Application - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.5.3.5
398

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 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
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 protection relay
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.
1MAC504801-IB E
Figure 183,
620 series ANSI
Technical Manual