Operator Interfaces, Switches & Relays


Eliminate impact torque conditions

April 2014 Operator Interfaces, Switches & Relays

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.

For more information contact Luc Dutrieux, NewElec, +27 (0)12 327 1729, sales@newelec.co.za, www.newelec.co.za


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