GE 469 Instruction Manual page 85

4 SETPOINTS
c) VOLTAGE DEPENDENT OVERLOAD CURVE
If the motor is called upon to drive a high inertia load, it is quite possible and acceptable that the acceleration time exceeds
the safe stall time (bearing in mind that a locked rotor condition is different than an acceleration condition). In this instance,
each distinct portion of the thermal limit curve must be known and protection must be coordinated against that curve. The
relay that is protecting the motor must be able to distinguish between a locked rotor condition, an accelerating condition
and a running condition. The 469 Voltage Dependent Overload Curve feature is tailored to protect these types of motors.
Voltage is continually monitored during motor starting and the acceleration thermal limit curve is adjusted accordingly.
The Voltage Dependent Overload Curve is comprised of the three characteristic shapes of thermal limit curves as deter-
mined by the stall or locked rotor condition, acceleration, and running overload. The curve is constructed by entering a cus-
tom curve shape for the running overload protection curve. Next, a point must be entered for the acceleration protection
curve at the point of intersection with the custom curve, based on the minimum allowable starting voltage as defined by the
minimum allowable line voltage. The locked rotor current and safe stall time must also be entered for that voltage. A second
point of intersection must be entered for 100% line voltage. Once again, the locked rotor current and the safe stall time
must be entered, this time for 100% line voltage. The protection curve created from the safe stall time and intersection point
will be dynamic based on the measured line voltage between the minimum allowable line voltage and the 100% line volt-
age. This method of protection inherently accounts for the change in motor speed as an impedance relay would. The
change in impedance is reflected by motor terminal voltage and line current. For any given speed at any given line voltage,
there is only one value of line current.
GE Multilin
Courtesy of NationalSwitchgear.com
g
HIGH INERTIA LOAD OVERLOAD CURVES
8800 HP, 13.2 kV, REACTOR COOLANT PUMP
1000
900
800
700
1
600
500
400
300
2
200
100
90
80
70
60
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40
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10
9
8
7
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4
3
2
1
1 2 3
MULTIPLES OF FULL LOAD AMPS
Figure 4–8: THERMAL LIMITS FOR HIGH INERTIAL LOAD
469 Motor Management Relay
GE Multilin
1- Running Overload Thermal Limit
2- Acceleration Thermal Limit at 80%V
3- Acceleration Thermal Limit at 100%V
4- Locked Rotor Thermal Limit
5- Motor Acceleration Curve at 80% V
6- Motor Acceleration Curve at 100%V
3
4
5
6
4 5 6 7
806821A4.CDR 806821A4.CDR
4.6 S5 THERMAL MODEL
8
4-35
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