circuit that turns equipment off when generator power is on?

Greetings all, and excuse me if this will be a long post
recently we are having alot of power outage, so most of ppl including family and friends got portable power generator or inverter to use mainly at night time for lighting and TV watching mainly, so power of the generation equipment is like 1.5 KVA to 2 KVA and sometime less
the sequence of things is something like: power go down , they disconnect heavy loads and fridges manually,they turn on generation equipment which is connect to home network via a double pole double throw knife switch (along with power grid mains).
the bad things happen when they forget to disconnect the fridges or other load manually, it is happening often that either fridge or generator (cheap ones) get damaged , or when inverter can bare the load , the battery go empty in 15 minutes or so ... or they forget to connect the fridge back when normal power is on and get everything is it spoiled .
i was wondering if there is any suggested circuit that can be installed on the fridge (or other equipment) plug that will automatically turn off the equipment off on reserve power and get it back on when normal power is back? Basically I am not looking to modify the home wiring or make separate wiring for the generator.
one thing to add, home wiring do not have earth / ground connection, so are all plugs.

thanks in advance and regards

There is no equipment I know of that can detect where the electricity is coming from so from the usage end of the wires it wouldn't be possible to tell if the generator or power company were producing it. It may be possible to do it by analysis of the AC waveform but it would be quite complicated and maybe not reliable.

There are things you can do though:
1. Easiest is wire the fridge and other 'vulnerable' appliances together on their own extension cable. The knife switch can then be used to link them to the generator or incoming AC while leaving other devices still running from the incoming AC alone. You can leave a light turned on on the old supply to show when power is present.

2. Use a relay circuit to automatically change over the supplies when the incoming AC fails. The problem with this method is the relays tend to be expensive and may cause problems in a brown-out situation where there is partial but not complete loss of power.

I suggest you research "automatic transfer switch" (ATS) which may give some ideas. Here I have a solar water heating system which will overheat if the pump fails so I use a relay based system to switch it to a battery/inverter during power outages. When the power is restored it recharges the battery.

Brian.

Thanks Brian , much appreciated
i will give look for ATS ,
an idea come on my mind to use some circuit i did while i was in my 4th year engineering ( long time ago ) that implement communication over mains , this will split the circuit into 2 parts , a control transmitter ( always on ) , to be connected at the generator/inverter side of the knife switch , and a control receiver that supply the fridge from grid AC when on rest status ( transmitter is off ) and will cut power from the fridge when generator power up and transmitter is on
im not sure if this will be reliable and cheap , i dont have the diagram of my previous circuit either

thanks again and if any have a clue it is more welcome
regards

betwixt said:

There is no equipment I know of that can detect where the electricity is coming from so from the usage end of the wires it wouldn't be possible to tell if the generator or power company were producing it. It may be possible to do it by analysis of the AC waveform but it would be quite complicated and maybe not reliable.
Brian.

Click to expand...

You're right of course Brian - but it is an interesting problem.... I can think of 2 applications where it might be invaluable...
Hmmm ... now you've set me thinking.... !

There are analytical methods - checking the frequency and amplitude for example but they would have difficulty distinguishing between a power station and a good sine wave inverter. Possibly a method of checking neutral to local ground voltage could be employed, based on the generator/inverter having them (hopefully) tied together in close proximity rather than some distance away.

My mains power is unreliable (off for 3 days in the storms of last week) and I have a back-up Diesel generator to keep things running but it still needs manual intervention to swap supplies. I use a 63A three way isolator switch, 1=Generator, 2=Isolated, 3=Incoming line. The middle position makes sure I cant join the other two together! Sadly, although I also have 2.5KW of PV, it disables itself because of the poor frequency stability of the generator output.

It's illegal to connect to the incoming supply upstream of the electricty meter so I have an LDR taped to the face of the meter which has an LED 'AC present' indicator. It lets me know if the power has been restored while I'm running off the generator.

Brian.

I lived in a house with a manual transfer switch so we could run on backup power. I had a backup system consisting of PV panels, 24V battery bank and inverter.

I wanted to know when power was restored, so I connected a radio to the incoming grid wires (or rather a 120V wire and a neutral). This required opening the switchbox and attaching jumpers at the correct contacts.

I wanted to avoid letting the refrigerator or microwave oven run down the battery bank. So I ran these appliances from a 12V inverter powered by the car. Every so often I started the engine to charge its battery.

I tried but I could not figure out an easy way to switch appliances so as not to run them unnecessarily, nor overload the battery bank.

Blackouts were infrequent but they could last several hours. I always had to stay on my toes, checking volt readings, etc.

A second independant system here stops the coolant boiling in a solar water heater if the pump can't run because of power loss. It's a bit complicated to wire up but it might be useful to the OP:

The control box contains a battery charger and an AC line powered relay.
Back-up is with a 20AH lead acid battery and a 300W modified sine inverter. (pure sine would be better)
The relay is normally energized which routes the charger to the battery so it trickle charges and it also isolates the DC feed to the inverter. Line AC goes to the pump
If the power fails, the relay disengages and connects the battery to the inverter input and its output to the pump.

It's slightly more complicated because it also has a control signal from a differential thermometer so the pump is only told to run if the heating panel is more than 6C warmer than the storage tank. When the power has failed and the pump isn't called to run, it also disconnects the DC to the inverter to prevent residual current draining the battery. The system can hold it's own for ~6 hours in sunshine, plenty long enough for me to get home if I'm away and start the Diesel generator if necessary.

