Coil relay switching 2 power supplies

[Zozew007]

Hey,

Total noob here so please be patient ?

I have two power supplies mains 220v in a type of small cabin/house every day around 8am they cut of the good supply and leave it off for a few hours then turn it back in again.

What I would love would be a contraption that switches between the power supplies.

Can a simple 220v 14pin relay do the trick?

The load is minimal.. Maybe 2A max more like 1A...but there are 2 inductive loads 2 60w fans...

The 2 power supplies are not the same in voltage as the one cut of runs some type of AVR. So there is sometimes a difference of 20v between the supplies.. 220 on one and 200 on the other..

I've been trying to read up on feedback and synchronization AC etc but I can't find something that kinda fits my situation... I'm not looking for a "certified" way of doing it... Just some info and how it could be done... Budget is a big factor..

Thanks

 

[Lucien Nunes]

You can buy this ready-made, it's called an automatic transfer switch (ATS). They are most often used to transfer between the incoming line and a backup generator or inverter.

In principle, a 2-pole changeover relay will do the job. Connect the coil and the normally open contacts to the priority supply, so that when it is live, the relay operates and selects it. Connect the normally closed contacts to the backup supply, so that it gets selected by default when the priority supply is unavailable.

However, there are practical safety issues with an ordinary relay. What must be prevented at all costs is interconnecting the two sources. At best, this would result in blown fuses, but at worst it could backfeed into a supposedly dead / disconnected circuit undergoing maintenance, and electrocute the person working on it. Backfeeding can occur if the relay contacts weld, or jam partway with one contact in NO and one in NC position, or the relay chatters in and out if the line voltage is low, and sets up an arc across all the contacts. Trust me, this happens. The relay also has to cope with twice the peak line voltage, in case it switches at a time when the two supplies are out of phase, which is 650V on a 230V supply.

Therefore commercial transfer switches and UPS circuits use additional safety measures such as positively guided contacts and a control circuit that stops the relay chattering etc.
I strongly recommend you use one of these rather than making your own.

 

[Zozew007]

Very informative thank you!

Yes I have seen these ATS they are about 30usd and have a motor/servo between the two switches and a big wheel for selecting the source manually...

But just to explore the cheaper 2usd coil relay version ( that is not advisable) for educational purposes.. Apart from the 650v off phase issue... Is there not a problem with backfeed from a fan motor running when the switch happens? Or can you maybe explain what the issue is and what the backfeed creates or doesn't create?

Thanks again for the very swift and detailed answer ?

 

[Lucien Nunes]

Fans are usually driven by induction motors. Their windings are inductive and can store a small amount of energy magnetically during each half-cycle of the AC line. Interrupting the current at a non can create a spark at the switch contacts (any switch - the fan's own or your transfer switch) if the current is interrupted at non-ideal point in the AC waveform. However, the motor will not regenerate, that is convert the kinetic energy of the moving parts back into electricity, because it requires the AC line to provide the magnetising current.

Other types of motor can regenerate more readily and some are intended to do so. A braking electric locomotive returns power to the track, which can cause over-voltage if the supply systems disconnect.