PTCR installed on Condensing Unit

[scarey8]

Went to a no cooling call on a 10 Seer 2.5 Ton Comfortaire/Heat Controller Air Conditioner,
Model RSD1030-Y, Serial LT027589364R

PTCR, Postive Temperature Coeffiecient Resistor had blown apart, cleaned all charring up that I could, replaced the capacitor and repaired burnt wiring to fan and compressor, initial inrush of compressor start was 47 but within fractions of seconds it settled to proper RLA on rating plate, I installed a hard start and reduced the inrush current to 28 amps.  Cooling back on.

My question is.... why would a PTCR be required or installed on the compressor?  only thing I could think of would be an electrical safety to prevent compressor windings from over heating.  PTCR was wired in series off the herm terminal of the cap to the compressor run winding.

Any advise, call it my need to know

SCAREY

[Admin]

PTCRs have been used in the HVACR industry for many years. They are made from a very pure semiconducting ceramic material.
Because the PTCR has a positive temperature coefficient, resistance is very low over a wide range of low temperatures. However, when reaching higher temperatures, the resistance increases. These characteristics can be used when starting certain motors.

Many times, PTCR devices are used in place of potential or current-type starting relays. PTCRs can provide starting torque in permanent split capacitance (PSC) motors, when the condensing and evaporating pressures have been equalized somewhat during the off cycle.

The main advantage of this device is that it has no contacts, coils, or moving parts, thus requiring less maintenance. Also, it is simpler to troubleshoot.

The PTCR is wired in parallel to the run capacitor. In other words, it is wired in series with the start winding and in parallel with the run winding.
When the cycling control closes, an inrush of starting current passes through the PTCR and the start winding because they are in series. This large starting current inrush happens because of the very low resistance of the PTCRs when they experience a low temperature. The starting current can actually bypass the run capacitor because it passes through the PTCR (which is in series with a run capacitor).

One function of the run capacitor is to cause a phase shift between the run and start windings during the starting and run cycles, which creates a torque. However, the run capacitor also has the ability to limit current flow through the start winding during the running cycle because of its capacitive reactance. With the PTCR in parallel with the run capacitor, large amounts of starting inrush current can pass through the PTCR and start winding for more starting power.

This high starting current passing through the PTCR causes it to increase in temperature rapidly. In less than half a second, the PTCR quickly increases in resistance, which pretty much stops the current flowing through it. The motor should now be up to speed and acting much like a PSC motor, with the run capacitor in series with the starting winding and in parallel with the run winding.

As long as there is voltage on the motor circuit, the PTCR stays hot and thus has very high resistance. It is actually out of the circuit when hot, and has no function but to block current through itself.