Switch from Inverter to Mains Automatically

Power a circuit directly from a power inverter, but with automatic mains power back up.

The majority of domestic renewable energy is generated to supplement an existing mains power supply - for example, an array of solar panels (1) used to reduce the costs of powering a home.

Unfortunately the cost of grid tie inverters (2) with UK G83 approval (and professional installation) puts them well outside the price range of most small to medium microgenerators. Therefore, if a microgenerator with let's say one 150 Watt solar panel and battery backup (3) wants to power a lighting circuit in their home, they will need to look for alternative means to achieve their goal.

One way would be to separate the chosen lighting circuit completely from the mains power supply, connecting it directly to a suitable power inverter (4) in turn connected to the renewable system battery bank (5). (See our article Dual Powered Lighting Circuit (6)).

However, is not an optimal solution since manual intervention is required when the batteries become discharged. Ideally we need a way to power the lighting circuit from the renewable source, but with an automatic way of switching back to mains power when necessary.

Using a Relay to Switch from Inverter to Mains Power

All power inverters have an automatic shut off function which prevents the attached battery bank from being overly discharged. When the voltage of the batteries falls to a certain fixed level, the inverter simply stops supplying power (sometimes sounding an alarm/buzzer at the same time). Using this information we can put together a simple automatic mains/inverter switch using just a 230VAC DPDT Relay (7).

A DPDT relay (pictured above) has eight connections - two for the coil which should be connected to the live and neutral outputs from the power inverter, two NO's, two NC's, and two COM's. When the relay is energised (i.e. when there is 230VAC across the coil from the inverter), NO1 is connected to COM1 and NO2 is connected to COM2. When the relay is not energised (when the power inverter is not putting 230VAC across the coil), NC1 is connected to COM1 and NC2 is connected to COM2.

Use a DPDT 230VAC relay to switch between inverter and mains electricity.

Therefore, connecting the relay up as shown in the schematic diagram above (and the connection list below), the lighting circuit will be powered by the inverter when it is on, and the rest of the time by mains electricity. The switchover occurs in the fractions of a second.

Connections:
Inverter Live to Relay Coil
Inverter Neutral to Relay Coil
Relay COM1 to Lighting Circuit Live
Relay COM2 to Lighting Circuit Neutral
Mains Live to Relay NC1
Mains Neutral to Relay NC2
Inverter Live to Relay NO1
Inverter Neutral to Relay NO2

Buy a DPDT Power Relay

The relay pictured in this article is available for around £2 plus £1 for the socket from Rapid (8), for around £4 from Maplin (9), or from £0.99 from this search on eBay UK: DPDT Relay (10).

Safety

DISCLAIMER - mains electricity can KILL. We recommend that you talk to a qualified electrician before attempting to implement any of the ideas discussed in this article.

Further details on the implentation of this scheme have not been included since if you do not know how to do it, you really should not be doing it!

Web Link References

(1) http://www.reuk.co.uk/solar.htm
(2) http://www.reuk.co.uk/Grid-Tie-Inverters.htm
(3) http://www.reuk.co.uk/storage.htm
(4) http://www.reuk.co.uk/Power-Inverters.htm
(5) http://www.reuk.co.uk/Interconnecting-Batteries-for-Battery-Bank.htm
(6) http://www.reuk.co.uk/Dual-Powered-Lighting-Circuit.htm
(7) http://www.reuk.co.uk/Relays-and-Renewable-Energy.htm
(8) http://www.awin1.com/cread.php?awinmid=1799&awinaffid=80045&clickref=&p=http%3A%2F%2Fwww.rapidonline.com%2Fsearchresults.aspx%3Fstyle%3D0%26kw%3D601676
(9) http://www.maplin.co.uk/Module.aspx?ModuleNo=2567&C=Maplin&U=SearchTop&T=dpdt
(10) http://rover.ebay.com/rover/1/710-53481-19255-0/1?type=3&campid=5335956244&toolid=10001&customid=&ext=dpdt+relay&satitle=dpdt+relay

Article from REUK.co.uk:
http://www.reuk.co.uk/Switch-from-Inverter-to-Mains-Automatically.htm
Published: 19th August 2008
© REUK 2009