General-purpose LV motor protection relays

November 2001 Motion Control & Drives

NewElec's locally designed and manufactured KA range of relays offer economical solutions for general-purpose LV motor protection. The KA relays are compact in size providing protection against overload, unbalance/single phasing, phase rotation, phase symmetry, overvoltage, undervoltage and minimum loads. Their over/under voltage and phase rotation features make the KA relays particularly suited for low voltage pump motor protection.

The integral current transformers allow for direct connection into the main circuit of the protected motor for currents up to 25 A. Above 25 A, external current transformers are used. The overcurrent level dial setting covers 10% to 100% of the specified current transformer.

Both cyclic and sustained overloads are protected by pre-loading with thermal memory utilising accurate hot and cold thermal curve characteristics to IEC 255-8. In the event of an overload trip, the entire thermal capacity of the motor has been activated. If auto reset has been selected, the motor overload LED will flash until the thermal capacity has integrated to 33%.

Phase unbalance protection is provided independent of motor load. In the event of the three line currents becoming unbalanced by more than 35%, a 5 s trip delay is initiated. Incorrect phase rotation will result in the corresponding LED switching on, while the 'relay healthy' LED will switch off and de-energise the main trip relay. In the event of the phase voltages becoming unbalanced by more than 10%, a 10 s trip delay is initiated, after which, the phase rotation LED is pulsed at 1s intervals. The 'relay healthy' LED will then switch off and trip the main relay.

If the main circuit voltage becomes greater than 120%, or less than 80% rated supply, a 10 s trip delay is initiated. During a minimum load condition, the main trip relay de-energises after 10 s, and if during this period the load current increases above minimum load % MLC dial setting, and drops below this value once more, a new 10 s trip delay period will occur.

NewElec

(012) 327 1729

[email protected]

www.newelec.co.za

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