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5 SETTINGS
In this instance, each distinct portion of the thermal limit curve must be known and protection coordinated against that
curve. The relay protecting the motor must be able to distinguish between a locked rotor condition (curve 4) and an
accelerating condition for different levels of the system voltage (curves 2 and 3). Voltage is continually monitored dur-
ing motor starting and the acceleration thermal limit portion of the relay overload curve is dynamically adjusted based
on motor voltage variations.
The acceleration thermal limit is a function of motor speed during the start. The dynamically shifted voltage dependent
overload curve inherently accounts for the change in motor speed as a function of motor impedance. The change in
impedance is reflected by motor terminal voltage and line current. This method aids to set dynamically the appropriate
value of the thermal limit time for any given line current at any given terminal voltage.
The
VOLTAGE DEPENDENT FUNCTION
tion of the programmed relay overload curve with respect to the acceleration thermal limits. These thermal limits are
typically available from the machine specifications provided by motor manufacturer.
•
VOLTAGE DEPENDENT MIN MOTOR VOLTS: This setting defines the minimum allowable line voltage applied to the
motor during the acceleration if
of the
SYSTEM SETUP
this setting during acceleration, the thermal curve is switched to one based on the programmed minimum voltage ther-
mal limit:
•
VD VOLTAGE LOSS. This setting is used to address situations when the voltage input into thermal model has been
lost. In this case, the voltage dependent algorithm readjusts the voltage dependent curve to avoid an inadequate ther-
mal protection response. The VT fuse failure function is typically used to detect a voltage loss condition. If a voltage
loss has been detected while motor accelerates, the thermal curve is switched to one based on the programmed 100%
voltage thermal limit:
•
VD STALL CURRENT @ MIN V: This setting defines the locked rotor current level at minimum motor voltage (I
•
VD SAFE STALL TIME @ MIN V: This setting defines the maximum time that motor is allowed to withstand the locked
rotor current at minimum motor voltage (t
•
VD ACCEL. INTERESECT @ MIN V: This setting defines the starting current level corresponding to the crossing point
between the acceleration thermal limit at minimum voltage and the programmed relay overload curve (I
can be typically determined from motor acceleration curves. The value at the breakdown torque for the minimum volt-
age start is recommended for this setting.
Figure 5–61: TYPICAL MOTOR ACCELERATION CHARACTERISTICS
GE Multilin
setpoint enables the voltage dependent feature and modifies the locked rotor por-
VOLTAGE DEPENDENT FUNCTION
MOTOR
MOTOR NAMEPLATE VOLTAGE
I
-------------- -
trip_time
=
I
-------------- -
trip_time
=
).
1
M60 Motor Protection System
is enabled. This voltage is expressed as a percentage
setting. If the measured line voltage drops below
2
t
1
1
2
I
2
t
3
3
2
I
5.6 GROUPED ELEMENTS
(EQ 5.24)
(EQ 5.25)
).
1
). This value
2
833714A1.CDR
5-155
5
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