Motor Protection (Motor Starting Protection 48, Motor Restart Inhibit 66); Motor Starting Protection 48; Description - Siemens siprotec SJ62 User Manual

2.8 Motor Protection (Motor Starting Protection 48, Motor Restart
Inhibit 66)
2.8.1

Motor Starting Protection 48

2.8.1.1

Description

General
Inverse Time Over-
current Element
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015

2.8 Motor Protection (Motor Starting Protection 48, Motor Restart Inhibit 66)

For protection of motors the devices 7SJ62/63/64 are provided with a motor starting
time monitoring feature and a restart inhibit. The first feature mentioned supplements
the overload protection (see Section 2.10) by protecting the motor from frequent start-
ing or extended starting durations. The restart inhibit described prevents from a restart
of the motor, when starting might exceed the permissible time the rotor can suffer heat-
ing.
By application of devices 7SJ62/63/64 to motors, the motor starting time monitoring
protects the motor from too long starting attempts and supplements the overload pro-
tection (see Section 2.10)
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 du-
rations of these starting attempts are lengthened e.g. by excessive voltage dips during
motor starting, by excessive load torques, or by blocked rotor conditions, a tripping
signal will be initiated by the device.
Motor starting is detected when a settable current threshold I MOTOR START is ex-
ceeded. Calculation of the tripping time is then initiated.
The protection function consists of one definite time and one inverse time tripping el-
ement.
The inverse time overcurrent element is designed to operate only when the rotor is not
blocked. With decreased starting current resulting from voltage dips when starting the
motor, prolonged starting times are evaluated correctly and tripping with appropriate
time delay. The tripping time is calculated based on the following equation:
with
t
TRIP
t
STARTUPmax
I
I
STARTUP
I
MOTOR START
– Actual tripping time for flowing current I
– Tripping time for nominal start-up current I
4103, STARTUP TIME)
– Current actually flowing (measurement value)
– Nominal starting current of the motor (address 4102,
STARTUP CURRENT)
– Pickup value for recognition of motor starting (ad-
dress 1107I MOTOR START),
(address
A
154