Siemens SIPROTEC 7SJ62 Instruction Manual

Functions
6-94 7SJ62 Manual
C53000-G1140-C121-1
6.8 Motor Starting Protection (48)
6.8.1 Description of Motor Starting Protection
General When the 7SJ62 relay is used to protect a motor, the starting time monitoring feature
supplements the overload protection described in Section 6.9 by protecting the motor
against the potential damage that might result from frequent starting or extended start-
ing durations. 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 by excessive voltage
surges during motor starting, by excessive load moments, or by blocked rotor condi-
tions, a tripping signal will be initiated by the device.
Motor starting protection consists of two time-overcurrent tripping characteristics initi-
ated by the motor starting recognition setting entered at address  ,02725
67$57
. One characteristic is definite time while the other one is inverse time. When
the motor phase currents exceed the setting entered at address , time delayed
tripping will be initiated. To gain a better understanding of how to set the motor starting
recognition setting at address 
, refer to Subsection 6.1.3.
Inverse Time
Time-Overcurrent
Tripping Character-
istic
The inverse time time-overcurrent characteristic is designed to operate only when the
rotor is not blocked. When the phase currents exceed the motor starting recognition
setting entered at address , time delayed tripping via the inverse time character-
istic is initiated. The inverse time characteristic allows motor starting protection to ad-
just for those situations where high starting voltages result in decreased starting cur-
rents. The tripping time is calculated based on the following formula:
t
TRIP
I
A
I
-----
2
t
Amax
⋅=
where I> I
MOT START
where t
TRIP
– Actual tripping time for flowing current I
t
A max
– Tripping time for nominal start-up current I
A
(set at address 4103)
I – Current actually flowing (measurement value)
I
A
– Nominal starting current of the motor (set at address 4102)
I
MOT START
– Pickup value for recognition of motor starting (set at address 1107)
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Siemens SIPROTEC 7SJ62 Specifications

General

Brand	Siemens
Model	SIPROTEC 7SJ62
Category	Relays
Language	English