Fast switching between power sources

Thread Starter

Addyroid

Joined Dec 9, 2017
4
Good day folks I need help designing a circuit and your input will be highly appreciated.

I have a deep freezer with 220V 100W running power.

I have a portable tiger generator of 220V 650VA.(it has never put out more than 350watts)

Based on the power rating the generator should be able to power the freezer. But it doesn't. Due to onrush power which may be up to 1000W.

I designed a 12V1000w inverter connected to a 12v 100amp car battery.

The idea is for the generator to continuously charge the car battery through a rectifier with a voltage regulator and a charge controller which supplies the inverter from which the freezer can draw it's. startup power

I don't need a deep cycle battery here because the batteries only job is to provide startup power.
Which will immediately be topped up by the generator.

I would like to design a circuit to to engage the inverter only when It detects current rising above 1.0A ac and by pass back to generator output as soon as the current reduces below 1.0A ac. This should be done so fast that the freezer compressor doesn't notice.
Can anyone out there suggest a circuit with appropriate components?
Thanks
 
Thought:

Can you get access to the thermostat?

Basically run the contact off of 12V. Do a delay on make for the inverter for say 10 seconds. When the inverter is OFF AND no line power, run off generator.
 

crutschow

Joined Mar 14, 2008
23,774
What about running the freezer off the inverter all the time, and just supply enough current from the charger to run the freezer and keep the battery charged?
That would require about a 10A charger.

It would eliminate the requirement for any switch-over circuitry.
But the switch-over could likely be done with a couple of solid-stage power relays by control circuitry triggered from the thermostat, as KISS suggested.
 
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Thread Starter

Addyroid

Joined Dec 9, 2017
4
Thought:

Can you get access to the thermostat?

Basically run the contact off of 12V. Do a delay on make for the inverter for say 10 seconds. When the inverter is OFF AND no line power, run off generator.
Are you saying that the thermostat should power the compressor for 10 seconds through the inverter and switch to generator after. That's an idea. But that means opening up the fridge and tampering with internal components, which I am trying to avoid.
 

Thread Starter

Addyroid

Joined Dec 9, 2017
4
I
What about running the freezer off the inverter all the time, and just supply enough current from the charger to run the freezer and keep the battery charged?
That would require about a 10A charger.

It would eliminate the requirement for any switch-over circuitry.
But the switch-over could likely be done with a couple of solid-stage power relays by control circuitry triggered from the thermostat, as KISS suggested.
I thought about that and may still end up doing that. I just want to see how i can reduce wear and tear on components.thanks
 

MisterBill2

Joined Jan 23, 2018
4,524
Not all inverters are able to power the induction motor in a freezer, and likewise, not all generators. I discovered that with a 5KW generator that had plenty of capacity but not a clean enough waveform. And you will need a circuit to decide when to switch on the inverter because there is no need to power it when the freezer is not running. That can be done externally with a simple circuit that detects that the thermostat has operated. The low power DC circuit then starts a timer cycle that connects the inverter to the freezer and then switches the inverter on. After another time delay it starts watching a current relay in series with the freezer power, and when the freezer thermostat switches off, it switches off the inverter. Fairly simple, and the whole package is available commercially, but not that complicated to build yourself.
 
But that means opening up the fridge and tampering with internal components, which I am trying to avoid.
The fridge could be a simple device or a complex device. The thermostat might be one way. Another, might be intercepting the power before it goes to the compressor. It might be an easier place overall to detect and make the switch, so only the compressor runs off inverter power for a few seconds.

I used what popped into my head first and it's not necessarily a bad idea to put any idea on the table, absurd or not.

This is brainstorming.
 

MrSoftware

Joined Oct 29, 2013
1,610
If your goal is to maintain the battery with the generator, then don't worry about the current that the inverter is drawing, just keep an eye on the battery voltage. An off the shelf charger would be one way to go here.
 

dendad

Joined Feb 20, 2016
3,167
If you are wanting to switch AC voltage sources without a break, then you will need to phase lock the supplies.
Otherwise, a short time with no power will hopefully allow the compressor motor to keep running so there will not be the high starting current. But, there may well be significant transients produced.
If your inverter can run the fridge, I too would vote for running it all the time and have the generator charge the battery.
That will be a lot simpler to implement.
 

crutschow

Joined Mar 14, 2008
23,774
If your inverter can run the fridge, I too would vote for running it all the time and have the generator charge the battery.
That will be a lot simpler to implement.
And depending upon the duty cycle of the freezer, the average power delivered to the battery could be significantly less than power required to run the compressor when it's on.

Also if you make the battery large enough, then you wouldn't have to run the generator all the time just for the freezer.
The battery could carry the load for awhile.
 

MisterBill2

Joined Jan 23, 2018
4,524
Just Use a current switch
current switch
You can't use a current switch to control the inverter because the refrigerator draws no current when it is switched off. I described the parts and the required functionality back in my post #6, the second part. It is not that complicated. And the best part is that it requires NO opening of the refrigerator. I realize that there are lots of folks terribly afraid to dig in to anything, and many others lacking any skill to dig in without destroying things. But even those folks should be able to assemble a simple external package and plug the refrigerator in to it. The only real concern is if that refrigerator has a defrost system that draws power constantly.
 

be80be

Joined Jul 5, 2008
1,961
We use current switch to start a pumps it draws no current till it started You can set it to change over when the refrigerator compressor start's up.
The current switch it fired on 2 mA rise. I'm sure you could unplug the compressor hot and set it up to catch it rise and switch over and not even break
the warranty on the refrigerator
 
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be80be

Joined Jul 5, 2008
1,961
Current switch you install on a wire that draws current when starting it then sends signal to switch over in my case powers up pumps to wash cars.
but in you case you be switching power from gen to inverter
There even a better way let the power to compressor switch a relay that powers the inverter and run the compressor from that.
both would do the job
 

MisterBill2

Joined Jan 23, 2018
4,524
Can " be80" explain how current would flow in the refrigerator power circuit when the inverter is switched off?
And then explain why he needs to use a vulgar term to express his opinion that a statement is incorrect?
I have used isolated current detection switches, and they tend to not trigger without any current flow, and if the inverter used to supply current is off it does not seem like there would be any current flow.
 

be80be

Joined Jul 5, 2008
1,961
The TS said
Fast switching between power sources
got be power there somewhere to say hey i'm going to need more then the generator can handle.

The compressor relay starts to close the current switch catch's that and switches on the inverter

But there are more was as I said the TS could install a relay that comes on when the compressor start and powers it from his inverter.
 
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MisterBill2

Joined Jan 23, 2018
4,524
The TS did state in post #1 that this is a deep freezer, meaning that it is not a refrigerator. Mt deep freezer has no light and no defroster. The TS will need to determine what their deep freezer does have. The system that I described would run off of the 12 volt battery that also supports the inverter, as described. The trigger circuit detecting the freezer needing to run can be a simple as a sensitive relay and the 12 volt source in series across the freezer line connection. After the demand is detected the timer starts running and keeps running until the inverter has switched on and then is connected to power the freezer. That relay change-over also disconnects the trigger sense circuit, isolating the AC power from the sense circuit. The current sensing relay is provided to switch off the inverter after the freezer switches off. But that must be a momentary signal so that the inverter can switch on the next time. The first versions of this scheme used a mechanical timer with a spring reset arrangement. Now it can all be done with just a few IC devices and some relays.
 
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