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Soft-start circuit for refrigerator?

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Posted by bob prohaska on December 12, 2021, 6:44 pm

Does anybody have experience with soft-start circuits for domestic
refrigerators? The goal would be to minimize the size of inverter
or generator needed to run a fridge during an outage.

My present fridge draws only about 100 watts in steady state operation,
but the nameplate power is around 800 watts, presumably because of the
defrost heater. On startup, a Kill-A-Watt reports initial draw of about
500 watts, but that declines within a few seconds. I can't easily measure
the initial surge that accelerates the compressor.

There's a considerable advantage in parasitic losses, especially for the
case of generators, in using the smallest possible unit. Near as I can
tell, gasoline consumption for a small generator is roughly

actual GPH = .1 x (rated kW + load kW)  

so oversizing the generator for tranisent loads costs dearly in overall
fuel consumption. Presumably the same reasoning applies to inverters.

If a soft-start circuit allows a 1 kW generator to replace a 2 kW unit
that would be a substantial reduction in fuel consumption.  

Any stories or references would be appreciated.

Thanks for reading,

bob prohaska


Posted by Jim Wilkins on December 13, 2021, 3:26 am

Does anybody have experience with soft-start circuits for domestic
refrigerators? The goal would be to minimize the size of inverter
or generator needed to run a fridge during an outage.




That's the sort of thing I experiment with, though not with changing the  
operation of the compressor because I don't know how much starting torque  
margin they have under all conditions, especially after a very brief outage  
when pressure is still high, and don't want to stall and burn out one.

I capture the starting surge with an AC current probe and a digital storage  
scope after other methods gave very inconsistent readings. The surge on a  
small fridge that draws 60~80W running was 12A for about 600mS. My APC1400  
UPS (900W, 1400VA) starts it until the battery is nearly drained. The APC  
also starts a compact 4.3 cubic foot Magic Chef refrigerator without  

If you can live with the small capacity a compact refrigerator may be the  
most cost-effective solution. Mine was $60 on sale. Its capacity was  
adequate for me until Covid forced me to stock up with more food less often.  
I also have a small Alpicool C20 to use in the car, which became my backup  
freezer with an extra week of food. It's shut down for every type of soft  
fault mentioned in the Amazon reviews but so far always worked again after  
disconnecting power.

I'm presently checking out an Alpicool T60 fridge and/or freezer that runs  
on 12V or 24V DC. Their inverter compressors, similar to the Danfoss/Secop  
BD35, soft start without overshoot. For a solar+battery system they have the  
advantage of not needing a true sine AC inverter with its significant idle  
power consumption. A KAW EZ shows the T60 as a freezer costing $.28 per  
month at $.187 per KWH. For reference the Magic Chef costs less than $ per  
month in winter and around $ in summer, at room temps between 75F and 80F.

60 liters is 2.1 cubic feet. Since the food is in wire baskets it can be  
more tightly packed than on shelves, assuming you choose containers that  
nest together well like mayo in flat squeeze bottles instead of round jars.

The C20 and T60 power connector appears to be an IEC C11, which ARB and  
other makers of twice-the-price portable DC refrigerators also use. The  
supplied cord drops several tenths of a volt and I couldn't find a bare  
connector to wire so I made snug-fitted socket pins from 3/16" brass tubing  
flattened to 2mm inside, and wired them to Anderson PP45s.

You would have to suffer long and frequent outages for the cost of gasoline  
to approach the price of another generator or refrigerator.
My Honda EU1000i is rated to run 3.9 hours on its 0.61 gallon tank at full  
load, 900W, and the ECO setting. That's .156 gallons/hour, $.69 per KWH at  
$ a gallon. The cost is around $0 to recharge my 2 KWH battery bank daily  
for a week of outage each year. That's the maximum, we don't have week-long  
outages every year and in sunny weather I don't need the generator at all.

My appliances are sized within the capacity of the APC inverter and low  
consumption from a generator is just an added bonus. For me these are hobby  
and educational expenses, I'll never recover the costs of "free" solar power  
if I store it in batteries. The Alpicools allows me to use solar electricity  
as it's generated to replace grid power but the savings is hardly $ per  
month. Their value is versatility and convenience, I can stock up during  
sales and shut them off after using the contents. The C20 is a 0F freezer  
right in the car.


Posted by Jim Wilkins on December 13, 2021, 3:06 pm

Does anybody have experience with soft-start circuits for domestic
refrigerators? The goal would be to minimize the size of inverter
or generator needed to run a fridge during an outage.

I've done some crude starting surge testing on older, cheaper generators and  
appliances I could afford to burn up, specifically a Coleman 3000/3750W  
generator and a 1970's Maytag washer. The motor inrush is on the high side  
of 40A which is at the generator's limit. If the belt is tight the start  
switch stays closed and the winding rapidly overheats and smells. Pushing  
the spring-loaded motor in with a foot to loosen the belts lets it start,  

Likewise I can get a 1/2HP air compressor to start by adding a lever  
operated relief valve to the compressor outlet. It's also at the generator's  
limit and won't start against back pressure, but will if the tank is empty  
or the relief valve open. I made the cam lever to replace the original wire  
loop on the valve stem.

I didn't measure the surge because I don't know how to characterize the  
generator's overload capacity. I have no way to measure engine BMEP, torque  
or rotational inertia and can only replace what broke when I exceed them, as  
on the sawmill. I could build a starting surge simulator with a time delay  
relay and large resistor and rheostat load bank but it might be a  
destructive test.
30A at 30V, continuously. I paid $2 for it and have driven it to its full  
rating while testing a homebrew 24V battery charger.

Unless someone else is paying I don't normally test anything of value beyond  
published specs, before the warranty ends. When someone else was paying they  
knew the expected limits but needed custom test equipment to confirm them.  
For gear I built I find the usually predictable hot spots with an IR  
thermometer and monitor them with thermocouples.

Posted by danny burstein on December 13, 2021, 3:13 pm

You all know that bad feeling when there's a terrific product
that, well, is just dropped by the manufacturer?

Yamaha, which has been fighting a losing battle with Honda
for high quality commodity portable generators, had a line
which was _perfectly_ suited for dealing with this issue.

These were inverter units, battery started, which... (sit
down for this one..)

 ... which kept the battery inline while the generator
was operating.  

If the load suddenly increased (as in, oh, that starting
surge we all know an hate), the generator, or I guess more
accurately the inverter, could tap off the battery so as
to get short term extra capacity.

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by bob prohaska on December 13, 2021, 6:50 pm
Is there a model name or number I could search for? I do see
Yamaha generators on craigslist from time to time. It never
hurts to look.....

Thanks for writing!

bob prohaska

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