Operator Interfaces, Switches & Relays


Eliminating downtime due to impact torque conditions

January 2015 Operator Interfaces, Switches & Relays

Preventing mechanical damage to chains, couplings, screw conveyors and a variety of other mechanical systems eliminates costly downtime. Impact torque occurs when an electric motor/gearbox combination attempts to deliver the necessary power and torque to overcome sudden jamming conditions.

By fitting a NewElec OLR 84 protection relay to the motor protection circuit these problems can potentially be avoided. The relay has two settings: the first designates the total start time for the given motor in seconds through a range of up to 50 seconds; the second designates the load trip threshold. The latter setting is percentage calibrated from 30-120% of the selected CT ratio.

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 use 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 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 where the motor enters a stall condition after the starting sequence. OLR 84 will interrupt the supply to the motor by means of the main holding contactor and prevent mechanical fractures in the production process.



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