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Overload Curve - GE MM300 Instruction Manual

Motor management system low voltage motor protection and control.
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CHAPTER 5: SETPOINTS

Overload curve

MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
The overload curve accounts for the rapid motor heating that occurs during stall,
acceleration, and overload. Specifically, the overload curve controls the rate of increase of
Thermal Capacity Used
whenever the equivalent motor heating current is greater than
1.01 times the full load current setpoint. The curve is defined by the following equation and
reflects that overload heating largely swamps the cooling, and this heating is primarily due
to resistive losses in the stator and the rotor windings (said losses being proportional to the
square of the current).
Trip
time
=
0.02530337 x (Pickup - 1) + 0.05054758 x (Pickup -1)
In the above equation,
The trip time represents the time (in seconds) for the MM300 to trip, given the motor
starts cold and the current is constant.
The multiplier represents the value of the
be used to adjust the curve to match the thermal characteristics of the motor.
I
represents the equivalent motor heating current in per-unit values on a full load
eq
current base. The value of I
from acting as an instantaneous element and responding to short circuits. Equivalent
motor heating current is discussed in the Unbalance biasing section.
For example, a motor with a stall current (also known as locked rotor current) of 8 times its
FLA, with a curve multiplier of 7, if stalled from a cold state, trips in the following amount of
time.
This would respect a safe stall cold time of 10 seconds.
The standard overload curves are displayed below.
Curve Multiplier
´
2.2116623
2
Curve Multiplier
is limited in this equation to 8.0 to prevent the overload
eq
PROTECTION ELEMENTS
Eq. 4
setpoint. This setpoint can
Eq. 5
5–37

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