ABB RELION REX640 Technical Manual page 430

Protection and control

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Protection functions

which cause the heating to be rapid. This prevents premature degeneration of the

rotor unit. Additionally, the function provides an estimate of the motor slip, which

allows identify locked rotor. This function is applicable for direct-on-line started

induction motors with the motor line voltages available.

The rotor thermal overload protection function should be used in parallel with the

motor thermal overload protection function MPTTR. This way both, the more rapid

heating of the rotor, and the stable heating of the entire motor are taken into

Abnormal conditions that can result in overheating the rotor include overload,

stalling, failure to start, high ambient temperature, restricted motor ventilation,

reduced speed operation, frequent starting or jogging, high or low line voltage

or frequency, mechanical failure of the driven load, improper installation, and

unbalanced line voltage or single phasing. Protecting from insulation failure by

implementation of current sensing alone cannot detect all of these conditions, such

as restricted ventilation. Similarly, protection by sensing temperature alone may be

inadequate, in cases like frequent starting or jogging. Thermal overload protection

addresses these deficiencies to a large extent by deploying a rotor thermal model

based on load current and the load voltage.

Rotor thermal level is calculated based on the rotor resistance factors, True RMS

phase currents and negative sequence current. The rotor resistance factor is based

on the estimated slip, that is calculated based on the measured line voltage and

measured load current.

In addition, motor ambient temperature is taken into account in the thermal level

calculation. Apart from current and the rotor resistance factor, the rate at which

rotor heats up or cools is governed by time constant of the rotor.

Selecting of weighing factor

Motor manufacturers usually define two thermal limit curves one for warm and

one for cold starts. The demands that these curves set are answered with the

corresponding relay curves

1MRS759142 F

Technical Manual