Alternative Power sources - Size of inverter to start a refrigerator?

Author

Size of inverter to start a refrigerator?

Roger Moss

2006-04-22, 7:21 pm

I want to get an inverter to run my refrigerator. It is a normal size
(850*600*600mm), "A" efficiency refrigerator. The running power is pretty
small - 70W I think - but I'm not sure how much current is needed to start
up the induction motor in it.

Will a 250W inverter get it started, or do I need something bigger?

Thanks

Roger

John Smith

2006-04-22, 7:21 pm

Once you get it started off an inverter check the servo motor for the defrost
timer. They are power picky and it may not run at all.

Roger Moss wrote:

> I want to get an inverter to run my refrigerator. It is a normal size
> (850*600*600mm), "A" efficiency refrigerator. The running power is pretty
> small - 70W I think - but I'm not sure how much current is needed to start
> up the induction motor in it.
>
> Will a 250W inverter get it started, or do I need something bigger?
>
> Thanks
>
> Roger

Vaughn Simon

2006-04-22, 9:21 pm

"Roger Moss" <rwm1@rwmoss dot gotadsl dot co.uk> wrote in message
news:X9mdnfBEYbUrAtfZnZ2dnUVZ8tydnZ2d@pipex.net...

> Will a 250W inverter get it started, or do I need something bigger?

I would be very surprised if it would start on that little inverter.
Refrigerators can easily take up to 10X their run power for starting. You did
not mention what type of inverter, many refrigerators do not seem to like square
wave power (or MSW). My 'fridge (about 200 watts run power) will not start on
my little 600-watt Honda "lunch box" genny, and it will not start reliably on my
1000-watt inverter, but does run very well on my 2000-watt Honda inverter genny.

Vaughn

>
> Thanks
>
> Roger
>
>

Eric Sears

2006-04-22, 10:21 pm

On Thu, 20 Apr 2006 16:47:27 +0100, "Roger Moss" <rwm1@rwmoss dot
gotadsl dot co.uk> wrote:

>I want to get an inverter to run my refrigerator. It is a normal size
>(850*600*600mm), "A" efficiency refrigerator. The running power is pretty
>small - 70W I think - but I'm not sure how much current is needed to start
>up the induction motor in it.
>
>Will a 250W inverter get it started, or do I need something bigger?
>
>Thanks
>
>Roger
>
>

A few comments first:
As far as I am concerned, refrigerators are the bane of renewable
energy system, simply because in general manufacturers won't make them
to run in any sensible fashion - especially the latest ones!
The problems are:
High start-up current - despite the current consumption being much
lower than previously. (Direct d.c. would be best - but genarally
prices and choice don't encourage buyers).
Various defrosters and electronics that need to run all the time, and
probably do NOT like mod square inverters (or at least - the
manufacturers will not honour warranties if run on them).
Still insufficient insulation.

My own observations:
To run a freezer with the 79 watt motor common in a lot of those units
today, I use an 800 watt mod sqr, which has a surge capability of
about 2200 watts.
A smaller 700 watt (el cheapo) with a surge of 1100 watts would
SOMETIMES start it - though it was improved by the addition of a small
400 volt, 2uf cap across the motor. It has no other electronics except
a switch to prevent it re-starting for 5 mins when power fails.
A similar small "bar fridge" (50 litre capacity) with same size motor,
also runs fine. In both cases, the motor runs slightly warmer than on
mains power, but nothing to be concerned about.
The current draw from a 12 volt battery peaks at more than 70 amps on
start-up, but then runs at about 9 amps initially, dropping to about 8
amps over the cycle.
(Its hard to tell the "start-up" - I have to guesstimate from an
analogue meter)

I have a new F&P fridge/freezer which it rated at just 384kwh a year,
but I have not been game to try it on my (1500watt) mod square
inverter (same size motor) - so I just run the little freezer on the
inverter.

My suggestion:
I wouldn't recommend anything other than a pure sine-wave inverter if
your fridge has the usual electronics in it. You also need to plan for
it to be running most/all of the time. I know of some people who have
thought they were doing the best thing by getting an inverter that
goes into standby mode - and turns on when it detects a load. They
also bought a super-efficient fridge (less than 300 watt-hr a day).
However, the "detection" system relies on sending a pulse every second
or so - which caused a relay to click on and off interminably! (and
would soon have destroyed it). It IS possible to run a timer on the
inverter - but that can cause other problems.
Assuming a sinewave inverter might start the fridge more easily than
the MSW (is that true?), then I still think you will need something no
less than 600 watts - and personally I suspect that as loads tend to
"grow" I would go for nothing less than about 1200 watts.
But of course, you haven't said what the situation is where you are
running this fridge - permanant home? motorhome? holiday cabin? etc.

