ICE MACHINES Q-Model Service Manual

Section 6Electrical SystemPTCR DIAGNOSTICSWhat is a PTCR?A PTCR (or Positive Temperature Coefficient Resistor) ismade from high-purity, semi-conducting ceramics.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 the conventionalstart relay/start capacitor, a simple PTCR provides thestarting torque assistance to PSC (Permanent SplitCapacitor) single-phase compressors, which canequalize pressures before starting.Compressor Start SequencePTCR’s provide additional starting torque by increasingthe current in the auxiliary (start) winding during starting.The PTCR is wired across the run capacitor (in serieswith the start winding).1. It is important for the refrigerant discharge andsuction pressures to be somewhat equalized prior tothe compressor starting. To assure equalization ofpressures the harvest valve (and HPR valve onremotes) will energize for 45 seconds prior tocompressor starting. The harvest valve (and HPRvalve on remotes) remains on for an additional 5seconds while the compressor is starting.2. When starting the compressor, the contactor closesand the PTCR, which is at a low resistance value,allows high starting current to flow in the startwinding.3. The current passing through the PTCR causes it torapidly heat up, and after approximately .25-1second it abruptly “switches” to a very highresistance, virtually stopping current flow through it.4. At this point the motor is up to speed and all currentgoing through the start winding will now passthrough 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 between compressorrestarts to allow the PTCR to cool down to near itsinitial temperature (low resistance). When thecontactor opens to stop the compressor, the PTCRcools down to its initial low resistance and is againready to provide starting torque assistance. Toassure the PTCR has cooled down, during anautomatic shut-off, the Q model ice machines have abuilt-in 3-minute off time before it can restart.Part No. 80-1100-3 6-41