Why cannot circuits like astable multivibrator, RC phase shift oscillator and wien bridge oscillators be used as inverters?

Question

I have been having this doubt for quite a while. An inverter basically is a power electronic equipment that converts DC voltage in AC voltage for powering loads that work on AC supply.So this means that the inverter switches operate at a frequency of 50Hz (for India).

Astable multivibrators also does the same job of converted a DC voltage into a alternating square wave voltage. Also I've found that research is going on in elimination of harmonics from inverters in order to obtain pure sine wave. Instead of that, why not use a simple RC phase shift oscillator or wien bridge oscillator designed to oscillate at 50 Hertz? Is it difficult to design? What are the complications faced in doing so?

Answer 1

The power level of the usual applications of inverters is the main obstacle.

With power levels of, as you say, 100W, for example, the inverter must be very efficient, otherwise you would have two problems:

1. lots of energy (i.e. money) wasted into heat;

2. lots of heat to get rid of (or the circuit will cook to death), and this means massive (i.e., expensive) heat sinking and complex heat flow management (even with fans and, to the extreme, liquid coolant).

Even if the circuit is 80% efficient (which is more or less the minimum for actual inverters), you have to get rid of 20W of heat. And 100W inverters are not so big. Imagine what would happen for a 1kW unit: you get a 200W heater for free!

The circuits you mention are used for low power signal, less than some milliwatts usually, so even if they have poor efficiency, it is not a problem. I never calculated the efficiency of, say, a one transistor phase-shift RC oscillator, but I suspect it could be very low (less than 30%), since the BJT operates as a class A amplifier.

For more details about inverters' efficiency you can see this site.

Answer 2

You can do what you suggest but you will need an efficient power amplifier to magnify the oscillator voltage and power in order to drive hundreds or thousands of watts to your loads and appliances. So you have a 30 cent oscillator and a twenty dollar power amp feeding a twenty dollar transformer and what seems an interesting thought about an oscillator topology becomes of hardly any importance at all in the bigger scheme.

It becomes so unimportant in the design that it is trivial in both cost, performance and complication. In other words it's like placing the importance of designing the pedals of a bycycle above that of the design of the frame, gears, tyres and saddle. Yes you need an oscillator and a bike needs pedals but let's not forget the really tricky bits.