Unbalance (Negative Sequence Current) Biasing Negative sequence current, which causes rotor heating, is not accounted for in the thermal limit curves supplied by the motor manufacturer. The 469 measures unbalance as the ratio of negative to positive sequence current. The thermal model is biased to reflect the additional heating. Motor derating due to current unbalance can be selected via the setpoint unbalance bias k factor. Unbalance voltage causes approximately 6 times higher level of current unbalance (1% of voltage unbalance equal to 6% of current unbalance). Note that the k=8 curve is almost identical to the NEMA derating curve.
Hot/Cold Safe Stall Ratio The Hot/Cold Safe Stall time ratio defines the steady state level of thermal capacity used (TCU) by the motor. This level corresponds to normal operating temperature of the fully loaded motor and will be adjusted proportionally if motor load is lower then rated. The Hot/Cold Safe Stall ratio is used by the relay to determine the lower limit of the running cool down curve, and also defines the thermal capacity level of the central point in RTD Biasing curve.