Ever wonder what's happenin' when you start your 12V camping fridge and how much current it sucks in during startup? Also about AC fridge inrush. (1 Viewer)

[e9999]

I did. So I had some fun.
This is an ancient baby Waeco. 18qts I think. Compressor.

LHS is zero current. Every grid interval vertically is 20mV or 2.7A (I'm using an in-line shunt between the battery and the fridge input connector). RHS is the fridge humming away at a bit over 3A. Each horizontal interval is 100ms.

So, looks to me like it starts (I can hear a relay) and for 0.2s it takes in about 3A fairly smoothly getting ready, then the compressor starts, sucks in up to about 11A for about 0.1s, and then settles down at 3A. All that not including the significant extra noise.

IOW, the startup peak current is over 3x the steady-state one. No big surprise, of course, but fun to see first-hand:

and then, looking at the pulsing on the RHS, and counting them, I get about 2,700 / min, presumably the rotation speed of the motor or some multiple thereof.

 

[peesalot]

Good stuff, I have been trying to determine where and when the low voltage protection measures the voltage because I have an inexpensive Aspensora ( seacop) fridge that will trip the low voltage when that start up spike hits, even if system voltage is above the threshold. That spike trips their low voltage code and you have to unplug and replug to clear code and it works fine after that. It doesnt do it every time but it will screw you if you dont notice. The Engel does not do that. The remote temp device is a great peace of mind.

 

[e9999]

If the cutoff is not adjustable, you may be able to avoid the problem if you increase the size of the wire running from the battery to the fridge. It's likely the voltage drop over that wire that does you in.

 

[peesalot]

Agreed, not the wire tho, its the plug, need to delete the socket style connections for good.

 

[surfpig]

Agreed, not the wire tho, its the plug, need to delete the socket style connections for good.

Those are giving me trouble as well.

When I used to have old refrigerators (40s - 50s) in the house, I added a soft-starter (?) which I think is mostly just a capacitor to help with that big amp draw because it tended to kill those old compressors.

 

[peesalot]

I mean, go figure, a cig lighter socket, a lot of bouncing, banging, dirt, left outside to rot and it failed, huh, who woulda guessed, its the old take it for granted issue. I will say that the Engel, on same socket has never failed, the little diode failed and caused fridge to draw stupid amps, sold it to Bill for parts then he tells me the solution is an easy fix and fridge works fine.

 

[surfpig]

I mean, go figure, a cig lighter socket, a lot of bouncing, banging, dirt, left outside to rot and it failed, huh, who woulda guessed, its the old take it for granted issue. I will say that the Engel, on same socket has never failed, the little diode failed and caused fridge to draw stupid amps, sold it to Bill for parts then he tells me the solution is an easy fix and fridge works fine.

That sounds about right.

 

[e9999]

AC musings now:

been doing some tests running my 120V kitchen fridge off an LFP battery.
For context, the fridge uses about 110W with the compressor running. Pretty efficient. (PSA: check yours, the old clunkers out there are costing a fortune to run. I got rid of my old one, bought a new one, and the payback time was only a year or 2 IIRC. And no. It does not save money to use the old one as a beer fridge in the garage, even though that may be a good idea overall... )
I have a Victron Smart Shunt on the DC side of the inverter.
Well, I saw some serious current spikes going into that inverter when the compressor is starting up. Specifically, I saw almost 1,300W draw during the inrush spike. Dang, 12x the continuous draw? 12x!! That is a serious spike. And it's not even a sure thing that the Smartshunt captured the actual max.
Now to be fair, I have ran a 1000W continuous inverter with that fridge and it seemed fine, but that may be thin ice territory. So I put on a 1500W inverter instead later on. Safer.

Dang, those inrush currents are something else...

