Question --- 12v to 240v inverter starting high surge star

N

Norman Webb

Guest
I have a 600W modified sine wave inverter and am trying to run a 70w bar
fridge.

It almost starts the fridge but overloads.
I have been told that a 600w pure sinewave inverter would probably do the
job but they are 2 to 3 times the price.

My 2000w inverter easily handles the load.

Pure sinewave inverters usually have a decent output transformer so my
question is if I could get a couple of cheap 240v transformers connected
back to back would this possibly help in starting the fridge motor.

Any ideas?

Regards
Norm
 
On Dec 23, 7:20 pm, "Norman Webb" <tek...@iinet.net.au> wrote:
I have  a  600W modified sine wave inverter  and am trying to run a 70w bar
fridge.

It almost starts the fridge but overloads.
I have been told that a 600w pure sinewave inverter would probably do the
job but they are 2 to 3 times the price.

My 2000w inverter easily handles the load.

Pure sinewave inverters usually have a decent output transformer so my
question is if I could get a couple of cheap 240v transformers connected
back to back would this possibly help in starting the fridge motor.

Any ideas?

Regards
Norm
Click to expand...
Hi, Norm. Compressor motor startup current is almost certainly the
culprit. That isn't going to be helped by supplying a more pure sine
wave.

Look at the inverter specs on peak current. You'll have to choose one
that has greater peak current than the startup current of your fridge
compressor.

Cheers
Chris
 
On Mon, 24 Dec 2007 09:20:14 +0800, "Norman Webb"
<tekrec@iinet.net.au> wrote:

I have a 600W modified sine wave inverter and am trying to run a
70w bar
fridge.

It almost starts the fridge but overloads.
I have been told that a 600w pure sinewave inverter would probably do
the
job but they are 2 to 3 times the price.

My 2000w inverter easily handles the load.

Pure sinewave inverters usually have a decent output transformer so
my
question is if I could get a couple of cheap 240v transformers
connected
back to back would this possibly help in starting the fridge motor.

Any ideas?

Regards
Norm
Click to expand...
At 70W input consumption the frig is most likely a Peltier cooled
device a-la Sicao BC46A
http://www.cnsicao.com/en/pview.asp?pid=106&cid=3

Since Peltier devices run from DC the likely culprit is the input
conversion circuitry to convert from mains power down to the required
DC. Connecting back-to-back transformers at the input of your frig is
not going to help the situation imo. A higher powered DC - AC
converter (not necessarily sine-wave) is the only way you can overcome
the switch-on surge requirement.
 
Ross Herbert wrote in message ...
On Mon, 24 Dec 2007 09:20:14 +0800, "Norman Webb"

At 70W input consumption the frig is most likely a Peltier cooled
device a-la Sicao BC46A
http://www.cnsicao.com/en/pview.asp?pid=106&cid=3

Since Peltier devices run from DC the likely culprit is the input
conversion circuitry to convert from mains power down to the required
DC. Connecting back-to-back transformers at the input of your frig is
not going to help the situation imo. A higher powered DC - AC
converter (not necessarily sine-wave) is the only way you can overcome
the switch-on surge requirement.
Click to expand...

No.

Definitely a compressor as I can hear that little motor run and the
compressor is visible at the back.
Regards

Norm
 
Chris wrote in message ...
Hi, Norm. Compressor motor startup current is almost certainly the
culprit. That isn't going to be helped by supplying a more pure sine
wave.

Look at the inverter specs on peak current. You'll have to choose one
that has greater peak current than the startup current of your fridge
compressor.

Cheers
Chris


Hi
The "salesman" told me that because the pure sine wave inverter had a larger
tranformer it would be better able to handle the surge. It definitely was
much heavier than my modified sinewave inverter.

The modified sine wave inverters have tiny ( and I mean tiny) transformers
which means there is less energy stored in the iron.

Regards
Norm
 
On Dec 25, 11:51 pm, "Norman Webb" <tek...@iinet.net.au> wrote:
The "salesman" told me that because the pure sine wave inverter had a larger
tranformer it would be better able to handle thesurge. It definitely was
much heavier than my modified sinewave inverter.
Click to expand...
Did he also tell you Saddam had WMDs? Same reasoning is being used
by the salesman.

