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What You Should Know About Power Inverters

 Inverters Change Voltage From DC to AC. Converters Change AC to DC

Part 1. Introduction to Inverters

Inverters basically come in 2 flavors. Modified and Pure. So what is the difference and more importantly, how does that effect the decision to purchase one over the other? First lets explain the difference.

To really understand the difference, we need to get just a little technical, but we'll keep it short and sweet and most will be able to follow along. If not, you can jump down the page to Part 2. "Choosing a Pure or Modified Inverter".

An Inverter is used to change a DC voltage to an AC voltage. In other words, it changes a flat steady DC level (usually 12 volts, 24 volts or 48 volts) to an AC sine wave that goes from zero to some positive peak value, then back to zero and continues to a negative peak value and then back to zero volts. In the US, it does this 60 times each second (referred to as 60 Hz). If you graph the voltages over time, they would look like the following image. The length of time the following sine wave shows is 1/60 of a second or 1 cycle.

DC to AC Current

 What an Inverter actually does, is take a flat DC voltage as an input and using electronic components, builds a sine wave at the output. As most who use an Inverter know, the flat DC voltage input comes from a battery, which is usually 12 volts. The output, which is AC, is generally provided by way of one or more AC outlets, similar to what you use in your home or office.

As mentioned above, Inverters are divided into 2 main types. They are Modified Sine Wave Inverters and Pure Sine Wave Inverters. As might be expected, the difference is in the shape (or purity) of the output Wave form, with the Pure Sine wave Inverter outputting a smother waveform, that looks similar to the waveform your electric company provides in your home or office.

There are 3 basic waveforms that are associated with Inverter outputs. They are referred to as, Square Wave, Modified Sine Wave and Pure Sine Wave. As you might expect the Square wave, a type of modified Sine Wave, looks like a square when graphed over time. The voltage is switched from plus to minus, which creates the square wave. Square wave Inverters are not very efficient and create harmonics which can causes problems with some equipment, but are easier and less expensive to manufacture.

The following picture shows a representative of a Modified Sine wave on the left and a Pure Sine Wave on the right. Pay Particular attention to the zero voltage points where the Voltage waveform switches from positive to negative and from negative to positive.

Modified & Pure Sine Wave

Apart from the obvious voltage waveform differences, the way the voltage waveform switches from plus to minus, and minus to plus is important. Notice, that the Modified wave lingers at zero volts before switching from Positive to Negative and vice versa, whereas the Pure Sine wave makes a very smooth transition through zero volts. Remember, that these waveforms represent voltage over time with Voltage being on the Vertical and time being on the Horizontal.

Some appliances and equipment, especially those, that use the zero voltage crossing for timing can have difficulty in operation. For example, clocks and variable speed drills. Equipment utilizing two electronic components in particular, SCRs (Silicon Rectifiers) and Thyristors, have difficulty with this and I suspect some stereos and Microwaves and others with poor filtering will get buzzing noises from the harmonics.

By now, you have probably guessed that choosing between a Modified and a Pure sine wave inverter is one of the decisions you will need to make.

Part 2. Choosing a Pure or Modified Inverter

Thor 1500 Watt Pure (left) and Modified (right) Sine Wave Power Inverter

Thor 1500 Watt Pure (left) and Modified (right) Sine Wave Power Inverter

If your budget allows, the Pure is the way to go. It can run just about all appliances and equipment, including variable speed tools and medical equipment like sleep apnea machines and oxygen concentrators. Less heat will be generated so your equipment will have a longer life-cycle. You will need to use a Pure sine wave Inverter for a grid tie system and they are generally needed for newer LED TVs, CFL lighting, rechargeable batteries, brushless motors and some other inductive devices like key machines. You may also need the Pure sine wave for some clocks, microwaves and audio equipment.

RoadTrucker recommends the Pure Sine Wave Inverter and Inverter kits for professional work trucks, since the total cost of ownership is more cost effective when you consider, cost of Inverter and wiring kit, installation cost and prolonged tool life. Those who will use their Inverter on a regular basis should also consider a Pure Sine wave Inverter.

For those on a tighter budget, or for those who do not want to spend as much money, a Modified Sine Wave Invert just may be all you need. Many Truckers and RVers use a modified sine wave inverter and are very happy with them. They will run most basic Appliances and Equipment, but you should check with your appliance or equipment manufacturer to see if they recommend, that you do not use a modified Inverter, before investing in them. Some of the things to watch out for are the more sophisticated electronics, and be extremely careful with medical devices, as a modified Inverter may cause damage.

Some other equipment, that may not work well with a modified Inverter are laser printers, photocopiers, some laptops, digital clocks with radios, motors with brushes, cordless tool battery chargers, sewing machines, key cutting machines, etc. In addition, you may hear buzzing from audio equipment, ceiling fans and microwaves. Equipment like motors will run hotter and fatigue over time faster then normal.

Once your choice is made for the Inverter type, your next decision is on the Inverter size.

Click here for Part 3, Introduction to Inverter Sizing.