You might wonder why your AC has trouble starting on hot days. The reason is often because of its capacitors. Start capacitors help the AC motor begin working fast. Run capacitors help the motor keep running well. If you know how capacitors work, you can find problems early. This helps you stop surprise breakdowns. Capacitor failures cause 31% of AC problems. This is more than any other part.
| AC Failure Type | Percentage of Failures |
|---|---|
| Capacitor Failures | 31% |
| Refrigerant Leaks | 21% |
| Compressor Burnouts | 15% |
| Clogged Drain Lines | 12% |
| Electrical Control Failures | 9% |
If you learn about these parts, you can care for your AC better. This helps keep your house cool all summer.
Key Takeaways
- Start capacitors give a big push to help your AC motor start fast. Run capacitors help the motor keep running well and save power. Start capacitors only work when the AC turns on, then they stop. Run capacitors stay on while the motor runs to keep power steady. Bad capacitors can make humming sounds, weak air, warm air, or stop the AC from starting. Finding these problems early can stop expensive repairs. Always match the capacitor’s voltage and microfarad numbers to your AC motor’s label. This keeps your system safe and working well. For safety, let a pro check and change capacitors. This is important if you see problems or do not know how to handle electric parts.
How Capacitors Work
What Is a Capacitor?
You may wonder what a capacitor does in your AC unit. A capacitor is a small part with two metal plates. These plates are separated by a special material called a dielectric. When you connect it to power, the plates get opposite charges. This lets the capacitor store energy. In a direct current (DC) circuit, the capacitor charges up and then stops the current. In an alternating current (AC) circuit, the capacitor keeps charging and discharging as the current changes direction. This action is important for how capacitors work in your air conditioner.
The main types of capacitors in HVAC systems are motor run, motor start, ceramic, and film capacitors. Most AC units use aluminum electrolytic capacitors to start and run the motor. These designs help the ac capacitor work well inside your air conditioner.
| Capacitor Type | Construction & Materials | Typical Use in HVAC Systems |
|---|---|---|
| Motor Run | Aluminum electrolytic, continuous-duty | Keeps AC motors running efficiently |
| Motor Start | Aluminum electrolytic, short-term use | Starts AC motors with high torque |
| Ceramic | Ceramic dielectric, small size | Used in control circuits |
| Film | Polypropylene or polyester film | Used in high-frequency circuits |
Why AC Units Need Capacitors
Capacitors are needed in your AC unit to help motors start and keep running. When you turn on your air conditioner, the start capacitor gives the motor a strong push. This helps the motor begin moving. After that, the run capacitor keeps the motor going at the right speed. This is how capacitors help your AC work well.
Tip: If your ac capacitor is not working, your air conditioner may not start or keep running, especially when it is hot outside.
Capacitors also protect your AC system. They control voltage, lower power surges, and stop the motor from getting too hot. Special materials, like metallized polypropylene film, help the ac capacitor last longer and handle heat and humidity. This design keeps your air conditioner working, even in tough weather.
Capacitors act like both buffers and boosters. They store and release energy when needed. This makes sure your AC motor gets the right power at the right time. That is how capacitors help keep your home cool and your AC running well.
Start Capacitor
What It Does
You count on your air conditioner to cool your home. The start capacitor is important when you turn on your AC. When you press the power button, the compressor motor meets resistance. The start capacitor gives a strong electrical push to help the motor start spinning. This push helps the motor beat inertia and begin to turn. The start capacitor makes a phase shift in the auxiliary winding. This phase shift creates a spinning magnetic field. The spinning field is needed for the motor to start. The start capacitor only works during the start phase. It disconnects when the motor reaches about 75% of its full speed. This protects the motor and helps it work well.
-
The start capacitor gives extra power to start the motor.
-
It helps lower the starting current, making things smoother.
-
The start capacitor stays in the circuit only at startup.
Note: If your AC has trouble starting, the start capacitor might be bad.
Capacitance and Design
You see that motor capacitors come in many shapes and sizes. The start capacitor is special because it has high capacitance and voltage. Most home AC units use a start capacitor rated at 75 microfarads (uF) and 440 volts. You may see labels like 75uF/440V on the motor. Manufacturers say to use a capacitor with a voltage rating higher than the supply voltage for safety. The start capacitor stores energy and lets it out fast when the motor starts. This design helps the compressor and fan motor start well. You should always match the capacitance to the motor label. Using a higher voltage is okay, but the capacitance must be right.
| Feature | Start Capacitor |
|---|---|
| Capacitance | 75 uF (common value) |
| Voltage Rating | 370V or 440V |
| Duty Cycle | Short-term (start only) |
| Shape | Cylindrical, plastic case |
When It Operates
You see the start capacitor work only when the motor starts. When you turn on your AC, the start capacitor gives an initial boost. This boost helps the motor shaft start spinning. The start capacitor turns off soon after the motor reaches the right speed. A potential relay or a PTC device makes this happen. The relay senses voltage in the start winding and disconnects the start capacitor. The PTC device heats up and raises resistance, opening the circuit. This timing makes sure the start capacitor does not stay on too long. If it stays on, it can break the motor. The start capacitor works for about 0.6 to 0.8 seconds each time the motor starts. You get this quick help every time your AC turns on.
