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Motor Protection Using Positive Temperature Coefficient (PTC) Thermistors

The temperature resistance characteristics of three PTC thermistors in the motor stator winding protect the motor from overheat.

The PTC thermistor must have the characteristics shown in Figure 1 for each motor phase.

Figure 1. PTC Thermistor Temperature and Resistance

When the PTC input signal input to the drive is more than the overload alarm level, the drive detects oH3 [Motor Overheat (PTC Input)]. The drive continues the operation set in L1-03 [Motor Thermistor oH Alarm Select]. By factory default, oH3 flashes on the keypad and the drive continues operation.

The overheat fault level triggers an oH4 [Motor Overheat Fault (PTC Input)] fault, and outputs a fault signal. The drive outputs a fault signal, and uses the stop method set in L1-04 [Motor Thermistor oH Fault Select] to stop the motor.

Note:

PTC is an acronym for Positive Temperature Coefficient.

Figure 2 shows the configuration procedure when you use terminal A3.

  1. Connect the PTC thermistor input from the motor to analog input terminal A3 on the drive.

    Figure 2. Connect Motor PTC

  2. Set drive DIP switch S1-3 to V (voltage) and set DIP switch S4 to PTC.

  3. Set these MFAI terminals:

    • Set H3-05 = 0 [Terminal A3 Signal Level Select = 0 to 10V (Lower Limit at 0)].

    • Set H3-06 = E [Terminal A3 Function Selection = Motor Temperature (PTC input)].

  4. Set these L1 parameters:

    • L1-03 [Motor Thermistor oH Alarm Select]

    • L1-04 [Motor Thermistor oH Fault Select]

    • L1-05 [Motor Thermistor Filter Time]