Scotsman CME306 - Manual (Page 34)

Service Diagnosis: PTCR
The CME306 and CME456 use a PTCR (Positive
Temperature Coefficient Resistor) in place of a
conventional start relay and start capacitor.
• Power from the contactor connects to the
PTCR and to the Run Capacitor.
• A wire connects the other terminal of the
PTCR to the compressor’s Start winding.
• Another wire connects the run capacitor to
the start winding.
• A parallel circuit connects power from the
contactor to the Run winding of the
compressor.
A PTCR changes resistance sharply when its
temperature changes. When the PTCR is cold, it
connects full current to the compressor’s start
winding. After a very short time the PTCR heats
up and shuts off the current flow. Under normal
operation, the PTCR’s case is at about 180 o F.
At that temperature the PTCR has very high
resistance and will not allow current flow. It must
cool down to 120
o
F. before current will pass
through it again.
Diagnosis:
1. Disconnect electrical power.
2. Check if the PTCR is cool enough to handle
safely. If not, wait 5 minutes for it to cool off.
3. Disconnect both leads to the PTCR and
measure its resistance with an ohmmeter. If the
PTCR reads more than 50 ohms, replace it.
CME306 & CME456
January 2000
Page 34
PTCR