ABB Relion 615 series Technical Manual Download

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4.1.9.5

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 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 147

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 start-up, the thermal level starts to decrease quite sharply, simulating the leveling

ABB Relion 615 series, Technical Manual

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