Modified Sine Wave Inverters vs Pure Sine Wave Inverters


Courtesy of Larson Electronics LLC

Most everyday appliances and commercial tools run on AC power. This is fine if you’re connecting the device to a wall outlet that provides AC power. But what if you want to use power from a DC source, like a solar-powered generator or a car battery?

In such cases, individuals need to use an inverter – a device that converts DC power to AC power. There are several types of inverters available on the market today, which provides generally similar results, while using different processes. The three kinds of inverters currently in circulation are: pure (or true) sine wave inverters, modified (or quasi) sine-wave inverters and square wave inverters. Out of the three, square wave inverters are considered to be the least efficient, while pure sine wave inverters are considered to be the most reliable due to the production of a cleaner, more superior electrical current (note: also the most expensive option).

Pure Sine Wave Inverters

Pure sine wave inverters offer smooth functionality and compatibility with virtually all electronics, because it generates a perfect sine wave (or a “multiple step sinusoidal AC waveform”) that is very similar to what one would get from a generator or electricity supplied from a utility company. During operation, the voltage “rises and falls” fluidly with a changing phase angle. Moreover, it shifts polarities quickly when crossing zero volts. This results in clean electrical output that does not produce any harmonic distortion in the frequency, while generating very little heat.

The power inverter’s ability to stimulate AC power from mainstream sources, such as wall outlets, in an accurate manner is key to increased adoption by electronics manufacturers – despite its more expensive price point (due to extra circuitry components). LED televisions, CFL bulbs, as well as grid-tie systems work efficiently well with pure sine wave inverters.

Modified Sine-Wave Inverters

Modified sine-wave inverters are designed to mimic pure sine waves during conversion from DC to AC power. However, it can only accomplish this up to a certain point, making it less efficient than true sine wave inverters. A suitable comparison would be a car with square wheels (modified sine-wave inverter) versus a car with round wheels (pure sine wave inverter). The wave (or “non-sinusoidal AC wave”) starts of as a flat positive voltage and drops suddenly to zero (almost straight down, not like a smooth wave). It sits at the zero crossing for a short amount of time before falling to a negative voltage, plateauing and rising back up to zero.

Visually, the wave resembles abrupt (or stepped) squared edges, as opposed smooth waves. In multi-step, modified sine-wave inverters, the steepness of the wave is lessened to closely resemble a rounded, true sine wave. While effective, such solutions still create blocky signals. Since there is more area under a square, compared to a smooth curve, the inverter is able to deliver more power to the unit. This is not necessarily desirable and at high, odd frequencies such properties can lead to radio interference.

Due the “forced” DC-to-AC conversion process, such inverters generate higher harmonic distortion and heat during operation. Additionally, electronics with this type of power inverter will consume up to 30 percent more energy. Old electronic devices and motors with brushes are examples of equipment that will work with modified sine-wave inverters. On the other hand, X-10 home automation platforms, equipment with silicon-controlled rectifiers (SCRs), lighting systems with electronic ballasts and some battery chargers are not suitable with this type of inverter.

Uses and Applications

The application of specific inverters highly depends on the internal control circuitry of the device. Generally, most units that rely on a control circuitry that detects the phase or instantaneous zero voltage crossing will not operate effectively when using a voltage with a modified sine wave form. Furthermore, modified sine-wave inverters are prone to the production of radio interference and excess heat due to high frequency harmonic content (since technically, a modified sine-wave is a form of a square wave with varying, odd harmonics).

With this in mind, the following types of electronics are least compatible with modified sine-wave inverters: photo copy machines, dimmers (output voltage control units), HID or metal halide lights, laser printers, devices with built-in clocks (such as radios, coffee makers and alarms), some fluorescent lamps and luminaries with capacitators that have power factor correction components (in such cases, the modified sine-wave inverter may fail or shut down with an indication of overload).

Electronics that do not function properly with modified sine-wave inverters may utilize pure sine wave inverters. As mentioned earlier, since the latter (pure inverter) is capable of decreasing both electrical and audible noise in devices, they are preferred for use in fans, audio equipment and iron ballasted fluorescent fixtures. Moreover, since pure sine wave inverters are considered to be the most reliable type of inverter (out of the three), they perform exceptionally well inside sensitive electronics, such as laptops and medical equipment. The units can even prevent computing machines from crashing or sudden failure, reduce noise levels in monitors and reduce print out errors in cutting-edge printers. Going back to modified sine-wave inverters, the devices can also work with computers (issues arise with some laptops) and monitors (but may cause interference in the form of distracting lines or “humming”).

What about Square Wave Inverters?

Square wave inverters are considered to be the cheapest option from the group. During operation, this unit is capable of withstanding heavy currents, while producing a square wave with low quality power. It is made up of four switches, a DC power source and the load. When creating a square waveform, the inverter flips the voltage (from plus to minus). Their main drawback is the presence of large amounts of power in high harmonics. This feature makes it difficult to use with commercial appliances. Such devices are used to operate electronics with universal motors, lights with low sensitivity and heating appliances. Like modified sine wave inverters, they should not be used with sensitive electronics and audio equipment, which can result in “humming,” a decrease in lifespan or premature failure.

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