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Crusher duty - What's it all about?

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:

1. Cast iron (resistant to deformation, but brittle) vs. steel (resistant to breakage, but more easily deformed) for enclosure components.

2. Ventilating fans -- fabricated steel vs. aluminum or plastic.

3. Shaft material: low carbon steel (less sensitive to propagation of surface cracks) vs. higher-strength alloys.

4. Shaft keyway configurations affecting fatigue stress.

5. Bearing bracket mounting bolt variations.

6. 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:

1. 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.

2. Solid-state reduced voltage starting.

3. A bypass contactor allowing the motor to be started across-the-line where possible, and where reduced voltage does not produce adequate accelerating torque.

4. Starter semiconductor capability of withstanding severe overload as well as limited cooling within a dust-tight enclosure.

5. 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

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