ADJUSTABLE SPEED DRIVES

By definition, adjustable speed drives of any type provide a means of variably changing speed to better match operating requirements. Such drives are available in mechanical, fluid and electrical types.

The most common mechanical versions use combinations of belts and sheaves, or chains and sprockets, to adjust speed in set, selectable ratios – 2:1, 4:1, 8:1 and so forth. Traction drives, a more sophisticated mechanical control scheme, allow incremental speed adjustments. Here, output speed is varied by changing the contact points between metallic disks, or between balls and cones.

Adjustable speed fluid drives provide smooth, step less adjustable speed control. There are three major types. Hydrostatic drives use electric motors or internal combustion engines as prime movers in combination with hydraulic pumps, which in turn drive hydraulic motors. Hydrokinetic and hydro viscous drives directly couple input and output shafts.

Hydrokinetic versions adjust speed by varying the amount of fluid in a vortex that serves as the input-to-output coupler. Hydro viscous drives, also called oil shear drives, adjust speed by controlling oil-film thickness, and therefore slippage, between rotating metallic disks.

An eddy current drive, while technically an electrical drive, nevertheless functions much like a hydrokinetic or hydro viscous fluid drive in that it serves as a coupler between a prime mover and driven load. In an eddy current drive, the coupling consists of a primary magnetic field and secondary fields created by induced eddy currents. The amount of magnetic slippage allowed among the fields controls the driving speed.

In most industrial applications, mechanical, fluid or eddy current drives are paired with constant-speed electric motors. On the other hand, solid state electrical drives (also termed electronic drives), create adjustable speed motors, allowing speeds from zero RPM to beyond the motor’s base speed.

Controlling the speed of the motor has several benefits, including increased energy efficiency by eliminating energy losses in mechanical speed changing devices. In addition, by reducing, or often eliminating, the need for wear-prone mechanical components, electrical drives foster increased overall system reliability, as well as lower maintenance costs.

For these and other reasons, electrical drives are the fastest growing type of adjustable speed drive.

There are two basic drive types related to the type of motor controlled DC and AC. A DC direct current drive controls the speed of a DC motor by varying the armature voltage (and sometimes also the field voltage). An alternating current drive controls the speed of an AC motor by varying the frequency and voltage supplied to the motor.