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Inverters

This article discusses how Inverters work and why they are so desirable to many travellers. It explains the difference between modifed sine wave inverters and pure sinewave inverters; contains a table showing the inverter requirement for a range of common appliances; and also explains the terminology associated with power and how to understand what it all means.
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An Inverter is an electronic device that converts one voltage to a higher voltage. Travellers usually want to convert 12 volts DC (Direct Current) to 240 volts AC (Alternating Current).
Because we have 240 volts at home, we already have many appliances that run off 240 volts, whether it's a very low power charger for a camera, or a very high powered microwave oven. An Inverter allows you to use the applicances you already have when you are away from Mains power - provided you have an Inverter that puts out enough power to suit the loads and that it puts out a waveform good enough for the load.
An Inverter only changes the voltage and current - the output power (in watts = voltage multiplied by current in Amps) is always less than the input power, due to Inverter Efficiency being less than 100% - usually between 80% and 90%. The power that is lost as electrical energy escapes as heat energy in the Inverter.

Electrical Power

Electrical power is always measured in Watts (1000 W = 1 kilowatts, 1 kW).

You may see Inverter output rated in VA (VoltAmps) rather than Watts and for many appliances this will equate to the number of Watts, but for those appliances with motors in them, the VA needed can be up to 50% more than the Watts needed.

Peak and Continuous

Two numbers are used to describe the output of an Inverter - Peak and Continuous.

Peak (or surge) describes the output deliverable for several minutes - the Inverter components won't blow up at that amount of load, but they will heat up quickly. Continuous ratings are usually limited by the heat that needs to be removed when running continuously. Ideally you would find out the Peak and the Instantaneous ratings of your appliances and check that they were less than the Inverter output, but you can't rely on this because most appliances don't state the start-up or peak power - and the length of the peak really needs to be known too. Unfortunately, the only way to be sure, is to test the appliance with the Inverter!

 Size of Inverter Needed

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 The following table is only a guide - there is a lot of variation in startup surge load and continuous load for similar appliances between makes/models.

Size of Inverter Required Type of Appliance
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Most small plugpack chargers and power supplies for cameras, laptops etc these days use Switchmode power supplies - feel the weight of it - if it feels like it is full of iron, then it has a conventional iron transformer in it (most probably needs Sinewave), if it is much lighter, then it is Switchmode. Theoretically the switchmode power supplies should work well on any waveshape input, because they firstly convert the AC to DC, however if the components inside aren’t rated adequately, then they may overheat from the inrush current 100 times a second, caused by the squarish waveshape.

Modified Square Wave (MSW) Inverter

Most appliances will work from these basic Inverters, but unfortunately unless you can look at the internal electronic design of the appliance, you can’t be sure if you will really need a Sinewave Inverter. The squarish waveshape can also confuse the timing circuits in some appliances that use the Mains Frequency to control a clock. If the appliance doesn’t work normally or makes unusual noises, then disconnect it or you may have expensive damage. These Inverters also generate more interference to TV and AM and HF radio reception than Sinewave Inverters and can cause buzzing in CD players.

Sine Wave Inverter

Sinewave Inverters are more expensive than Modified Square Wave (up to twice as much) because more components are needed to electronically generate the Sinewave. If the appliance has a Transformer or an Induction Motor (refrigerators and non-Inverter Air-conditioners), then you’ll most probably need a Sinewave Inverter to prevent overheating. Speed-controls on drills, Light Dimmers, Computers and Laser Printers usually need a Sinewave input.

TIP
If your Autostart Inverter won’t start with small loads like a Plugpack Charger, connect a 60-100 watt lamp to the output.
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Optional Features

Below is a list of optional features that you may encounter in regards to inverters.

Isolated Output

If you are using only double-insulated appliances (i.e. they have no third Earth-pin on the Mains plug) with your Inverter, then it doesn’t matter if the Inverter has total electrical isolation between the DC input and AC Output (provided there is no fault to earth). In larger installations with an earthed neutral, or if you have earthed appliances such as Washing Machines, then if the Inverter is not isolated, the DC input will be up to 120 volts AC above the Mains Earth - a very dangerous situation!

Soft-start

Some loads draw much more current when starting up compared with steady operation e.g. a Halogen bulb draws ten times its normal current when first turned on, and motors can also draw very high currents as they come up to operating speed. To reduce these massive current surges, some Inverters bring the output voltage up to 240 volts progressively over many seconds. This means you will be able to use an Inverter with a smaller continuous power rating, for a particular load.

