Run Capacitor vs Start Capacitor

Understanding the difference between a run capacitor vs start capacitor is a big part of working with an HVAC system, air conditioner, or AC motor. You need to know what’s what! Both types of motor capacitors play critical roles in motor operation, but they work in completely different ways. Whether you’re troubleshooting capacitor failure, planning a capacitor replacement, or just trying to understand how capacitors work for your system’s efficiency, a little information can help. When you know how each one functions, you can avoid mistakes and protect your equipment with the right parts. Along the way, remember that Surplus City has exactly what you need—all at rock-bottom prices!

What is the Difference Between a Capacitor Run and a Capacitor Start?

The core difference in the run capacitor vs start capacitor comparison lies in how each works with the motor. A start capacitor gives a motor the initial boost of power. It’s responsible for high starting torque and this is what gets the motor to begin spinning. A run capacitor, on the other hand, is a continuous duty capacitor. It keeps the motor’s operation stable once it reaches maximum speed.

A start capacitor vs run capacitor setup is most common in a single phase induction motor. Think of your air conditioner, fan motor, or other HVAC equipment—that’s where you’ll find these parts. Some systems use two separate capacitors, while others use a dual run capacitor. In the second case, this is a combination of the start cap and run cap in one cylindrical device.

How Do Run Capacitors Work?

A running capacitor, or run cap, works during the motor run cycle. It stores energy and regulates the electrical phase. Basically, it’s operating between the run winding and start winding. The running capacitor is responsible for increasing motor efficiency and delivering smooth, quiet operation.

One key thing about run capacitors is that they have lower microfarad ratings than start caps. This is because they are designed for continuous use without overheating in an HVAC system. When working like they should, you won’t notice a thing. But if something’s wrong, your run capacitor can cause humming, weak airflow, or even a bad compressor or fan motor.

How Do Start Capacitors Work?

A motor start capacitor works to provide a short burst of energy. This is what gets a motor up to full voltage and speed. It has just one job. Once the motor reaches proper RPM, the start capacitor disengages.

Start capacitors are not designed for continuous use. They have higher microfarad ratings, which means they can generate strong starting torque. Because of that though, they would overheat if left energized. This is the key difference in the run capacitor vs start capacitor comparison—one is continuous, and one is temporary.

How Do I Know If I Need a Run Capacitor or Start Capacitor?

To determine which type you need, begin with capacitor identification. Look at the physical shape, label information, and the motor’s wiring setup. Start capacitors are usually larger, black, and mounted in plastic cases. Run capacitors are typically metal and cylindrical.

In systems using separate capacitors, the motor will have separate terminals to connect to each part. If your HVAC system uses a dual run capacitor, both functions will be combined into one device.

What Do the Ratings Mean on Capacitors?

Every capacitor in a run capacitor vs start capacitor comparison includes two key ratings. First you have the Microfarad Rating (µF). This is used to indicate energy storage. It’s higher for start capacitors, and lower for run capacitors.

After that you have Voltage Classification. You might see these as 370V or 440V. The key here is to match or exceed the original voltage rating for your capacitor replacement. You want the right efficiency. Otherwise, if you go with the wrong ratings, you can damage the AC unit. You’ll also have worse motor efficiency, or even system failures.

When shopping, be sure to check the inventory at Surplus City. We carry all types of run capacitor vs start capacitor parts so you can get exactly what you need to keep your system running at its best, for less!

Can I Use a Start Capacitor as a Run Capacitor?

No. In the run capacitor vs start capacitor discussion, this is one rule you cannot bend. A start capacitor is not built for continuous operation. It’s designed for one thing only and will quickly overheat and fail if used in the wrong spot. From there, it can even damage the AC motor or other components.

What’s worse, using a start capacitor as a run capacitor can cause electrical hazards. This will start with a loud humming noise, and ultimately lead to permanent motor failure. Always replace each capacitor with the correct type.

 

What Are the Symptoms of a Bad Run Capacitor?

Since run capacitors operate continuously, they are more prone to capacitor failure over time. You might start by noticing problems where the motor won’t start. It could also stall soon after starting. From there you might notice a humming noise from the unit.

If you’re getting weak airflow or warm air from your air conditioner, that’s a sure sign that something isn’t working correctly. Sometimes this is because of a bad run capacitor, but not always. If the fan motor is spinning slowly or not at all, you might be on the right track to upgrade your run capacitor. Another thing to check for are leaks from the capacitor’s housing.

You don’t want your system to overheat. A bad run capacitor just leads to other problems, so be sure to take care of it quickly. If you notice any issues, disconnect power immediately and call a professional. They’ll be able to check the HVAC capacitor and take care of any problems with the right parts. Check Surplus City to see how much you can save on those upgrades!