I need a backup power source for my aquarium tank filtration system. A pure sine wave UPS seems to be an expensive overkill, so I'm looking for alternatives.
The filtration system pump runs of 230V, 50Hz AC motor with magnetic coupling (I assume synchronous?) and consumes somewhat less than 10W of power.
My idea was to buy a cheap modified sine wave power inverter from eBay, rated for 100W, and use a spare 12V sealed lead-acid motorcycle battery I already have.
The inverter came yesterday, I've tried it with the battery and my pump. It seems to work, at least in sense the motor pumps water properly, but it does not seem to like the modified sine wave much – it's very noisy and produces more heat than normal. I stopped it after a minute worrying about damaging my equipment. I suspect this problem is caused by sharp edges and high harmonics produced by the modified sine wave inverter.
Wouldn't it be possible to simply smooth the output of a modified sine wave inverter somewhat to make it more acceptable for the motor? How about using a some capacitor on the 230V output?
I've done some research and found some people arguing it was inefficient but possible, in principle, by using a capacitor on the output, parallel to the load. I don't need it to be very efficient, it's just 10W, nor I need the smoothing to be perfect, but I definitely need to make the output more sine-like. Is this idea viable? If so, how I compute the optimal capacity of the filtration capacitor?
Best Answer
I would suggest that an inductor in series with the load may help.
Most times, a Modified Sine Wave inverter produces a simple square wave output with significant OFF-time. The peak voltage matches that of a sine-wave and the off-time is increased such that the average voltage is equal to that of a sine wave.
But it's still a square (rectangular) wave.
Placing a capacitor across the load is a bad idea because the peak current will be very high. Look at the leading edge of the waveform - it's almost vertical.
However, an inductor in series with the load may slow the edges sufficiently that the motor is "more happy" than with the plain square wave from the inverter.
Size, current rating, inductance value is "left as an exercise for the student".