Product Overview 1-17
Thermistor/PTC Protection
The SMC-Flex provides terminals 23 and 24 for the connection of
positive temperature coefficient (PTC) thermistor sensors. PTC
sensors are commonly embedded in motor stator windings to monitor
the motor winding temperature. When the motor winding temperature
reaches the PTC sensor’s temperature rating, the PTC sensor’s
resistance transitions from a low to high value. Since PTC sensors
react to actual temperature, enhanced motor protection can be
provided to address such conditions as obstructed cooling and high
ambient temperatures.
The following table defines the SMC-Flex PTC thermistor input and
response ratings:
Table 1.A PTC Input Ratings
The following figure illustrates the required PTC sensor
characteristics, per IEC-34-11-2.
Figure 1.18 PTC Sensor Characteristics per IEC-34-11-2
PTC Trip
The SMC-Flex will trip with a PTC indication if:
• No other fault currently exists
• PTC protection is enabled
Response resistance 3400 Ω ±150 Ω
Reset resistance 1600 Ω ±100 Ω
Short-circuit Trip Resistance 25 Ω ±10 Ω
Maximum Voltage at PTC Terminals (R
PTC
= 4kΩ) < 7.5V
Maximum Voltage at PTC Terminals (R
PTC
= open) 30V
Maximum Number of Sensors 6
Maximum Cold Resistance of PTC Sensor Chain 1500 Ω
Response Time 800 ms
10
20
100
250
550
1330
4000
-20°C TNF-20K
0°C TNF- 5K
TNF+15K
TNF+ 5K
TNF
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