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The Purpose of a Start Capacitor
A start capacitor can be found in the electrical compartment of most single speed heat pumps and some outdoor air conditioning units. It can be identified by its cylindrical shape and black plastic body. These are often used in a system design where the unit must operate during low outdoor temperature conditions and/or when there is a non-bleed TXV installed in the refrigerant circuit.
A non-bleed TXV does not allow the refrigerant pressures to equalize on the sides of the compressor. Rather, high pressure refrigerant remains downstream of the compressor and low pressure refrigerant remains upstream of the compressor. When the pressures are not equalized, it makes it hard for the compressor to start up. If a hard start kit (such as a potential relay and start capacitor) are not installed on these units, the compressor may draw excessive current with an inability to start rotating. Over time, this situation causes the compressor windings to overheat and burn out.
The start capacitor stays in the electrical circuit for roughly the first quarter second of run time to allow more current flow to the start winding of the compressor during startup. This allows the compressor motor to overcome the pressure imbalance across the compressor pump in order to start running. The amount of current allowed at the start winding is based on the capacitor's microfarad rating. The higher the MFD, the higher the current. Compressors usually have a manufacturer specified start cap size found in the manufacturer's literature for each model compressor.
Testing of a Start Capacitor
To determine if a capacitor is good, use a digital multimeter that has an MFD settting to measure capacitance.
Turn the power off to the system.
Open the electrical compartment shroud.
Test for voltage to confirm the power is off.
Take a picture of the wiring to the start capacitor.
Disconnect the wires from the start capacitor terminals.
Put the multimeter setting on MFD.
Connect the test leads of a digital multimeter to the capacitor terminals. (NOTE: A bleed resistor should be installed on a start capacitor in order to bleed any residual voltage off the capacitor. If there is one presently, it must be uninstalled prior to testing and reinstalled after the testing is complete.)
Measure the MFD using a digital multimeter.
Compare the actual MFD measurement to the range listed on the side of the capacitor. If it is outside of this range, the capacitor is bad.
The start capacitor below tests good by measuring 154 Microfarads while the capacitor's range is listed as145-175 MFD.
The capacitor below tests bad by measuring only .19 Microfarads while the capacitor's range is listed as 161-193 MFD.
How the Potential Relay Works with the Start Capacitor
The start capacitor is connected to the compressor's start winding prior to and while the compressor is starting.
A potential relay shown below, (white rectangular box, commonly referred to as 5,2,1 relay) or a thermistor (pill-type typically in a cylindrical plastic shell), opens up the electrical circuit after the first quarter second of run time. Therefore, the start capacitor should only be in the electrical circuit until the compressor gets up to about 75% of its rotation.
At this point the compressor motor generates a back emf (back voltage created due to the motor spinning) that powers the coil within the potential relay (see photo below), which changes the coil into an electrical magnet.
In the pic below, the coil is not powered and the contacts are closed.
In the pic below, I am holding the metal plate inward as if the coil was powered, this opens the small electrical contacts
On a 5,2,1 Potential relay, powering the electromagnetic coil between terminals 5 and 2 causes the normally closed contacts between terminals 1 and 2 to open. When these contacts open, it removes the start capacitor from the electrical circuit (see photo below). The compressor's motor creates a high enough voltage (back EMF) to power the coil after the rotational speed of the motor passes roughly 75% of its rated rpm.
Below is a wiring diagram of a heat pump with the start capacitor noted as SC and the potential relay noted as SR.
If the potential relay contacts do not open, due to a failed potential relay coil, corrosion at the terminals, or if the contacts within the potential relay have melted together, the result is that the start capacitor will overheat and fail.
Start capacitors are designed to fail if left in the circuit in order to protect the compressor. The compressor will fail from excessive current at the start winding unless the start cap fails first, which it is designed to do!
Visual Signs of a Bad Start Capacitor or Bad Potential Relay
Visual signs that a start capacitor is bad (has failed) are in the photos below:
The top of a start capacitor is only partially attached or is completely missing.
The membrane is completely blown off.
The capacitor is leaking fluid.
The relief port at the top of the capacitor is open.
The pic below shows what it looks like for the start capacitor to fail during the actual event.
Takeaways
In the event of a bad potential relay, the start capacitor is actually designed to fail so the compressor is not completely destroyed.
When a start capacitor fails, it is absolutely essential to replace the potential relay/start thermistor as well as the start capacitor.
Potential relays usually fail closed (see image below) and may have burn marks on the inside of the case. Start PTC thermistors usually fail open.
Looking for a video on the Main Reason Why the Start Capacitors Fail in HVAC Units? Check out our "The Main Reason Why Start Capacitors Fail in HVAC Units! This Cap Blows Out!" video below!
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Published: 7/6/2022 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel
