Start Thermistors

Start thermistors are used to provide additional starting torque for PSC compressors. The start thermistor is a temperature sensitive device whose electrical resistance changes as a result o f a change in temperature. Positive temperature coefficient (PTC) thermistors increase their resistance with an increase in temperature. PTC thermistors are commonly used in the start circuit of PSC compressor motors.

Figure SP-9-12 shows a P SC compressor motor with a PTC start thermistor. A s shown, the PTC thermistor is placed across the run capacitor. A t room temperature, the PTC thermistor resistance is very low, about 25 to 50 ohms. When the compressor is turned on, the application of voltage provides an initial surge of high current through the start winding because the low resistance of the PTC thermistor is effectively bypassing (shorting) the run capacitor. This surge results in increased motor starting torque due to increased current flow in the start winding. The temperature increase created by the high current causes the PTC thermistor resistance to increase very rapidly to several thousand ohms, blocking current flow and effectively removing the thermistor from across the run capacitor. However, a small leakage current through the thermistor is enough to keep the thermistor heated and its resistance high. The compressor then runs as a normal PSC compressor. Circuit operation remains this way until the compressor is turned off. When turned off and after a cool-down period, the thermistor’s resistance will once again be the low value needed to start the compressor.

Testing the status of a thermistor (good or bad) is done by taking a resistance measurement o f the thermistor. The cold resistance of any PTC thermistor should be about 100 to 180 percent of the thermistor ohm rating. For example, a thermistor rated at 25 ohms should have a cold resistance of 25 to 45 ohms. If the PTC thermistor resistance is much lower or more than 200% higher than its rating, the thermistor should be replaced. The procedure for testing a start thermistor is provided in the detailed procedure at the end of this section.

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