Section 6
1MAC050144-MB C
Supervision functions
464
615 series ANSI
Technical Manual
• In the “IIt” or “IIt & stall” modes, the aim of this setting is to check for the completion
of the motor startup period. The purpose of this time delay setting is to allow for short
interruptions in the current without changing the state of the
MOT_START
output. In
this mode of operation, the value of the setting is in the range of around 100
milliseconds.
• In the “IIt, CB” or “IIt & stall, CB” modes, the purpose of this setting is to check for
the life of the protection scheme after the
CB_CLOSED
input has been activated.
Based on the values of the phase currents, the completion of the startup period cannot
be judged. So in this mode of operation, the value of the time delay setting can even
be as high as within the range of seconds, for example around 30 seconds.
The
BLOCK
input signal is used to block the operation of the
MOT_START
output. The
activation of the
BLOCK
input signal deactivates the
MOT_START
output.
Thermal stress calculator
Because of the high current surges during the startup period, a thermal stress is imposed
on the rotor. With less air circulation in the ventilation of the rotor before it reaches its full
speed, the situation becomes even worse. Consequently, a long startup causes a rapid
heating of the rotor.
This module calculates the thermal stress developed in the motor during startup. The heat
developed during the starting can be calculated using the formula,
(Equation 55)
R
s
combined rotor and stator resistance
i
s
starting current of the motor
t
starting time of the motor
This equation is normally represented as the integral of I²t. It is a commonly used method
in protective relays to protect the motor from thermal stress during starting. The advantage
of this method over the traditional definite time overcurrent protection is that when the
motor is started with a reduced voltage as in the star-delta starting method, the starting
current is lower. This allows more starting time for the motor since the module is
monitoring the integral of I²t.
The module calculates the accumulated heat continuously and compares it to the limiting
value obtained from the product of the square of the values of the
Motor start-up A
and
Motor start-up time
settings. When the calculated value of the thermal stress exceeds this
limit, the
OPR_IIT
output is activated.
The module also measures the time
START_TIME
required by the motor to attain the rated
speed and the relative thermal stress
IIT_RL
. The values are available through the
monitored data view.
The
BLOCK
input is used to reset the operation of thermal stress calculator. The activation
of the
BLOCK
input signal blocks the operation of the
OPR_IIT
output.
W
R i t dt
s
s
t
=
( )
∫
2
0
Содержание 615 Series ANSI
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Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
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Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
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