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K8DT-PM
Questions and Answers
Q
Load-side Phase Loss
In principle, phase loss cannot be detected on the load side
because the K8DT-PM measures three-phase voltage to
determine phase loss.
Q
Motor Load Phase Loss during Operation
Motor load phase loss cannot be detected during operation. It
can be used to detect phase loss at startup.
Normally, three-phase motors will continue to rotate even if
one phase is open. The three-phase voltage will be induced
at the motor terminals. The diagram shows voltage induction
at the motor terminals when phase R has been lost with a
load applied to a three-phase motor. The horizontal axis
shows the motor load as a percentage of the rated load, and
the vertical axis shows voltage as a percentage of the rated
voltage. The lines in the graph show the voltage induced at
the motor terminals for each load phase loss occurs during
operation. As the graph shows, phase loss cannot be
detected because the motor terminal voltage does not drop
very much even if a phase is lost when the load on the motor
is light. To detect motor load phase loss during operation, use
the undervoltage detection function to detect the motor
terminal voltages at phase loss. Set the operating time
carefully because it will affect the time from when the phase
loss occurs until tripping when this function is used.
Characteristic Curve Diagram
Note: This characteristic curve shows the approximate
values only.
100
95
90
85
80
75
70
65
60
55
50
45
0
10
20
Motor load (as a percentage of rated load)
Note: For phase loss of phase R. V
indicate the motor terminal voltage at phase
loss.
8
V
ST
V
TR
V
RS
30
40
50
60
70
80
90 100 110
, V
, and V
ST
TR
RS
Is an overvoltage detected if only one of the three-phase
Q
phase-to-phase voltages exceeds the overvoltage set
value?
The K8DT monitors all three phase voltages. Therefore, an
overvoltage is detected if only one of the phase-to-phase
voltages exceeds the set value. The same is true for
undervoltages.
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