Figure 326: Functional module diagram
Level detector
The level detector compares the negative-sequence current to the set
Start value. If
the I
2
value exceeds the set
Start value, the level detector sends an enabling signal
to the timer module.
Timer
Once activated, the timer activates the
START
output. When the operation timer
has reached the set
Operate delay time value, the
OPERATE
output is activated. If
the fault disappears before the module operates, the reset timer is activated. If the
reset timer reaches the value of 200 ms, the operation timer resets and the
START
output is deactivated.
The timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the start situation and the set operation time. The value is
available in the monitored data view.
4.4.3.5
Application
The rotation of a motor in the reverse direction is not a desirable operating
condition. When the motor drives fans and pumps, for example, and the rotation
direction is reversed due to a wrong phase sequence, the driven process can be
disturbed and the flow of the cooling air of the motor can become reversed too.
With a motor designed only for a particular rotation direction, the reversed rotation
direction can lead to an inefficient cooling of the motor due to the fan design.
In a motor, the value of the negative-sequence component of the phase currents is
very negligible when compared to the positive-sequence component of the current
during a healthy operating condition of the motor. But when the motor is started
with the phase connections in the reverse order, the magnitude of I
2
is very high.
So whenever the value of I
2
exceeds the start value, the function detects the reverse
rotation direction and provides an operating signal that disconnects the motor
from the supply.
4.4.3.6
Signals
Table 556: PREVPTOC Input signals
Name
Type
Default
Description
I
2
SIGNAL
0
Negative sequence current
BLOCK
BOOLEAN
0=False
Block signal for activating the
blocking mode
1MRS757644 H
Protection functions
620 series
Technical Manual
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