Low Voltage cut-out

To prevent damage to your battery, it’s important that the Inverter switch itself off and remove all current drain, when the battery voltage drops below a set limit. It helps if the cut-off voltage can be set by the user, based on the type of battery, the depth of discharge desired and the voltage drop in the wiring. Keep in mind that as battery voltage drops, the Inverter will draw more current to keep input power constant, so this means that voltage will drop fairly quickly towards the end, and therefore manual monitoring of battery voltage is not really practical.

Automatic Fan Cooling

To get rid of waste heat in Inverters over 200 watts, it really helps to have an internal fan and usually these are automatically switched on only when needed, to minimise power used.

Earth leakage protection

Bigger Inverters may have these built-in, but they can be easily added.

Meters

You can get by without voltage and current meters, but with them it’s easier to check if your system and appliances are healthy, or if you’re close to overloading your Inverter.

Autostart

Some larger Inverters automatically sense when any appliance is connected or switched on, so that the Inverter only draws significant power when it’s actually needed. This is particularly useful in large vans, mobile homes or fixed buildings, because you don’t have to switch the Inverter off and on when using appliances.

Remote Control

For installations in large vans, mobile homes or fixed buildings, it’s handy to be able to control it remotely, because the Inverter may be mounted in a cabinet near the batteries.

Inverter/Charger

For permanent Installations, the Inverter may be able to charge the batteries when mains is available, but there is no solar power available.

Grid-Feed Inverter

For permanent installations, when there is surplus solar power available, the Inverter can feed power back into the 240volt mains, resulting in electricity being ‘sold’ to the electricity company.

Overload cut-out

Not really optional, but the more sophisticated ones will protect against over-temperature as well as over-current, over-voltage or under-voltage (that can damage some appliances).

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TIP
Always use a Safety Switch at the Inverter if one isn’t built in - it’s very cheap protection against a fatality.

Overheating of Inverter & Wiring

Inverters typically have an efficiency of 80% which means that for 1000 watts input, only 800 watts gets output as 240 volts, so the remaining 200 watts gets dissipated as heat in the Inverter. So it’s important not to install Inverters in confined spaces where the heat can’t escape, through air movement or conduction. It’s also important not to cover the case of Inverters or obstruct any fan intake e.g. by letting clothing fall on it. If there is no fan in the Inverter, then the Inverter housing is the heatsink that must get rid of this waste heat. Never mount an Inverter in the engine compartment.

Inverters draw high current from the battery - for 12 volt Inverters, the general rule is to divide the output power by 10 to understand how much current will need to be supplied. So a 1000 watt Inverter will draw 100 amps from the battery and this means using battery cables which can supply this level of current without overheating, while also keeping total voltage-drop in the positive and earth lead to less than 1.5% in each - i.e. less than 0.2 volts.

TIP
By installing the Inverter close to the battery, you can minimise the amount of heavy cabling that is needed in the 12 volt feed - since the 240 volt cable can be much thinner at the same power level.
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Battery capacity

All the power for the load has to come from the battery, so with high power Inverters or appliances you want to run for a long time, you need massive battery banks. If you wanted to run a small Air Conditioner that used 1000 watts on average, it would draw 100 amps - so you would need to supply 2400 amphours over a day. That means big, heavy and expensive batteries, and a way of recharging them!

Use 12 volt appliances

Many appliances are available as both 12 or 240 volt versions (fridges, microwaves etc), but the 12 volt version always uses less power and is always more expensive ! A 12 volt fridge will have thicker insulation and use a more efficient motor because mains power is cheaper than 12 volt power ! A 12 volt microwave will use less power because there are no losses in converting the 12 volts to 240 volts first. Using only 12 volt appliances, may reduce your power needs, so you won’t need a generator or Solar power to recharge your batteries on shorter outings.

Generator

These can easily generate the high power level often needed and they produce a 240 volt output directly. However an Inverter is totally silent and smaller than a Generator.

DC-DC Inverter

If you want to operate a laptop, you will already have a mains supply for it, so usually the cheapest way to run it off 12 volts is to buy a low-power 240 volt Inverter to power the laptops Mains supply pack. This means you will have two boxes and you will waste more power in the double conversion, compared with buying a DC-DC Inverter. A DC-DC Inverter directly converts the 11 to 15 volts that a car electrical system puts out to the 12 to 21 volts DC that your laptop needs to run and recharge its internal batteries.
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What Others have Posted

Member - Alex K (NSW) : 27 Nov 07 22:13
Hi All, Probably an easy question for the electrical mined. I'...
Rusty_lux : 13 Nov 07 23:24
Hi everyone, I was wanting to know if I need a pure sine wave or mo...
JERRYNIC : 15 Aug 07 17:47
I ONLY WANT TO RUN A FREEZER (940W) IN MY CARAVAN. CAN I RUN A CHARG...

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