Functions
2.8 Motor Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
178
2.8.3.2 Setting Notes
Elements
A warning and a tripping element can be configured. The threshold value of the tripping element 4402 Load
Jam I> is usually configured below motor startup at double motor ampere rating. Warning element 4404 I
Alarm is naturally set below the tripping element, to approx. 75% of the tripping element, with a longer delay
time (parameter 4405 ALARM DELAY). If the warning element is not required, the pickup value can be set to
its maximum value and the respective message from the buffers can be removed.
Motor Standstill and Motor Startup
Due to the threshold setting below the motor startup current, the load jam protection during motor startup must
be blocked. Via parameters 212 BkrClosed I MIN the open circuit breaker is detected during current-flow
measurement (motor standstill). In this condition the load change protection is blocked. After having closed the
circuit breaker, the blocking is maintained during motor startup by the setting 4406 T Start Blk.. In order
to avoid malfunctioning, the T Start Blk. is set to the double startup time.
Motor Protection Example
Figure 2-61 illustrates an example of a complete motor protection characteristic. Such characteristic usually
consists of different protection elements, and each element is responsible for special motor malfunctions.
These are:
• Thermal overload protection: to avoid overheating of the motor due to inadmissible load
• Load jam protection: to prevent overheating and mechanical damage due to a locked rotor
• Motor starting protection: protects the motor against prolonged startup procedures and the consequent
thermal overload of the rotor
• Overcurrent and high-current elements: for motor shutdown due to electrical faults
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