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
account.
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
.
Protection functions
1MRS759142 F
430
REX640
Technical Manual