Product Manual

CHAPTER X
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, stepless 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 hydroviscous drives directly couple input and output shafts.
Hydrokintetic versions adjust speed by varying the amount of fluid in a
vortex that serves as the input-to-output coupler. Hydroviscous 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 hydroviscous 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 mainte-
nance costs. For these and other reasons, electrical drives are the fastest
growing type of adjustable speed drive.
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