GE Masoneilan 469 Series, Instruction Manual

Page: 208 / 348

5–64 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
When setting the
CURRENT UNBALANCE ALARM(TRIP) PICKUP
level, note that a 1% voltage
unbalance typically translates into a 6% current unbalance. Therefore, to prevent nuisance
trips or alarms, the pickup level should not be set too low. Also, since short term
unbalances are common, a reasonable delay should be set to avoid nuisance trips or
alarms. The unbalance bias feature is recommended to bias the thermal model for motor
heating caused by cyclic short term unbalances (see page –55 for details).
NOTE
Note
Unusually high unbalance levels may be caused by incorrect phase CT wiring.
For example, fluctuations of current unbalance levels are typically caused by the supply
voltage. It may be desirable to have a latched alarm to capture any such fluctuations that
go beyond the Unbalance Alarm parameters. Also, a trip is recommended.
If the supply voltage is normally unbalanced up to 2%, the current unbalance seen by a
typical motor is 2 × 6 = 12%. In this case, set the
CURRENT UNBALANCE ALARM PICKUP
to
“15” and the
CURRENT UNBALANCE TRIP PICKUP
to “20” to prevent nuisance tripping; 5 or
10 seconds is a reasonable delay.