Run Capacitor
Purpose in AC Units
Your air conditioner keeps your home cool and comfy. The run capacitor is very important for this job. When the motor starts, the run capacitor stays on. It helps the motor keep spinning at the right speed. This gives steady power and stops the motor from starting or stopping suddenly. The run capacitor makes the motor stronger and helps it use less energy. This means your AC works better and saves power. If you have a dual run capacitor, it helps both the compressor and fan motor together. This keeps the air moving and the temperature just right in your house.
-
The run capacitor works the whole time the motor is on.
-
It balances the current and stops voltage spikes.
-
Your motor runs smoother and makes less noise.
-
The run capacitor keeps the AC from getting too hot and helps it last longer.
Tip: If your AC is noisy or turns on and off too fast, check the run capacitor.
Capacitance and Features
You must use the right run capacitor for your AC motor. Most run capacitors are between 2.5 and 100 microfarads (µF). They usually have 370 or 440 volts. Run capacitors can be round or oval and are made of steel or aluminum. The CBB65 motor run capacitor is the most common type. It uses a special film inside to work well and last long. A dual run capacitor has two capacitors in one case. This saves space and makes it easier to put in. Always look at the motor label and match the numbers for safety and good performance.
| Attribute | Details |
|---|---|
| Capacitance Range | 2.5 to 100 µF |
| Voltage Ratings | 370 or 440 VAC |
| Frequency Rating | 50 or 60 Hz |
| Case Styles | Round or oval; steel or aluminum |
| Terminal Type | 1/4" push-on tabs, 2 to 4 terminals |
| Common Type | CBB65 motor run capacitor, metalized polypropylene film, aluminum casing |
| Dual Run Capacitor | Supports both compressor and fan motor |
Continuous Operation
The run capacitor helps your AC all the time it is running. It stays in the circuit and controls the power. The run capacitor makes a phase shift so the motor runs quietly and smoothly. This keeps the air moving and the temperature steady. The run capacitor also stops the motor from getting too hot or breaking early. It cuts down on wasted energy and noise, so your AC works better and lasts longer. If the run capacitor breaks, you might see weak air, too much heat, or the AC turning on and off a lot. You should change a bad run capacitor fast to keep your AC safe.
-
The run capacitor keeps the current steady.
-
It helps save energy and makes the AC work better.
-
Your motor lasts longer and works well.
-
The run capacitor helps the motor spin smoothly and stops overloads.
Note: Most run capacitors last about ten years. Check them often so your AC does not break suddenly.
Run Capacitor vs Start Capacitor
Key Differences
You might wonder what makes a start capacitor different from a run capacitor in your AC. The main difference is how and when each one works. The start capacitor gives a big push to help the motor start turning. The run capacitor keeps the motor running smoothly after it starts. Each one has its own job in the motor circuit.
Here is a table to show how they are different:
| Feature | Start Capacitor | Run Capacitor |
|---|---|---|
| Function | Gives extra current to start the motor | Keeps the motor running efficiently |
| Operation Time | Works only during startup | Works the whole time the motor runs |
| Capacitance Value | High (50-300 μF) | Lower (5-50 μF) |
| Duty Cycle | Short-term use only | Continuous use |
| Connection Mode | Disconnects after startup | Stays connected during operation |
| Starting Torque | Provides strong starting torque | Maintains torque while running |
| Usage Restriction | Not for continuous use | Can run continuously |
-
The start capacitor only helps when the motor starts.
-
The run capacitor stays on and helps the motor keep running.
-
Both are important, but they do different things.
Physical Characteristics
You can tell start capacitors and run capacitors apart by their looks. The start capacitor is usually round and made of black plastic or Bakelite. It is bigger because it needs to hold more energy for a short time. The run capacitor is round or oval and has a metal shell made of steel or aluminum. This metal case helps it last longer and handle heat since it works all the time.
-
Start capacitors are bigger and use plastic cases for short bursts.
-
Run capacitors are smaller and have metal cases for steady use.
-
The shape of the run capacitor just helps it fit in your AC.
Run capacitors have 2 to 4 terminals and are made for long use. Start capacitors have fewer terminals and are made for short use. These differences match what each one does in your air conditioner.
