Organization: Queen's University at Kingston
Date: Tue, 15 Sep 1992 11:27:21 EDT
From: <LEEK@QUCDN.QueensU.CA>
Newsgroups: sci.electronics
Subject: Re: AC power inverter questions?
Distribution: world
 <1945d7INNneo@grapevine.EBay.Sun.COM>

In article <1945d7INNneo@grapevine.EBay.Sun.COM>,
smckinty@sunicnc.France.Sun.COM (Steve McKinty - Sun ICNC) says:
>
>I don't think you're missing anything, square waves are used simply because
>they're easier & cheaper to create. As far as efficiency goes, an oscillator

Yeap.  The only problem with square wave design is that the harmonics
contents are high.  It has to be removed so that excess harmonics does
not leak bak into the power lines.  Power Utilities companies and your
neighbours are usually not to happy when that happens.

>producing square waves will have switching elements (usually transistors)
>either on or off, so they run cool. Generating sine waves requires the
>transistors to operate in a linear fashion, and for most of the cycle they

Very few power supplies except for really low power or low noise
requirement are operating in linear mode.  PWM is widely used (I think
judging from the availability of PWM chips)

>As a thought, could you do do something creative with pulse-width-modulation
>to create pseudo sinewaves, with only switching transistors? Integrate them
>in the transformer?

That's pretty much what they do.  The transformer acts as a filter in
this aspect.  I would doubt that's the way to do it if you are thinking
of a UPS or battery -> 120V AC inverter.  The problem has to do with the
bulky 60Hz transformer required (& cost).  I haven't looked into the
designs of commerical units, but the following block diagram is what I
would design for my own use.

|<-- DC-DC converter       -->|             | <- PWM Inverter ->|
Battery (24V) --> High Freq    --> +170V DC --> PWM --> Filter --> Load
     ^            Switching        -170V
     |            power supply        ^
 Recharger                            |
     ^----------- 120V AC ------> Rectifier (or Power factor correction)

The design might look unnecessarily complicated.  It is as it is right
off the top of my head and hasn't gone through the process of making it
economical.  It does have the advantage of light weight and compact
design.  Normally the inverter runs off rectified 120V AC into a + -
170V DC bus. The PWM is continuously operating to convert the DC to AC.
If the input AC power drops below a certain point, the DC-DC converter
kicks in automatically.  The DC-DC unit runs on high frequency square
wave and uses small & light weighted transformer core commonly used in
switching power supply.  This simplifies on the switching over to
backup power (the diodes in the rectifier also acts as the power switch
to cut off AC if it drops belows a certain point - may it be brown out or
black out) and eliminates the switching delay of standby UPS.

>I'd expect motors to be OK, they have enough inertia to ride out the
>square ends of the waveform. Anything which relied on sinewave input
>for phase-control of voltage (drill controllers, light dimmers, some TV's)
>would have problems.

The industry has been using square wave inverters.  The inductance in
electric machines (large 3 phase motors) helps to filters out the
upper harmonics so that the current waveform is very close to sine
wave (the voltage waveform is still square wave).   Audio - Hi-Fi
stuff might have a problem with the extra noise.  (BTW mordern TV set
rectifier 120V AC into DC to run the flyback circuit.  The onboard
DC source comes from rectifing a low voltage secondary off the flyback.
The remote control & fancy microprocessor stuff probably runs off yet
another small power supply as they have to be on at all times.)

>Steve McKinty

K. C. Lee
Elec. Eng. Grad. Student