I would value comments from those who have run fridges on MSW
inverters - especially anyone who has run any of the latest F&P
fridges in that way.
Other suggestions about the best ways to run "renewable refrigeration"
would be helpful, as I find myself in the position of advising people
at times.
My MSW inverter (1500w) draws just 0.5 amp when it has no load (but
running) - which is reasonably acceptable.
Many sinewave inverters only quote the "standby" mode - which isn't
much use if it needs to run "electronics" all the time.
I have even thought of reconnecting a fridge so that the electronics
runs on a very small sinewave inverter (or better - directly on dc -
since that is what it generally needs), and have an MSW inverter
running the compressor - which can then go into standby mode.
Has anyone done this?
Of course it would invalidate any warranty.

Regards to all

Eric Sears

Neon John

2006-04-23, 4:21 am

JoeSP

2006-04-23, 11:21 am

"John Smith" <nooneshome@shaw.ca> wrote in message
news:444AA378.72546E15@shaw.ca...[color=darkred]
> Once you get it started off an inverter check the servo motor for the
> defrost
> timer. They are power picky and it may not run at all.
>
> Roger Moss wrote:
>

This and many other similar postings points to the necessity for a
demand-surge capacitor bank. A very common problem with alternate power is
the need for a much higher load for a brief period of time for things like
starting motors.

I'm not aware of any such devices on the market, but I'm sure a good
engineer could enlighten us.

Harry Chickpea

2006-04-23, 11:21 am

phoneme@025379386.for.email.address (Eric Sears) wrote:

>I wouldn't recommend anything other than a pure sine-wave inverter if
>your fridge has the usual electronics in it. You also need to plan for
>it to be running most/all of the time. I know of some people who have
>thought they were doing the best thing by getting an inverter that
>goes into standby mode - and turns on when it detects a load. They
>also bought a super-efficient fridge (less than 300 watt-hr a day).
>However, the "detection" system relies on sending a pulse every second
>or so - which caused a relay to click on and off interminably! (and
>would soon have destroyed it). It IS possible to run a timer on the
>inverter - but that can cause other problems.

Dunno about super high efficiency refrigerators, but after Wilma I ran
my 1680 watt-hr per day refrigerator for a couple of weeks off my
Trace 2000W MSW inverter, with daily doses of generator power to
recharge the batteries. I also run a small "dorm" type refrigerator
in my van using the same inverter. The clicking doesn't destroy
anything, but what does become noticable is the noise when the battery
power can't supply the startup power, and the compressor and overload
circuitry fight. Other than that, both refrigerators ran quite
happily.

Dave Nay

2006-04-23, 1:21 pm

JoeSP wrote:
> This and many other similar postings points to the necessity for a
> demand-surge capacitor bank. A very common problem with alternate power is
> the need for a much higher load for a brief period of time for things like
> starting motors.

The problem there is that capacitors are a DC device, and actually will
filter AC. For a capacitor bank to be of any use, it needs to be on the
DC side of an inverter, and therefore still limited by the capacity of
the inverter.

JoeSP

2006-04-23, 2:25 pm

"Dave Nay" <davedotnay@vidanaydotcom> wrote in message
news:_sOdnbrQ5IcLB9bZnZ2dneKdnZydnZ2d@mssinternet.com...
> JoeSP wrote:
>
>
> The problem there is that capacitors are a DC device, and actually will
> filter AC. For a capacitor bank to be of any use, it needs to be on the
> DC side of an inverter, and therefore still limited by the capacity of the
> inverter.

And therein lies the engineering challenge.

Neon John

2006-04-23, 8:21 pm

On Sun, 23 Apr 2006 16:57:46 GMT, "JoeSP" <olegp@telus.net> wrote:

>
>"Dave Nay" <davedotnay@vidanaydotcom> wrote in message
>news:_sOdnbrQ5IcLB9bZnZ2dneKdnZydnZ2d@mssinternet.com...
>
>And therein lies the engineering challenge.

Actually no challenge at all. Amble down to your friendly local
electric motor repair shop, your friendly local HVAC wholesaler or
your friendly local appliance parts store and get a "hard start kit",
AKA "Kick Starter".

Nothing more than a start capacitor and either a potential relay or a
polyswitch thermistor. This gadget will significantly improve the
starting torque of the motor while decreasing the inrush current.

Not necessary in this application, of course, since the common garden
variety of cheap ChiCom inverter will easily start a 'fridge motor.

John
>
---
John De Armond
See my website for my current email address
http://www.johngsbbq.com
Cleveland, Occupied TN
Don't let your schooling interfere with your education-Mark Twain

surfnturf

2006-04-24, 1:21 pm

Joh, thanks for the kick starter tip. May want this some time in the future
for some larger drives in my future full sized application.

