UPS sine wave vs square wave output?

sort of

Do a fourier transform on a square wave and you will find that it represents the sum of the sine wave with same period, plus all the odd hamonics of that sine wave, with amplitudes proportional to the harmonic number - a 60Hz square wave is the sum of a 60Hz sine wave, plus a 180Hz sine wave with one third of the amplitude, plus a 300Hz sine wave with one fifth of the amplitude, and so on.

Put this into a transformer or a motor that is intended to be driven by a 60Hz sine wave, and the higher harmonics excite extra eddy current in the magnetic path, making the motor or the transformern run hotter than it would have done if excited by a pure 60Hz sine wave.

The harmonics extend up to a frequency defined by the rise and fall times of the square wave, which can be quite fast - fast enough to interfere with local radio reception for a really cheap and nasty UPS.

-------- Bill Sloman, Nijmegen

Basically because there are things attached to them that assume that they have sine waves, and behave very badly when when they get other waveforms...

Motors, transformers, some electronics, esp. power supplies are designed around low harmonic content sine waves. Give them bad waveforms, and they loose efficiency, get warm, release magic smoke from components...

--
Charlie
--
Edmondson Engineering
Unique Solutions to Unusual Problems

Probably the same marketing company that claims you need 'pure rocky-mountain spring water and high country barley'.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

That's a poor example, in the steady state (implied by mentioning RMS). Consider instead the doubler used in most off-line switching power supplies when strapped to run off of 115 VAC. Your square waves will induce much higher peak currents than the less steep wavefronts of a sinusoid will.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

But seriously folks, I ran a complete hardware/software engineering lab out of a motorhome for several years. My Trace 810 inverter (modified square wave) ran everything just fine. Thats not to say that some electonic equipent may have problems - especially AM radios.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

Why such insistence on a UPS having a "pure" sinewave vs any other sort of output wave form such as square wave?

of

-------------------------------------------------------> >

-----------------------------------------------------------Question:

How does one design a power supply to accept a sine wave but not a square wave??

of

-------------------------------------------------------->

-----------------------------------------------------------Question: Wouldn't the filter capacitor enjoy receiving what amounts to alternating DC in the form of a square wave?

Because the power supplies in the equipment it is powering have all been designed to accept a sine wave from the mains supply. There is no guarantee how they would behave when presented with a square wave.

d

Pearce Consulting

sort

square

will

in

sine RMS

the

force

So why not lower the peak voltage of the square wave to a safe level??

John,

You can probably get a particular device to work with a square wave input. For instance, if the load has a capacitor input filter that normally charges up to near the peak of a sine wave, you could feed it a square wave of about

165 V peak. If you now fed this same square wave into a 120 V light bulb, it would not last long. The manufacturer does not know what kind of load you are going to connect to his UPS. Also, if the load uses power factor correction, the square wave would probably mess it up.

Tam

Start by considering the difference between the rms and peak voltage values for each waveform.

Graham

You don't. Not intentionally anyway.

A square wave input of the same RMS value of an equivalent sine wave will have a much higher crest factor. That is, the peak value is *much* higher in relation to the RMS value, as compared to a sine wave.

Most power supplies simply bridge rectify the AC input, and feed that somewhat pulsating DC into the regulator that does the bulk of the work.

With a sine input, that peak DC at the filters will be 1.4 times the sine RMS value. With a stepped square wave, or worse still, a pure square wave input of the same RMS value as your sine input will have a peak much higher.

Thus, if your components are only expecting a peak of X volts, and you force feed it something significantly higher than X, it'll blow.

--
Linux Registered User # 302622

Its not just the peak level which matters. Imagine a disharged capacitor connected to a low impedance squarewave generator. The squarewave goes up fast and the capacitor voltage must follow. This means a large current flows through the diode and the capacitor. Now imagine a sinewave : the capacitor has to follow the sinewave as it climbs up. The current pulse is stretched out and not as large in amplitude.

Result : Transformer, diode and capacitor RMS current is less, which means less heating of these parts.

Roger

That's not really possible.

If you reduce the on (peak) voltage, you'll have to increase the pulse width to cater for the resulting lower RMS voltage.

That in itself isn't a problem. What *is* the problem is that by reducing the peak, you're limiting your maximum power output.

A stepped square output is a reasonable comprimise, in that it's cheap to do, and it brings the crest factor down to a reasonable value that won't kill most (hopefully any) power supplies.

A sine inverter would remove all these issues. However, while it's reasonably cheap to produce a good facsimile of a sine output, the filters to smooth the output add to the weight and cost of the supply.

I'm still not convinced that it adds _that_ much to the cost of the inverter, but mass production wins every time: square or stepped square inverters are much more popular and plentiful than sine, thus their cost is artificially brought down relative to sine inverters.

--
Linux Registered User # 302622

Which is why the 900W OEM UPS' I worked on in MA had extremely long switching times - the "square" waves had edges of around 500us - 1ms. Think I=C*dV/dt. In our case the "problem" that steep edges caused was the X capacitor in the downstream equipment, just like Larrys voltage doubler.

Cheers Terry

It also helps that many "square wave" UPSs don't just go from full pluss to full minus, but instead go plus -> zero -> minus -> zero

-> plus -> and so forth. I seem to remember reading about a few that would have five or seven voltage levels instead of three.

And the magnetic materials used in these motors, transformers etc. are designed for 60Hz sinusoidal operation. Lamination thickness is chosen based on 50/60Hz, so harmonics have much higher eddy current losses than the fundamental. Hysteresis losses also skyrocket with frequency.

I once did an interesting job for a customer when I built AC drives. The customer was a panelbeater (auto body shop? unsure of USian term) had a large 400Hz angle grinder, all Al chassis. very rugged. They bought an AC drive, and used that to run the angle grinder. The AC drive generated a PWM voltage output (the load is usually an inductor, which integrates the PWM voltage to get a nice sinusoidal (ish) current), the harmonics of which made the grinder get too hot to hold after a few minutes use. So we slapped some chokes on the output of the drive, and a few caps, and voila - away went the heating problem. Well it did once we replaced the 50Hz chokes (which caught fire) with something better designed for the harmonics we jammed up it (much thinner lamination, grain-oriented Si-steel set up such that the flux path followed the grain)

Cheers Terry

John Tserkezis ha scritto:

However for medium/high power (>3-4 kVA) they are always sine wave inverters (PWM bridge + LC filter)

-- Per rispondermi via email sostituisci il risultato dell'operazione (in lettere) dall'indirizzo