There are 2 ways to adjust the speed of decanter centrifuge:
1 Use 3 extra pulleys, when you want to use different speed, you can change the pulley accordingly. for example, use middle speed to recycle the barite, use high speed to separate fine particles. By this way, you save the cost of VFD control panel, but it takes a long time to replace the pulleys. 
2 Use VFD control panel, it is the common way we used now. The variable speed drive control panel can be made with PLC control, with IEC ex or ATEX or UL certificate for explosion proof. When you start the centrifuge, you can adjust a lower frequency to protect the motors, after it works smoothly, you can adjust the frequency to your required speed. Even though it costs more compared with the 1st way, but it is much more convenient for operation.
The photo above is GN designed new VFD control panel for VFD decanter centrifuge, it is with small footprint and safe, easy operation.