7THEORY OF OPERATION
Motor Overload
7.1 Solid State Motor Overload Protection
7.1.1 Overview
The MX
2
contains an advanced I
2
t electronic motor overload (OL) protection function. For optimal motor protection, the MX
2
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
2
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
2
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.
7.1.2 Setting Up The MX
2
Motor Overload
Motor overload protection is easily configured through seven parameters (please refer to the descriptions of each parameter in section 6
of this manual for additional parameter information):
1. Motor FLA (QST 01)
2. Motor Service Factor (QST 02)
3. Motor Running Overload Class (PFN 15)
4. Motor Starting Overload Class (PFN 14)
5. Independent Starting/Running Overload (PFN 13)
6. Motor Overload Hot/Cold Ratio (PFN 16)
7. Motor Overload Cooling Time (PFN 17)
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
2
standard overload curves after the "pick-up" point has been reached is:
132
7 - THEORY OF OPERATION
1
Motor FLA
Factor DerateImbalCurrent
1
*Current Measured
Class * seconds 35
(seconds) Trip toTime
2
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