How Does a VSD (frequency converter, VFD) Work (part 2)?
Continued from the previous article: How Does a VSD (frequency converter, VFD) Work (part 1), we explain the other 2 stages of the conversion process of frequency changers below.
2. Inverter stage:
Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter.
3. Control system:
An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions.
Converting direct current (DC) to variable frequency alternating current (AC) is accomplished using an inverter. Most currently available inverters use pulse width modulation (PWM) because the output current waveform closely approximates a sine wave. Power semiconductors switch direct current (DC) voltage at high speed, producing a series of short-duration pulses of constant amplitude. Output voltage is varied by changing the width and polarity of the switched pulses. Output frequency is adjusted by changing the switching cycle time. The resulting current in an inductive motor simulates a sine wave of the desired output frequency. The high-speed switching of a pulse width modulation (PWM) inverter results in less waveform distortion and, therefore, lower harmonic losses.
The availability of low-cost, high-speed switching power transistors has made pulse width modulation (PWM) the dominant inverter type.
