4.1.9.5 Application
MPTTR is intended to limit the motor thermal level to predetermined values during
the abnormal motor operating conditions. This prevents a premature motor
insulation failure.
The abnormal conditions result in overheating and 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. The protection of insulation failure by the
implementation of current sensing cannot detect some of these conditions, such as
restricted ventilation. Similarly, the protection by sensing temperature alone can be
inadequate in cases like frequent starting or jogging. The thermal overload
protection addresses these deficiencies to a larger extent by deploying a motor
thermal model based on load current.
The thermal load is calculated using the true RMS phase value and negative
sequence value of the current. The heating up of the motor is determined by the
square value of the load current. However, while calculating the thermal level, the
rated current should be re-rated or de-rated depending on the value of the ambient
temperature. Apart from current, the rate at which motor heats up or cools is
governed by the time constant of the motor.
Setting the weighting factor
There are two thermal curves: one which characterizes the short-time loads and long-
time overloads and which is also used for tripping and another which is used for
monitoring the thermal condition of the motor. The value of the Weighting factor p
setting determines the ratio of the thermal increase of the two curves.
When the Weighting factor p setting is 100 percent, a pure single time constant
thermal unit is produced which is used for application with the cables. As presented
in
Figure 130, the hot curve with the value of Weighting factor p being 100 percent
only allows an operate time which is about 10 percent of that with no prior load.
For example, when the set time constant is 640 seconds, the operate time with the
prior load 1 x FLC (full Load Current) and overload factor 1.05 is only 2 seconds,
even if the motor could withstand at least 5 to 6 seconds. To allow the use of the
full capacity of the motor, a lower value of Weighting factor p should be used.
Normally, an approximate value of half of the thermal capacity is used when the
motor is running at full load. Thus by setting Weighting factor p to 50 percent, the
IED notifies a 45 to 50 percent thermal capacity use at full load.
For direct-on-line started motors with hot spot tendencies, the value of Weighting
factor p is typically set to 50 percent, which will properly distinguish between short-
time thermal stress and long-time thermal history. After a short period of thermal
stress, for example a motor startup, the thermal level starts to decrease quite
sharply, simulating the leveling out of the hot spots. Consequently, the probability
of successive allowed startups increases.
Section 4 1YHT530004D05 D
Protection functions
256 615 series
Technical Manual
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