Variable Frequency Drives (4) – FINISH

4. Basic Operation of a PWM Inverter (VFD)

In this section we will discuss how the five basic drive system components work together. After this discussion we shall include a detailed, component level, discussion of operation.

The rectifier circuit of a pulse width modulated drive normally consists of a three phase diode bridge rectifier and capacitor filter. The rectifier converts the three phase AC voltage into DC voltage with a slight ripple. This ripple is removed by using a capacitor filter. (Note: The average
DC voltage is higher than the RMS value of incoming voltage by: AC (RMS) x 1.35 = VDC)
The control section of the AFD accepts external inputs which are used to determine the inverter output. The inputs are used in conjunction with the installed software package and a microprocessor. The control board sends signals to the driver circuit which is used to fire the inverter.
The driver circuit sends low-level signals to the base of the transistors to tell them when to turn on. The output signal is a series of pulses, in both the positive and negative direction, that vary in duration. However, the amplitude of the pulses are the same. The sign wave is created as the average voltage of each pulse, the duration of each set of pulses dictates the frequency.
By adjusting the frequency and voltage of the power entering the motor, the speed and torque may be controlled. The actual speed of the motor, as previously indicated, is determined as:

Ns = ((120 x f) / P) x (1 – S)

where: N = Motor speed; f = Frequency (Hz); P = Number of Poles; and S = Slip.


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