Agree that fridge should start with small inverter for the OP. Just need to
use decently sized wiring and securely connect inverter to battery. Then the
battery is the "capacitor" and most inverters will handle the temporary
surge. Of course, better inverters will handle more for longer.

surfnturf

"Neon John" <no@never.com> wrote in message
news:s70o42pj6q5u8o6ft0in1eihogls1odv05@4ax.com...
> On Sun, 23 Apr 2006 16:57:46 GMT, "JoeSP" <snip>
> Actually no challenge at all. Amble down to your friendly local
> electric motor repair shop, your friendly local HVAC wholesaler or
> your friendly local appliance parts store and get a "hard start kit",
> AKA "Kick Starter".
>
> Nothing more than a start capacitor and either a potential relay or a
> polyswitch thermistor. This gadget will significantly improve the
> starting torque of the motor while decreasing the inrush current.
>
> Not necessary in this application, of course, since the common garden
> variety of cheap ChiCom inverter will easily start a 'fridge motor.
>
> John
> ---
> John De Armond
> See my website for my current email address
> http://www.johngsbbq.com
> Cleveland, Occupied TN
> Don't let your schooling interfere with your education-Mark Twain

Eric Sears

2006-04-25, 10:21 am

On Sun, 23 Apr 2006 14:06:31 GMT, hchickpeaREMOVEME@hotmail.com (Harry
Chickpea) wrote:

>Dunno about super high efficiency refrigerators, but after Wilma I ran
>my 1680 watt-hr per day refrigerator for a couple of weeks off my
>Trace 2000W MSW inverter, with daily doses of generator power to
>recharge the batteries. I also run a small "dorm" type refrigerator
>in my van using the same inverter. The clicking doesn't destroy
>anything, but what does become noticable is the noise when the battery
>power can't supply the startup power, and the compressor and overload
>circuitry fight. Other than that, both refrigerators ran quite
>happily.

Thanks for your comments Harry.
I wonder if you could further comment on these questions:

Does the Trace send a pulse about every second - which clicks a relay
in the fridge on/off? (I'm not talking about the normal clicking
associated with turning the motor on/off at the usual intervals)

Does the inverter shut down or go into "standby" mode at intervals
when the motor is not running?

What make is the fridge?

Does it seem to have electronics which run all the time (things such
as timers, fan etc).

Generally the small "dorm" type fridges are no problem because they
are just a compressor with a thermostat.

Any further comments appreciated.

(PS - the "super-effficient" fridge I have just remembered is a Gram)

Eric Sears.

Harry Chickpea

2006-04-25, 11:21 am

phoneme@025379386.for.email.address (Eric Sears) wrote:

>On Sun, 23 Apr 2006 14:06:31 GMT, hchickpeaREMOVEME@hotmail.com (Harry
>Chickpea) wrote:
>
>
>Thanks for your comments Harry.
>I wonder if you could further comment on these questions:
>
>Does the Trace send a pulse about every second - which clicks a relay
>in the fridge on/off? (I'm not talking about the normal clicking
>associated with turning the motor on/off at the usual intervals)

There is a click within the inverter about every second, but it
doesn't click anything in the fridge.

>Does the inverter shut down or go into "standby" mode at intervals
>when the motor is not running?

I wasn't paying a whole lot of attention at the time on the big
refrigerator. I'm fairly sure it did, but I had other items being
powered, so other than overnight it didn't have much of an opportunity
to go into that mode. It definitely does go into standby on the dorm
refrigerator. If you re-frame the ticking of the inverter to be a
hypnotic and restful sound of no power being used, it is easy to fall
asleep next to it.

>What make is the fridge?

The big one is a GE. Bought it at Sears after checking out a
half-dozen other places. We chock a refrigerator full, and some of
the selling points for us were the low energy usage and the deep door
shelves and roll out shelves inside, which make things more
accessable. The little probably has a name, but my guess it was made
in China somewhere.

>Does it seem to have electronics which run all the time (things such
>as timers, fan etc).

Depends on what you mean by run all the time. There is an icemaker,
but there are long periods when the fridge is dead silent - no fans,
no compressor, nothing. Yet if I open the door, that darned light is
still on. :-)

>Generally the small "dorm" type fridges are no problem because they
>are just a compressor with a thermostat.
>
>Any further comments appreciated.

Use a short hefty extension cord. This isn't like mains power. At a
minimum, you need two group 27 marine batteries to get through a night
and not destroy the batteries, three or four is better; also reset the
low voltage cutout on the Trace to something higher than the default
10.5 volts if you plan on keeping the batteries. Even though the
resting voltage of the batteries doesn't get down to 10.5, it gets low
enough to heavily sulphate the cells.

>(PS - the "super-effficient" fridge I have just remembered is a Gram)
>
>Eric Sears.

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