1.7 times the previous load in each healthy phase and zero current in the open
phase.
The negative-sequence currents flow through the stator windings inducing
negative-sequence voltage in the rotor windings. This can result in a high rotor
current that damages the rotor winding. The frequency of the induced current is
approximately twice the supply frequency. Due to skin effect, the induced current
with a frequency double the supply frequency encounters high rotor resistance
which leads to excessive heating even with phase currents with value less than the
rated current of the motor.
The negative-sequence impedance of induction or a synchronous motor is
approximately equal to the locked rotor impedance, which is approximately one-
sixth of the normal motor impedance, considering that the motor has a locked-
rotor current of six times the rated current. Therefore, even a three percent voltage
unbalance can lead to 18 percent stator negative sequence current in windings. The
severity of this is indicated by a 30-40 percent increase in the motor temperature
due to the extra current.
4.4.4.7
Signals
Table 563: MNSPTOC Input signals
Name
Type
Default
Description
I
2
SIGNAL
0
Negative sequence
current
BLOCK
BOOLEAN
0=False
Block signal for acti-
vating the blocking
mode
Table 564: MNSPTOC Output signals
Name
Type
Description
OPERATE
BOOLEAN
Operate
START
BOOLEAN
Start
BLK_RESTART
BOOLEAN
Overheated machine recon-
nection blocking
4.4.4.8
Settings
Table 565: MNSPTOC Group settings (Basic)
Parameter
Values (Range)
Unit
Step
Default
Description
Start value
0.01...0.50
xIn
0.01
0.20
Start value
Operating curve
type
5=ANSI Def. Time
15=IEC Def. Time
17=Inv. Curve A
18=Inv. Curve B
15=IEC Def. Time
Selection of time
delay curve type
Machine time Mult
5.0...100.0
0.1
5.0
Machine related
time constant
Operate delay time 100...120000
ms
10
1000
Operate delay time
Protection functions
1MRS757644 H
628
620 series
Technical Manual
