Technical information

8 – THEORY OF OPERATION
Motor Overload
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8.1.3 Motor Overload Operation
Overload Heating
When the motor is operating in the overloaded condition (motor current greater than FLAxSF), the motor overload content
accumulates based on the starter’s operating mode at a rate established by the overload protection class chosen. The accumulated
overload content can be viewed on the display or over the communications network.
Overload Alarm
When the accumulated motor overload content reaches 90%, an overload alarm condition is declared. A relay output can be
programmed to change state when a motor overload alarm condition is present to warn of an impending motor overload fault.
Overload Trip
When the motor overload content reaches 100%, the MX starter trips, protecting the motor from damage. If the controlled fault stop
feature of the MX is enabled, the starter first performs the defined deceleration or DC braking profile before stopping the motor. The
motor overload trip time accuracy is ± 0.2 seconds or ± 3% of total trip time.
Overload Start Lockout
After tripping on an overload, restarting is prevented and the starter is “locked out” until the accumulated motor overload content has
cooled below 15%.
8.1.4 Current Imbalance / Negative Sequence Current Compensation
The MX motor overload calculations automatically compensate for the additional motor heating which results from the presence of
unbalanced phase currents. When a current imbalance is present, there can be significant negative sequence currents present in the
motor. These negative sequence currents have a rotation opposite the motor rotation and are typically at two times the line
frequency. Due to the negative sequence currents opposite rotation and higher frequency, these currents can cause a significant
increase in rotor heating.
The overload curves provided by a motor manufacturer are based on balanced motor operation. Therefore, if a current imbalance is
present, the MX motor overload compensates for the additional heating effect by accumulating overload content faster and tripping
sooner to protect the motor. The current imbalance compensation also adjusts the Hot / Cold motor protection as described below in
section 8.1.6 – Hot / Cold Motor Overload Compensation. The MX’s derating factor is based on NEMA MG-1 14.35 specifications
and is shown in the following Figure 14 – Overload Derated for Current Imbalance.