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Soft starters for electric motors.

The 'Electronic Shear Pin' facility integrated into Fairford's QFE series of soft starters eliminates the need for a mechanical shear pin entirely, according to the manufacturer. The speed and extent of a sudden and rapid rise in motor torque is immediately detected by the QFE which will then decide on a course of actions ranging from instantaneous shutdown to monitoring for recurrences if the blockage is released rapidly.

Soft starters provide an electronic solution to mechanical problems at relatively low cost. They protect and extend the life of belts, chains, gearboxes, shafts, bearings and machine mountings. In addition, soft starters can achieve real savings in energy for users by providing the facility to switch motors off, without fears for the torque stresses they will cause upon restarting. Added to these factors, the traditional fit-and-forget reliability of soft starters ensures security of operation even in the most critical of tasks.

The traditional method of achieving this safe guarding was via a mechanical 'shear pin' that consisted of a pin of a deliberately weak material inserted into two concentric shafts at a convenient point in the drive train to the load. If the load became jammed, the sudden rise in torque would cause the pin to shear so that the two shafts could then rotate independently, thereby disconnecting the motor from its load. Before the load could be re-started, the old pin would need to be removed and a new one inserted.

Statistics show that over 90% of the motors in UK industrial have no form of control, other than simple electro-mechanical switching. As a result, the sudden impact at start-up of these motors, followed by the rapid acceleration to full speed, causes problems across a wide range of equipment types. Sudden torque stresses cause excessive wear on belts, pulleys, gears, chains, couplings and bearings, and also cavitation in pumps, which reduces their efficiency and life. In conveyor systems, too, loads may be displaced or damaged upon start-up, and products may become contaminated.


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Publication:Plant & Works Engineering
Date:Aug 1, 2006
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