Refrigerator induction motor: peak current draw

[eccerr0r]

I was wondering about my refrigerator: It's a fairly old model and according to Mr. Kill-a-Watt, I get around 180W when the compressor is on (without the 40W lamp and without the defrost heaters). This is about 1.5 amperes. Now I wonder what the typical startup power for such motor would be?
Do refrigerators average more like 3x or 10x peak startup power?

Now I wonder if there's a way to reduce the starting power for a small generator, I don't think I can put an NTC inrush resistor on it Maybe over the years the starting capacitor has degraded a bit? Would it affect starting power?

Maybe there's some other way to reduce inrush power?

[RCA2000]

Re: Refrigerator induction motor: peak current draw

Most refrigerators I have seen do NOT use a starting capacitor. Usually either a split-phase motor, with a start relay--or a "PSC" motor--that uses a RUN capacitor and NO start relay. A FEW MIGHT use a start cap on that relay--but not real common.

If it draws 180 watts at run--I would say likely it would not exceed 800 or so watts to start. So, an 1KW continuous generator --or inverter-should suffice.

[delaware74b]

Re: Refrigerator induction motor: peak current draw

Eccerror - I don't think 180 watts running is too bad for a refrigerator, especially one that's 6-10 years old. The compressor has a thermal start device (unless it's one of those %^$$ GE side by sides with a motherboard) that switches between the start and run windings. There is nothing you can do to reduce the start current. A 'hard start' kit will ruin most newer compressors.

IMO, for overall efficiency, keep the coils clean, keep the doors closed, keep it at least half full (both fresh food and freezer sections). Keeping it full will actually drop compressor run time due to thermal mass in the compartments.

BTW, what make and model refrigerator? Conventional top freezer, side by side, or french-door?

[budm]

Re: Refrigerator induction motor: peak current draw




http://www05.abb.com/global/scot/scot234.nsf/veritydisplay/18cb6349632fe21583257861003d9507/$file/technical%20note%20tm008%20low.pdf


http://powertips-elec.blogspot.com/2...rents-and.html

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

Hmm...This is a really old Montgomery Ward (that went out of business...) and not sure what model. It's a conventional freezer-on-top refrigerator. There is a schematic printed on the back and the compressor motor has three wires, and has a capacitor hooked up to it, so I thought it was a capacitive start induction motor. Sometimes I think that 180W number seems a bit low because I've seen on some generator sites that refrigerators use more than that when running, and much much more than that when starting up. But does Mr. Kill-a-Watt lie?

Unfortunately Mr. Kill-a-Watt can't tell me what the instantaneous power draw is upon startup. However one thing that I do worry about: Power factor. That 180W is "real" power not "apparent" and this could make a big difference for generators. Then again I don't recall the PF being less than 0.5, it was in the 0.7 range, which does increase current required a bit...

But hopefully if the generator can get over the startup hump, that PF detail should be minor...

[delaware74b]

Re: Refrigerator induction motor: peak current draw

If the capacitor is a small rectangular box, it's used across the run winding and the neutral. It's main function is to help the refrigerator's efficiency.

If the capacitor is a round can, it's a start capacitor. I believe your refrigerator does not use this type of capacitor.

Can you post pics of the cap and compressor? The model info tag is usually inside the fresh food compartment.

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

Yes it says "Run Capacitor" on the schematic along with the 'ptc relay'. I wonder if over the years these could go bad and thus require more power to start up, or will a dead unit just stop the compressor motor from running...

[delaware74b]

Re: Refrigerator induction motor: peak current draw

The ptc relay is used to switch from start to run windings after the compressor has reached a certain load level.

The run capacitor would have nothing to do with starting current, only efficiency while it's running.

I'd be more worried about the refrigerator being 30+ years old. Not trying to sound negative, but this refrigerator is really past its prime. I would leave it alone and be prepared for its 'retirement.'

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

It's more like 20 years old right now, saw a 1995 date somewhere. The refrigerator still works, I just wonder how small of a generator I should be able to get away with.

But of course I'm going to try to run this thing into the ground. The cost of a new refrigerator these days is way too high, the earnback period measures in decades if I calculate it correctly...

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

For the curious:

Say I want to get one off those 60W high efficiency refrigerators. It will cost $1000 or so. I measured my average power consumption (counting compressor-off time) to be around 120W. The savings is 60W.
At $1000 at the current power rate where I live, which is 11 cents per kWh, will buy 9090 kWh. This is 9.09 MWh = 378.75kW*days = 1037 Watt*years. Now the saving is 60W, so the payback is 1037/60=17 years.

17 years?!?!

Run it into the ground, then buy.

Then again these 60W refrigerators are easier to power with a small generator.

[budm]

Re: Refrigerator induction motor: peak current draw

how big is this fridge that only consumes 60W.

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

Unsure if it's really 60W or not (this is overall, not compressor-on), it was a guesstimate based on the numbers for a high efficiency unit. I was only looking at full sized refrigerators for the heck of it.

If it's more than 60W, then the payoff for these high efficiency refrigerators will take more than 2 decades...

Still... run my old one into the ground.

Here is one webpage that seems to indicate there do exist 18 ft^3 refrigerators that only take 60W
https://www.efficiencyvermont.com/Fo...ic-Usage-Chart

[Agent24]

Re: Refrigerator induction motor: peak current draw

I'd be more worried about the refrigerator being 30+ years old.

