PTC (Positive Temperature Coefficient)
The PTC is used as an alternative start device for compressors to previously used relays. The main property of the PTC is that it only allows current to pass freely when it is cold, but this passage of current also causes a rise in temperature which alters its resistance. It is the higher resistance that in turn reduces the voltage flow. Therefore by placing it in the start winding circuit, it will initially start the motor running until the heat produced by the current passing through it causes the temperature to rise, increasing the resistance within the PTC and restrict the voltage. Although at this point there is a trickle current getting through this is not enough to keep the windings energise but it is sufficient to keep the temperature of the PTC at a high enough level to render it inoperable. The start windings are now out of the equation and the motor continues on the run windings alone.
Associated Faults
  • Compressor does not start
  • Compressor hums and tries to start
  • Compressor overload tripping