Compressor hard-start devices are a luxury item for service technicians to use in rectifying a myriad of compressor start problems.

It is true that the majority of hard-start device applications result from the marginal voltages delivered by electric utilities during peak demand periods.

As the predominant application is for air conditioning, the hard-start device can serve as an insurance policy for compressor starts when voltages drop to 90% of rated line conditions.

The ability to ensure that a compressor will start under low-voltage conditions can minimize the number of nuisance service calls, allowing a service contractor to focus on true problem events.

As the air conditioning industry has expanded and diversified, numerous types and models of air conditioning units and compressors have entered the marketplace. This diverse proliferation has resulted in the need to provide a one-size-fits-all compressor start device.

Unfortunately, as will be discussed in this article, a start device should be closely matched to the compressor, and a one-size-for-all approach may actually cause damage to a compressor if the device is applied incorrectly.


Before delving into specifics, it is useful to discuss the general application and function of a hard-start device. A capacitor in conjunction with a switching device (typically a relay) is introduced across the start windings of a single-phase compressor. The typical wiring arrangement for two-wire and three-wire connections is shown in Figure 1. (See page 28.)

When the compressor is called upon to start, the start capacitor provides a voltage boost to the start winding of the motor, effectively simulating the phase or lead/lag of a three-phase motor, and causes the motor rotor to turn. At some point, when the capacitor is released from the start winding, the motor continues to run.

In a three-wire configuration, the potential relay opens at a manufacturer’s specified voltage across the start winding of the motor, effectively removing the start capacitor from the circuit. A third wire is necessary to connect to the run winding.

In a two-wire configuration, the potential relay and start capacitor are connected across the run and start winding. The potential relay opens at a specified increment above line voltage, thus removing the start capacitor from the circuit. There is no need for a third wire.

The size of the capacitor significantly impacts the characteristics of the start winding. Figure 2 shows the generalized impedances for the compressor motor and start devices. (See page 28.) As such, the start capacitor should be carefully matched to the specific compressor.


Two main types of start devices exist in the marketplace today. Each has its own desirable applications and each specific advantages. The two types of start devices are:

PTC — Positive temperature coefficient devices; and

Potential relay devices — voltage sensing and current sensing.

The PTC device has been successfully employed in a number of applications for many years. This device uses a ceramic element with a predictable thermal response to the introduction of electric current. As current is introduced across the start windings, the PTC element begins to warm.

When the PTC device reaches approximately 250 degrees F (corresponding to 0.6 to 0.8 seconds), the resistance in the element increases and creates an open switch, releasing the start winding from the circuit. The 0.6 to 0.8 seconds that the PTC device allows the start windings to be engaged is generally enough time to enable the compressor to start.

This device’s chief advantage is its simplicity. A two-wire connection between the run and start terminals on the compressor is all that is required to provide reliable starts in most cases. However, this device has several limitations that should be considered if the application is critical:

  • The PTC device has no ability to sense whether the compressor has actually started.

  • The amount of time provided for a start boost is dictated solely by the temperature of the ceramic device, which has warmed due to the introduction of the starting current.

  • If the compressor does not start before the temperature threshold has been reached, it will not start until the PTC device cycles through a cool-down period (usually 2 to 3 minutes). Many view this start approach as an appropriate safety measure. The PTC effectively limits the continued unsuccessful cycling of the start windings, which can often result in a motor burnout. Others will argue that a start device should be able to re-cycle immediately. If this feature is desired, a PTC is not the correct start device for the application.

    The potential relay start device has recently been the subject of considerable attention in the market. Several manufacturers are promoting products with a variety of technologies. The primary distinction between these potential relay devices relates to a voltage-sensing or current-sensing capability.

    The voltage-sensing method monitors start winding-developed voltage and actuates a mechanical or electronic potential relay to disengage the start capacitor. The electronic potential relay is inherently more reliable and precise than the older type, mechanical potential relay.

    The current-sensing approach senses current through the run winding and drops the start capacitor out of the circuit based upon a threshold value. Both methods have proven effective in providing devices that are able to “sense” when a compressor has started, thus providing more reliable compressor starts in marginal conditions.

    However, the current-sensing method must employ an internal fuse to protect the motor from potential damage — and it is more difficult to connect than the two-wire, voltage-sensing type.


    The proliferation of potential relay-type devices has resulted in the notion that one capacitor can be employed to start all compressors. The idea is, use the biggest capacitor and give the compressor a big kick to get it started. The sensing characteristic will drop the capacitor out of the start circuit when necessary, so the compressor will not be harmed.

    This idea, however, is flawed. The use of a capacitor that is too large for the impedance characteristics of the windings in some compressors can actually result in significant compressor damage. Recent investigations indicate that this situation is particularly evident in voltage-sensing devices.

    In a successful compressor start, the run-start and start-common voltages increase to a maximum value and the total supply current drops to operating conditions when the start device is dropped from the circuit. In an unsuccessful (locked-rotor) compressor start, the run-start voltage never increases to a point that would indicate a motor start. The total supply current remains at a maximum and the motor never starts.

    If the start capacitor is too large for the application, the capacitor can actually mask the developed voltage in the start windings and keep the start capacitor in the circuit continuously. In a compressor start with a capacitor that is too large, the motor is actually running, but the run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs. A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit. This event can be seen at the end of the time duration of the run-start current.

    A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start windings in the compressor.

    When an oversized capacitor without a safety timing circuit is used, the run-start voltage is suppressed by the combined characteristics of the motor windings and the extra-large capacitor. It never reaches the prescribed threshold voltage defined by the potential relay for removing the start capacitor from the circuit. The total supply current remains near the locked rotor value even after the motor has started.

    If the capacitor is never removed from the start windings, premature winding failure can occur. Since this is so, care should be taken when selecting capacitor sizes for an application.

    Some devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode. In these cases, the start winding voltage drops appropriately after the start capacitor has been removed.


    Compressor start devices are available in a variety of forms. Specific applications call for specific products.

    PTC devices will continue to fulfill specific needs in the industry. Potential relay devices are available in a wide assortment. Care should be employed in selecting potential relay devices to ensure that all state-of-the-art developments are included.

    The features that comprise the most advanced developments in start device technology can be summarized thus:

  • Voltage-sensing technology that monitors for motor start (current-sensing devices require internal fuse protection);

  • A two-wire connection that simplifies installation;

  • A secondary timing circuit that ensures the capacitor is not permanently left in the start winding circuit;

  • A fully electronic device, minimizing the limitations of mechanical devices and secondary fusing associated with triac devices; and

  • A start device matched with an appropriately sized capacitor to cover the range of compressors for the intended application. One size does not fit all.

    The use of compressor start devices results from a need to ensure that a compressor — usually in air conditioning — will start under voltage conditions that are less than ideal. Several options exist in the market to address compressor start concerns.

    Start devices exist in many forms for specific applications. Take care and choose a device that meets the requirements of the job.

    Doyle is principal of Doyle Engineering Co., 101 N. Adams St., Rockville, MD 20850; 301- 979-0013; 301-674-8579 (fax); (e-mail).

    Publication date: 06/24/2002