Section 6 Electrical System6-39PTCR DIAGNOSTICSWhat is a PTCR?A PTCR (or Positive Temperature CoefficientResistor) is made from high-purity, semi-conductingceramics.A PTCR is useful because of its resistance versustemperature characteristic. The PTCR has a lowresistance over a wide (low) temperature range, butupon reaching a certain higher temperature, itsresistance greatly increases, virtually stopping currentflow. When the source of heat is removed, the PTCRreturns to its initial base resistance.In severe duty cycles, it can be used to repeatedlyswitch (virtually stop) large currents at line voltages.PTCR’s have been used for many years in millions ofHVAC applications. In place of using theconventional start relay/start capacitor, a simplePTCR provides the starting torque assistance to PSC(Permanent Split Capacitor) single-phasecompressors, which can equalize pressures beforestarting.Compressor Start SequencePTCR’s provide additional starting torque byincreasing the current in the auxiliary (start) windingduring starting. The PTCR is wired across the runcapacitor (in series with the start winding).1. It is important for the refrigerant discharge andsuction pressures to be somewhat equalized priorto the compressor starting. To assure equalizationof pressures the hot gas valve (and HPR valve onremotes) will energize for 45 seconds prior tocompressor starting. The hot gas valve (and HPRvalve on remotes) remains on for an additional 5seconds while the compressor is starting.2. When starting the compressor, the contactorcloses and the PTCR, which is at a low resistancevalue, allows high starting current to flow in thestart winding.3. The current passing through the PTCR causes itto rapidly heat up, and after approximately .25-1second it abruptly “switches” to a very highresistance, virtually stopping current flow throughit.4. At this point the motor is up to speed and allcurrent going through the start winding will nowpass through the run capacitor.5. The PTCR remains hot and at a high resistance aslong as voltage remains on the circuit.6. It is important to provide time betweencompressor restarts to allow the PTCR to cooldown to near its initial temperature (lowresistance). When the contactor opens to stop thecompressor, the PTCR cools down to its initiallow resistance and is again ready to providestarting torque assistance. To assure the PTCRhas cooled down, during an automatic shut-off,the J model ice machines have a built-in 3-minuteoff time before it can restart.
