General purpose AC drive controllers are manufactured adjustable frequency in three types: Variable Input Voltage (VVI), Power Source Input (CSI) and Pulse Width Modulation (PWM). Each has advantages specific characteristics.

Variable Input Voltage (VVI) .-

Although this design was common in the 70s and early 80s, is today limited to special applications such as high-speed drives that develop (400 to 3 000 Hz).

VVI design receives AC voltage of the plant, rectified and controls, developing a variable DC voltage to the power amplifier (inverter stage). The power amplifier inverts the DC variable voltage variable frequency and variable voltage AC. This can be performed by power transistors or SCRs.

The voltage output from a VVI unit is often called "six-pulse wave." The VVI was one of the first solid-state AC drive that was generally accepted. See Figure 3-5.

Current Source Inverter (CSI) .-

It is used in inverters with powers greater than 50HP. CSI units are well placed to handle pumps and fans as an alternative to saving energy for flow control.

Capable of working with efficiencies close to DC drives, the CSI design offers economy of VVI and PWM units for applications in pumps, fans and the like. The CSI provides capacity for regeneration. With an overload, the controller feeds back energy to the AC system. See Figure 3-5.

Pulse Width Modulation (PWM) .-

Many PWM units (often called "drives V / Hz") provide operation at zero speed. Some provide frequency range close to 200:1. This wide range is possible because the driver converts AC input voltage to a fixed DC voltage through the power rectifier.

After this amplifier, the DC voltage is modulated by means of an inverter to produce pulses of various widths, to vary the effective voltage. Although the voltage is modulated, the waveform of the current is close to a sine wave, much better than any other system. PWM drives using IGBT power transistors.

Noting the current waveforms of Figure 3-5, we conclude that the PWM drive is one that provides better quality of AC power to the motor, making work with better efficiency and producing a finer control of torque. They are therefore the most currently used

The voltage waveform produced by the PWM drive is called "Sine PWM" and is the work at high speed (reaching 20 kHz) of the IGBT transistors, which are controlled by means of a sophisticated micro computerized control circuit.