Manitowoc J-1300, Service Manual

Page: 90 / 165

Section 6 Electrical System
6-39
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 hot gas valve (and HPR valve on
remotes) will energize for 45 seconds prior to
compressor starting. The hot gas 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 J model ice machines have a built-in 3-minute
off time before it can restart.