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Motor protection – direct temperature sensing 235
The main advantages of thermistors are:
• Their small size allows them to be installed in direct contact with the stator
winding.
• Their low thermal inertia gives rapid and accurate response to winding
temperature changes.
• They measure temperature directly irrespective of how these temperatures are
initiated.
• They can be used to detect overload conditions in motors driven by frequency
converters.
The temperature coefficient can be positive (PTC – positive temperature coefficient),
where the resistance increases with temperature, or negative (NTC – negative temperature
coefficient), where the resistance decreases with temperature. The type most commonly
used in industry is the PTC thermistor, whose typical resistance characteristic is shown in
the curve below.
The resistance at normal temperatures is relatively low and remains nearly constant up
to the rated response temperature (RRT). As the RRT is approached and exceeded, the
gradient of the resistance increases sharply, giving the PTC thermistor a high sensitivity
to small changes of temperature. At the set point, a temperature rise of a few degrees
results in a large increase in resistance. The resistance is monitored by a thermistor
protection relay (TPR) and, when the sharp change in resistance is detected by the
thermistor protection relay (TPR), it operates a contact to initiate an alarm or to trip the
protected device.
Thermistor protection relays are required to trip reliably when the sensor resistance
rises above about 3 kΩ. They will also respond to an open circuit, either in the cable or
the thermistor sensor, thus providing fail-safe protection. Modern TPRs are also designed
to detect a thermistor sensor short circuit, when sensor resistance falls below about 50 Ω.
The specified operating levels are:
• Thermistor over-temperature protection according to IEC
− Response level = 3300 Ω ± 100 Ω
− Reset level = 1650 Ω ± 100 Ω
• Thermistor short-circuit protection according to IEC
− Response level ≤ 15 Ω
In AC variable speed drives, PTC thermistors are commonly used to protect the AC
squirrel cage motor fed from inverters. Many modern AC converters have a thermistor
protection unit built into the converter, avoiding the requirement for a separate thermistor
protection relay.
In DC motors, PTC thermistor sensors are increasingly used instead of microtherms,
which are described in the section above.
The rated response temperatures (RRT), which are commonly selected for the various
classes of insulation on electric motors, are summarized in the table in Figure A.2.