Functions
2.8 Motor Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
162
Switching of Startup Times
The motor manufacturer provides startup time curves for both cold and warm motor conditions (see Figure 2-
52). The function Motor Starting Protection automatically performs a switching. The "warm motor" condition is
derived from the thermal storage of the restart inhibit (see Section 2.8.2). Therefore, this function must be en-
abled. The condition for the switching is determined by the parameter 4106 TEMP.COLD MOTOR. If the motor
temperature (actually the rotor temperature) exceeds the threshold value, then a switching from "cold motor"
to "warm motor" takes place (see logic diagram 2-53). The threshold values can be derived from the permitted
number of cold (n
cold
) and warm (n
warm
) motor startups. By means of the following formula an approximate limit
value can be determined.
(Parameter 4106 TEMP.COLD MOTOR)
The setting value should always be lower than the limit value (see Setting Notes2.8.1.2).
2.8.1.2 Setting Notes
General
Motor starting protection is only effective and accessible if address 141 48 = Enabled is set. If the function is
not required Disabled is set. The function can be turned ON or OFF under address 4101 48.
Startup Parameter
The device is informed of the startup current values under normal conditions at address 4102 STARTUP
CURRENT, the startup time at address 4103 STARTUP TIME. At all times this enables timely tripping if the value
I
2
t calculated in the protection device is exceeded.
If the startup time is longer than the permissible blocked rotor time, an external rpm-counter can initiate the
definite-time tripping characteristic via binary input („>Rotor locked“). A locked rotor leads to a loss of ven-
tilation and therefore to a reduced thermal load capacity of the machine. For this reason, the motor starting time
function is to issue a tripping command before reaching the thermal tripping characteristic valid for normal op-
eration.
A current above the current threshold 1107 I MOTOR START is interpreted as motor startup. Consequently,
this value must be chosen such that it is reliably attained by the actual starting current under any load or voltage
conditions during motor startup, but not during a permissible short-time overload.
Example:
Motor with the following data:
Rated Voltage V
Nom
= 6600 V
Nominal current I
MOTNom
= 126 A
Startup current (primary) I
STARTUPw
= 624 A
Long-term current rating I
max
= 135 A
Startup time (cold condition) T
TMax.STARTUPc
= 15 s
Startup time (warm condition) T
TMax.STARTUPw
= 8.5 s
Current transformer I
NomCTprim
/ I
NomCTsec
= 200 A / 1 A
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