I tested my refrigerator to see how much current it drew.
On three tries it drew 13a for about a second, and dropped down to 1a.
Then I tried to start it off my generator.
Twice it did exactly the same as on line current; 13a for 1 second, then
dropping to 1a.
Twice it drew 13a for 5 seconds when the generator tripped off since it was
exceeding the generator's capacity.
What accounts for the two prolonged 13a draws?
I can't imagine what it could have to do with the generator; AFAIK it
produces 120v until it trips out.
So, is there something about refrigerators that they sometimes take longer
to start, and I just didn't happen to get one of those times when I was
testing on line current?
I plugged the meter in a few feet from the refrigerator.
On line current two things happened.
Sometimes it went to 118v for a few seconds then to 122v
Othertimes it just went to 122v
On the generator two things also happened.
Sometimes it went to 95v until the generator tripped out.
Other times it went to 122v and the refrigerator ran normally.
My interpretation is that sometimes the refrigerator starts easily, and the
generator can handle it.
Other times the refrigerator starts hard and it is too much for the
Does that make sense? Why would that be? I (obviously) don't know much
I also suspect my meter reads 2% high, since both line and generator
probably aren't 122v!
I wouldn't rely on their statement about this. Even if they are trying
to be honest (and I assume that they are), they've either reached this
conclusion by theory, and theory does't always match reality unless
the theortician is really smart; or by testing, and they can't
possibly test all situations, and in practice test even fewer than
Did we ever find out if this is a generator making DC current that is
inverted, or if it is an AC generator. I don't know what differences
that makes but some people do. Try cross posting to
sci.electronics.repair or some other specifically electric group.
It's pretty clear -- I think you said -- that they are using a circuit
to go from 120 to zero. First, they can't make it trip at 119.9.
That would be counter productive. Did they choose 119, 115, 106.9 to
trip? And does the circuit trip at exactly the design point, or is
there a plus or minus factor. The latter, so they have to make it so
that the entire range of likely trip points is below the intended trip
point, which makes the most likely trip point to be even lower than
the hoped for trip point.
Measure, like he says to. What is so hard about that.
Oops. Sorry. I hadn't read this line when I answered the parts above.
Yes, that would do it. Hard to watch two things at once, maybe enlist
a family member.
(When we were taking a trip, I wanted to read all the historical
markers in Virginia without slowing down, so I assigned one person for
the first three lines, one for the second three, and one for the last
3, and then we would all repeat them after we passed the sign. It
worked well till people got tired.)
The longer (5 second / 13 amp) surge current may be entirely due to the
fridge and have nothing to do with the generator. If you didn't allow enough
time between your three successive startups to allow the refrigerant loop
pressures to stabilize, you may, on the latter attempt, have a compressor
working against a much higher load, or a start capacitor which has not yet
recovered from the previous start, and / or some other fan or other demands
which would not otherwise occur.
It will be important to do this experiment allowing enough time for the
fridge to recover, and I am merely guessing when I say this would be a few
minutes, and certainly not a few seconds.
Since you did not indicate the intervening wait times between startup
attempts, I thought it best to alert you to this entirely different and
entirely plausible explanation.
The difference is pretty straight forward: "real" generators have
significant rotational inertia. There's energy stored in the rotation
of the motor and generator. The instantaneous power they can deliver
is limited only by the resistance of the generator windings and
power cord - on overload, the motor starts to slow down (on a heavy
overload slow down _real_ rapidly), and eventually (hopefully) the
protective circuits (mostly temperature in the generator and mechanical
circuit breakers) will trip. Otherwise, the generator stalls and/or
Electronic converters, on the other hand, are going thru semiconductors
which are (virtually) instantaneously controlled, and actively limited
to a "safe region" including semi-conductor based thermal overloads
which are more directly part of the circuitry.
What this means is that "real" generators will "operate thru" rather
larger overloads than electronic converters will, unless the converters
have been designed for it.
In an old car, I had an active demonstration of this. Generally,
a car alternator can produce something on the order of 1000W (1HP).
Something went wrong electrically, and the alternator effectively
dead-shorted. The car abruptly lost power due to the loading of the
shorted alternator - which would imply that the alternator was sinking
10HP or more. For about a second (accompanied by screeching
from the alternator drive belt)... The alternator then burned out,
and everything worked just fine for a while ... until the battery
ran out of juice...
Age and Treachery will Triumph over Youth and Skill
Motor starting currents can be up to five to eight times the normal
running current. That's why one sometimes will see a voltage dip or
drop when some motor operated device start up. Used to be quite common
with rural well-pumps.
It very much sounds as though the generator has not got enough oomph
(capacity) to provide the starting current for the fridge motor. Thus
it shuts off in order to protect itself against burn out.
Might be rather like saying that once you get a tractor trailer moving
along the road it only takes 100 horsepower to keep it rolling. But it
takes 500 to 600 HP to get it moving!
If the fridge can't get started there is usually a heat device on the
motor that will click out; then cool down and motor will try again to
start. That device is to protect the fridge motor from sitting there,
stalled and unable to start on the amount of current available,
possibly overheat and burn out.
Without further info it does sound as though there is a considerable
danger of burning out either the generator or the fridge?
Fridge motor starting load depends very strongly on whether there remains any
pressure rise across the compressor - and it typically takes about 2 minutes
for the full steady-state pressure difference to drop to zero across the
refrigerant-expansion "valve". Repeated restarting within 2-3 minutes of
successive shutdowns is increasingly risky to the compressor motor, and even
more risky to any generator or inverter supporting it, because their output
impedance (the ratio at which output voltage decreases as output current
increases) is 10-100 times greater than that of your utility service.
Initial current surge is due to no back-EMF being produced by a non-spinning,
"locked-rotor"motor. To save mf'g cost, fridge compressor motors are NOT
designed to start against any pressure difference. So forcing restarts too soon
can (and apparently did) result in the motor being unable to start spinning
against the non-zero pressure difference. And the 13A lock rotor current
eventually tripped out your generator breaker. Had you been doing the same
repeated testing using grid power, you might well have toasted the motor winding
before the 15A or 20A house circuit breaker tripped.
FWIW, we have two refrigerators: "Old" and "New"
The new one has never tripped our 2KW gennie even when everything else
was running. The old one wants something like 1,700 watts to start up
and I wound up with a smart transfer switch that senses that demand and
drops all the other circuits for a few minutes until it is satisfied.
Maybe somebody who knows can chime in, but my sense is that
refrigerators newer than a certain date have greatly-reduced startup
requirements while older ones have quite large startup requirements.
On Wednesday, April 16, 2014 7:25:32 PM UTC-4, (PeteCresswell) wrote:
From observation and listening to the generator and the fridge, I agree
that it appears new ones don't have a very high starting load. I did have
a kill-a-watt on it and saw a brief number of I think about 300W, but that
may not be peak. But whatever peak is, it doesn't last long and the
generator doesn't change speed or seem to even notice it.
The refrigerator's compressor's starting effort will vary depending upon the
pressure differential of the coolant. If the fridge is left off for 10~15
minutes before attempting to start, you should see minimal effort. If the
refrigerator's compressor has just stopped or was stopped just prior to plugging
it into the generator, the compressor will have to work hard to start since
there will still be pressure in the cooling system.
I'm not sure how the new linear compressors operate under this, but starting
should still be a bit harder.
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