detected, the operate timer is reset and the saved values of start time and
integration are cleared.
Inv. curve B
The inverse time equation for curve type B is:
t s
k
I
I
I
I
r
S
r
[ ]
=
−
2
2 2
GUID-805DCB50-71D2-4721-830B-3343E1A5500B V1 EN (Equation 71)
t[s] Operate time in seconds
k
Machine time Mult
I
2
Negative-sequence current
I
S
Set
Start value
I
r
Set
Rated current
If the fault disappears, the negative-sequence current drops below the Start value
setting and the START output is deactivated. However, the function does not reset
instantaneously, but instead it depends on the equation or the Cooling time setting.
The timer can be reset in two ways:
• With a drop in the negative-sequence current below start value, the subtraction
in the denominator becomes negative and the cumulative sum starts to
decrease. The decrease in the sum indicates the cooling of the machine and the
cooling speed depends on the value of the negative-sequence current. If the
sum reaches zero without a fault being detected, the accumulation stops and
the timer is reset.
• If the reset time set through the Cooling time setting elapses without a fault
being detected, the timer is reset.
The reset period thus continues for a time equal to the Cooling time setting or until
the operate time decreases to zero, whichever is less.
4.4.4.6 Application
In a three-phase motor, the conditions that can lead to unbalance are single
phasing, voltage unbalance from the supply and single-phase fault. The negative
sequence current damages the motor during the unbalanced voltage condition, and
therefore the negative sequence current is monitored to check the unbalance
condition.
When the voltages supplied to an operating motor become unbalanced, the positive-
sequence current remains substantially unchanged, but the negative-sequence
current flows due to the unbalance. For example, if the unbalance is caused by an
1YHT530004D05 D Section 4
Protection functions
615 series 463
Technical Manual
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