 

[FrazzledHunter]

AC musings now:

been doing some tests running my 120V kitchen fridge off an LFP battery.
For context, the fridge uses about 110W with the compressor running. Pretty efficient. (PSA: check yours, the old clunkers out there are costing a fortune to run. I got rid of my old one, bought a new one, and the payback time was only a year or 2 IIRC. And no. It does not save money to use the old one as a beer fridge in the garage, even though that may be a good idea overall... )
I have a Victron Smart Shunt on the DC side of the inverter.
Well, I saw some serious current spikes going into that inverter when the compressor is starting up. Specifically, I saw almost 1,300W draw during the inrush spike. Dang, 12x the continuous draw? 12x!! That is a serious spike. And it's not even a sure thing that the Smartshunt captured the actual max.
Now to be fair, I have ran a 1000W continuous inverter with that fridge and it seemed fine, but that may be thin ice territory. So I put on a 1500W inverter instead later on. Safer.

Dang, those inrush currents are something else...

Air conditioners have the same issue. On RV's the higher end A/C units come with a feature called, among other things, "start assist" or "soft start". For those that don't come with it you can often add one:
REF:
https://www.etrailer.com/Accessories-and-Parts/SoftStartRV/SSR54FR.html

 

[e9999]

Sheesh. Did some more testing with my AC kitchen fridge running on a battery fed by a PV panel. As stated above, that fridge is a fairly new (about 7 years old) model using about 100-110W (dang good!) when the compressor is running. In that test, I had an LFP battery feeding into my 1,500W continuous inverter. Specs on that inverter state that it can do 3,000W intermittently. So I figured I should be good.

Well, first thing that happened is that I was unexpectedly reminded that this fridge will occasionally defrost. No big surprise in retrospect, of course, but TBH I had forgotten about that. Well, it does draw 600W when defrosting, so that does a number on the battery charge and the DC wiring. Should plan for that when sizing the system up, but I don't know how often that happens. The problem is that this is going on for a bit, so the DC wiring does need to handle a lot more on a "continuous" basis than the 110+W I was originally planning for. Fortunately, I had overkilled the DC wiring. [OTOH, I have not seen a typical camping fridge with a defrost feature, so likely not an issue for those?].

Then I found out that a 32A DC breaker I put in between the battery and the inverter to protect the wiring did not disconnect when it was drawing well over 30A (over 50A for a bit, actually) for several minutes. Was not expecting that. Cheap chinese breaker...? Defective? So much for thinking I was doing this safely for a change.

But finally, and worst, the inverter totally shut down because of an overload. I recorded about 1,500W during that event, but that was with the Victron connect app and it does not have a very good time resolution, so it was likely higher, but how much more I don't know at this time. I am surprised it would have overloaded the inverter which is capable of 3,000W supposedly, as I can't imagine it would have drawn more than 3,000 W inrush. Maybe there is something off with the inverter protection system?


The shutdown is concerning in practice because if I did not know about it, I could have left my fridge on the inverter, feeling all smug cuz I got it running off solar (Yea! Sustainability!), take off for the day, but the inverter trips because of a huge unexpected inrush current and the fridge spends the day with no power (I don't believe the inverter would reset itself automatically and get going again). That would not be good. So, there you have it, curse again those inrush currents!

 

[FrazzledHunter]

Sounds like you need a data logging voltage/current/power meter.
REF:
McMaster-Carr - https://www.mcmaster.com/products/voltage-data-loggers/

 

[e9999]

thanks for the link. I do have meters and loggers actually, although AFAIK you can only record the inrush properly with a scope or a dedicated inrush meter (expensive), but didn't use them because I did not expect any problem with the inverter. And don't feel like setting that all up for now. Plus, I gotta dial down the testing of the kitchen fridge as my wife is getting nervous about it (hints about "frying" the fridge etc) and that could get ugly (and I mean besides wires laid everywhere as is the case now...)

 

[FrazzledHunter]

thanks for the link. I do have meters and loggers actually, although AFAIK you can only record the inrush properly with a scope or a dedicated inrush meter (expensive), but didn't use them because I did not expect any problem with the inverter. And don't feel like setting that all up for now. Plus, I gotta dial down the testing of the kitchen fridge as my wife is getting nervous about it (hints about "frying" the fridge etc) and that could get ugly (and I mean besides wires laid everywhere as is the case now...)

I can completely empathize! You're ****ing around and finding out. Sometimes you just gotta do that.

 

[Land Shark]

leave it to @e9999 to conjure things like this up. Good stuff and to most not interesting at all but some like this shizzle.