Manitowoc QD0202A, Service Manual

Page: 97 / 175

Section 6 Electrical System
Part No. 80-1100-3
6-41
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 the
contactor 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 Q model ice machines have a
built-in 3-minute off time before it can restart.