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WARNING Dec
07 Be very wary purchasing DC/AC inverters from a nation wide
auto parts supplier of imported Chinese junk. These inverters, rated
at 150, 300 and 600 W, not only do they not work properly due to very
poor design but are dangerously assembled as well.
Only purchase this type of product from an electronics supplier who
hopefully has done some basic acceptance testing of the product as
opposed to the auto parts company who it would seem to me just markets
container loads of anything from which they can make a quick $$$.
Generally, made in Taiwan is a better label than made in China or
PRC. |
Feb 2003
Inverters
are electronic devices that convert your 12 or 24 volt battery power into 240 volts AC to enable you to operate many domestic appliances when no mains power is available and without the need for a generator.
Inverters are available in three basic types characterised by their output waveform. They are
square wave, modified square wave and sine wave types.
The above illustration shows an approximate comparison between the waveforms.
(The square and modified
square waves are not as "square" as shown above in practice but
it illustrates the difference)
The modified square waveform can take a variety of shapes, including the
above, depending on manufacturer.
Many manufacturers and retailers try to pass off the modified
square wave as "almost a sine wave" by referring to it as a
modified sine wave or similar but don't be fooled, only a sine wave is a
sine wave, anything else is a square wave or a variation of a square wave
and don't let them convince you otherwise.
These are the earliest types, characterised by an AC output waveform which when viewed on an oscilloscope as shown above is, as the name suggests, square.
There are several problems with a square wave.
All AC appliances are designed to operate from a sine wave of a specified voltage (e.g. 240V) such is available from the supply grid. This voltage is known as an RMS voltage and does in fact have a peak value of 340V.
A square wave can also be 240 V RMS but its peak value is only 240V.
Appliances incorporating induction motors (refrigeration and air conditioning compressors, range hood fans etc.) generally do not perform as well, sometimes not at all, on a square waveform.
Motor speed controls, such as on your variable speed drill and light dimmers generally do not work since they rely on triggering on the sine waves' sloping waveform.
The speed control or dimmer adjustment will usually need to be set to maximum
to work.
Interference to AM radio and TV can be a problem since a square wave inverters' output contains a high order of harmonics.
(The highest frequency is directly mathematically related to the "steepness" or rise time of the
waveform. The time it takes for a wave form to rise from 10% above the zero line to 90% of its maximum voltage is defined as the rise time.
For a square wave, the dominant frequencies in its makeup are the odd harmonics of the
fundamental, three times the fundamental, five times the fundamental, and
so on.)
Advantages & Suitability
-
- Cheap to manufacture and therefore cheap to purchase.
- Suitable for universal (brush type) motors, (e.g. power tools - variable speed tools will need to be set to maximum, vacuum cleaners etc.) Check appliance wattage does not exceed inverter rating, and be aware of high in-rush or starting current with motors and to a lesser extent, lights.
- May be suitable for TV, VCRs and mobile phone / camcorder type battery charging.
(Check with the manufacturer or supplier, although from my experience it is most unlikely any manufacturer, supplier or retailer would authorise operation from any supply other than that stamped on the appliance. If they do, get it in writing. When a manufacturer specifies 240 VAC 50 Hz, they are
always
referring to a sine wave voltage.) Maybe the appliance will work OK for years, but the risk is yours.
(That said, before the availability of affordable sine wave inverters, I have used many square and modified square wave inverters over the years without damage to any appliances. In many cases things didn't work as well as they could and in some cases not at all, but none were damaged. If, for example, a motor doesn't sound, feel or perform quite right, don't persevere, switch it off before it is damaged.)
Disadvantages -
- Not suitable for most microwave ovens (ranges from worst case of not working at all to best case of increased cooking times)
- Generally not suited to SCR and Triac type speed controllers and light dimmers.
- Electrically noisy.
Also known as Modified Sine Wave
and Quasi Sine Wave
by sales and marketing people - makes it sound like you have almost got a
real sine wave inverter for a half to one third of the
price. I personally believe it is misleading to refer
to a mildly doctored square wave in the same breath as a sine
wave and pretend they are essentially the same but I seem to be in the minority.
In reality, there
are only two types of inverter,
- the sine wave type and
- the rest (no matter how sugar coated the alternative names for the
rest may be)
I have recently encountered a Pure Modified Sine Wave Inverter at a
caravan & camping show. The
salesman spent five minutes trying to convince me that it was as good
as a sine wave inverter but he was unable to explain in technical
terms how the "pure" bit fitted in but then again, he was unable
to explain anything in technical terms.
A sine wave by definition for our purpose is a single frequency (50Hz in
our case), in other words a pure undistorted single frequency with no
harmonics.
In practice, a sine wave generator will suffer some distortion but
providing it doesn't exceed 5%, it isn't a problem.
A modified sine wave (a misnomer in any case since the waveform was never a
sine wave to start with, and if it was, why would you want to modify it ?)
cannot by definition be pure.
(In fact if for some pointless reason you started with a sine wave and
modified it, you would be converting something that was pure into something
impure.)
Modified Square
Wave Inverters are a big improvement on the square wave. The waveform varies dependent on the manufacturer, and partly addresses the difference between the RMS value and the peak value and may
make some induction motors happier.
The fact remains that all mains operated appliances and devices were designed to operate from a sine wave supply and the degree to which they will satisfactorily perform on a square wave, modified square wave, or high distortion sine wave will vary.
Advantages & Suitability and Disadvantages
Pretty much the same as for Square Wave above.
Since a reputable brand of sine wave inverter has a low distortion sine wave output with stable voltage and frequency, these inverters will operate any appliance or device that was designed to operate from the mains grid up to their design rating. (Keep in mind the high starting current of motors particularly in compressor applications.)
Advantages & Suitability -
Ability to operate any appliance / device within inverter design rating.
Disadvantages
-
Only one, higher purchase price.
Square wave inverters are becoming less common and I believe should be overlooked in favour of either modified square wave or preferably sine wave types.
Your budget and appliances will determine your final choice, for a comparison, an
1000W modified square wave inverter currently sells for around
$500 and an 1000W sine wave inverter around $1200 (June 2003).
*****
But, you don't get something for nothing.
If you want to run an appliance rated at 1200W, for example a microwave oven, you must supply 1300W from the battery (this is because good inverters are
typically in the vicinity of 85-95% efficient in the conversion process). This equates to around 108A from your 12V battery or 54A from your 24V battery.
*****
Speaking of microwave ovens, when calculating the size inverter you would need to run it, you need to appreciate that the advertised power rating is the RF cooking power, the total power consumed by the microwave oven is approximately double that. For example, a 600W microwave will consume
almost
1200W. *****
Most manufacturers incorporate safety devices including a 'low battery voltage cutout', which will turn the inverter off when the battery falls below approximately 10.5V for more than a small preset time limit in order to protect the battery.
*****
Be aware of the inverter's rated output. They have various ratings . . .
Continuous rating - an output which can be sustained indefinitely
1/2 hour rating (sometimes called maximum rating) - an output which can be sustained for 30 min
Surge rating - a brief momentary burst (e.g. starting electric motor)
*****
You need to ensure that you have adequate battery type and capacity to match the inverter's capability and your requirements. For example, you will be disappointed if you run your 1Kva inverter from a 60Ah cranking battery, when in reality you may really need a
minimum
of 2x120Ah deep cycle batteries.
Then of course, you must consider how you are going to charge them, but that's another story.
*****
See Batteries, Battery Chargers,
Batteries & 3-Way Fridge Problems, Solar and Generator
pages.
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