PWM Adjustable Frequency Drives

Figure 1 illustrates the basic principles of a PWM drive. The rectifier converts input line power, which has a nominally fixed voltage and frequency, to fixed voltage DC power. The fixed voltage
DC power is then filtered to reduce the ripple voltage resulting from the rectification of the AC line. The inverter then changes the fixed voltage DC power to AC output power, with adjustable voltage and frequency.

The output waveform consists of a series of rectangular pulses with a fixed height and adjustable width. The overall pattern of positive vs. negative pulses is adjusted to control the output frequency. The width of the individual pulses is modulated so that the effective voltage of the fundamental frequency is regulated in proportion to the fundamental frequency.

One (1) cycle of the output waveform at a given output voltage can be made from many narrow pulses or fewer wider pulses. To generate a waveform containing more pulses, the transistors or other switching devices in the inverter must switch more often. The rate at which the switches operate is called the switching frequency or carrier frequency.


Figure 1 Principles of Operation for PWM Adjustable Frequency Drives

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