Group 3.9.2: Motor Thermal Protection
The motor thermal protection is to protect the motor
from overheating. The AC drive is capable of
supplying higher than nominal current to the motor. If
the load requires this high current there is a risk that
the motor will be thermally overloaded. This is the
case especially at low frequencies. At low frequencies
the cooling effect of the motor is reduced as well as
its capacity. If the motor is equipped with an external
fan the load reduction at low speeds is small.
The motor thermal protection is based on a calculated
model and it uses the output current of the drive to
determine the load on the motor.
The motor thermal protection can be adjusted with
parameters. The thermal current IT specifies the load
current above which the motor is overloaded. This
current limit is a function of the output frequency.
The thermal stage of the motor can be monitored on
the control keypad display. See chapter Application
parameter lists.
If you use long motor cables (max. 100 m)
together with small drives (≤ 1.5 kW) the
motor current measured by the drive can
be much higher than the actual motor
current due to capacitive currents in the
motor cable. Consider this when setting up
the motor thermal protection functions.
CAUTION
Minor or moderate personal injury
‐ The calculated model does not protect
the motor if the airflow to the motor is
reduced by blocked air intake grill. The
model starts from zero if the control
board is powered off.
Code Parameter Min Max Unit Default ID Description
P3.9.2.1 Motor Therm Prot 0 3 2 704
0 = No action
1 = Alarm
2 = Fault (Stop according to stop mode)
3 = Fault (Stop by coasting)
If available, use the motor thermistor to
protect the motor. Choose then value 0 for this
parameter.
P3.9.2.2 MotAmbient Temp -20.0 100.0 °C 40.0 705 Ambient temperature in °C
P3.9.2.3 ZeroSpeedCooling 5.0 150.0 % Varies 706
Defines the cooling factor at zero speed in
relation to the point where the motor is
running at nominal speed without external
cooling.
P3.9.2.4 ThermTimeConst 1 200 min Varies 707
The time constant is the time within which the
calculated thermal stage has reached 63 % of
its final value.
P3.9.2.5 MotThermLoadbil 10 150 % 100 708
Motor thermal Loadability states how much
the motor can be thermally loaded. E.g. 90 %
means that the motor temperature will settle
around 100 % when continuously running at
90 % of nominal load.
P3.9.2.6 MotTempInitialMode 0 3 2 777
The estimated temperature rise is added to a
motor ambient temperature set by the
parameter P3.9.2.2 MotAmbientTemp.
0 = Disabled
2 = Last value
3 = Real time clock
P3.9.2.7 MotTempInitialAdjust 0.0 100.0 % 33.0 778
Motor thermal memory adjust [%].
Depending on the parameter P3.9.2.6
MotTempInitialMode, this parameter is used
as a constant or scaling coefficient.
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