Let's first look at numbers. For example, this modified sine wave
output is two 200 volt square waves with a spike of up to 270 volts
between those square waves. This is perfectly fine for computers
because computers are so robust. But this may also damage some small
electric motors due to what is in those output waves - ie harmonics.

There is no pure sine wave inverter. There are just 'cleaner'
outputs. Increased harmonics means hot spots inside a motor. That is
completely irrelevant to your problem - starting that fridge. It may
be 70 watts. But it also has a massive 'power on' surge. Electric
motors (unlike electronics) don't like low voltage during power up - a
slow response to a load change. Can your inverter supply new current
demands quickly?

Meanwhile, other problems include what controls that motor. Are its
electronics made upset by electricity provided with multiple
frequencies (60 Hz, 180 Hz, 300 Hz - all provided simultaneously)?

Long before anyone can answer, first the problem must be
identified. It does not start? If symptoms don't come with 'whys',
then every action will only be speculation. Is this fridge nothing
more than a motor and a mechanical thermostat? Or does it contain
other components? What do specs say for that fridge? Unfortunately,
most numeric specs for refriges are poor. Still, required are those
numbers and facts.

There is no bigger transformer. If the unit is outputing 200 watts,
then it has a 200 watt transformer in both modified or pure sine
wave. Inverter must supply not just 70 watts but also the massively
higher power required, fast on demand, during startup. The pure sine
wave inverter is only three times more? Then what is it missing, or
is that modified inverter better than most? We don't know since even
numbers such as THD for both units were not provided. Start with
manufacture specs for everything. What kind of load is this refrig?
 
On Dec 24, 12:20 pm, "Norman Webb" <tek...@iinet.net.au> wrote:
I have a 600W modified sine wave inverter and am trying to run a 70w bar
fridge.

It almost starts the fridge but overloads.
I have been told that a 600w pure sinewave inverter would probably do the
job but they are 2 to 3 times the price.

My 2000w inverter easily handles the load.

Pure sinewave inverters usually have a decent output transformer so my
question is if I could get a couple of cheap 240v transformers connected
back to back would this possibly help in starting the fridge motor.

Any ideas?

Regards
Norm
Click to expand...
Norm,

It all comes down to the surge capacity of your inverter. They are
rated for this i.e. how much peak wattage they can output for a short
period of time. Generally most 600W inverters can handle a surge of
about 1000W but you need to spec your inverter to check that. The
modified vs pure sine is most likly not an issue in this case as a
fridge should run without to much trouble on a modified wave. It is
only generally important to have a "clean" wave when running more
sensitive equipment such as computers, printers, and home
entertainment systems. As for a pure sine wave inverter it being
heavier doesn't mean it can handle a big surge. It's heavier because
it takes a more complicated transformation process to produce the wave
form, therefore more components.

Again this is a case of a salesman not really knowing the physics of
what is going on in the product he sells. They read a product sheet
and spit it out with out really knowing what your needs may be.
Bottom line is to spec out the component and match it to the specs on
your loads, generally a little online research can do this much more
than some guy who just needs to make the sale.

Anyway just run the fridge off your larger inverter as it can handle
the surge.

Kurt
 
On Wed, 26 Dec 2007 12:46:03 +0800, "Norman Webb"
<tekrec@iinet.net.au> wrote:

Ross Herbert wrote in message ...
On Mon, 24 Dec 2007 09:20:14 +0800, "Norman Webb"

At 70W input consumption the frig is most likely a Peltier cooled
device a-la Sicao BC46A
http://www.cnsicao.com/en/pview.asp?pid=106&cid=3

Since Peltier devices run from DC the likely culprit is the input
conversion circuitry to convert from mains power down to the
required
DC. Connecting back-to-back transformers at the input of your frig
is
not going to help the situation imo. A higher powered DC - AC
converter (not necessarily sine-wave) is the only way you can
overcome
the switch-on surge requirement.


No.

Definitely a compressor as I can hear that little motor run and the
compressor is visible at the back.
Regards

Norm
Click to expand...
Does it have a brand and model number?

70W for a compressor type frig is unusual. Are you sure the motor you
can hear running is not simply the fan which cools the Peltier
heatsink?
 
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