How to Tell The Difference Between a Start and Run Capacitor

When your single-phase motor is giving trouble, the first thing you should check for is the condition of the capacitors, but how can you tell the difference between the start and run capacitors when they are nearly identical to look at?

The way to tell the difference between a start and run capacitor is by checking the rating printed on the label of the capacitor, the start capacitor will have a capacitance of between 70 – 800 micro farads (uF) whereas the run capacitor will be rated between 1-100 uF depending on the motor size.

Sometimes you will open up the terminal box only to discover that the rating label is either worn away or melted off, either way, it’s not readable. If this happens, not to worry, there are a few other ways of determining which capacitor is which.

blown capacitor

Terminal Block Markings

If you are having trouble finding the rated capacitance of your motor’s capacitors, the sure way to know which is the start and which is run is by looking at the markings that are on the terminal block of the motor.

motor maintenance pdf

The auxiliary winding and the start capacitor will always be connected to the terminals marked Z1 and Z2 whereas the main winding and the run capacitor will always be connected to the terminals marked U1 and U2.

Wiring Configuration

Examining the wiring setup can help identify the capacitor type. A start capacitor will always be wired in series with the motor’s start winding to provide the extra energy needed to overcome the initial inertia of the rotor, this is then disconnected after the motor reaches a predetermined speed.

In contrast, a run capacitor is connected in parallel with the run winding and remains active throughout the motor operation.

To find out more about how it all works you should check out this article on how single-phase motors actually work.

Capacitance Value

Capacitors have different capacitance values depending on their purpose. Start capacitors tend to have higher capacitance values, typically ranging from 70 to 800 microfarads, these capacitors are specifically designed to cope with the high starting load of the motor, typically the bigger the motor, the higher the capacitance value of the start capacitor, due to being able to hold more electrical charge.

capacitor rating plate

As you can see from the capacitor in the picture it is an 8 microfarad (uF) capacitor with a voltage rating of 440 volts AC.

Run capacitors, on the other hand, have lower capacitance values, often between 1 and 100 microfarads, because the run capacitor is only used to improve efficiency and power factor correction, meaning not a lot of charge is needed to be stored.

Physical Size

While not always a definitive indicator, there is often a correlation between capacitor size and its intended function. Start capacitors are typically larger in size due to their higher capacitance value, while run capacitors are relatively smaller.

1 farad capacitor big
1 Farad
15 micro farad capacitor
15 Micro farads

Just to put it into perspective, a one-Farad capacitor is roughly the size of a beer can whereas a 15 micro Farad capacitor is about the size of a yogurt bottle. But large capacitors like that are only used in large-scale power factor correction installations.

Labeling or Markings

Examining the capacitor itself can provide valuable information. Capacitors may be labeled or marked with their capacitance value, voltage rating, and sometimes even their purpose (e.g., “start capacitor” or “run capacitor”).

By considering these factors, you can rather easily differentiate between a run capacitor and a start capacitor. Understanding their unique roles is crucial for proper motor operation, maintenance, and troubleshooting thats why I would suggest checking out this article which covers exactly what each capacitor does and what can happen if there not maintained and replaced as needs be.

Conclusion

In conclusion, distinguishing between start and run capacitors involves considering several key factors. Not every time will you be lucky enough to have a rating plate in a readable condition on the capacitor but now that you are armed with the knowledge of what else to look for you will easily be able to tell the difference in the start and run capacitor.

Getting the correct information is key when it comes to fault finding and electrical maintenance, that’s why I would suggest taking a read of this article I wrote recently covering in basic language how motors work, I think you’ll enjoy it.

Gavin

I'm Gavin and Iv been teaching electrical science to apprentice electricians in a local technological university since 2022. I hold an Electrical Level 6 QQI Qualification along with several NZEB Certifications.

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