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?

  • \$\begingroup\$ From "the motor should be silent" you can't specify a filter. \$\endgroup\$
    – peter
    Commented Nov 30, 2016 at 22:02
  • \$\begingroup\$ I agree and never said so. I'm not asking for a solution, I know I will have to experiment myself. I'm just asking for a hint, if the idea with a capacitor is viable for this kind of 10W motor and possibly for some starting values for the capacitor I might try. I've done some digital electronics and microcontrollers before, but never anything like this. \$\endgroup\$ Commented Nov 30, 2016 at 22:09
  • \$\begingroup\$ Sorry, I ment to delete that comment... \$\endgroup\$
    – peter
    Commented Nov 30, 2016 at 22:12

2 Answers 2


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".

  • \$\begingroup\$ Okay, thank you for a hint, especially pointing out it is a bad idea to place the capacitor across the load. I will try what you suggest and see what happens, possibly borrow a scope somewhere, to check the wave. \$\endgroup\$ Commented Dec 2, 2016 at 14:50

A modified sine wave can be transformed to a pure sine wave by a simple LC filter.

The only "problem" is the large inductor, e.g. 1 H for a 300 Ω load at 50 Hz. The capacitor should be 10 μF which is large as well, but not as heavy as the coil.

I did some simple simulation here.


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