I tested just turning the contactor coil off, and boom the inverter died, it is arcing really bad. Another indication is that half of the H bridge always dies. Meaning that there was such a huge inrush thru the fets that were on at the moment of the switching that they died, if it was the inverter overshooting its HVDC rail it wouldn't result in such damage as instead one mosfet would breakdown thru the other that was on at the moment.

And we are still switching the inverter with heavy loads, just that as a temporal solution what I have is that the inverter is isolated with a switch, when the power goes out I turn the subpanel breaker off and then with the inverter on I turn the switch that is isolating the inverter, that has worked perfectly for a month since the incident, and now the inverter also has to charge up all the capacitors of all the LED lights and power supplies in the apartment at once.
 

HammaMan

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Most hair dryers are 1800w so when you talk of hair dryer and fans on a 1kw inverter, that sounds a little odd. If the inverter and coil are powered by the same switch (dpdt), adding a diode and capacitor to the relay coil will keep it energized slightly longer than the inverter which would then power off right before the coil switches, in a perfect world at least. A larger cap will keep the coil engaged longer. Consider revising your design with this delay in place to remove arc potential. If the coil is the only thing on that side of the circuit past the switch, a diode isn't needed. A cap connected to the coil would act as a very time limited battery holding for whatever time is needed based on sizing for the inverter to power down.
 
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robby

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Keep your relay idea intact but add a “Fridge Guard” to the fridges outlet. These devices can detect even a flicker in the power and will disengage the fridge power for 5 minutes. The will prevent out of phase isssue as well as save wear on your compressor and relay contacts.
Last one I bought was by GE and it cost under $20.
 
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Samueru

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Most hair dryers are 1800w so when you talk of hair dryer and fans on a 1kw inverter, that sounds a little odd. If the inverter and coil are powered by the same switch (dpdt), adding a diode and capacitor to the relay coil will keep it energized slightly longer than the inverter which would then power off right before the coil switches, in a perfect world at least. A larger cap will keep the coil engaged longer. Consider revising your design with this delay in place to remove arc potential. If the coil is the only thing on that side of the circuit past the switch, a diode isn't needed. A cap connected to the coil would act as a very time limited battery holding for whatever time is needed based on sizing for the inverter to power down.

The inverter can power it no problems, it is one of the reasons why I have that specific model, there's a nice 4 part review of it on youtube:

We've been using it since 2018 and has performed great, and it has overloaded just once and it was because we tried to run the coffee maker with the hair dryer at the same time, nothing bad happened to it, just had to restart it (both the AC and DC side have a NTC monitoring the temp to the fets). Also it can do it because its rms voltage drops to about 108V so the hair dryer draws about 1.2 kW instead.

I don't know what you are saying about inverter being powered by the same switch? Its output goes to the 2NC of the 2NC 2NO contactor, when the contactor is energized power goes from city power right to the subpanel thru 2NO (the coil is wired to city power), and when power goes out the coil deenergizes, 2NO disconnect, 2NC make contact and now the inverter powers the subpanel, similar to a dpdt relay. The extra switch it has at this moment isolating it is to prevent arcing from damaging it again if the contactor switches while there is city power.

Even if I manage to add a delay that will not work, because mains voltage is not stable at all, sometimes we've had constant 20V AC, other times it changes rapidly between 100 to 60 to 150V!!!! So in order to prevent the coil of the contactor from dying, I wired an over/under voltage comparator (it is also in din rail format) that cuts power to the coil if the voltage is outside the range, meaning that it will still switch with voltage present.

Check this:

Mains went from 105V (it was low already) to 60V, then jumped back 138V with a lot of distortion (iirc that distortion is the distribution transformer saturating due to the overvoltage).

I will just get one of the transfer switches that I was talking about in the first message here, there is no way that one is going to arc across with a +3 cm gap.
 

HammaMan

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Okay, so you've got no qualms pullling ~2kw on a 1kw product. C'mon man to quote former VP joe biteme.

I've pulled 30a through a 16ga extension cord. It 'handled it just fine' until it turned into a heating element and caught on fire.
 

efficientPV

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I bought one of those Bestek inverters for 99 cents + shipping (under $15 total and of course it didn't work). Most inverters have one fuse for each individual inverter transformer. Indeed, this had eight 40A fuses. When I opened it up they were all wires in parallel. More likely the wire to the battery would be in flames before those fuses worked. In reality, the board traces would vaporize long before that. I cut the fuse board trace in two and fed each board with half the fuses. I also reduced the fuse size. All that might give me a chance in a catastrophic failure. I'll pass on the next one I see.
 

Samueru

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Okay, so you've got no qualms pullling ~2kw on a 1kw product. C'mon man to quote former VP joe biteme.

I've pulled 30a through a 16ga extension cord. It 'handled it just fine' until it turned into a heating element and caught on fire.

More like 1.5 kW total, why should I have to worry? Did you not see the review? The thing is built better than most inverters of that range, similar inverters would just use the thin metal casing as heatsink for both sides for example, the H bridge fets would be less capable as well.

'''I've pulled 30a through a 16ga extension cord. It 'handled it just fine' until it turned into a heating element and caught on fire'''

And this is why it is important to make sure that the protections are well calibrated, following your example, 16 AWG can take 30A for about a minute before the 13A ocp opens, but if you don't have it, what you said will eventually happen.
 
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