Brian.

I am thinking that each unit that has to be "turned off" when on generator power has a box that goes in series with its mains supply (mains plug on a lead and a socket on the box). Inside the box is a triac which has its gate powered via a resistor and diode from the incoming live (perhaps with a small capacitor), so with mains applied the box is a short circuit and the mains is fed out to the appliance. Now for the magic bit!, from the incoming live, there is a filter network which supplies a diode to produce a negative voltage to turn the triac off. So when on the generator, you feed the mains via some sort of network which injects your control frequency. I would suggest allowing one watt per outlet, so a 20 W squarewave oscillator at a suitable frequency , say, 1 - 20 KHZ.
Food for thought?
Frank

Interesting but I'm not sure it would work. Wouldn't it be more sensible to superimpose a HF signal (with low harminic content - no square waves!) on the AC to the socket which is itself generated by a line powered unit. Filtering to prevent it leaving the premises would be a problem. When the line fails and the generator/inverter is running, the signal would be absent at the outlet but on restoring the line, even if the generator/inverter is still providing the power, the HF signal would be restored and could be used to alert the user. Care would be needed to prevent harmonics or switching interference from an inverter being recognised as the signal.

Brian.

I did think about this hence the HF is only injected when on the generator, rather lessens the possibility of it getting on the grid. I was thinking that to simplify the filter to perhaps a simple high pass series small HV working cap + small inductor to neutral, a wave form with a lot of HF content would help along with a rubbish waveform output from the generator .
Frank

The problem I see is that it doesn't monitor for the line AC being restored. The HF would only be present on the isolated supply and nothing would indicate the main line AC was present again. Maybe it would work if the HF generator was powered from incoming AC but applied to the consumer socket regardless of source, it probably doesn't matter which way around it goes as long as it provides an indication of incoming power status. Of course, if this method was adopted, it would be much easier to use a low power RF signal rather than modulating the power lines.

Brian.

Hello again and thanks for the feedback
i see Brian point of indicating mains power back on , so yep i think if the transmitter is located at the mains part of the network so it does send "On" signal to the devices that should run on mains power while the absence of the control signal means the concerned devices will be off .
as for indication , an indication lamp or similar mean can be plugged into some free wall plug to indicate power is back but this will mean extra receiving control
and yes , i remember my study year circuit was working on like 36 KHz , and the receiver used some kind of active band filter but the performance varied from place to place , at some places with a lot of electric equipment it didn't even recognize the control signal
i see now that there are dedicated ICs work as power modem lines but this will make the whole thing very expensive because i don't think they are cheap.
i've spotted 2 simpler circuits online , one uses RC filter on transmitter and receiver side , works at about 120 KHz and the second use LC tuning circuit using 445 Khz IF transformer on both side , however i couldnt test any of them yet as i have to buy the parts
what would affect the HF as i think is the loads that have some kind of capacitive nature in the home , maybe fluorescent lamps that have starter with power factor correction caps because they will kinda short the HF signal
will come back if i find other thoughts
Regards

- - - Updated - - -

one thing to note in my case , though the back up power source will be turned off when mains is back, the starting of the backup power run is not automatically done when mains is out itis done manually and mostly at evening /night power outage

As the generator change over is manual, I think that the mains present and generating in use is pretty trivial. How about a buzzer running of the generator output, but with normally open relay contacts in series with it with the coil powered by the incoming mains. So no generator , no buzz, no mains , no buzz, generator AND mains = buzz.
The reason I suggested a high power oscillator was to keep the component count down at the controlled outputs - 6+?
Frank

I don't think I saw this, but why not a power-held latching relay, with a pushbutton reset, on the problem devices. When the line power drops the relays open. An indicator on the relay could show that the relay had tripped. This would be important for brief outages that no one noticed. Then when line power was restored you could reset the relays.

Ken

KMoffett, I think that's the exact opposite of what the OP is asking for. They need something to tell them the incoming AC has been restored while they are running on their own power. When their appliances are running on their generator/inverter they are looking for something that tells them they can safely switch back to normal power to preseve their battery/fuel.

My system works, a relay energized from the incoming line is used to sense line voltage presence. When line is present, the relay is energized and the contacts route the line voltage to the appliance. A charger also operates to keep the battery charged up. When the line fails, the relay changes to connect the battery to an inverter input and its output to the appliances. It is automatic with only a few mS change-over time.

Brian.

Looks like two issues:

the sequence of things is something like : power go down , they disconnect heavy loads and fridges manually , they turn on generation equipment which is connect to home network via a double pole double throw knife switch ( along with power grid mains ).
the bad things happen when they forget to disconnect the fridges or other load manually , it is happening often that either fridge or generator ( cheap ones ) get damaged , or when inverter can bare the load , the battery go empty in 15 minutes or so ... or they forget to connect the fridge back when normal power is on and get everything is it spoiled .

Click to expand...

1. Heavy loads (Fridge, etc.) on the system when they switch to backup power. They need a way to have them automatically disconnected when line power drops. >>>> My solution.
2. An alert to remind them that the heavy loads were disconnected after they return to line power. >>>>Your solution.

Ken