Impact torque conditions can result in broken chains, couplings and screw conveyors as well as an assortment of mechanical fractures. When a NewElec OLR 84 S protection relay is mounted on a motor circuit, impact torque conditions can be totally avoided. The relay has two settings: the first designates the total start time for the given motor in seconds through a range up to 50 seconds, while the second designates the load trip threshold. The latter setting is calibrated from 30 to 120% of the selected CT ratio XXX:1. The user must 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 3 x 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 permits the starting current for the motor start but will cause a trip in 100 ms if the motor load suddenly exceeds the upper trip threshold when the motor enters a stall condition after the starting sequence. In doing so, it interrupts the supply to the motor by means of the main holding contactor and prevents mechanical fractures.
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