AC Repair and fixes - AC Capacitor and Contactor
What do they do and why do you need them
The temperature outside is unbearable, and the inside of your home is quickly warming up. Your thermostat is turned on, but your ac unit isn't blowing cold air. What could it be and what can you do about it? Two of the most common air conditioner problems are a bad capacitor and a malfunctioning AC contactor. These may be foreign words to you, but that is why you are here reading this article. We will explain these crucial components, how they work, and why you need them.
Where are they and what do they look like
Most air conditioning systems have two capacitors, one connected to your indoor fan motor, and once connected to your outdoor unit. These will be cylindrical or oblong-shaped objects that have two or three terminal clusters on the top. Picture a soda can with terminals and wires on top. The smaller ones, like what you will find attached to your indoor fan motor, are called run capacitors. AC capacitors on outdoor units will be wired into the condenser fan and air conditioning compressor called a dual capacitor. Sometimes if the condenser fan motor has been replaced, it will have a second capacitor inside the outdoor AC unit. Dual capacitors will usually be much larger in size and rating. Air conditioner capacitors have two general ratings voltage and capacitance which are measured in microfarads.
What do they do for your air conditioning system?
A capacitor is wired in line with the voltage going to your motor. This means that the electricity feeding your fan motor or compressor will have to go through the capacitor before it can reach the device. The capacitor slows down part of the voltage as it passes through it, which creates two opposing magnetic fields. The interior of the motor becomes magnetized and begins to spin. It works similarly to when you try and put two magnets together in a certain way, but they repel each other. The magnetism on each side of the motor repels each other and spins as it tries to escape the magnetism. The capacitor is what gives it the initial jolt that it needs to start moving. Capacitors are used to accomplish two goals. The first goal is to provide more energy to help with starting torque. The second is to improve the power factor which improves the efficiency.
How to tell if you have a bad capacitor
Caution: Capacitors store energy and could shock you even if power to the air conditioning system has been disconnected. Additionally, the correct capacitor must always be used, with the proper size, and voltage rating. Always contact an HVAC technician for assistance with service on your AC unit.
The symptoms of failure for a capacitor are very clear, meaning that your condenser unit fan will not turn or is turning very slowly, or the fan supplying your home with air is not working properly. You may also hear a humming noise or notice a burning smell. Maybe the compressor, the big black cylinder at the bottom of your outside air conditioner, isn’t running. If the capacitor has failed, likely, the unit that it is providing power to will no longer start moving on its own. A way to check this is to spin the fan blade, without getting your hands near the fan blade, while it should be moving. If it starts spinning on its own and begins running normally, then you most likely have a bad or failing capacitor. A bad AC capacitor will sometimes puff up indicating that it has failed and needs to be replaced.
Why should I replace my capacitor if it is working?
Capacitors are a piece of electrical equipment, which means that they can get weak just like any other piece of electrical equipment. A qualified AC company will be able to check the readings of the old capacitor and tell you whether this has happened. A weak capacitor could result in the motor no longer starting on its own, or spinning far slower than its normal speed. Bad capacitors can cause motors to draw higher than normal currents. This could cause them to trip on overload protection. This higher than normal current can also cause high energy bills. A weak, failing capacitor needs to be replaced for this reason.
Hard Start AC Capacitors
A hard start air conditioner capacitor can save your air conditioner. When your air conditioner starts up it uses far more amperage than when it is running, you may have noticed that your lights flicker when this happens. A hard start capacitor is designed to help your unit start quicker and easier, especially under strenuous circumstances, such as extreme temperatures, high head pressure, or low voltage. This will help you save money and lower the risk of failure from your unit. A qualified HVAC technician will need to check the system for weak components before installing a hard start. Weak capacitors can cause the hard start capacitor to increase the amperage on startup causing damage to your compressor rather than preventing it.
The main AC contactor opens and closes to supply electricity to your outdoor air conditioning unit. The contactor is shut by a coil that becomes energized from the low voltage supplied by your thermostat. When cold air is no longer needed, the magnetic field on the coil is de-energized and opens by spring tension. It will only allow voltage to pass through whenever the unit receives a signal to run. The contactor is a black square inside of your condenser unit with metal spring-loaded electrical contacts and multiple wires coming off it. Once the connection closes, the main voltage is free to pass through, and your air conditioner begins to run.
Contactors have very distinctive symptoms of failure. Because of its design, it can malfunction because of an electrical failure or mechanical failure. When they fail electrically, they are designed to fail in the open position to remove power from the heat pump or air conditioner. If they fail mechanically, they could be open or shut depending on the failure. The first step of any troubleshooting is to determine if the air conditioning unit is getting power. If breakers are closed and the disconnect is properly seated, the contactor would be a good thing to check first. You may also note a loud chattering noise from the contactor cycling open and shut. If the AC contactor is closing and you still don't have power, it could be caused by debris or insects getting into the contactor limiting its connection. During routine maintenance checks, you could find that the main contact surfaces have become pitted or that spring tension has weakened. If the coil is not energizing to shut the contactor, the problem could be the thermostat, thermostat wiring, control board, etc.
Caution: Always contact an HVAC technician for assistance with service on your AC unit. To prevent electrical shock power should be secured to the system before working on the contactor. An air conditioner contactor could be failing for multiple reasons. Contact an HVAC contractor before replacing.
The next time your air conditioner fails you and you need to call an ac repair company you may already have an idea of what the problem is. You now know that if one of the fans or the compressor isn’t running you may need a new capacitor. If the whole outdoor unit isn’t running, then you know it could be the contactor. Now, when the ac repair company comes out to repair your system you will know to ask them if your system is a good candidate for a hard start to be added to your system.
AC Repair Series
Capacitors and contactors
Condenser fan motors
Why is my AC not blowing cold air?
Why is my AC not turning on?
Written by Jeremy James
Jeremy is the founder and CEO of Blue Water Climate Control. He is a Navy veteran with 24 years of naval service in the Navy's Nuclear Propulsion Program. He has broad engineering experience having worked on submarines and aircraft carriers. LinkedIn