Does raising the switching frequency, reduce the harmonic content in output current of inverter. If yes ,how?
inverter's output is not a sinusoidal wave. from fundamental switching frequency to higher switching frequency - outputs are having less harmonics. Because the wave was sliced with a strategy (PWM), these sliced wave having zero magnitude at particular periods. all these particular periods helps to reduce harmonic content in the wave, so the harmonic content was reduced from lower to higher switching frequencies.
the reduction of harmonic content is depends on switching frequency & logic to generate pulses, i.e., only the higher (MHz) frequencies are not helps to reduce the harmonics, but also lower switching frequencies with a different logic & different number in switching frequency gets better performance.
By increasing the swwitching frequency, the harmonic in current can be reduced, but the power loss of the switches will increase. In comercial two level inverter, the allowable switching Freq is normally from 3kHz to 10kHz. By using multilevel inverter, the harmonic can be reduced significantly with even with much lower switching Freq.
Generally we kept the switching frequency 5 KHz i.e. sampling time is 0.2 mSec, this gives the best result for two level inverter...I have done all my simulation for two level inverter with this switching frequency.
Often, the switching frequency of about 20 KHz is used. This puts it above the human hearing range, and keeps distortion down even on relatively high frequency outputs like 400 Hz. Of course, a small, LC output filter is needed.
The output current of inverter has to flow through a series inductance which is part of its output filter. The Reactance of this inductor (L Henry) is given by 2*pi*(n*f)*L, where f is the fundamental frequency and n is the order of harmonic. Higher switching frequency pushes the dominant harmonic order to higher value, hence the Reactance value for same inductor is higher for the dominant harmonic, reducing dominant harmonic current flow. Hence harmonic content in output current reduces.
Although one would like to switch the devices at the highest possible switching frequency, the practical switching frequency limit depends on the type of device used (causing heat generation) and the nature of cooling, due to temperature rise limits. MOSFETs can switch comfortably upto 70kHz with natural air cooling, and 125kHz under forced air cooling. IGBTs will switch upto 10kHz under natural air cooling and upto 15kHz under forced air cooling. Note that the above values are approximate.
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