AC Drives and Soft Starter (3)

Variable Speed Drive Operation
The ac line voltage, figure 3, is rectified with a passive diode bridge. This means that the diode(s) conduct whenever the line voltage is greater than the voltage on the capacitor section. The resulting current waveform has two pulses during each half-cycle, one for each diode conduction
The waveform, figure 4, shows some continuous current when the conduction transitions from one diode to the next. This is typical when a reactor is used in the dc link of the drive and some load is present. Inverters use pulse width modulation to create the output waveforms. A triangle waveform is generated at the carrier frequency where the inverter IGBT’s will switch.
This waveform is compared with a sinusoidal waveform at the fundamental frequency that is to be delivered to the motor. The result is the voltage waveform shown in figure 5.
Figure 6 shows the resulting current waveform at the motor with a PWM signal applied.
The inverter output can be any frequency below or above the line frequency up to the limits of the inverter and/or the mechanical limits of the motor. Note that the drive is always operating within the motor slip rating


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