Inverter with Auto Switch to AC Power

D. Abram · 2010-07-02 14:56

I'm working on a small solar project for my house to run a few items.

Solar panel, charge controller, battery and inverter are installed and work well.....until the battery drops below min. charge and the whole system shuts down [noparse]:([/noparse]

What I'm trying to find is an inverter with a built in switch back to utility provided AC when the battery drops below min. charge level.

I've found several units that work the opposite, if utility AC fails, inverter switches to battery.

Any thoughts or suggestions are much appreciated.

Spiral_72 · 2010-07-02 15:06

Could you use your existing setup with a contactor or large relay with a 110V coil? You'd run the inverter into the coil of the contactor. When the inverter fails, the contactor / relay drops out to supply utility AC through the N.C. contacts...... the inverter would supply the N.O. contacts.

Simple but effective!?

The only thing I see with this is.... it'd be nice if there were a time delay maybe, so the batteries would charge a little before coming back on-line in the morning.

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"puff"...... Smile, there went another one.

Post Edited (Spiral_72) : 7/2/2010 3:12:01 PM GMT

Spiral_72 · 2010-07-02 15:09

Also you'd probably need to oversize the switch to handle the inrush.... maybe, depending on what you're running with it.

Maybe there won't be an inrush though..... the switch time would / should be very small.... 10ms or less I would guess for a big relay.

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"puff"...... Smile, there went another one.

Phil Pilgrim (PhiPi) · 2010-07-02 17:04

If "the whole system shuts down" all at once (i.e. the inverter output instantly drops from 120VAC to zero), you could use the inverter output to hold a DPDT relay closed, with the poles connected to the load, the NO contacts connected to the inverter, and the NC contacts connected to the mains. Be sure to consider all the possible failure modes and include sufficient protections against them.

-Phil

WBA Consulting · 2010-07-02 17:16

Here's an idea from a funny source. I noticed something in one of the outlets in the mens' bathroom at Parallax during UPEW: one of those rechargeable emergency flashlights that comes on when AC power goes out. You may be able to hack something like that for your project or maybe search google for that circuit.

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Andrew Williams
WBA Consulting
PowerTwig Dual Output Power Supply Module
My Prop projects: Reverse Geo-Cache Box, Custom Metronome, Micro Plunge Logger

Peter KG6LSE · 2010-07-02 17:40

to add Most modern inverters shut off at a low voltage as to keep eonugh juce to sstart your car .

I had a Idea .. some inverters have a set of LEDs taht one could use as a batt Low Siganal to switch a relay

EG one of my UPSes has a BAT LOW led . thats green then red .. With some clever hacking I could use it with a Opto Iso to drive a relay .........

Peter KG6LSE

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LOL

D. Abram · 2010-07-02 17:50

Thanks for all the great ideas. I think I may be able to put together a device to switch the power as many of you have described.

I've started to rough out the circuit for this, but I'm stuck on the ground leg of the AC side of the project.

The relay will switch the hot wire from the inverter to the utility AC. Can I split the common and have it connect to both the inverter and the utility AC?

D. Abram · 2010-07-02 17:55

thought i'd better clarify my use of the term ground in the previous post....I think common is a more accurate term for "the white wire" that I'm referring to and not the green ground wire.

Phil Pilgrim (PhiPi) · 2010-07-02 18:05

No. Switch both of them. Use a DPDT relay.

-Phil

Spiral_72 · 2010-07-02 18:34

Phil Pilgrim (PhiPi) said...
No. Switch both of them. Use a DPDT relay.

-Phil

I'd agree 100%. Pretty simple solution eh?

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"puff"...... Smile, there went another one.

D. Abram · 2010-07-02 19:03

attached PDF is my design based on the discussion to this point

need additional details on the control voltage for the relay. where to get it and where to connect it.

Spiral_72 · 2010-07-02 19:28

This has a built in on-delay timer that can be set from 0s to 60hrs... rated for 5A at 240VAC with 100VAC - 240VAC input power.
I love these, but it may not be big enough for you:
http://www.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/Fuji_Timer_Relays_1-z-16_DIN_(MS_Series)/MS4SA-AP-ADC

This is just the relay. I've never used this one at 15A, but the spec says it'll do it:
http://www.automationdirect.com/adc/Shopping/Catalog/Relays_-z-_Timers/15A_Electro-Mechanical_Cube_Relays/782-2C-120A

You'll have to look around but the timer relay would be perfect (I think). Surely Fuji... Koyo.... (Square-D MIGHT make one) would have a timer relay rated for 15A or so.

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"puff"...... Smile, there went another one.

Post Edited (Spiral_72) : 7/2/2010 7:36:57 PM GMT

Desy2820 · 2010-07-04 16:26

An expensive, but better way to do this may be a transfer switch.· They're normally used to switch over to a generator or other backup power when "normal" power fails.· I think you can work it the other way, connect your inverter as·"normal" power, as far as the transfer switch is concerned, when the inverter fails, it will switch to "backup" (which is "normal" power).

Anyway, hope this makes sense and gives you some ideas.

Inverter with Auto Switch to AC Power

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