Talking About PTCRs

October 2, 2006
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PTCRs are made from a pure semiconducting ceramic material.

Positive Temperature Coefficient Resistors (PTCRs) are variable resistors that vary their resistance when their surrounding temperature changes.

Because they have a positive temperature coefficient, as their temperature increases so does their resistance. Also, as their temperature decreases, so does their resistance. PTCRs have a low resistance over a wide temperature range.

However, when they reach a certain higher temperature, their resistance greatly increases. This very high resistance can act as an open circuit and stop all current flow through a circuit. When the heat source on the PTCR is removed, its resistance will return to its initial room temperature resistance. PTCRs are made from a very pure semiconducting ceramic material.

This type of resistor is being used often in the HVACR industry in place of current and potential relays. In fact, the ice making industry is using PTCRs in place of potential relays in commercial ice machines. They make wiring much easier and get rid of a lot of nonessential, high-maintenance moving parts like relay contacts and springs.

The PTCR is wired in parallel with the run capacitor and in series with the start winding in a Permanent Split Capacitance (PSC) motor. PTCRs can supply the extra starting torque needed with a single-phase, PSC compressor motor on systems that can equalize pressure before starting.

When the cycling control brings power to the motor, both the run and start winding experience a heavy inrush of current. The run (main) winding sees heavy current flow because its low resistance winding is wired across Line 1 and Line 2 directly.

The start (auxiliary) winding also sees heavy current flow because its low resistance winding also sees Line 1 and Line 2 directly for a split instant.

By increasing the current flow in the start winding, additional starting torque is provided by the compressor motor. The reason being is the PTCR is at room temperature and its resistance is very low. It actually shunts out the run capacitor because it acts like a straight wire with very low resistance when it is at room temperature.

Since both the run and start winding see high current that is a bit out of phase from their different inductance values, the motor will have torque to start.

The high current flow through the PTCR will now increase its temperature and will have a very high resistance in less than one second. This will act as an open circuit in the PTCR circuit. The run capacitor will no longer be shunted out and the motor will now run as a PSC motor for the remainder of its running cycle. The PTCR will remain hot and at a high resistance value as long as voltage remains on the circuit.

PTCR in a commercial ice machine.

To help this PSC motor starting situation, some ice machine manufacturers use PTCRs as starting relays energizing a hot gas solenoid (harvest) valve before a compressor motor start-up.

Energizing a hot gas solenoid valve located between the compressor's discharge line and the entrance to the evaporator prior to starting will somewhat equalize system pressures and cause much easier compressor starting.

Many times manufacturers will allow the hot gas solenoid valve to remain energized for up to 45 seconds to ensure system equalization before compressor starting. To ensure the compressor is started and will not stall, often the hot gas solenoid valve is energized for 5 to 10 seconds after the compressor has started.

It is important that the PTCR is cooled down to near the ambient temperature before attempting to restart the compressor. If the cool down process is not accomplished, the high starting torque for the PSC compressor motor may not last long enough. Because of this, the ice machine's logic may be schemed to remain off for at least three minutes before it can automatically restart.

Often, a good PTCR may be too hot to operate properly at start-up because the compressor motor has short-cycled or the compressor overload has opened. In these situations, move the toggle switch to the "off" position and allow the compressor motor and the PTCR to cool down. A low voltage situation at the compressor at start-up may also cause the compressor not to have enough starting torque and stall.

In normal operation, a PTCR may reach over 200°F while the compressor is running. To check for a bad PTCR, visually inspect it for physical signs of damage.

Next, wait for at least 10 to 15 minutes for the PTCR to cool down to room temperature with the machine off. Isolate the PTCR from the ice machine and use an ohmmeter to measure its resistance. The manufacturer should have resistance values for room temperature PTCRs. If the resistance falls outside of the acceptance range, replace the PTCR.

Publication date: 10/02/2006

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