The VFD input rectifier is the first of the three main stages on a VFD. The other two being the DC bus and the inverter. The rectifier stage converts AC line voltage supplied to the drive to DC.

The rectifier is usually a silicon controller rectifier ( SCR) or a diode. The difference between the two being that the SCR types can increase switching gradually thus increasing the voltage applied to charge the DC bus (second stage of the three stages of an AC drive). The diode types rely on a pre-charge circuit to perform the gradual voltage ramp up of the DC bus capacitors.

For drive rectifier stages, most commonly, there is a 6 diode or  6 SCR arrangement that makes up what is called a 6-pulse rectifier. A good illustration of this process from a TranspowerNZ video on YouTube

There are 12 pulse and 18 pulse diode arrangements that can be used to make up the rectifier stage of a drive. The main purpose of the 12 diode and 18 diode arrangements are to achieve lower harmonic distortion on the line side. The 12 pulse would be made up of 2 sets of 6 pulse rectifiers supplying the DC bus in parallel and the 18 pulse with 3 sets of 6 pulse rectifiers. Understandably, the higher order arrangements take up more space and cost more.

Considering that the main purpose of these arrangements is to reduce harmonic distortion, there are other options besides 12 or 18 –pulse rectifiers that can be considered for the same or better results. These options are to include a filter on the line side of the drive or by adding an active front end (AFE). MTE (maker of filter and power quality equipment) has a good paper comparing 18-pulse VFD’s with a 6-pulse VFD and their Matrix filter. The essence of it is that it costs less, takes up less space and consumes less energy (specifically for the 100hp test case).