Costly downtime and repairs caused by impact torques can now be avoided. Impact torque occurs when an electric motor/gearbox combination attempts to deliver the necessary power to overcome a condition of sudden jamming. This can result in broken chains, couplings, screw conveyors and an assortment of mechanical fractures in a variety of industrial applications.
These can be avoided by fitting a NewElec OLR 84 S protection relay to the motor circuit. The relay has two settings. The first designates the total start time for the given motor through a range up to 50 seconds. The other designates the load trip threshold. The latter setting is percentage calibrated from 30 to 120% of the selected CT ratio XXX:1. It is necessary for the user to select an appropriate ring CT that will cover the expected trip threshold current. A general rule is that if the motor load maximum current is x A, the user must select a current transformer where 3x A is about mid scale. The reason for this is that a stall or jam condition usually manifests itself at about three times motor full load when the motor develops an output torque in the region of 2,5 to 3 times its normal rated torque. This output torque will then be amplified through a factor dependent on the gearbox ratio.
The OLR 84 S is set to permit the starting current, but will cause a trip in 100 ms if the motor load suddenly exceeds the upper trip threshold should the motor stall anytime after the start sequence. It will interrupt supply to the motor by means of the main contactor, thus preventing excess torque and eliminating mechanical fractures.
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