Frequency Drives, friend or foe? Inverter Refrigerator use off grid

mcgivor

The use of inverter appliances off grid is commonly viewed as a God send, lowering inrush current on start up, allowing smaller DC-AC inverters to be used and saving precious battery capacity due to better overall efficiency. On the surface this is true but are there hidden factors that are overlooked, such as the generation of harmonic distortion which possibly could damage equipment?

Whilst tending to battery maintenance the inverter was turned off, upon restoring power, the only AC load, the inverter refrigerator started, the DC-AC inverter had pulses of about 0.5 second duration, very low but audible buzz. This occurs only during the initial start, disappearing when up and running. Nothing unusual can be measured using a DVM (true RMS) on the AC side, voltage remains steady at 230V, no inrush current, 0.6A dropping to0.3A, on the DC side nothing unusual.

Thought it was a little strange and the thought of harmonics came to mind as in the past I have installed frequency drives, sometimes as many as 20 per site, the energy savings were noticeable but transformer overheating became an issue, all the drives were on the 600V side of the transformer the building side was 120/208V. Using a power quality analyzer it was determined that harmonics, 5th, 7th, 11th, 13th, 17th and 19th, were manifesting as heat in the transformer, since harmonics cannot pass through the transformer, the general house loads were not affected.

The other day I plugged in a 500w  microwave, first time, it worked fine, until the refrigerator decided to cycle on. The inverter shut down and restarted, it repeated this, so turned off the microwave and refrigerator ran as normal. The inverter is 24V 2000W surge 4000W, so should have worked, tried a 800W hair dryer and a 500W rice cooker together  no issues, so it would seem the combination of capacitive loads didn't play well with the inverter. Unfortunately I no longer have a power quality analyzer to test but my thoughts are harmonics are a factor, perhaps an isolation transformer would work or just run the refrigerator on its own inverter as I have done so far, albeit a smaller one is planned in the future.

westbranch

I am wondering if the fridge had stopped mid cycle and the down period was not long enough for the back pressure to drop and the compressor was working against that residual pressure.
What brand of fridge and inverter are involved?  We have a GE 18 cu ft.fridge and Cotek 1500W inverter.

mcgivor

westbranch said:

I am wondering if the fridge had stopped mid cycle and the down period was not long enough for the back pressure to drop and the compressor was working against that residual pressure.
What brand of fridge and inverter are involved?  We have a GE 18 cu ft.fridge and Cotek 1500W inverter.

Refrigerator is Panasonic, it was not running when the microwave was turned on, microwave ran for 2 minutes so my belief is it started during as opposed to before. Have you tried a microwave and your inverter refrigerator with the Cotek? My Cotek didn't have the same buzzing I referred to, the inverter in question is a cheaper model, perhaps I'll experiment and attempt to replicate the problem using the Cotek.

westbranch

Microwave was never in the picture from the start... winter is wood cook stove that cooks us out if fed too many times in daytime...but we do have an induction cook-top that has an 1800W max...essentially for summer use only. so far the cook is undecided if it is faster than the outside propane 2 burner due to not going to the max yet (1200W used so far)  I'll try to keep it in mind for an Opportunity-Load power test when the first good sunny day comes along.

mike95490

Power Factor is a BIG issue with inverters and generators and can easily push a system into instability.  Add in all the CFL & LED bulbs with little drivers in them and you may need 100% headroom in the inverter

jonr

You can add a load reactor to smooth things out somewhat.

Dave Angelini

It is not a friend and it is a foe if you can't easily get a spare. If you are in a place like that, less is much more!

Less to go wrong and you just have to do what Mike said above.

One of the things that I have noticed over the decades living offgrid is how long appliances and devices last offgrid compared to equipment connected to the grid with neighbors doing arc welding or surges getting spread around.

An easy way to simulate grid-life in my opinion, is by having too small of an inverter and shortening the life of your loads offgrid!