Impact on Performance
Both the run capacitor and start capacitor help your AC work well. The start capacitor gives a quick, strong burst of energy. This helps the motor start fast and use less energy to get moving. The run capacitor keeps the power steady while the motor runs. This helps the motor use less energy, run quietly, and last longer.
If the start capacitor breaks, your AC might not start at all. If the run capacitor breaks, your AC might run badly, make noise, or get too hot. Both are needed for your AC to work right and save energy.
-
The start capacitor helps the motor start quickly and safely.
-
The run capacitor helps the motor run better and stay cool.
-
If either one fails, you might pay more for energy or have poor cooling.
Tip: Always change a bad start capacitor or run capacitor fast. This keeps your AC working well and saves money.
How Do Start and Run Capacitors Differ
Function in AC System
You might wonder how these two parts work together in your air conditioner. The start capacitor and run capacitor are both important, but they do different things. When you turn on your AC, the start capacitor gives the motor a big push. This helps the motor start spinning. The start capacitor only works for a short time. When the motor gets to about 75% speed, the start capacitor turns off. This keeps the motor and capacitor safe from harm.
After the motor starts, the run capacitor takes over. It stays on while the motor is running. The run capacitor keeps the power steady and helps the motor run smoothly. This gives you steady air and less noise. The run capacitor also stops the motor from getting too hot or turning off too soon.
Here is a simple way to see how start and run capacitors are different in your AC:
-
The start capacitor gives a big burst of energy to start the motor.
-
The run capacitor stays on to help the motor keep running well.
-
The start capacitor turns off after the motor starts, but the run capacitor stays on.
-
Both work together: the start capacitor gets things moving, and the run capacitor keeps them going.
| Aspect | Start Capacitor | Run Capacitor |
|---|---|---|
| Operational Role | Gives a strong push to start the motor; only works at startup and then turns off. | Stays on while the motor runs, giving steady power and helping it work better. |
| Capacitance | High capacitance (50-300 μF), made for short use to give enough starting power. | Lower capacitance (5-50 μF), made for long use to help the motor work well. |
| Connection Mode | Connected with the starting wires; turns off after the motor gets to about 75% speed. | Connected with the running wires; stays on the whole time the motor runs. |
| Purpose | Helps the motor start by giving extra power and making a phase shift. | Makes the motor use power better, gives more strength, keeps voltage and current steady, and saves energy. |
Tip: If your AC motor will not start, the start capacitor might be bad. If your AC runs but does not cool well or makes noise, check the run capacitor.
Efficiency and Power Factor
You want your air conditioner to use less energy. The start capacitor and run capacitor both help, but in different ways. The start capacitor gives a quick, strong boost to get the motor going. This uses a lot of energy, but only for a short time. After the motor starts, the run capacitor takes over. It stays on and keeps the motor running at the right speed.
The run capacitor does more than just keep the motor spinning. It makes a phase shift in the electric current. This helps the motor use power better. The run capacitor makes the power factor better, so your AC uses less energy to do its job. A good power factor means you pay less for electricity and waste less energy.
If the run capacitor breaks, your motor might get too hot, use more power, or stop working. You might hear strange sounds or feel weak air. If the start capacitor breaks, your AC might not start at all. Keeping both capacitors in good shape helps your AC work better and saves money.
| Feature | Start Capacitor | Run Capacitor |
|---|---|---|
| Purpose | Gives a strong jolt of power to start the motor | Gives steady power to keep the motor running well |
| Operational Duration | Only works at startup, then turns off | Stays on while the motor is running |
| Electrical Support | Gives a big burst of energy to get the motor moving | Keeps the phase shift, so the motor works well and uses less energy |
| Failure Impact | Motor will not start | Motor works poorly, gets too hot, or stops working |
You can see how both types work together to help your AC use less energy. The start capacitor helps the motor begin, and the run capacitor keeps it running well. Both give extra power that helps the motor work better. This saves energy and lowers your bills. If either capacitor breaks, your AC will use more power and cost more to run. Always check both capacitors if your air conditioner has problems.
Signs of Capacitor Failure
Common Symptoms
You can find signs of bad capacitors in your AC if you pay attention. When a run capacitor starts to go bad, your air conditioner may act strange. You might feel weak air or notice your house is not as cool. Sometimes, the compressor and fan do not work as strong as they should. This makes your AC cool less and your home feels uneven.
Here are some signs you might notice:
-
Humming or clicking sounds from the outside unit
-
Air conditioner takes longer to start or will not turn on
-
Warm air comes out of the vents instead of cold air
-
AC shuts off suddenly while running
-
Your energy bill goes up because the compressor works harder
-
The capacitor looks damaged, like bulging, leaking, or rust
-
You smell something burning near the condenser unit
-
Airflow is weak or the temperature is hard to control
-
Cooling is not steady or changes a lot
If you see any sign of a bad ac capacitor, like a burning smell or a bulging case, turn off your AC and call a technician. Ignoring these signs can cause bigger problems later.
