Siemens SIPROTEC 4 7SJ61 Manual page 104

Functions
2.6 Motor Protection
NOTE
i
i
When function settings of the motor restart inhibit are changed, the thermal replica of this function is
reset.
The motor restart inhibit acts on the starting process of a motor that is shut down. A motor is considered shut
down when its current consumption falls below the settable threshold 212 BkrClosed I MIN. Therefore,
this threshold must be set lower than the motor idle current.
The motor restart inhibit concludes the condition "warm motor" from the thermal replica of the restart inhibit.
For this function, address 4301FCT 66 has to be enabled.
Additionally, the function can trip directly if the rotor temperature exceeds the maximum admissible tempera-
ture (100%). For this purpose address 4311 ROTOR OVERLOAD is set to ON. If only monitoring is desired, set
to Alarm Only, otherwise to OFF.
Characteristic Values
Many of the variables needed to calculate the rotor temperature are supplied by the motor manufacturer.
Among these variables are the starting current Ι
allowable starting time T START MAX (address 4303), the number of allowable starts from cold conditions
(n ), and the number of allowable starts from warm conditions (n
cold
The starting current is entered at address 4302 IStart/IMOTnom , expressed as a multiple of nominal motor
current. In contrast, the nominal motor current is entered as a secondary value, directly in amperes, at address
4305 I MOTOR NOMINAL. The number of warm starts allowed is entered at address 4306 (MAX.WARM
STARTS and the difference #COLD-#WARM between the number of allowable cold and warm starts is entered
at address 4307.
For motors without separate ventilation, the reduced cooling at motor stop can be accounted for by entering
the factor Kτ at STOP at address 4308. As soon as the current no longer exceeds the setting value entered
at address 212 BkrClosed I MIN, motor standstill is detected and the time constant is increased by the
extension factor configured.
If no difference between the time constants is to be used (e.g. externally-ventilated motors), then the exten-
sion factor Kτ at STOP should be set to 1.
The cooling with the motor running is influenced by the extension factor 4309 Kτ at RUNNING. This factor
considers that motor running under load and a stopped motor do not cool down at the same speed. It
becomes effective as soon as the current exceeds the value set at address 212 BkrClosed I MIN. With Kτ
at RUNNING = 1 the heating and the cooling time constant are the same at operating conditions (Ι >
BkrClosed I MIN).
Example: Motor with the following data:
Rated Voltage
Nominal current
Startup current
Startup duration
Permitted starts with cold motor
Permitted starts with warm motor
Current transformer
The following settings are derived from these data:
[formel-ianl-260602-kn, 1, en_US]
[formel-imotnenn-260602-kn, 1, en_US]
104
, the nominal motor current Ι
STARTUP
warm
V = 6600 V
Nom
= 126 A
Ι
Nom
= 624 A
Ι
SRARTUP
T = 8,5 s
Start max.
n = 3
cold
n = 2
warm
200 A/1 A
, the maximum
MOT. NOM
).
SIPROTEC 4, 7SJ61, Manual
C53000-G1140-C210-6, Edition 05.2016