These are semi conductor devices,
which have a property of suddenly
changing their resistance at a definite temperature known as 'curie point'.
Thermistors that may be provided on the motors are those having 'Positive
Temperature Coefficient' (PTC); where the resistance suddenly increases at a
'Curie Point'. The thermistors generally provided are at 130°c (PTC 130) for
Class B temperature rise, 150°C (PTC 150) For class F motors. A combination of
different ratings of thermistors can be provided in same motor for 'Alarm &
Trip' facilities.
MODE OF OPERATION OF
THERMISTORS
Three thermistors connected in series
are placed inside each of the phase windings of the motor. This gives
protections against single phasing and /or over heating due to excess load on
the motor. During normal operation the thermistors carry a current of few mA,
which is sufficient to actuate a relay in control unit. This in turn allows the
contactor operating coil to hold the starter in the 'Run position. If the
winding of the motor heats up to such an extent so as to bring the temperature
of the thermistors up to 'Curie point'. The increase in the resistance causes
the relay to open and the contactor to disconnect the motor supply.
