amazing ARB pricing

On Sunday, October 28, 2018 at 7:43:22 PM UTC+11, Tom Gardner wrote:
> On 27/10/18 23:07, Johnny B Good wrote:
> >   It turns out the purity of the UK's national grid 50Hz sine wave supply
> > is somewhat lacking even when compared to that of a cheap 2.8KVA generator
> > [1]. For reference, it's more accurately best described as having a 'down
> > sloping' flat topped appearance (with respect to absolute amplitude -
> > it's an 'up-slope' on the negative peaks). I make this reference for the
> > benefit of those armed only with a cheap digital 'scope[2] that simply
> > shows a flat topping effect without any hint of a slope on the mains
> > waveform versus the more pure looking sine wave from a cheap 1KW Parkside
> > inverter generator (PGI 1200 B2).
> 
> The "clipped sine wave" appearance surprised me, when I looked
> at it for the first time since the 70s.
> 
> I believe it is because there has been a change in the loads
> on the grid. Nowadays there are a large number of devices
> with SMPSs. Many presume a more-or-less constant input voltage
> throughout the mains cycle, so they rectify the and smooth the
> mains, "topping up" the charge in the capacitor only at the
> peak voltages. Hence significantly more current is drawn at
> the peaks than at other parts of the cycle.

I'm not sure where the limit is now, but it used to be that if you were drawing more than 100W from the grid you had to build "power factor correction" into the front end of your power supply, which extracted a more or less sinusoidal current into a capacitor which fed your high-frequency inverter with a more or less constant high DC voltage - it obviously had to end up with enough 100Hz ripple to cope with delivering a more or less constant current while getting something like a rectified mains sine wave current.

Smaller stuff - like the ubiquitous wall warts - could get away with clipping the top off the sine wave.
 
> As for inverters (i.e. not petrol-driven generators), I
> would be wary about applying a "modified sine wave" output
> to valuable equipment. The PSU might be unfazed by such
> rectangular input, but in cheap equipment, who knows.

A proper modified sine wave has no third harmonic component, not a lot of fifth harmonic component but everything above that is just as bad as it would be in a square wave. You could filter quite a bit of that out, but a fancier inverter might be cheaper.

Higher harmonics are good for warming up iron cores.

-- 
Bill Sloman, Sydney