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?
Try measuring voltage at the same time, I bet voltage drops out due to your generator's inability to produce enough power. That woudl explain prolonged motor startup.
My understanding is that the generator produces 120v or nothing; when it can't produce 120v it trips off. But I can test that easily enough; putting a voltage meter in another outlet on the same circuit as the fridge would do that, wouldn't it?
Yes, it would -- try it, and let us know what happens. My prediction: you're going to see it drop. A lot. I'm guessing down around 95-100V before the genny shuts down.
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 they can.
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 something fries.
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...
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?
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 generator.
Does that make sense? Why would that be? I (obviously) don't know much about refrigerators. I also suspect my meter reads 2% high, since both line and generator probably aren't 122v!
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.
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.
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.
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 price of your 'smart' transfer switch probably would have bought you several more kw in generator. My 4kw generator never kicks out from the fridge even if the microwave is running too.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.