Question about switching between 220v and 12v

Thread Starter

ignacio.ivb

Joined Dec 20, 2020
4
Hi,
I'm an engineer but not in the field of electronics. I live in the countryside and here the water comes from a 0.5HP electric water pump to the house, from a water tank. There are power outages quite often in winter, so I have some batteries with a solar panel and a inverter to manually connect the pump in case of power outage.

So, I would like to have some device that senses when power outages occur and automatically changes from the 220V AC from the grid to 220V AC from the battery inverter.

I know there are solar system controllers or UPS that have this characteristic incorporated, but I would like to know if there is a more simple and cheaper device in the market and how is it called. The first thing I thought was a DPDT relay but I believe that it's not safe to wire two different power supplies there in case it fails. Or is it?
 

jpanhalt

Joined Jan 18, 2008
11,088
I would use a purpose designed switch. The initial pain for the cost will be offset by peace of mind and knowing it meets code. One thing you need to be sure of is to not back feed the mains. That is, when your motor is supplied from your batteries, it needs to be completely isolated from the mains.

As a DIY, I don't see any reason a DPDT switch/relay wouldn't work if appropriately rated. Motor to center poles, mains to one end, and your supply to the other.
 
A UPS would probably be ideal. May or may not need a battery expansion.

A relay or automatic transfer switch alone would require a traditional inverter to be left permanently on unless it has on/off remote control signal terminals.

Ostensibly you could wire a suitably sized DC contactor normally closed held-open when mains power is present. When the mains fail, the DC contactor closes and provides power to the inverter... inverter produces 240VAC to energize a DPDT relay acting as a rudimentary transfer switch to cut over the pump to inverter power. Just choose quality, brand-name contactors and relays - Chinese garbage off of Amazon usually isn't listed and sometimes has an annoying habit of becoming an ignition source.
 

Thread Starter

ignacio.ivb

Joined Dec 20, 2020
4
jpanhalt, what do you mean by "purpose designed switch"?

For the DPDT switch it seems like a simple solution. I just wasn´t sure is they are normally used this way in the industry. What kind of circuit or mechanism do they use in this situations? for example, in automatic emergency lights? I would like to read about it but I don´t know where to find it.

Thank you very much for the replies.
 

Ian0

Joined Aug 7, 2020
2,178
Emergency lighting is switched electronically because it has to meet strict criteria about when it what mains voltage constitues "mains failure" and how long it can take to switch over. However, the ultimate failsafe backup is a contactor wired like this.
Unlike a relay, a contactor has sufficient gaps between the "off" contacts to meet safety standard for insulation, and is designed for switching between two different mains supplies.
You probably don't want to run the inverter all the time, so, if it has an input for an external on/off control you could switch it with a set of auxiliary contacts on the contactor.
 

Attachments

jpanhalt

Joined Jan 18, 2008
11,088
jpanhalt, what do you mean by "purpose designed switch"?

For the DPDT switch it seems like a simple solution. I just wasn´t sure is they are normally used this way in the industry. What kind of circuit or mechanism do they use in this situations? for example, in automatic emergency lights? I would like to read about it but I don´t know where to find it.

Thank you very much for the replies.
My brother lives in SoCal and has enough solar cells on his home that the electric company pays him a monthly amount. He was required to install an approved switch to prevent backfeeding when the mains went down as in being serviced. That is all I meant.
 

Thread Starter

ignacio.ivb

Joined Dec 20, 2020
4
Interesting video, thanks.
I have one last question about contactors that confuses me. I have seen that its common that, for example, 3-pole contactors to have an auxiliary contact. Can the auxiliary contact be used excatly the same as the other contacts, as if it were a 4-pole contactor? Is it just a thing of labeling or 3-pole contactors are different than 3-auxiliary contacts contactors?
 
Auxiliary contacts on NEMA and IEC standard contactors are intended for control purposes only - they are not rated to handle high currents or switch highly inductive loads, e.g. induction motors. If you disassemble a contactor, you'll find that the main contacts are an order of magnitude more heavily built than the auxiliaries.
 

Ian0

Joined Aug 7, 2020
2,178
The auxiliary contact is usually for feedback to the control circuit, so that a contactor failure can be detected. It switches "logic" signals, though in the world of power control, "logic" tends to be 24V at tens of milliamps (enough current to operate a 24V signal relay), rather than 3.3V or 5V at microamps.
Perhaps I should have explained that when I suggested using the aux. contacts to enable your inverter. Many inverters have an input for a "volt free contact" to switch them on and off.
 

Ian0

Joined Aug 7, 2020
2,178
Also, very important is that the contactor opens if the mains fails, removing the potentially dangerous situation of the motor starting unexpectedly that would occur when the mains returns.
 

Lo_volt

Joined Apr 3, 2014
189
There are purpose-built charger/inverters on the market. I have a similar situation at my house in that, when utility power fails, we have only the water pressure left in the storage tank. My storage tank is only a 10 gallon tank, so that's not a whole lot of water.

I put my system together in 2008 and since then, technology has improved and prices are much lower. I used a Xantrex modified sine wave inverter/charger. It connects directly to my battery bank of 4 batteries. When utility power is on, it charges the batteries. On losing utility power it automatically transfers over to the inverter and supplies the AC for my water pump. Xantrex no longer sells the unit that I used but there are better ones on the market today.

Note that startup surge from a water pump can be 15-20 amps or more at 230 volts. Your inverter must be able to handle that surge. My water pump (3/4 horsepower) levels out at about 6 amps after startup.

Note, as well, that your DC wiring must be able to handle that much power. 6 amps at 230 volts translates to nearly 60 amps at 24 volts (my battery bank voltage). To accommodate that much current and with some fudge factor for startup current, I used 2 AWG for my DC wiring.

Lastly, don't forget to put a fuse or circuit breaker near the battery bank to disconnect the inverter and protect your system if the need should arise. Keep in mind that AC circuit breakers are not rated for DC use. Make sure that you purchase a DC rated breaker or fuse.
 
NFPA-70 310.16 will provide guidance on suitable conductor sizes. Use 60 degree ampacities and copper conductors unless you are familiar with the ins- and outs of the code.

An RK5 fuse or similar with an adequate DC voltage rating will be more than sufficient for this appliation. If your battery bank is negative- or positive-grounded you will want to fuse the ungrounded conductor as near to the battery terminals as practical. If it is floating, fuse both conductors in this same fashion.
 
Last edited:
Top