Omron K8AB-PM Series Manual page 10

Questions and Answers
Q
Load-side Phase Loss
In principle, phase loss cannot be detected on the load side
because the K8AB-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 30 40 50 60 70 80 90 100 110
Motor load (as a percentage of rated load)
Note: For phase loss of phase R. V
terminal voltage at phase loss.
10
Three-phase Phase-sequence Phase-loss Relay
V
ST
V
TR
V
RS
, V , and V indicate the motor
ST TR RS
Q
Overvoltage Detection When Only One Phase Exceeds
the Overvoltage Threshold
The K8AB-PM@ monitors each of the three-phase voltages.
This means an overvoltage is detected if even only one
phase exceeds the threshold value. The same applies to
undervoltages.
K8AB-PM
Cat. No. N146-E1-02