Being ancient might be a good thing, probably built better than the new stuff!
Our freezer is probably that old and shows no signs of stopping.

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

Well, I don't think I can get refrigerant for it anymore, nor parts... but who cares, until it breaks

I just was a bit dismayed my 300W AC inverter wasn't enough to overcome startup power of the refrigerator. Just wonder how much more is needed...

[Longbow]

Re: Refrigerator induction motor: peak current draw

Aside from the academic question of what the motor start current is, I am missing the importance of why it is an issue? Motor starting on a compressor might take all of one second, so attempts to minimize start current would be pointless unless it was so large that it was tripping the breaker. Start current for a specific motor type under a known load really can't be "improved" because the motor has to generate enough torque to start rotating and move the load during the starting process.

If you can get a digital storage scope hooked to a current clamp, you will easily see what you are looking for - let us know if you get this info. The trend in everything right now is to use brushless motors with imbedded electronics. At neat improvement in motor operation at triple the price.

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

It is an issue when dealing with designing minimalistic backup power sources.

Grid power is plenty, but when the grid goes down and you have to resort to backup power - whether a generator or a battery system - getting past that hump makes a big difference on whether or not your stored food goes bad.

I still do not have a DSO nor a CT to measure, but based on just a DMM I can tell the voltage droops a lot during startup, and the inverter gives up trying to supply enough power to run the motor.

Do modern refrigerators now use electronically commutated compressor motors now? That's something I haven't heard of before, not sure if it's that or just pure insulation that gives modern refrigerators its low power requirements.

[budm]

Re: Refrigerator induction motor: peak current draw

Is your inverter putting out sine wave or step/modified sine wave?
Can your meter do peak hold? I use my FLUKE 87 and CT for capturing peak current after period of times.

http://www.theinverterstore.com/tips.html
http://www.powerstream.com/inFAQ.htm

[eccerr0r]

Re: Refrigerator induction motor: peak current draw

The inverter is modified sine wave, which may add additional issues.

I think my fluke 77 does have a peak hold function but not sure if it can deal with the current...

[delaware74b]

Re: Refrigerator induction motor: peak current draw

Is there a LRA (locked rotor amp) rating on the compressor? It will be several times higher than your running amps. The LRA will be in the circuit for the first second or 2 during start-up.

For example. I attached the data tag of the outdoor unit of my 3-ton a/c system. At 93 amps, the LRA is 5.5x more than the RLA of 16.4 amps (running loaded amps). The thing to remember is motor loads are inductive and have short-lived high current start-ups.

[kaboom]

Re: Refrigerator induction motor: peak current draw

Do modern refrigerators now use electronically commutated compressor motors now? That's something I haven't heard of before, not sure if it's that or just pure insulation that gives modern refrigerators its low power requirements.

Oh this is a good point- I almost forgot about this completely.

Stay away from GE! Any of their fridges with that "access panel" on the back behind the fresh food compartment (Arctica, others) use an SMPS deliberately designed to fail within a year from bad/fakecaps.

The whole machine depends on this, depending on "features," in one way or another.

The temp controls are on a board just inside the door are all digital, with a com interface to the main board. When temp control board first gets its 12V supply, its MPU sends a pulse train talking to the main CPU, apparently for the ADC'd values of the fridge and freezer temp. Upon receiving this data, it gets converted to the "1-9" temperature number displays; or on middle models, the actual temperature is displayed.

When changing the temp, the small display board comm's to the main, "commanding" it to change setpoint(s). Remember, there's yet another section, either on the main or individual boards, containing thermistors, in the fridge & freezer, which talk to the main.

There are "monitor" switches for both doors that generate interrupts for the CPU. This data is used to determine the cycles-per-hour, defrosts-per-day, and, of course, turning the lights on and off. There are stepper-motorized dampers, like on auto instrument clusters, to control dampers in the ducts between the evap, freezer, and food compartments, also controlled by CPH/door openings.

IOW, we run "algorithms" in our "fridge."

All fans are 12V-PWM controlled, and similar to high-performance computer fans. Basic models power everything from the derived supplies of that shitty SMPS, except the lights, defrost heater(s), and compressor.

"High end junk" models have a variable speed compressor, a 'la Carrier, but unlike a real HVAC system, the power board for the GE compressor is almost as bad as the SMPS/main.

If RoHS doesn't kill the VFD, the comm link between it and the main is fried when (not if) the Hermeis die.

And they claim the VFD-compressor "saves energy." A proper implementation will, with an easy to clean condensor coil, but when the "barrel coil" condensor clogs and the POE cooks, you throw food away. And there's the "minor" issue of replacing boards every now and then...

GE= Garbage Equipment

Not any surprise really, they're just a vector for money, disguised as "appliances," and even more so after Holder and Soetoro's "leveraged takeover."

http://forum.appliancepartspros.com/...ain-board.html

From my understanding, these things keep a local appliance store (Good's) quite busy. They're always doing board swaps (at least) on warranty fridges. Look at the board in the link- it's quite deliberately and well designed with the layout, SMTs, etc- do not tell me those "Na-circle" and Ltecs are there "by accident."