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) as shown below, and a 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
3 Phase Grid to Generator Changeover Circuit
The following diagram shows how a 3 phase grid to generator changeover can be implemented using a couple of 3 phase contactors.
How the Circuit Works
Let's assume mains AC is not available, and generator is switched ON by the left side relay.
In this situation the center relay will be deactivated, and its pole will be connected with its N/C contact, so that the +12V DC from the generator passes through the N/C contact and actuates the bottom/right 3 phase generator contactor.
The top/right grid mains contactor remains switched OFF due to the absence of a +12V DC.
Therefore, the generator AC flows through this bottom/right contactor and operates the connected appliances or load.
Now, suppose the mains grid AC restores.
The left side relay activates and turns OFF the generator. Also, simultaneously the center relay switches ON through the +12V from the grid mains.
The center relay pole now shifts from N/C to N/O, so that the +12V from the mains grid AC passes through the N/O contacts and actuates the top/right contactor. The bottom/right generator contactor is simultaneously switched OFF.
With the top/right contactor switched ON, the grid AC now becomes available to the load.
Again, if the mains AC fails, the left side relay deactivates, switching ON the generator procedures, the center relay connects with its N/C contacts, turning on the generator 3 phase contactors and turning OFF the grid contactors.
If you are unable to get the above 12V electromechanical relay/contactor, you can go for a 3 phase SSR contactor instead, as shown below.
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