Testing Methods
You can use a few ways to test an hvac capacitor and see if it works. HVAC pros often check the run capacitor while the AC is running. First, you measure the start winding amps with a clamp meter on the wire going to the capacitor. Then, you use a voltmeter to check the voltage across the capacitor ends. After that, you use this formula to find the microfarad value:
Capacitor F = (Start Winding Amps × 2,650) ÷ Capacitor Voltage
Check this number against the value written on the capacitor. If your answer is within 5-6% of the label, your run capacitor is fine. If it is not, you have a bad run capacitor and need to replace it. Testing an hvac capacitor while it is working helps you see how it really performs without turning off the AC.
Always use tools with insulation and wear safety gear when you test run capacitor or any other part. Safety is most important when working with electricity.
If you want to know if your ac capacitor is bad, watch for the signs above and try these tests. Finding problems early can stop bigger issues from happening.
What to Do If a Capacitor Fails
Safety Tips
You need to be careful when working with AC unit capacitors. These parts can still have electricity inside after you turn off the power. Always do these things before touching a capacitor: Turn off the power at the main breaker. Use a multimeter to make sure the power is really off. Discharge the capacitor so you do not get shocked. Wear gloves and safety glasses to protect yourself. Use tools with insulation for extra safety. Read the instructions from the manufacturer. Learn about local electrical rules so you follow them.
⚠️ If you are not sure or see damage, call an electrician. Your safety is the most important thing.
DIY vs Professional Help
You might want to fix your AC capacitor by yourself. Doing it yourself can save money, but it can be dangerous. Capacitors can shock you if you do not handle them right. If you change start capacitors without training, you could lose your warranty or break your AC. Professionals have special tools and know how to work safely. They finish repairs faster and look for other problems too. Certified technicians follow the rules and keep your warranty safe. They also have insurance in case something goes wrong.
-
Professionals find and fix problems quickly.
-
Doing it yourself can cause injury, more damage, or lost warranty.
-
Technicians help your AC stay safe and work well for a long time.
Replacement Steps
If you want to change a start run capacitor, follow these steps to stay safe: First, turn off the power at the main breaker. Next, use a multimeter to check that the power is off. Discharge the old capacitor with an insulated screwdriver. Take a picture of the wires so you remember where they go. Carefully remove the old capacitor. Make sure the new capacitor has the same voltage and microfarad numbers as the old one. Put in the new capacitor and connect the wires like in your photo. Secure the new capacitor in place. Turn the power back on and test your AC.
🛠️ If you do not feel sure, ask a professional for help. A safe repair keeps your home cool and your AC working well.
Start capacitors help your AC motor start quickly. Run capacitors keep the motor working well and save energy. Start capacitors only work when the AC turns on. Run capacitors stay on while the AC is running. Both are needed for cooling and saving power.
If you notice humming, bulging, or weak airflow, your AC might have a problem. Finding these signs early can stop bigger repairs. If you are not sure what to do, call a professional for help.
Tip: Get your AC checked every year and keep the area around it clean. This helps your capacitors last longer.
FAQ
What happens if you use the wrong capacitor in your AC unit?
You could hurt your AC motor. Using the wrong capacitor might make it get too hot. Your AC may not work well or could even stop working. Always use a capacitor with the right voltage and microfarad numbers for your AC.
How often should you check your AC capacitors?
Check your capacitors once every year when you do AC maintenance. Checking early helps you find problems before they get worse.
Tip: Get your AC checked every year to keep it working well.
Can you run your AC if the start capacitor fails?
No, your AC will not turn on if the start capacitor is bad. The motor needs a boost from the start capacitor to begin spinning. Change a broken start capacitor before you use your AC again.
Why does your AC make a humming noise when a capacitor is bad?
A bad capacitor makes the motor have a hard time. You hear a humming sound because the motor cannot start or run right. This noise means you should check or change the capacitor.
⚠️ If you ignore this sound, you might have to pay for big repairs.
Written by Jack Elliott from AIChipLink.
AIChipLink, one of the fastest-growing global independent electronic components distributors in the world, offers millions of products from thousands of manufacturers, and many of our in-stock parts is available to ship same day.
We mainly source and distribute integrated circuit (IC) products of brands such as Broadcom, Microchip, Texas Instruments, Infineon, NXP, Analog Devices, Qualcomm, Intel, etc., which are widely used in communication & network, telecom, industrial control, new energy and automotive electronics.
Empowered by AI, Linked to the Future. Get started on AIChipLink.com and submit your RFQ online today!
