When the need is allowing a progressive rotation speed change in any equipment, there is two options to get there: Mechanical Speed Variators or Electronic Frequency Variators.
Mechanical Speed Variators can operate by a Variable Chain or friction system. The Variators from ELVEM and TRAMEC work by a friction satellite system. VARMEC Variators work by a Friction Wheel.
They are reliable systems, easy to operate and allowing tramission ratios between 1:1,5 and 1:7,5 (in addition to the other selected transmission ratio).