Technical information

8 – THEORY OF OPERATION
Motor Overload
116
Frame Size Cooling Time
180 30 min
280 60 min
360 90 min
400/440 120 min
500 180 min
Larger frames Consult
Manufacturer
For motors less than 300hp, another approximation based on allowable motor starts per hour can also be used to set an initial value of
the Motor Overload Cooling Time parameter:
hourper Starts
minutes 60
(minutes) Time CoolingMotor ≈
The Motor Overload Cooling Time parameter is defined as the time that it takes for the motor to cool from 100% overload content to
less than 1% overload content. Sometimes a motor manufacturer may provide a cooling time constant (τ or tau) value. In these
cases, the Motor Overload Cooling Time parameter should be set to five (5) times the specified time constant value.
8.1.9 Motor Cooling While Running
When the motor is running, the Motor Overload Cooling Time parameter and the Motor Overload Hot/Cold Ratio parameter settings
control the motor OL content. If the motor overload content is above the steady state OL running level (See section 8.1.6, Hot / Cold
Motor Overload Compensation for more details) the motor OL exponentially cools to the appropriate steady state OL level. When
the motor is running, the cooling time is adjusted based on the measured current level and current imbalance level at which the motor
is operating.
Factor Derate ImbalanceCurrent
1
*
FLAMotor
Current Running Measured
* Stopped Time CoolingRunning Time Cooling =
In all cases, the running motor cooling time is shorter (motor will cool faster) than when the motor is stopped. The faster cooling
results because it is assumed that when a motor is running, cooling air is being applied to the motor.
8.1.10 Emergency Motor Overload Reset
The MX has an emergency motor overload reset feature that allows the user to override the overload starter lockout. This resets the
motor overload content to 0%. It does not reset the overload fault.
To perform an emergency overload reset, simultaneously press the RESET and DOWN buttons on the keypad. An emergency overload
reset may also be performed by applying 120 Volts to a digital input that is configured as an emergency overload reset input or by
setting the emergency overload reset bit in the starter control Modbus register
CAUTION: This feature should only be used in an emergency. Before an emergency reset is performed the cause of the motor
overload should be investigated to ensure that the motor is capable of restarting without causing undesired motor or load damage.
When the emergency motor overload reset is used, the accumulated motor overload content is reset back to zero (0%). Therefore, the
MX’s motor protection functions may not be able to fully protect the motor from damage during a restart after performing an
emergency motor overload reset.