Fridge startup issue

[msalari]

Hey guys, cabin solar is going very well - Thanks to SunKing and Mike90250 and everyone else for your help. Only remaining (and very annoying) issue is when my fridge starts up, the inverter screams, coughs a few times so all my LED lighting turns on and off a few times, and then eventually everything is OK.
Some more information: Inverter is a 1000W pure sine wave. 2 X 100Ahr FLA batteries. Fridge is a modified chest freezer (installed a bulb thermostat so I can set to 38F or so). KillaWatt mater reads about 90-100W while it is running, but it isn't fast enough to show me whats going on during the startup cycle.

As I said, everything else in the cabin is great, but the fridge (compressor?) must be putting a helluva drag on the inverter. I understand that startup for the compressor will be a lot but the peak for the inverter is supposedly 1500 or 2000W I forget. I tried another inverter (thanks to Costco's return policy) I bought a 1500W inverter with a 3000W peak in hopes of ruling that out. That particular inverter just shut down completely when the fridge came on. C'mon, I find it very hard to believe that this little chest fridge is pulling THAAAAT much power at startup.
But oh well, maybe it is. My solution for the past year or so has been to put the fridge on a timer so that at night it shuts down and at least lets us sleep.

Question: Is there such a device that would somehow slow down the huge startup current that the fridge compressor is pulling, and calm it down or somehow spread it out over a few seconds? Before I just disable the beeper on the inverter I thought I would check with you guys

Does anyone else have a fridge or freezer on solar, and does the startup cause problems?


thanks guys

[SunEagle]

Any type of compressor motor found in refrigerators or something similar usually have an extremely high start up current requirements. The inrush current draw can be very high depending on the pressure that needs to be overcome on startup. That is one reason you want to keep the compressor from starting if there was an unexpected shutdown or power loss. Usually it takes a few minutes for the built up pressure in the system to reduce which makes it easier to start back up again.

Now adding to the issue may be the lack of available power from the batteries which could drop the voltage lower then what the inverter likes so it gets stressed out.

[msalari]

Thanks
- the issue seems to happen even when the batteries are fully charged.
So, no such thing as a "slow down inrush current for 1-2 seconds" Device ?

[SunEagle]

There are "soft start" and variable frequency type starters which can reduce the inrush event. Although that type of hardware is designed more for 3 phase motors but maybe there is a device that will work on your fridge.

[Mike90250]

What gauge are the inverter wires (to the battery) and how long ? This is critical, if too small, your 12.5 battery volts turns into 10.5 volts with loss in the cables and you get shutdowns. Should be at least #4, but #2 would be even better.
1,000 watts surge at 11V is going to be 110A and that will give you some serious ohmic copper loss, till the compressor comes up to speed, 2-5 seconds. Or it's possible the battery internal resistance is affecting how much surge it can supply. Add wires and battery losses and you get LED's browning out !

The easy alternative is a "resistor" in the AC power cord to the fridge. You could try a 100 foot 14 ga ext cord !!

[PNPmacnab]

I have run a chest fridge for several years and use a dedicated 2000/4000W inverter. I have found nothing smaller that works. Startup current runs 131A @ 12V and it could even be higher as my data logger may not catch the peak. Settles down to about 9A. My cable runs are less than a foot. It is microprocessor controlled to prevent hot starts and the system monitors the lowest battery voltage for each startup. As this is a fairly uniform high current load on the battery, it serves as a test of connections and battery state. Most of these compressors use a thermal resistor that increases resistance as it heats up disconnecting the starter winding. That can cause a longer high current load till it warms up. It's cheap and the grid doesn't care. Since my inverter is dedicated just to the fridge, I remove the fans and save a few watts with just convection cooling

[msalari]

Mike,
Per your previous good instruction, my battery to inverter is about 16" and uses 1 gauge wire.
Were you serious about using an extension cord? Could this act as a resistor?

[msalari]

PNPmacnab, thanks - wow 4000W peak is what that took? yikes. Confirms what I am seeing at the cabin too.

[inetdog]

PNPmacnab, thanks - wow 4000W peak is what that took? yikes. Confirms what I am seeing at the cabin too.

Switching for a moment from power requirement to current requirement, if started from a stiff source, that is one whose voltage does not drop dramatically when more current is drawn, a typical hermetically sealed compressor motor takes a minimum of 8 times the normal Full Load Amps (FLA) for a significant time (fractions of a second to seconds) when it starts.

If the inverter tries its hardest to maintain full voltage and then finally just shuts down when overcurrent is required (your second choice inverter) it will not be able to start the motor.
If, on the other hand, the inverter (or generator, or whatever) will allow the voltage to sink as overload approaches (your first inverter) then between the surge current capacity of the inverter and the reduced starting current drawn by the motor when the voltage goes down, you are able to scrape by, get the motor started, and recover to normal operation.

The reason that I am using current here is that the power consumed by the motor is NOT proportional to the current draw during the starting process.
You can have a generator with a small motor with limited power output but a coil and regulator system that can deliver more current at a lower power factor and start a compressor fairly easily.
To some extent you are also pulling power from the rotational kinetic energy of the motor-generator pair. There is no equivalent of that when using an inverter.

[Mike90250]

Yes, a long cable acting as a small resistor. $30 to try it out, leave it coiled up, and return it later that afternoon if it doesn't work. 100' of 16 ga or 14 ga cord.

[inetdog]

Yes, a long cable acting as a small resistor. $30 to try it out, leave it coiled up, and return it later that afternoon if it doesn't work. 100' of 16 ga or 14 ga cord.

One small caution:

If you leave the flexible cord tightly coiled up, limiting heat transfer, the turns in the center of the coil may get very hot. Possibly even hot enough to sustain damage if you use 16ga wires.
Keep the coil loose to allow air circulation.

[Sunking]

Some more information: Inverter is a 1000W pure sine wave. 2 X 100Ahr FLA batteries.

I bet you do have a problem and would expect a problem. When you Fridge motor starts, your batteries cannot supply the current demand without excessive Voltage Sag. A 12 volt 200 AH battery can only supply 400 watt Inverter. When your fridge starts up demand a lot more power than the batteries can supply. You have made a very common mistake.

[msalari]

Thanks again guys.

[Logan5]

If you are truly off grid, you should consider a native 24volt chest refrigeration unit, one made to run directly from 24v battery bank and solar. ie: Sunfrost, Dometic and others especially ones employing (dan foss) compressors. They are designed to soft start and use as little energy as possible. I have found it seldom makes sense to run an inverter, 24/7 just to maintain a cyclical refrigeration load. Depending on your cabin spread, you could run everything 24 volts, light's and 19v stepdown for TV's and Laptops. The higher initial cost of a 24v refrigeration unit is more than made up for with a much smaller battery bank and reoccurring replacement cost of batteries.