How Does a VSD (frequency converter, VFD) Work (part 1)?
Induction motors, which are the workhorses of industry, rotate at a speed that is determined by the frequency of the supply voltage. Alternating current (AC) applied to the stator windings produces a magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line frequency by the number of magnetic pole pairs in the motor winding.
A four-pole motor, for example, has two pole pairs, and therefore the magnetic field will rotate 60 Hz / 2 = 30 revolutions per second, or 1800 rpm. The rotor of an induction motor will attempt to follow this rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field. This small slip speed generates an induced current, and the resulting magnetic field in the rotor produces torque.
Since an induction motor rotates near synchronous speed, the most effective and energy-efficient method to change the motor speed is to change the frequency of the applied voltage. Variable frequency drives (frequency changers) convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.
A VSD (frequency converter, variable speed drive, variable frequency drive, VFD) converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process of frequency changers incorporates three functions:
1. Rectifier stage:
A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher supply voltages are used. (to be continued)
