Technical information

8 – THEORY OF OPERATION
Motor Overload
113
Figure 14 – Overload Derated for Current Imbalance
0 5 10 15 20 25 30
0.75
0.8
0.85
0.9
0.95
1
MX Motor OL derating vs. current imbalance
Current imbalance %
Derating Factor
8.1.5 Harmonic Compensation
The MX motor overload calculation automatically compensates for the additional motor heating that can result from the presence of
harmonics. Harmonics can be generated by other loads connected to the supply such as DC drives, AC variable frequency drives, arc
lighting, uninterruptable power supplies, and other similar loads.
8.1.6 Hot / Cold Motor Overload Compensation
If a motor has been in operation for some time, it will have heated up to some point. Therefore, there is typically less overload
content available in the case where a motor is restarted immediately after it has been running when compared to the situation where a
motor has been allowed to cool down before restarting. The MX provides adjustable hot motor overload compensation to fully
protect the motor in these cases.
If the hot and cold maximum locked rotor times are provided, the MX Hot/Cold Ratio parameter value can be calculated as follows:
%100x
TimeRotor Locked ColdMax
TimeRotor LockedHot Max
1Ratio H/C OL
⎟
⎠
⎞
⎜
⎝
⎛
−=
If no motor information is available, a Hot/Cold ratio value of 60% is usually a good starting point.
The MX adjusts the actual motor overload content based on the programmed Hot/Cold Ratio set point and the present running current
of the motor so that the accumulated motor overload content accurately tracks the thermal condition of the motor. If the motor
current is constant, the overload content eventually reaches a steady state value. This value is derived as follows:
Facto
r
DerateImbalanceCurrent
1
FLA
Current
Ratio H/C OLOL
ss ××=
The running OL content is also adjusted based on the derating factor due to the presence of any current imbalances and or harmonics.
If the existing motor overload content is less than the calculated running OL content, the motor overload exponentially increases the
overload content until the appropriate running overload content level is achieved. If the existing motor overload content is greater
than the calculated running OL content level, the overload exponentially cools down or decreases to the appropriate running overload