12vdc to 240vac inverter problems

[Chris Giles]

Hi guys, I have a small overshot water wheel driving a low rpm PMA which charges batteries. These power a 1200w square wave inverter which happily runs my house with a little bit of sensible power management. BUT - mt fride freezer will not start and cuases the inverter to trip. Obviously a very high inrush current required to get the compresor going ! Anybody got any ideas on how I can overcome this without spending a fortune on a high output inverter ? Some sort of capasitive device on the feed to the fridge ? I am no electrican so dont know if this is possible. Thanks for any help received.

[Joe Blake]

Have you considered using something like an Engel camping fridge/freezer? They use a "swing motor" which has only one moving part.

http://www.engelaustralia.com.au/uploads/image/N21Swing%20Motor%20Diagram%20colour%20ver6_400.jpg

It has a reciprocating action rather than rotary, so there's no start-up burst power.

They have 12v/24v/240v options. With both of them running flat out (-18 degrees C) they draw about 75 watts total.

I've had two of them for a few years now, and they're pretty good (I use them as freezers and make ice in one of them - see my thread on "Cold Statistics".)
http://www.greenpowertalk.org/showthread.php?t=16383


Joe

[Rob Beckers]

Hi guys, I have a small overshot water wheel driving a low rpm PMA which charges batteries. These power a 1200w square wave inverter which happily runs my house with a little bit of sensible power management. BUT - mt fride freezer will not start and cuases the inverter to trip. Obviously a very high inrush current required to get the compresor going ! Anybody got any ideas on how I can overcome this without spending a fortune on a high output inverter ? Some sort of capasitive device on the feed to the fridge ? I am no electrican so dont know if this is possible. Thanks for any help received.

Hi Chris,

Welcome to the forum!
The simplest thing I can come up with is to try a series resistor in line with the motor. This will limit the inrush current, but it will also limit the starting torque, so its value will need to be determined by trial-and-error (and it's entirely possible there is no value that will keep the inverter from tripping while still starting the compressor).

-RoB-

[Chris Giles]

Thanks guys, any input is great. Replacing the fridge is just too expensive. I only bought it 4 years ago ! I like the idea of resistors to limit the input current but dont know how to do this ! Any more input very welcome !! Cheers !

[Ralph Day]

Hi Chris
Here's a link to a fine thread on the NAWS forum. It deals with freezer to fridge conversion, but the requirement for the start circuit is what you should see. Wayne (the thread originator) has done this to a number of freezers, all of which run on small inverters requiring modifying the start circuit.

http://www.wind-sun.com/ForumVB/showthread.php?16252-My-latest-freezer-converted-to-fridge-showing-VERY-good-results&highlight=waynefromnscanada

Ralph

[Chris Giles]

Thanks Ralph for pointing me to thread on the NAWS forum, great stuff ! I am going to get a electrician buddy of mine (I am not sure how to wire them onto the supply to an AC circuit) to check out the capacitor specs they mention and we will try fitting same to my fridge/freezer. I will let you guys know how I get on. Thanks for all your help. Cheers.

[George stanoukis]

You can’t use a resistor in the circuit, that’ll only make you lose power. A resistor does resist the flow of current but to overcome that the fridge will eventually draw more power. You can use stabilizers that are easily available in the market; they stabilize the current and voltage to the fridge. You can also use them with any other electrical device.

[Rob Beckers]

George, what is a "stabilizer"? I've never heard of such a device. Can you point to a Web link that explains this?

As mentioned, using a resistor in series with the compresses walks a fine line between limiting the inrush current, while still allowing the compressor to run. The problem is not the running current, but the startup current, that's what a resistor would attempt to shave a bit so the inverter doesn't trip.

Using a capacitor works if the compressor does not yet contain one (or its value is utterly off). What a capacitor does is improve the power factor, so the compressor runs a bit more efficiently: The motor in the compressor is an inductive load, causing current to flow out-of-phase with the voltage, drawing more current from the inverter than strictly needed for the power that's produced by the compressor. A capacitor helps by providing current to the compressor during those out-of-phase moments, while getting recharged on the other side of the AC sine-wave, if dimensioned correctly the capacitive behavior of the capacitor and the inductive behavior of the motor will cancel out, resulting in a lesser current draw from the inverter.

-RoB-

[Cor van Houtum]

modified sin wave inverter ?
I have had a customer with the same problem

changed to true sin wave inverter
problem solved

cor

[David MPierce]

The only solution described by Cor. I do have the same situation handled by an electrician but I think he also done the same.:idea: