GE 469 Instruction Manual page 136

469
Motor Management Relay
5–30
reactance of the rotor cage causes the current to flow at the outer edges of the rotor
bars. The effective resistance of the rotor is therefore at a maximum during a locked
rotor condition as is rotor heating. When the motor is running at rated speed, the
voltage induced in the rotor is at a low frequency (approximately 1 Hz) and
therefore, the effective resistance of the rotor is reduced quite dramatically. During
running overloads, the motor thermal limit is typically dictated by stator
parameters. Some special motors might be all stator or all rotor limited. During
acceleration, the dynamic nature of the motor slip dictates that rotor impedance is
also dynamic, and a third overload thermal limit characteristic is necessary.
The figure below illustrates typical thermal limit curves. The motor starting
characteristic is shown for a high inertia load at 80% voltage. If the motor started
quicker, the distinct characteristics of the thermal limit curves would not be required
and the running overload curve would be joined with locked rotor safe stall times to
produce a single overload curve.
The motor manufacturer should provide a safe stall time or thermal limit curves for
any motor they sell. To program the 469 for maximum protection, it is necessary to
ask for these items when the motor is out for bid. These thermal limits are intended
to be used as guidelines and their definition is not always precise. When operation of
the motor exceeds the thermal limit, the motor insulation does not immediately
melt. Rather, the rate of insulation degradation has reached a point that motor life
will be significantly reduced if it is run any longer in that condition.
400
HIGH
300
INERTIA
MOTOR
200
100
80
60
40
20
10
8
6
4
E,F, AND G ARE THE
SAFE STALL THERMAL LIMIT
TIMES AT 100%, 90%, AND
2
80%VOLTAGE, REPECTIVELY
1
0 100
FIGURE 5–5: Typical Time-Current and Thermal Limit Curves
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RUNNING OVERLOAD
A,B,AND C ARE THE
ACCELERATION THERMAL LIMIT
CURVES AT 100%, 90%, AND
80%VOLTAGE, REPECTIVELY
C
B
G
F
200 300 400 500
(ANSI/IEEE C37.96)
S5 Thermal Model
A
E
600 % CURRENT
806827A1.CDR
GE Multilin