DSM Pocket Guidebook Volume 5: Renewable and Related ... - NREL

output waveform. The output of a square wove inverter consists of the
sinusoidal (fundamental) component and the nonsinusoidol component
(which is called the ripple or higher harmonics component).
For on AC electric motor, on the fundamental component is useful to
produce the torque. The higher harmonics will on produce the torque
pulsation (which translates into vibration and noise) and resistive losses
(which translate into higher temperatures that con lead to insulation
failure and then to a short circuit).
MODIFIED SQUARE WAVE INVERTER Another method of turning
the switch on and off is by modulating the switching pattern in a
different way. The modified square wove is one way to reduce the
harmonics.
PULSE WIDTH MODUlATED (PWM) WAVE INVERTER PWM
inverters ore the most expensive, but produce a high quality of output
signal at minimum current harmonics. The switching pattern for this
inverter normally follows what is called pulse width modulationwhere
the switching pattern follows the crossing point of a triangular
waveform (at a carrier frequency) and a sine waveform at a fundamental
frequency. (The carrier frequency is normal much higher than
the fundamental frequency.) By doing so, the higher harmonics voltage
occurs at much higher than the fundamental frequency. As a result, the
higher harmonics output current will be much smaller than the square
wove inverter and the modified square wave. In general, a series
inductor and capacitor filter is also included in the system to filter out
the higher harmonics; thus, the output voltage after the filter will be
very close to sinusoidal.
• DEFINITIONS AND TERMS
TOTAL HARMONIC DISTORTION The measure of closeness in
shape between a waveform and its fundomenjal component.
POWER FAaOR The ratio of the average power and the apparent
volt-amperes.
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