Soft-start circuit for refrigerator?

5 views
Skip to first unread message

bob prohaska

unread,
Dec 12, 2021, 1:44:04 PM12/12/21
to

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





Jim Wilkins

unread,
Dec 12, 2021, 10:27:05 PM12/12/21
to
"bob prohaska" wrote in message news:sp5fti$69s$1...@dont-email.me...

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.
...

----------

https://electronics.stackexchange.com/questions/256887/can-i-reduce-the-start-current-of-a-fridge-compressor

https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting

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
complaint.

If you can live with the small capacity a compact refrigerator may be the
most cost-effective solution. Mine was $160 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 $2.28 per
month at $0.187 per KWH. For reference the Magic Chef costs less than $3 per
month in winter and around $5 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, $0.69 per KWH at
$4 a gallon. The cost is around $10 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 $1 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.

jsw



Jim Wilkins

unread,
Dec 13, 2021, 10:06:35 AM12/13/21
to


"Jim Wilkins" wrote in message news:sp6ei6$rj9$1...@dont-email.me...

"bob prohaska" wrote in message news:sp5fti$69s$1...@dont-email.me...

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,
barely.

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.
https://www.galco.com/buy/Ohmite/RUS1R0
30A at 30V, continuously. I paid $22 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.
jsw

danny burstein

unread,
Dec 13, 2021, 10:13:57 AM12/13/21
to
[snip]

>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.
>...

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
dan...@panix.com
[to foil spammers, my address has been double rot-13 encoded]

bob prohaska

unread,
Dec 13, 2021, 1:46:40 PM12/13/21
to
Jim Wilkins <murat...@gmail.com> wrote:
> "bob prohaska" wrote in message news:sp5fti$69s$1...@dont-email.me...
>
> 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.
> ...
>
> ----------
>
> https://electronics.stackexchange.com/questions/256887/can-i-reduce-the-start-current-of-a-fridge-compressor
>
That's an informative if somewhat discouraging link.

> https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting
>




> 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.
>

Indeed, nor do I.


> 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
> complaint.
>

The only part of the starting surge that can profitably be suppressed is
the inertial stage, before differential pressure builds. Do your measurements
show a double peak? 600 mS seems too long for motor acceleration alone,
by then I'd think head pressure would be coming up, which is real work
that has to be done.


> If you can live with the small capacity a compact refrigerator may be the
> most cost-effective solution. Mine was $160 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.
>

My problem isn't urgent enough to justify buying anything just yet. The
fridge I have is a very ordinary Amana ~15 cu ft bottom freezer. If I want
to spend money the most productive thing seems like a current probe and
a scope.

>
> You would have to suffer long and frequent outages for the cost of gasoline
> to approach the price of another generator or refrigerator.

Cost of fuel isn't really an issue. Storage and availability of fuel are
bigger concerns. I'm in the 'burbs, so gas in my vehicles is the easiest
supply. Space is kinda tight, so a second fridge isn't attractive.

> 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, $0.69 per KWH at
> $4 a gallon.

If I'm not mistaken, an EU1000i is generally not considered suitable for a
normal domestic fridge, leaving the EU2200i as the smallest choice. If I
could figure out how to use an EU1000i with my fridge that would, I think,
cut fuel consumption nearly in half.

>
> For me these are hobby and educational expenses,

To a great extent the same is true of me. I acquired a generator to play
with, began to appreciate the very great penalty of appliances that need
high starting currents and started to look for remedies, if they exist.

It's looking like maybe they don't.

Thanks for writing!

bob prohaska

>
>
>

bob prohaska

unread,
Dec 13, 2021, 1:50:58 PM12/13/21
to
danny burstein <dan...@panix.com> wrote:
>
> 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.

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.....

danny burstein

unread,
Dec 13, 2021, 2:00:17 PM12/13/21
to
[snippeth]

>> 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.
>>
>> ... 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.

>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.....

Here's one which, of course, is no longer available.

My guess [TM] is to look for any model
numbers with an "EB" at the end.

https://www.yamahagenerators.com/EF3000iSEB-p/ef3000iseb.htm

Jim Wilkins

unread,
Dec 13, 2021, 3:55:02 PM12/13/21
to
"bob prohaska" wrote in message news:sp84ed$dnb$1...@dont-email.me...

Jim Wilkins <murat...@gmail.com> wrote:
................

The only part of the starting surge that can profitably be suppressed is
the inertial stage, before differential pressure builds. Do your
measurements
show a double peak? 600 mS seems too long for motor acceleration alone,
by then I'd think head pressure would be coming up, which is real work
that has to be done.

{{{{{{{{{{{{{{{
IIRC the current surge was constant, then very rapidly decayed to the run
current. The scope is an 80's HP boat anchor with HPIB output I can't save.
I recently acquired a Tenma DSO that saves screenshots to USB but I haven't
captured a motor startup with it yet.
}}}}}}}}}}}}}}}

If I'm not mistaken, an EU1000i is generally not considered suitable for a
normal domestic fridge, leaving the EU2200i as the smallest choice. If I
could figure out how to use an EU1000i with my fridge that would, I think,
cut fuel consumption nearly in half.

