Switch from Inverter to Mains Automatically
Switch From Inverter To Mains Automatically
Power a circuit directly from a power inverter, but with automatic mains power back up.
electric circuit | storage | generalThe majority of domestic
renewable energy is generated to supplement an existing mains power supply - for example, an array of
solar panels used to reduce the costs of powering a home.
Unfortunately the cost of
grid tie inverters 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 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 direclty to a suitable
power inverter in turn connected to the renewable system
battery bank. However, this would not be a very good solution since the lights cannot be used when the batteries are discharged. 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.
DISCLAIMER - 240 Volt AC electricity can KILL. We recommend that you talk to a qualified electrician before attempting to implement any of the ideas discussed in this article.
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 will stop supplying power. Using this information we can put together a simple automatic mains/inverter switch using just a
240V AC Relay.
The relay has three connections,
Common,
NO, and
NC. The live wire from the
lighting circuit is connected to the Common, the live wire from the mains supply to NC, and the live output from the
power inverter to NO. The live and neutral outputs from the power inverter are then connected across the
relay coil. This is all shown in the schematic diagram below:
When the
power inverter is switched on and the batteries are charged, the relay coil has 240VAC current flowing through it. This moves the contact within the relay from the NC to the NO position connecting the live wire from the inverter to the live of the lighting circuit -
lights are powered from renewable energy. When the batteries are discharged, or if the inverter is switched off, the lighting circuit live will be connected to the live of the mains supply via the NC of the relay -
lights are powered by mains electricity.
This is a fully
automatic system with switching from inverter to mains power in a
fraction of a second.
To make it safe, a
circuit breaker should be used (rated the same as the mains circuit breaker for this circuit), and a manual ON/OFF switch and LED indicator included to enable the user to see which power source is being used and manually switch to mains power if desired.
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!
Article Last Modified: 16:55, 4th Sep 2007
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