ABB 615 Series ANSI, Техническое руководство

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Section 4
1MAC050144-MB C
Protection functions
314 615 series ANSI
Technical Manual
4.5.3.5 Application
49M 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 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.
The " Weighting factor p = 100 percent", it produces a pure single time constant thermal
unit, which is used for application with the cables. As presented in Figure 151, the hot
curve with the value of "
Weighting factor p = 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 = 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.
When protecting the objects without hot spot tendencies, for example motors started with
soft starters, and cables, the value of
Weighting factor p is set to "100 percent". With the
value of Weighting factor p set to "100 percent", the thermal level decreases slowly after
a heavy load condition. This makes the protection suitable for applications where no hot
spots are expected. Only in special cases where the thermal overload protection is required