Cost of Inverters

[Guest Bill]

Does anybody know why inverters cost so much? And why “big” ones (i.e. high-wattage) cost so much more than small ones? From my dimly-remembered high school physics, all that should be needed is a trembler coil, and that should cost about 50p.

[Guest Clive]

Oh Bill, Is this a tongue in cheek question?. However let’s play it straight. In an WW2 army back pack or the B2 spy briefcase there was a Mallory trembler coil and a transformer that generated the high voltage required by the anode’s of the thermionic valves. The trembler would tremble at a slow mechanical rate, even 50 Hz perhaps. The output of this electro-mechanical inverter was no more that 10 watts or so. Plus the weight of this lot was not insignificant. Then technology advanced through valves to transistors that did not require high voltages on their collectors to be able to generate similar powers but a much lower voltage. Today, Inverters are used where we want a “mains” type supply when there isn’t one available. Even a modern laptop computer will require more watts than the WW2 radio back pack. Since transistors have been around these have been used as the switching devices for first generation inverters. These inverters would use a pair of transistors to chop up the dc voltage at 50Hz, this chopped voltage then applied to the low voltage of a iron cored transformer which will have a secondary voltage of many more turns which will provide the 240 volt AC squarish wave supply. In simple terms the more watts you need the bigger transistors you need and the bigger and heavier transformer you need. All these bits cost money. Now solid state electronics has gone on another step and uses Field Effect Transistors and switches these ON and OFF very quickly, sometimes above 20 thousand Hz (20 Khz) so that much smaller light weight lower cost ferrite cored mini transformers can be used. However the clever bit is that it uses two sets of high frequency switching FET,s alternately to generate an output of 50 Hz. These second generation solid state inverters cost less per watt than first generation and are very much lighter but the basic costing is still related to the output capability of the inverter. The output of these inverters is rectangular but with gaps and are frequently referred to as “modified sine wave” Third generation high frequency inverters use a similar power arrangement as the modified sine wave type but have a control circuit that varies the duty ratio of each FET within each 50 Hz half cycle so that the output 50 Hz waveform is a true sine wave. These cost more as more technology and control is required. Having said all that I don’t think £19.99 for a 300 watt modified sine wave inverter from Maplin electronics is expensive at all. I suggest that these are “as cheap as chips”. An asthmatic member of our family uses one of these to run a mains nebulizer quite effectively.

[Guest PeteC]

Hi bill What Clive is saying in a roundabout & Semi technical way is:- The modern inverters have to supply a higher output wattage - my lap top for instance has a 240V AC input to 19V DC output Power Pack giving 120W output To handle this wattage one requires more expensive transistors and some means of cooling them such as 1 a heatsink (basically a lump of aluminium on which the power transitors are mounted - The aluminium transmits the heat away from the transistors. Otherwise they will melt) or 2 A fan (as in the case of my lap top power pack) Also as the price increases there mabe more cleaning of the output wave form Many electronic devices (including TV's) require a sine wave form input in order to opperate correctly Whereas the Basic WW2 device would produce a Square wave output (as do some of the 'cheaper' inverters0 In other words you have to pay extra for a pure sine wave output and/also for a higher watttage output personally I dont run the TV if we dont have a mains supply to opperate it from

[Guest Clive]

But my Grundig TV runs perfectly from my DIY square wave 50(ish)Hz inverter and tolerably from the more modern "modified sine save" inverter with a small amount of interferance. If you can try before you buy. Pure sine waves cost quite a bit more. Wasn,t it a lot simpler with steam radio!

[Guest Bill]

Thanks to both of you for your replies. And no, the question wasn't tongue in cheek, though by hindsight it could have been better phrased. I guess I was aware of the general answer: "because the box is full of expensive transistors". The real question was: why should it be? Reverting to my dimly-remembered high school physics, if you wrap a coil of wire - call it Coil A - round a bar of soft iron and then pass DC through it, the bar will be temporarily magnetised. Switch off the current and the magnetism will cease. If you now wrap another coil of wire - call it Coil B - round the bar, a current will be induced in it in one direction when the bar is magnetised, and in the opposite direction when it is de-magnetised - i.e. it will be an alternating current. So, if you switch the magnet on and off 50 times per second Coil B will carry AC at 50hz. If the number of coils in Coil A and the number of coils in Coil B are in the ratio 12:220 and the DC in Coil A is at 12 volts, then the AC in Coil B will be 220 volts. Is this right so far? (I did say my physics was dimly-remembered!) Assuming I am, where do the transistors (or thermionic valves) come into it? Clive seems to be saying that the iron bar would need to be heavy but, within reason, would we not be prepared to put up with a bit of weight to save the £350 cost of an inverter powerful enough to support a microwave oven? (My reason for wanting one.) Moving on from that, I am interested in PeteC's comments about the type of "mains current" required to e.g. charge a laptop. Just think about what we are doing: we take 12v DC from the vehicle (or leisure) battery, put it through an inverter to make 220v AC, then put that through the charger, which contains a rectifier to turn it back into DC, which is what the computer's chips actually require, as well as being what you need to charge a battery. If the geniuses who designed the laptop (and cordless toothbrushes, vacuum cleaners etc) had thought for a moment -Duh! people who buy a portable device just might want to use it on the move - they could have designed them to work at 12v, and then we could have powered them directly from our vehicle batteries. If you want to support a microwave oven in your (motor) caravan your choices are to spend £350 on an inverter big enough to do the job, or a petrol generator. Quadpower, who advertise regularly in MMM, advertise one for £300. It would be a no-brainer, except that it's a pain to have to start a petrol engine every time you want power. Anyone who has ever bought a lawnmower will know that, other things being equal, petrol mowers are a good deal more expensive than electric ones. Presumably this means that petrol motors cost more than electric ones. So, if Quadpower could be persuaded to rip out the petrol motor and replace it with a 12v DC motor, they could sell it for a good deal less than £300. I understand that motor-generators are widely used when AC is to be converted to DC (or vice versa) on an industrial scale. Is there anything wrong with my reasoning?

[Clive]

A rotary inverter with a 240v 50 Hz sine wave output would cost more and weigh more than a solid state inverter. It would also weigh probably in the order of 30 Kg, a lot more than the equivalent solid state model.