Switching Sources To Load?

Soulearner

Quick question: If -
I have an inverter which will power a refrigerator's running needs. But not it's start needs.
a generator which will power it's start (and, obviously, running) needs;

Can I make a set of DPDT switches in a box that inputs from both sources, delivering to the one load, that will allow me to start the 'fridge on the generator, and immediately switch to the inverter as the supply once the unit is running?

Additional info: the inverter is a Samlex 300w, supplying 60 volts to each pole.

BB.

Re: Switching Sources To Load?

Yea--Sort of...

There are several schools of thought on this... You have two 60 Hz AC sources that are not in frequency sync (generator and inverter in this example).

Start the AC motor on one (Generator) then switch over to the second (Inverter)--Some/Many/Most everyone (but me?) has a concern that because the motor and sources are not "in-phase" you could get high current surges on the transfers from one AC source to the other AC source.

Personally, I never had a big issue with that (switched motors on/off/on all the time and never noticed any big current draw/motor strain to "re-sync").

Could it be an issue on certain specially motors (such as synchronous motors)--I guess so. But for average induction motors around the house, I have never seen any issue.

Of course, when the compressor is shut off (via thermostat) and needs to restart, you have the whole generator+inverter transfer issue again. Not sure it is worth the effort for any long term operation.

-Bill

jcheil

Re: Switching Sources To Load?

BB. wrote: »

Yea--Sort of...

There are several schools of thought on this... You have two 60 Hz AC sources that are not in frequency sync (generator and inverter in this example).

Start the AC motor on one (Generator) then switch over to the second (Inverter)--Some/Many/Most everyone (but me?) has a concern that because the motor and sources are not "in-phase" you could get high current surges on the transfers from one AC source to the other AC source.

Personally, I never had a big issue with that (switched motors on/off/on all the time and never noticed any big current draw/motor strain to "re-sync").

Could it be an issue on certain specially motors (such as synchronous motors)--I guess so. But for average induction motors around the house, I have never seen any issue.

Of course, when the compressor is shut off (via thermostat) and needs to restart, you have the whole generator+inverter transfer issue again. Not sure it is worth the effort for any long term operation.

-Bill

Someone on here did a really amazing (and complicated) circuit to allow a fridge to run and start on that same inverter. It might have been a chest freezer conversion, can't remember. But bottom line was it WAS possible, but very complicated.

BB.

Re: Switching Sources To Load?

Wayne from NS Canada did this...

Basically, his trick was to experiment with the motor capacitor sizing, using a step-down transformer to boost starting current, and a relay to switch between step-down transformer (starting current) and full AC inverter power (running current) (a small 300/600 watt MorningStar PSW inverter).

-Bill

Soulearner

Re: Switching Sources To Load?

Thanks for the great advice folks. It gives me some new ideas and cautions. Had not thought about the surge possibility, just phasing. Whether the compressor is split-phase or capacitor induction, I just don't see how a few milliseconds of phase-change could be damaging. Perhaps I'm mistaken.
For surging, I could use a trick I've used successfully before; put a small incandescent bulb in-line with the load.
Maybe I'll make such a box a winter project.

Cariboocoot

Re: Switching Sources To Load?

Most likely the problem will be you can't switch fast enough from one source to the other before the compressor slows down and requires a restart. Compressor under load will slow very quickly, unlike a motor free spinning.

westbranch

Re: Switching Sources To Load?

Just to add, IIRC Wayne now uses a larger inverter for his fridge...

Soulearner

Re: Switching Sources To Load?

Thanks, I looked up some of his posts looking for a reference to this subject, but, he is using a circuit I can't build, and, he is using a different approach.
Still, for grins-n-giggles I might build a box to try it. I don't see any way it can actually cause harm...

notes

Re: Switching Sources To Load?

Find your self a refrigeration tech. This is what I am currently testing at my shop. I have a top mount freezer, frig. I cut out all electric defrost, frame heaters and timer. Installed a 115 volt selinoid valve between static condenser and evaporator coil to stop migration of refrigerant to compressor in off cycle. On start up from thermostat compressor draws minimal amps, maybe 1.5, I have a time delay relay that times out at 1 minute to energize coil when t stat calls for cooling. Still in test mode, playing with timer and start and run capacitors to get efficient starting. Manual defrost after running a few days.

Soulearner

Re: Switching Sources To Load?

@notes,
thank you so much for posting! Yours is parallel to my considerations. I could do the relay timing by hand - no imposition. Disconnecting the heaters / timers crossed my mind, since, as a kid, we defrosted the 'fridge every couple weeks anyway - I don't mind. I would be concerned about stopping the coolant migration to the compressor though for it will be starting dry - or nearly so.