Motor Protection
For the protection of motors, devices 7SJ62/64 are provided with a motor starting protection function, a
restart inhibit and a load jam protection. The starting protection function protects the motor from prolonged
startup procedures thus supplementing the overload protection (see Section 2.11 Thermal Overload Protec-
tion 49). The restart inhibit prevents restarting of the motor when this restart may cause the permissible
thermal limits of the rotor to be exceeded. The load jam protection serves to protect the motor during sudden
rotor blocking.
Motor Starting Protection 48
When devices 7SJ62/64 are used to protect a motor, the starting protection feature supplements the overload
protection and protects the motor against prolonged starting operations (see Section 2.11 Thermal Overload
Protection 49).
Description
General
In particular, rotor-critical high-voltage motors can quickly be heated above their thermal limits when multiple
starting attempts occur in a short period of time. If the durations of these starting attempts are lengthened
e.g. by excessive voltage surges during motor starting, by excessive load moments, or by blocked rotor condi-
tions, a trip signal will be initiated by the protective relay.
Motor starting is detected when a settable current threshold I MOTOR START is exceeded. Calculation of the
tripping time is then initiated. It should be noted that this timer starts every time the motor is started. This is
therefore a normal operating condition that is neither indicated in the fault log nor causes the creation of a
fault record. Only when the locked rotor time has elapsed is the trip command issued.
The protection function consists of one definite time and one inverse time tripping element.
Inverse Time Overcurrent Element
The inverse time overcurrent element is designed to operate only when the rotor is not blocked. With a
decreased startup current resulting from voltage dips when starting the motor, prolonged startup times are
evaluated correctly and tripping with an appropriate time is enabled. The characteristic (see formula below)
can be ideally adjusted to the condition of the motor by using different startup times depending on the cold or
warm condition of the motor (see Figure 2-55).
The tripping time is calculated based on the following equation:
[formel-taus-150502-kn, 1, en_US]
with
t
TRIP
Actual tripping time for flowing currentΙ
t
maxSTARTUP
Tripping time for nominal startup current Ι
STARTUP
(address 4103,
STARTUP TIME or 4105, STARTUP T WARM)
Ι
Current actually flowing (measurement value)
Ι
STARTUP
Nominal startup current of the motor (address 4102, STARTUP
CURRENT)
Ι
MOTOR START
Pickup value for recognition of motor startup (address 1107, I MOTOR
START)
2.8
2.8.1
2.8.1.1
Functions
2.8 Motor Protection
144 SIPROTEC 4, 7SJ62/64, Manual
C53000-G1140-C207-8, Edition 08.2016
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