Grid Mains to Generator Changeover Relay

The post explains a simple configuration which can be used as a automatic changeover circuit for switching AC grid mains to generator mains, during power failures or outages.

The explained circuit will effectively switch the connected appliances to the generator mains during power failure however it won't be able to switch start the generator automatically, this will need to be done manually, because most generators involve a difficult mechanical actuation procedure.

How it Works


Referring to the given diagram  we can see a simple circuit comprising of a TP relay (triple pole relay) and an transformerless power supply circuit.

The input of the transformerless power supply circuit is connected to the mains 220V or 120V input.

When mains power is present, the connected relay activates with this power and switches ON the load or the appliances via its N/O contacts.

Conversely when mains power fails, the relay deactivates and connects with the N/C contacts which may be wired up with the generator mains.

Now as soon as the generator is pulled started, the mains finds its way through the connected N/O contacts of the relay to the appliances.

The third set of contacts is used for enabling and disabling of the CDI unit of the generator so that when mains is restored, the generator is automatically halted.

Simple yet effective.....

Circuit Diagram


Grid Mains to Generator Changeover Relay Circuit

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Comments

preston jose said…
If I need it to connect it to an inverter for mains and when the current goes, the inverter would handle
What will be the modifications ti do?

Swagatam said…
You will just need a single DPDT relay, the generator AC will get replaced with inverter AC and the lower CDI section will not be required.
Alfred Emasoga said…
Sir, can one use three SPDT relay connected in parallel, in place of a 3P (three pole) relay? Or would series connection of the three individual coils be a better option?
Swagatam said…
Alfred, it may be done with a parallel connection, series connection would require three times the coil voltage as the input.
Thanks. How do you connect your CDI GND and cdi gnd with relay.
RAHUL V C IFS said…
What is the wattage of 1 megaohm resistance
Henry Ndukwe said…
Hi swagatam can a 373j/400v go in place of the 474/400v in the circuit
thanks
Swagatam said…
Hi Henry, if your relay is able to trigger comfortably with this cap then it would be fine
please i really need your assistance to build and automatic changeover switch of 90amps 220 volts single phase and it should have the ability to switch off the generator set once there is power from the grid
Swagatam said…
you can use the circuit that's shown in the above article, it exactly matches your requirement.
thanx for the reply and assistance rendered so far. a have a contactor of LC1D09, it has L1 to L3, T1 to T3 and A1 and A2 two NO n NC that are in the opposite sides of the contactor. i dont know how to use it as a changeover for that your circuit. pls how do i go about it. pls i really need your expectrics. pls come to my rescure
how many pole contactor is the one used above? can i use the above changeover system for heavy amperage as in to be used for an entire flat that has up to 3 deep freezers and two ac units with other appliances? put me through please
Swagatam said…
the shown diagram uses it's own recommended relays, it's not meant to be used with a contactor.

I a sorry I won't be able to interpret the details of your contactor.

If you are interested to build the above circuit then you may have to incorporate separate relays as per the power specs of the connected load.
Swagatam said…
yes you can use the above circuit for any desired high current application, you just need to change the relays as per the ampere rating of the load, and instead of a capacitive power supply you may have to use a 12V AC/DC adapter for powering the relay coils
Qaisar Hussain said…
can you tell how it is possible to overcome changeover time.
Swagatam said…
use a heavy duty relay with low coil resistance, an decrease the value of the input cap to 2.2uF/400V
FredoRicko said…
What is the maximum load that a 30A 250VAC can accommodate if used in the circuit above?? Can it power a house with all the basic appliances?? Thanks
Swagatam said…
yes this much power is more than enough for powering a house with basic appliances.

The relay will need to be appropriately selected for handling 30A, and also the 474 capacitor which also may need to be upgraded
Unknown said…
Hi Sir can I use this above circuit for 40 amp and 63 amp single phase/3phase
Unknown said…
If not then what I have to change in this circuit
Swagatam said…
yes it can be used but the upper DPDT relay will need to be selected appropriately to handle the large amp.

also for such huge relay, the activation will need to be done through a AC/DC adapter, a capacitive power supply may not be strong enough....
Norie Coquilla said…
Mr. EKE. I have a design of ATS using two Contactors. I'll be glad to share it with if you wish.
VINCENT MWENDE said…
great work Swagatam,have learned a lot in the past 6 months.
Swagatam said…
It's my pleasure Vincent!
I will be glad if you would share yours with me. I really appreciate your time and support in rendering assistance to me. Thanks alot.
priti said…
if this circuit is used for changeover relay, then how much power losses will take place by the circuit.
priti said…
sir, please tell me about power losses by this changeover circuit.
Swagatam said…
power consumption will be as per the rating of the relay coil, could be around 50mA, which will be consumed from the AC mains
Nnaka Stanley said…
Sir, great work on the circuit. But please I need a changeover circuit, this time with three different power sources; generator, AC mains and an inverter supply. So far, what I've come up with is a manual changeover using a bypass, I need one which is automatic. Thanks
Swagatam said…
Hi NNaka, I already have this design in my website, please search it in the following link

https://homemade-circuits.com/search/label/Relay%20Changeovers
gbenga comrade said…
hi sir what kind of relay that can be used for this particular change over or i can make use of 3phase contactor to do it
Swagatam said…
Gbenga, due to low current contactor cannot be used in this circuit, you must use ordinary 400 ohm type relay here....you can probably use the contacts of this relay to operate the contactor and wire the contactor contacts for the overcharge actions
Swagatam said…
....it's not "overcharge", it should be "changeover" actions...
Bakare Emmanuel said…
i would want to request and upgrade to this system, to include a buzzer (using ic 555 timer set at 1minute ), two led indicator (one for main one for generator) The system will do two main function, switch from main to generator when power fails and switch back to main when power is restored . It will sound an alarm that will automatically go off after a period of time , say 1 minutes.
It will have some fail safe features, like , circuit breaker (when main is having low current and one needs to use the generator) and fuse ( for over current protection).

thanks.
looking forward to your quick response..
Swag said…
I already have many such upgraded circuits, you can type "ATS" in the search box and find them, you may also search by typing "generator changeover" in the search box for the same.
Bakare Emmanuel said…
sorry to disturb you . none of the ''ATS'', has anything like i requested. being a newbie and yet to be fully capable of design but undergoing training, this idea came to mind and i will want to build it ..
please help.
Swag said…
Only the LED indicators are not included in my other similar articles, which can be simply done by adding an LED parallel with the existing relay coil. LED must include a 1K series resistor

alarm can be made using a small ready made buzzer and wiring it again in parallel with the relay coil. however one of the wires of the buzzer must have a series 100uF capacitor, and a 220K resistor in parallel with this capacitor.

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