Dec 132013

Very often when choosing an inverter we ask ourselves the question whether to buy a pure sine-wave inverter or choose a modified (quasi) sine-wave one, which is much cheaper. Here we have already explained how to select the power of the inverter, depending on the nature of the load. Now we will explain in details when it is necessary to use a pure sine-wave inverter.

1. What is a pure sine-wave inverter?



The output voltage of this type of inverter has the form of the sine-wave of the alternating voltage into the public electric network. At sinusoidal voltage change, it rises and falls gradually (the phase angle smoothly changing) and also instantly changes its polarity when the sine-wave crosses 0 volts.

2. What is a modified sine-wave inverter?


With this type of inverters the voltage rises and falls sharply (the phase angle changes abruptly) and it remains at 0 volts for some time before it changes its polarity.

3. What are the benefits of pure sine-wave inverters?
The output waveform is a sine-wave with very low harmonic distortion and effective transmission of electric power as in the public electric network.
Inductive loads such as microwave ovens and motors run faster, quieter and more efficiently and don’t heat up much while operating.
The acoustic and electrical noise of fans, fluorescent lights, audio amplifiers, TV sets, fax machines and answering machines is reduced.
Computer crashes, quirky prints from printers and irregular images on monitors are prevented.

4. Which devices will not function properly with a modified sine-wave inverter?
Any device that uses a control mechanism that reads the phase voltage (voltage/speed control) or the current zero crossing of the voltage (for time control) will not work properly with a modified sine-wave inverter. Also, since the modified sine-wave is a form of a square wave, it comprises of a plurality of sine-waves of odd harmonics (multiples) of the basic frequency of the sine-wave. For example, for a 60 Hz basic frequency the modified sine-wave will consist of waves with odd harmonic frequencies of the third (180 Hz), fifth (300 Hz.), seventh (420 Hz.) and so on harmonics.

The high-frequency content of harmonics in the modified sine-wave leads to increased radio interference, stronger heating effect in electric motors / microwaves and overload due to the reduced resistance of filter capacitors, capacitors for improving the power factor.

Some examples of devices which cannot operate correctly with a modified sine wave and also may be damaged, are given below:
– Laser printers, copiers, magneto-optical discs;
– Embedded clocks in devices such as radio-clocks, alarm clocks, coffee makers, bakeries, microwaves, etc. may not read or save time correctly;
– Devices for controlling output voltage such as dimmers, regulators of fans and speed of electric motors may not work properly or not work at all;
– Sewing machines with speed control and / or microprocessor control;
– Devices without a transformer with a capacitive power input (switching power supplies) such as:

(I) razors, flashlights, reading lamps, smoke detectors, etc.
(II) chargers for rechargeable batteries, hand electric tools can be damaged.
– Devices that use radio frequency signals carried in the electrical network;
– Some new furnaces with microprocessor control;
– HID lamps such as metal halide lamps can be damaged;
– Some fluorescent lamps / lighting fixtures that have capacitors correcting the power factor.
Moreover, the inverter can go off and show overloading.

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