Figure 1. PTC relay with start capacitor

One type of starting relay used on fractional horsepower compressors is the positive temperature coefficient (PTC) relay. It performs the same function as a current relay or potential relay but in a much different manner. A PTC relay uses a positive temperature coefficient thermistor to remove the start winding and/or start capacitor from the circuit. A PTC thermistor is basically a resistor whose resistance increases on an increase in temperature.

The circuit is wired so that the PTC thermistor is wired in series with the start winding of the motor (Fig. 1). If the motor requires a start capacitor, it will also be wired in series with the start winding and PTC relay. When the motor starts, the resistance of the thermistor will initially be very low and sufficient current will flow through the PTC thermistor, the start winding and/or start capacitor to allow the motor to start normally. As the current flows through the PTC thermistor, it will quickly heat up and its resistance will quickly increase to a point where the current flow will dramatically reduce, essentially removing the start winding and/or start capacitor from the circuit.

Another use for a PTC relay is with a permanent split capacitor (PSC)-style compressor. These compressors are designed to operate with a run capacitor in the motor circuit to give them better running efficiency. Sometimes these compressors will require the addition of a start capacitor to assist the starting of the compressor. Normally a potential relay will be used to disconnect the start capacitor from the circuit. However, a PTC relay can also be used for the same purpose. There are two ways in which a PTC relay can be used to assist in starting a PSC-style compressor.

Figure 2. PTC relay with run capacitor

A PTC relay can simply be wired in parallel with a run capacitor (Fig. 2). During starting, the PTC relay causes a short across the run capacitor. This will allow full-line voltage to the start winding during start-up, giving the motor a stronger phase shift to get started. Once the thermistor warms up, the short across the run capacitor is removed and the motor runs normally.

The other method is to use the PTC relay with a start capacitor (Fig. 3). The PTC relay is wired in series with a start capacitor, and both the PTC relay and start capacitor are wired in parallel with the run capacitor. During the starting of the motor, both the start and run capacitors are in the circuit to assist in starting the motor. Once the thermistor heats up, the start capacitor is essentially removed from the circuit and the motor runs normally.

Figure 3. PTC relay with start and run capacitor

The one disadvantage to using this type of relay is once the motor circuit is shut down, it takes about two to five minutes for it to cool down to a point that its resistance reaches a point where the motor could be started again. If the motor tries to start before the PTC relay has cooled enough, the motor’s overload protector will trip the motor circuit. Once the motor’s protector has reset and thermistor cools down sufficiently, the motor should start normally.

Troubleshooting these relays is normally simple. Remove the relay from the circuit and allow it to cool to room temperature. Place an ohmmeter across the terminals of the relay. At room temperature, the measured resistance should be low. If an infinite resistance is measured the relay needs to be replaced.

Publication date:09/01/2008