June 2012 Electrical Apparatus

June 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2012 featured technical article,  by Richard L. Nailen, P.E.   

Safe acceleration of an a-c motor drive depends upon the combined inertia of motor and connected load. That value will have a specific effect on motor heating. The required load torque during acceleration also influences motor heating but is often imprecisely defined. Data from the manufacturer of the driven machine may be inaccurate or unavailable. Furthermore, the speed and torque of the load at its intended operating point may not match the motor speed-torque characteristic, so that the quoted motor efficiency doesn’t apply.

For the most common loads (pumps and fans), “typical” speed-torque curves vary widely. No standard motor design suits them all. Motors are therefore often misapplied.

Motor selection can become still more complex when loads do not exhibit a single speed-torque curve, or for which torque at full speed varies throughout each revolution. One example is the large ball mill used in minerals processing. Rotating at 15 to 30 RPM, the mill drum contains a mass or “charge” of material tending to become compacted in the bottom of the drum at standstill. Motor speed is typically 400 to 514 RPM. As mill rotation commences, the charge is lifted through an angle, requiring high motor torque. As that angle approaches 90 degrees, the charge begins to tumble or “cascade,” reducing the torque. That “cascade point” occurs when the motor reaches 20% to 40% of its full speed value. Determining the required motor accelerating torque and evaluating motor heating requires a complex calculation involving the relationship of mill torque to angular position.

A second type of load exhibiting a torque vs. angle variation is the reciprocating compressor. As the pistons move throughout compression cycles during each crankshaft revolution, the required motor torque pulsates, as does the associated motor current, causing rapid variation in system voltage that may create objectionable light flicker. Increased drive inertia can dampen the oscillations. The need for added flywheel effect must be determined by calculation involving harmonic analysis of the torque-angle curve provided by the compressor manufacturer.

Even a conventional centrifugal pump may exhibit torque changes during acceleration, when equipped with an automatic discharge valve that begins to open before full speed is reached. The resulting increase in torque may require special motor design.

To order a back issue with the full article, “Understanding the load speed-torque curve” call 312-321-9440 or visit our online webstore.

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