How VVVF Drive works?

Answer 1

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)

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.

Tags: vsd frequency converter vfd ac drive variable speed drive variable frequency drive Written by :Ben | Category :AC Drive | Comments :0 | Trackback :0 | Read :305