GE MULTILIN 269 MOTOR MANAGEMENT RELAY Series Instruction Manual Download

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3 SETUP AND USE

3-45

breaker). Enabling the 52B contact setpoint in
page 5 of setpoints will allow the 269 to determine
a 'STOP' condition if motor current is less than 5%
CT primary and the 52B contact is closed.

It is recommended that the trip functions and inhibit
features be assigned to different relays. For example,
all the trip functions may be assigned to activate the
TRIP relay when a trip condition is met. The Inhibit
Lockout should then be assigned to activate the AUX1
relay when the motor stops and an inhibit is issued by
the 269. Separating TRIPs and INHIBITs in this manner
makes it easier for operators to properly diagnose
problems and take appropriate corrective action.

Also, the “CAUSE OF LAST EVENT” message seen on
page 5 of Actual Values clearly shows whether the last
event was a TRIP or an INHIBIT.

Note: Inhibit lockouts are assigned to the AUX1

relay as a factory default. Ensure that AUX1 contac-

tors are properly wired in your control circuit. See

Figure 3.2 and Figure 3.3 for wiring details.

3.10 Unbalance Setpoints

Unbalanced three phase supply voltages are a major
cause of induction motor thermal damage. Unbalance
can be caused by a variety of factors and is common in
industrial environments. Causes can include increased
resistance in one phase due to pitted or faulty contac-
tors, transformer faults and unequal tap settings, or
non-uniformly distributed three phase loads. The in-
coming supply to a plant may be balanced but varying
single phase loads within the plant can cause voltage
unbalance at the motor terminals. The most serious
case of unbalance is single phasing which is the com-
plete loss of one phase of the incoming supply. This
can be caused by a utility supply problem or by a blown
fuse in one phase and can seriously damage a three
phase motor.

Unbalance at the motor terminals means an increase in
the applied negative sequence voltage. This results in
a large increase in the negative sequence current
drawn by the motor due to the relatively small negative
sequence impedance of the rotor. This current is nor-
mally at about twice the power supply frequency and
produces a torque in the opposite direction to the de-
sired motor output. For small unbalances the overall
output torque will remain constant, but the motor will be
developing a large positive torque to overcome the
negative sequence torque. These opposing torques and
the high negative sequence current produce much
higher rotor losses and consequently greatly increased
rotor heating effects. Stator heating is increased as
well, but to a much smaller extent. The amount of un-
balance that a given motor can tolerate is therefore
dependent on the rotor design and heat dissipation
characteristics.

Persistent, minor voltage unbalance can thus lead to
rotor thermal damage while severe unbalance such as
single phasing can very quickly lead to a motor burn-
out.

For phase currents above 100% FLC, the 269 relay
calculates the ratio of the negative to positive sequence
currents (In/Ip) for unbalance protection. The method
of determining In/Ip is independent of actual line fre-
quency or phase current lead/lag characteristics. This
negative sequence unbalance method provides read-
ings similar to the NEMA unbalance calculation but
gives more realistic results for the thermal effect of
unbalance on the motor (for a 269 unbalance example
see Appendix A). For phase currents below 100% FLC,
the relay calculates the ratio of In to full load current
(In/IFLC) and uses this to provide protection. This
avoids nuisance trips due to relatively high levels of In
with lower levels of Ip that may create high U/B levels
at low loads.

For unbalance protection, trip and alarm In/Ip ratios
may be chosen along with appropriate persistence
times (time delays) in SETPOINTS mode, page 1. If
no separate unbalance protection is desired, the trip
and alarm levels should be set to "OFF". The delay
times will then be disregarded by the relay. Above
100% FLC, if an unbalance of more than 30% persists
for more than 4 seconds, a "SINGLE PHASE TRIP" will
result. Below 100% FLC, if motor load is >25%, and
any one phase reads zero, this will also be consid-
ered a single phase condition. The single phase time
delay can be adjusted by contacting the factory.

Note: If the "UNBALANCE TRIP LEVEL" is set to
"OFF," single phase protection will be turned off.

It should be noted that a 1% voltage unbalance typi-
cally translates into a 6% current unbalance. So, if for
example the supply voltage is normally unbalanced up
to 2%, the current unbalance seen by a typical motor
would be 2

6 = 12%. Set the alarm pickup at 15% and

the trip at 20% to prevent nuisance tripping. 5 or 10
seconds is a reasonable delay.

Other factors may produce unbalanced phase currents.
Cyclic, pulsating and rapidly changing loads have been
observed to create unbalance in motors driving ma-
chines such as ball mill grinders, shredders, crushers,
and centrifugal compressors, where the load charac-
teristics are constantly and rapidly changing.

Under such circumstances, and in order to prevent
nuisance unbalance trips or alarms, the pickup level
should not be set too low. Also, a reasonable time de-
lay should be set to avoid nuisance trips or alarms. It is
recommended that the unbalance input to thermal
memory be used to bias the thermal model, thus ac-
counting for motor heating that may be caused by cy-
clic short term unbalances.