132
PTC Thermistor Input for Motor Overheat Protection
Motor protection is performed using the temperature-resistance charac-
teristics of the positive temperature coefficient (PTC) thermistor, which
is embedded in the coil for each motor phase.
The following graph shows the characteristics of the PTC temperature-
resistance value.
The voltage across the ends of three PTC thermistors connected in series
is input to an analog input terminal (FR), and motor OH alarms and
motor OH errors are detected according to the voltage in respect to the
temperature-resistance characteristics of the PTC thermistor.
After a motor OH alarm is detected (FR input > 0.94 V), operation con-
tinues according to the n141 Motor Overheat Operation Selection (and
the OH8 indicator on the Digital Operator will flash).
After a motor error is detected (FR input > 1.87 V), the motor stops
according to the n141 Motor Overheat Operation Selection (and the
OH9 indicator on the Digital Operator will flash).
1330
550
Class F
Class H
Resistance (
Ω
)
Tr: Temperature Threshold
Varies depending on the PTC type, but
the resistance value is nearly the same
For example Tr = Class F, 150˚C
Tr' = Class H, 180˚C
Temperature
Tr – 5
Tr Tr
'
Tr + 5
