MM300 MOTOR MANAGEMENT SYSTEM – QUICKSTART GUIDE 1
MM300 Motor Management System
Chapter 5: Protection elements
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Grid Solutions
Protection elements
Thermal protection
The primary protective function of the MM300 is the thermal model. The MM300 integrates
stator and rotor heating into a single model. The rate of motor heating is gauged by
measuring the terminal currents. The present value of the accumulated motor heating is
maintained in the
Thermal Capacity Used actual value register. When the motor is in
overload, the motor temperature and thermal capacity used will rise. A trip occurs when
the thermal capacity used reaches 100%. When the motor is stopped and is cooling to
ambient, the thermal capacity used decays to zero. If the motor is running normally, the
motor temperature will eventually stabilize at some steady state temperature, and the
thermal capacity used increases or decreases to some corresponding intermediate value,
which accounts for the reduced amount of thermal capacity left to accommodate
transient overloads.
The thermal model consists of six key elements.
• Unbalance current biasing that accounts for negative-sequence heating.
• Hot/cold biasing that accounts for normal temperature rise.
• RTD biasing that accounts for ambient variation and cooling problems
• An overload curve that accounts for the rapid heating that occurs during stall,
acceleration, and overload.
• Cooling rate that accounts for heat dissipation.
• Thermal protection reset that controls recovery from thermal trips and lockouts.
Each of these categories are described in the following sub-sections.
Overload curve
The overload curve accounts for the rapid motor heating that occurs during stall,
acceleration, and overload. Specifically, the overload curve controls the rate of increase of
Thermal Capacity Used whenever the equivalent motor heating current is greater than
1.01 times the full load current setpoint. The curve is defined by the following equation and
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