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4.3 S2 PROTECTION
MM
PATH: SETPOINTS
FULL LOAD CURRENT:
100 A
MESSAGE
HIGH SPEED FULL LOAD
MESSAGE
CURRENT:
OVERLOAD PICKUP
MESSAGE
LEVEL: 1.00
4
OVERLOAD CURVE
MESSAGE
NUMBER:
HOT/COLD CURVE
MESSAGE
RATIO: 75
For example, given the following motor information: motor FLC = 100 A, actual motor current = 80 A, LRT Hot
= 7 seconds, and LRT Cold = 10 seconds, the thermal capacity used can be determined from the HOT/COLD
CURVE RATIO as follows:
HOT/COLD CURVE RATIO
Thermal Capacity Used
Therefore:
4-10
M
S2 PROTECTION
MOTOR PROTECTION THERMAL
Range: 0.1 to 1000.0 A or OFF; Step: 0.1 A
Usually the rated current on the motor nameplate is entered as the full
load current value. A lower value effectively overprotects the motor. It is
not advisable to enter a value higher than the motor nameplate rating.
When the actual motor current exceeds this value, the thermal capacity
starts to be used up and the motor will eventually trip according to the
overload curve selected.
Range: 0.1 to 1000.0 A or OFF; Step: 0.1 A
100 A
This setpoint functions the same way as FULL LOAD CURRENT, but refers to
the high speed FLC of a two-speed motor, and is only in effect while
contactor B is energized.
Range: 1.00 to 1.25; Step: 0.01
This setpoint dictates where the overload curve begins as the motor
enters an overload condition. This is useful for service factor motors as it
allows the pickup level to be defined. The curve is effectively cut off at
current values below this pickup.
Range: 1 to 8, CLASS 10, CLASS 15, CLASS 20, CLASS 30
4
Select 1 of 12 different I
overload curve figures and tables in this manual (see pages 4–34 to 4–
37) to match the curve number to a particular motor. If no motor curve
data is available, select the curve which has a 6 times overload trip time
equal to the motor nameplate stall time. The MM3 also has four NEMA
class curves which can be selected should these curves be suggested by
the manufacturer.
Range: 20 to 100%; Step: 1%
Defines the ratio of motor "hot" thermal characteristic to the motor "cold"
characteristic. It is used to thermally model the heating in the motor when
running at or below full load current. This is often determined from motor
thermal damage curves or Locked Rotor Time Hot and Locked Rotor
Time Cold data. HOT/COLD CURVE RATIO determines the thermal capacity
used when a motor has run at its full load current long enough for the
motor temperature to reach its steady state value, which is defined as the
hot temperature. Note that the thermal capacity used is reduced
according to the actual motor current.
LRT Hot
×
=
------------------------ -
100
LRT Cold
actual motor current
=
---------------------------------------------------- -
FULL LOAD CURRENT
80 A
=
--------------- -
100 A
MM3 Motor Manager 3
4.3.2 MOTOR PROTECTION – THERMAL
2
t time-overcurrent overload curves. Consult the
7 sec.
×
=
------------------ -
100
=
70%
10 sec.
×
(
100% HOT/COLD CURVE RATIO
–
×
(
)
100% 70%
–
=
24%
4 SETPOINTS
)
GE Power Management
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