Technical information

8 – THEORY OF OPERATION
Motor Overload
110
8.1 Solid State Motor Overload Protection
8.1.1 Overview
The MX contains an advanced I
2
t electronic motor overload (OL) protection function. For optimal motor protection, the MX has
forty standard NEMA style overload curves (in steps of one) available for use. Separate overload classes can be programmed for
acceleration and for normal running operation and individually or completely disabled if necessary. The MX motor overload
function also implements a NEMA based current imbalance overload compensation, adjustable hot and cold motor compensation,
and adjustable exponential motor cooling.
CAUTION: If the MX motor overload protection is disabled during any mode of operation, external motor overload protection must
be provided to prevent motor damage and/or the risk of fire in the case of a motor overload.
8.1.2 Setting up the MX Motor Overload
Motor overload protection is easily configured through seven parameters (please refer to the descriptions of each parameter in section
5 of this manual for additional parameter information):
1. Motor FLA
2. Motor Service Factor
3. Motor Running Overload Class
4. Motor Starting Overload Class
5. Independent Starting/Running Overload
6. Motor Overload Hot/Cold Ratio
7. Motor Overload Cooling Time
The Motor FLA and Service Factor parameter settings define the motor overload “pickup” point. For example, if the motor service
factor is set to 1.00, the motor overload begins accumulating or incrementing when the measured motor current is >100% FLA
(100% * 1.00). The overload will NOT trip if the motor current is <100%. If the motor service factor is set to 1.15, the overload
starts accumulating content when the motor current >115% FLA (100% * 1.15). The overload will NOT trip if the measured motor
current is <115% of rated FLA.
The available overload classes are based on the trip time when operating at 600% of rated motor current. For example, a Class 10
overload trips in 10 seconds when the motor is operating at 600% rated current; a Class 20 overload trips in 20 seconds when the
motor is operating at 600% rated current.
The equation for the MX standard overload curves after the “pick-up” point has been reached is:
1
FLAMotor
Factor Derate ImbalCurrent
1
*Current Measured
Class * seconds 35
(seconds) Trip toTime
2
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