42 VLT
®
Soft Starter Pocket Guide
Thermal Model Protection: How is a motor thermal model
diff erent from other forms of overload protection?
The motor thermal model used in MCD 202 and MCD3000 soft starters
off ers precise motor protection normally only available from high-
end motor protection relays. The thermal model constantly models
motor temperature, based on information on the motor’s design
characteristics and actual operation. The thermal model accounts for
both iron and copper losses, as well as diff erent heating and cooling
rates when the motor is starting, running or stopped. Accurate
modelling allows the motor to be used to its maximum potential
without nuisance tripping.
Compared with a motor thermal model, thermal overload relays are
less precise.
They do not account for iron loss or for diff erent cooling rates at
diff erent stages of motor operation, and cannot be adjusted to match
the characteristics of the individual motor because the mass of the
bimetal strips is fi xed. The bimetal strips are also aff ected by their
own ambient temperature, which may be diff erent from the motor’s
ambient temperature.
Thermal modelling is also superior to inverse time-current and I2T
electronic overloads, which do not account for iron loss or for diff erent
cooling rates at diff erent stages of motor operation. They off er only
limited adjustment and the trip curves do not closely match the motors
thermal capability.. Inverse time-current protection also does not allow
for motor temperature before the overload.
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