|
 Summary:
Crusher duty - What's it all about? With
trilingual summary
Published standards define the motor
and control design and construction features required for certain
applications. A "severe duty motor," for example, is increasingly
well-understood as one complying with IEEE Standard 841.
In mining and quarrying industries,
however, no standard defines the characteristics of "crusher duty"
motors and controls. Such apparatus is being marketed by several
suppliers, offering various individual design approaches. Among the
differences:
-
Cast iron (resistant to deformation,
but brittle) vs. steel (resistant to breakage, but more easily
deformed) for enclosure components.
-
Ventilating fans -- fabricated steel
vs. aluminum or plastic.
-
Shaft material: low carbon steel (less
sensitive to propagation of surface cracks) vs. higher-strength
alloys.
-
Shaft keyway configurations affecting
fatigue stress.
-
Bearing bracket mounting bolt
variations.
-
Bearing mounting and sealing to suit
specific crusher configurations.
Squirrel cage rotor design also varies
widely. For high accelerating torque, double cage construction is a
common option for normal service. For crusher drives, imposing high
torsional shock loading and sometimes severe starting duty, the
double cage is physically weak. A single, high resistance, unusually
deep bar design is mechanically stronger but more highly stressed
thermally during acceleration. Whatever the design, particular
attention must be paid to tightness of rotor bars in the slots.
In some crusher duty machines, end
rings are slotted to fully contain the bar ends, and are clamped
against the rotor core to eliminate bar bending stress. But this
practice is not universal.
Although "crusher duty" starters are
also not standardized, the commonly encountered features include:
-
Circuit breaker and grounding terminals
allowing the starter to meet the National Electrical Code definition
of "service equipment" for direct connection to the incoming line.
-
Solid-state reduced voltage starting.
-
A bypass contactor allowing the motor
to be started across-the-line where possible, and where reduced
voltage does not produce adequate accelerating torque.
-
Starter semiconductor capability of
withstanding severe overload as well as limited cooling within a
dust-tight enclosure.
-
Extra voltage boost for breakaway
torque.
Applications for such apparatus include
cone, jaw, and gyratory crushers; granulators; hammermills;
pulverizers; and impactors. Simply requesting "crusher duty" does
not ensure motor or control suitability to all such loads
From
"Crusher Duty - What's It All About?" ...by
Richard L. Nailen, EA Engineering Editor -
published in Electrical Apparatus March 2004
© 2004
Barks Publications, Inc. All Rights Reserved.
|
