Manitowoc Ice S1800 service manual Ptcr Diagnostics, What is a PTCR?, Compressor Start Sequence

Section 6 Electrical System

PTCR DIAGNOSTICS

What is a PTCR?

A PTCR (or Positive Temperature Coefficient Resistor) is made from high-purity, semi-conducting ceramics.

A PTCR is useful because of its resistance versus temperature characteristic. The PTCR has a low resistance over a wide (low) temperature range, but upon reaching a certain higher temperature, its resistance greatly increases, virtually stopping current flow. When the source of heat is removed, the PTCR returns to its initial base resistance.

In severe duty cycles, it can be used to repeatedly switch (virtually stop) large currents at line voltages.

PTCR’s have been used for many years in millions of HVAC applications. In place of using the conventional start relay/start capacitor, a simple PTCR provides the starting torque assistance to PSC (Permanent Split Capacitor) single-phase compressors, which can equalize pressures before starting.

Compressor Start Sequence

PTCR’s provide additional starting torque by increasing the current in the auxiliary (start) winding during starting. The PTCR is wired across the run capacitor (in series with the start winding).

1.It is important for the refrigerant discharge and suction pressures to be somewhat equalized prior to the compressor starting. To assure equalization of pressures the harvest valve (and HPR valve on remotes) will energize for 45 seconds prior to compressor starting. The harvest valve (and HPR valve on remotes) remains on for an additional 5 seconds while the compressor is starting.

2.When starting the compressor, the contactor closes and the PTCR, which is at a low resistance value, allows high starting current to flow in the start winding.

3.The current passing through the PTCR causes it to rapidly heat up, and after approximately .25-1 second it abruptly “switches” to a very high resistance, virtually stopping current flow through it.

4.At this point the motor is up to speed and all current going through the start winding will now pass through the run capacitor.

5.The PTCR remains hot and at a high resistance as long as voltage remains on the circuit.

6.It is important to provide time between compressor restarts to allow the PTCR to cool down to near its initial temperature (low resistance). When theSV3158-6contactor opens to stop the compressor, the PTCR cools down to its initial low resistance and is again ready to provide starting torque assistance. To assure the PTCR has cooled down, during an automatic shut-off, the S model ice machines have a built-in 3-minute off time before it can restart.

Part Number 80-1634-3 6-15