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Tuesday, December 14, 2010

Appliance Burnout on RV Inverter Power

Do you have any information on a coffee maker that does not short out or burn out when the inverter is used? We have gone through six coffee makers in three years of full-timing in our motorhome. It burns up when shutting off the 50-amp power and operating on the inverter when on the road. I have a Newmar with a 2000-watt inverter and a 7500-watt generator. The inverter is a Xantrex, which was installed by Newmar at the factory when the rig was built. I just installed four new Trojan 6-volt deep cycle batteries. R., (Spring Lake, MI)

Ron, two words; iced tea. I think I’d give up on coffee at the rate you’re going through coffee makers! Just joking! But I’d focus on the inverter itself since you do not have a problem while on generator or shore power. If the coffee makers were the issue, you’d be having the same problems regardless of the power source.

To start, be sure all the connections for the positive and negative battery terminals, the remote panel, the incoming and outgoing AC conductors, etc., are all clean and tight at the inverter. Those terminal blocks with the small screw clamps can sometimes vibrate loose. Loose connections will always present problems, or at least opportunities for eventual problems.

One other item to check – and this is a common problem with some of the lighter loads that may be put on that inverter; be sure another load is on at the same time. There’s an internal function inside the inverter called the “search sense threshold.” If the inverter’s search sense threshold is set too high or too low, it could cause erratic output voltages, especially when the inverter is powering a relatively light load, such as a coffee maker. Measure the output voltage of the inverter with a light (small) AC load applied to help stabilize the output reading. 

If the voltage output at the coffee maker receptacle is upwards of 129 or more RMS volts AC, I would suggest calling the Xantrex customer service line and see if any firmware upgrades are available for your model. As improvements to the algorhythms are effectuated, the upgrade will update your older unit. Contact them at: 800-670-0707. I hope this, at least, points you in the right direction. It’s not easy trying to diagnose long distance like this. I often wish my VOM leads were a little longer!

Once the above question was first published, I received more than a couple of rebuttals to my response to Ron. It seems I completely missed an obvious condition relative to his question. I totally missed the fact that Ron's inverter might not be a true sine wave inverter. I have long recommended the use of a true sine wave inverter and simply assumed his was one of them. The output waveform of Ron's inverter is apparently a modified sine wave, or as some people identify it, a modified square wave output. Many light, non-resistive loads like coffee makers, electric blankets, portable power tool chargers, etc., are not appropriate to be powered by a modified square wave DC to AC inverter. In fact many small appliances are labeled as such. The fact that some coffee makers like Ron's are not, leads to the confusion and the common burnout issue with some of them.

Most low load devices, like coffee makers, clocks and electric blankets that employ electronics or are outfitted with a timer or a clock of some sort will likely not operate correctly when powered by a square wave inverter. One reader, Gene Arnott, pointed out that he has had success with at least one model of Black & Decker coffee maker; one that does not contain electronics. This, after burning through a handful of Mr. Coffee machines.

Another reader, Alfred Oxton, has a Craftsman 19-volt power tool that has a warning tag on the charger cord that specifically says to not use that charger with a DC to AC inverter.

Xantrex states in their manual that the best devices powered by an inverter be of the "resistive load" type. Here's their reasoning: "These are the loads that the inverter finds the simplest and most efficient to drive. Voltage and current are in phase, or, in this case, in step with one another. Resistive loads usually generate heat in order to accomplish their tasks. Toasters, coffee pots and incandescent lights are typical resistive loads. Larger resistive loads—such as electric stoves and water heaters—are usually impractical to run off an inverter. Even if the inverter could accommodate the load, the size of battery bank required would be impractical." 

So many thanks to Gene, Alfred, John Leitch, Bernie Dobrin and Dan & Carylin Larson for their input and sharp eyes! Bottom line, be aware of the output waveform of your inverter and also check the specifications and limitations of any device being powered by that inverter output.


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