Pure sine or modified sine for freezers

[Mary B]

Adding another 1200 watts of panels next spring.  Enough to run the fridge(modified freezer) and freezer. What are you using, modified sine or pure sine inverters? I know modified and motors sometimes don't agree. This will be on basically brand new freezers.

[dnix71]

Startup current is the problem. I couldn't get a small box freezer to run even by adding a large start cap to the compressor circuit. Gave up and went with an Engel that runs on 12vdc.

[Frank S]

My 1800w psw inverter has no problems running our 21cf side by side Roper fridge even while the coffee maker is brewing normally the television and 1 or 2 laptops are on at the same time with everything else we only have 1100w of solar and 880 Ah worth of 12v battery bank Normally only have to run the generator 2 possibly 3 hrs in evenings to top off the bank using a Schumaker 30/60 charger
 the microwave is a combo convection oven. can use the microwave even if the fridge cycles on but coffee maker must be off, but if using as combo or convection  it would suck the juice right out of the bank
 the inverter will go into overload while the oven is on preheat
 I have a Zantrex 3000+ MSW wired in but so far have only used it for running the drill press in the shop. & to test a few other things out there.
  I have a 1000w MSW mounted in the tool box in my Toyota pickup  it will run a 5" grinder or a 1/2" drill with no problems and as ling as the engine is running it will power a coffee maker

[OperaHouse]

I've been running a chest fridge on a 2000W HF MSW inverter for three years with no problem.   It is under UNO control  and it is the only thing on the inverter.  Fans are disconnected abd it draws 123A at startup on 12V.

[mab]

I've been running chest freezers off my 1400va 24v APC UPS which is MSW without problem. It will start the small one (75w) and the medium sized one (270w) OK. It can actually start both together - just - but won't start one whilst running the other. It won't start the big freezer (375w).

[OperaHouse]

Some things to add.  When I bought my freezer I tried to make it work with a fuel efficient 400W Onan generator and a 800W inverter.  Nothing worked including adding a capacitor to the start winding.  That didn't seem to make any difference but the capacitor is still in place.  One thing I remember was the wattqage of the fridge running was only about 100-110W.  Three years later I made a measurement of the power going into the inverter when the fridge was running on a MSW inverter.  It was about 150W.  There was only a loose calibration on that power meter and the value could be 10-15% off but I think you can say that the MSW waveform added maybe 30W of heat to the motor.    The inverter is only used for the fridge so after startup I could reduce the on time pulse width.  This was something pushed from NASA 30 years ago.  Remember the little box called the Green Machine sold to reduce motor power.  It was a TRIAC device used to reduce effective voltage going to the motor based on motor current phase angle.   Anyway it was a hot summer and thought about the motor case.  That has a lot of surface area inches and I added a computer muffin fan that operated when the 12V relay was on.  Had to make it a little more efficient.  Lots of things to try if I can get around to it this winter.

[Mary B]

Thanks everyone, plan on replacing my current ailing fridge this winter with a chest freezer with outboard temp control. Deep freeze is maybe 4 years old and one of the lower current draw models. I have to much in it to pull it out to get at the nameplate right now.

[clockmanFRA]

"I've been running chest freezers off my 1400va 24v APC UPS without problem."
Yes me as well.

But I understand that it does have a good sine wave.

My APC UPS 48v 5000va will not run the fridge or freezer, now its supposed to be a good sine wave, but I think its a modified.

However to complicate things some folk who know about these things inform me that its the Negative to Earth problem and my 5000va UPS is not earthed properly because I am using a domestic set up where the negative is connected to the appliance Earth?

But this APC 24v UPS does work.

[mab]

Er.. no - mine is one of the earlier ones and it is definitely modified sine wave  - it's a simple push-pull with clamping circuit driving a transformer and I've seen it's output on an oscilloscope.

My APC UPS 48v 5000va will not run the fridge or freezer, now its supposed to be a good sine wave, but I think its a modified.

bear in mind that MSW have a much better surge capability than pure sine (well, the double conversion pure sine type), which tends to make them much better at starting induction motors than an equivalently rated pure sine.  So your 5kva unit might well be pure sine - though with a 5kva rating I'm surprised it won't run a fridge or freezer.

[Mary B]

Any excess motor noise/extra heating? That is one of my big concerns. Losing my hearing and background noises can mask things like the TV and phone.

[mab]

no excess noise from the freezers - as far as I can tell they aren't running any hotter than on grid power but TBH I haven't really tried to find out.

I've lined my freezer(s) with storage heater bricks and try to run them when the sun's out and leave them off overnight, so they work hard in the morning getting down to temp then, once they're down to temp, they are running cooler.

[mab]

I've just tried running one of the freezers through a wattmeter:-

the power reading was about the same for both grid and MSW - actually slightly higher for grid power, but then the grid voltage is a bit high here - I've never seen it below 252volts. So in theory the freezer shouldn't be any hotter on MSW.