{{{{{{{{{{{{{{{
I haven't tried running the Magic Chef directly from the EU1000, which
tripped off at slightly over 1100 Watts. The APC1400 starts the fridge and
the Honda recharges its battery bank if the sun isn't out. My plan is to run
the EU1000i in the driveway while I'm clearing it and the path around the
back to the shed, then switch to the 2200W HF inverter genny, assuming it
still works. At my age that could take all day, especially if there are
ice-covered fallen trees to clear. The EU1000i is stored in the house empty
and I've practiced filling and draining it without spilling gas. The carb
drain hose is a big help there. I made a carb bowl drain screw hose spigot
for the HF but gas spills while I'm installing it. The issue is fumes
reaching the wood stove.

Perhaps you could try a 2KW sine inverter with enough battery backup to give
you time to connect your generator to charge it. I think some of them can
sense demand and turn on only when needed. Then you have a good start on a
solar system. A sine UPS like I have might work but they are usually more
expensive and recharge the battery slowly at the float voltage, to avoid
gassing.
}}}}}}}}}}}}}}}

Jim Wilkins

unread,
Dec 13, 2021, 4:11:00 PM12/13/21
to
"danny burstein" wrote in message news:sp857v$7qk$1...@reader1.panix.com...

[snippeth]

>> 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.
>>
>> ... 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.

>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.....

Here's one which, of course, is no longer available.

My guess [TM] is to look for any model
numbers with an "EB" at the end.

https://www.yamahagenerators.com/EF3000iSEB-p/ef3000iseb.htm
------------------------------

The special cord that enables two inverter Hondas to parallel is simply a
double jumper with shielded banana plugs on the ends, which connect directly
to the AC outputs. The only special part is the shield that prevents
unplugged ends from shorting. It looks to me like any other true sine 120VAC
source might work.

Jim Wilkins

unread,
Dec 14, 2021, 8:52:12 AM12/14/21
to
"bob prohaska" wrote in message news:sp5fti$69s$1...@dont-email.me...


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.

---------------------------

Stepping back from what should work by spec to what might by experiment, if
you buy the DSO and current probe you can determine the starting current
capacity you really need which is the first step toward finding the most
economical generator that can handle it. I assume it would be an inverter
model in ECO mode, the problem then is finding one that can respond to the
surge fast enough, which you might have to determine by test if no one else
has measured and posted it. I used a clamp-on current probe meant for a DVM,
with a sense resistor to convert to voltage. For my 80i-400 Fluke specifies
a shunt resistance of less than 12 Ohms to maintain accuracy. The output is
1mA/A so 12A gives 120mV thru 10 Ohms. RMS is peak-to-peak times 0.35, if
the scope can't do that for you.

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29964567/print/true.cfm
" it looks like people haven't really done your type of testing on it. Or
at least uploaded it for viewing."

I loaded my 2200W HF inverter generator to its 19A overload trip with coffee
pots and a hotplate, with one coffee pot on a Variac to adjust the load
current. Unlike a compressor motor this mostly resistive load isn't harmed
by low voltage. I could have set the load to the 12A surge of the small
fridge and switched it on in ECO mode to see how fast the genny responded.
If the waveform looks promising you could try the fridge. I'd keep any added
extension cords short and thick and be ready to pull the plug if the fridge
current surge doesn't quickly fall to its normal running value.

I didn't do this possibly risky surge test because by then the 2200W model
had been replaced with an 1800W one, with no explanation why. I didn't want
to replace a generator that can run my microwave with one that might not.

If my overload simulator performs to spec and burns up the customer's
product that's his problem, and I've watched it happen. I try not to destroy
my own stuff, especially the emergency backups which will be irreplaceable
when needed. I test it to its written limits and then stay below them.

AFAIK HF will accept returns without question, as long as you have drained
the gas and oil. I don't know other stores' return policies.

Jim Wilkins

unread,
Dec 14, 2021, 1:24:53 PM12/14/21
to
"Jim Wilkins" wrote in message news:spa7ia$9kb$1...@dont-email.me...

Here's a possible answer.
https://petepowerblog.wordpress.com/2017/10/24/eco-throttle-and-heavily-inductive-loads-like-refrigerators/

I designed a functionally similar circuit to wake my APC inverter when the
fridge wants to run. Mine requires some logic circuitry because it needs a
hack to handshake with the APC Start button and buzzer to wake it when on
battery power, so the start circuit is more complex than you need. APC
declares that waking on battery power, while possible, is not a "normal"
operation. They also scolded me for finding and using their "proprietary"
command language.

A current sensing relay will detect when the fridge shuts off and command
the APC to sleep, or it could return a genny to ECO if combined into Pete's
and Ron's circuit The relay has an LED that indicates when the fridge is
drawing current, which tells me to wait a few minutes when switching its
plug between grid and APC power.

Reply all
Reply to author
Forward
0 new messages