11
Motor
Manual
6 Thermal protection
Thermal protection is used to prevent motor overheating,
and to monitoring purposes.
6.1 Thermal protection with thermistors (PTC)
All standard motors (IE1 and IE2 only from 160 and up) have
PTC thermistors located in the stator windings.
It is recommended to connect the PTC thermistors to the
variable speed drive or other protective equipment by ap-
propriate means.
PTC thermistors are used to protect the motor from over-
heating. PTC thermistors are provided in sets of three, fitted
in each phase, unless otherwise stated. The PTC resistance
is 60 to 750 Ω at 20°C. The resistance rises in proportion to
the temperature. At the peak temperature the resistance
approaches infinity. This type of sensor is used in combina-
tion with a PLC or a relay for motor protection.
6.2 Thermal protection with bimetal switches
Thermal protection with bimetal switches is not stand-
ard and can be added in the motors upon request. Bimetal
switches are used to protect the motor from overheating.
Bimetal switches are normally fitted in sets of three, wired
in series. Each switch is placed on a phase winding and
opens or closes at a specific winding temperature. The bi-
metal switches are intended to be connected to an external
protective relay that switches off power if the motor over-
heats.
6.3 Temperature monitoring with Pt100 sensors
Pt100 sensors are not standard and can be added upon re-
quest. Pt100 sensors can be fitted in the motor both in the
windings and in the bearings. Pt100 sensors can track the
temperature rise where they are fitted. This is often used for
monitoring motor temperatures, for example in windings
or bearings, and it can indicate overheating. The signals can
be processed and used in PLCs.
6.4 Heater strip
A heater strip can be fitted in a motor. A heater strip allows
a minimum temperature to be maintained inside the mo-
tor. This prevent moisture condensation during cool-down
of the motor. Heater strips are available with two different
rated voltages (220–240 V and 110–120 V). The heater strip
can be energised when the motor is not running. Heater
strips are standard for marine motors with frame size 160
or larger.
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