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PROTECTION
NOTE:
4–130
8.
The line connecting points 1, 3 and 5 represent the motor safe stall conditions for any
system voltage from the minimum to 110% of rated. Ideally, all the points on this line
are characterized by the same thermal limit (I
impedance at reduced voltage is greater than the impedance at full voltage. As such,
the higher terminal voltages tend to reduce I
the VD Stall Current and VD Safe Stall Time setpoints for rated and minimum voltage
conditions. For voltage conditions above rated, the locked rotor thermal limit and
acceleration curve are extrapolated up to 110% of the terminal voltage. The point
coordinates (I
, T
) for 110% are extrapolated based on the I
s
s
starting currents at voltages higher than 110%, the trip time computed from 110% V
thermal limit value is used.
The voltage dependent curve for current values above 8 times pickup (OL x FLA) are
clamped and the time to trip is frozen at the level calculated for the 8 times pickup current.
The following three figures (a), (b) and (c) illustrate the resultant overload protection
curve for minimum, 100%, and maximum line voltages. For voltages between these
limits, the 869 shifts the acceleration curve linearly and constantly, based on the
measured line voltage during a motor start. Figures (d), (e) and (f) illustrate the motor
starting curves for the following abnormal conditions: line voltages below the minimum,
above 110%, and the situation for voltage loss.
For the Voltage Dependant Overload Curve Protection figure: (a) At Minimum Voltage, (b)
At 100% Voltage, (c) At 110% Voltage, (d) At Less Than Minimum Voltage, (e) At Voltage
Loss Condition, (f) At More Than 110% Voltage
Figure 4-39: Voltage Dependent Overload Curve Protection
2
t), but the equivalent starting
2
2
t. The rate of I
t reduction is dictated by
, T
1
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
, I
, and T
values. For
1
3
3
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