How to Size a Starting and Running Capacitor for Motors

Capacitors play a critical role in motor performance, particularly in single-phase motors. They provide the necessary starting torque and help maintain smooth operation during runtime. Proper capacitor sizing is essential to ensure efficient motor startup, prevent overheating, and maximize energy efficiency.

In this guide, we will explore how to correctly size both starting capacitors and running capacitors, covering key calculations and factors that influence selection.

Starting Capacitors

Starting capacitors are electrolytic capacitors designed for temporary operation during motor startup. They provide the high torque needed to overcome initial inertia and are automatically disconnected once the motor reaches approximately 75% of its rated speed of centrifugal switch or relay.

Key Characteristics:

High capacitance values (typically 70-400 µF)
Short duty cycle (2-3 seconds maximum)
Electrolytic construction for compact size
Voltage ratings: 125V, 250V, 330V AC

Running Capacitors

Running capacitors are film capacitors that remain connected to motor operation. They provide continuous power factor correction, improve efficiency, and maintain balanced current in the motor windings.

Key Characteristics:

Lower capacitance values (typically 5-70 µF)
Continuous duty operation
Film construction for reliability
Higher voltage ratings for safety margin

Fundamental :

How to Size a Starting Capacitor

Starting capacitors are used temporarily to provide the necessary torque for the initial rotation of the motor. Once the motor reaches a specific speed (typically around 75% of its rated RPM), the capacitor is disconnected by a centrifugal switch or relay.

Start-capacitor sizing

Same formula structure:

Start capacitors are sized to give the needed starting torque and are only in circuit for a brief interval; typical values are 70 µF and up

Factors Affecting Starting Capacitor Selection

✅ Voltage De-rating Factor: Ensures capacitor longevity under high loads.
✅ Safety Standards Compliance: Meets industry regulations for electrical safety.
✅ Maximum Operating Temperature: Avoids overheating issues.
✅ Physical Space Availability: Ensures the capacitor fits within the motor housing.

For quick and accurate calculations, try our Capacitor Sizing Calculator Online.

How to Size a Running Capacitor

Running capacitors, also known as continuous-duty capacitors, help maintain power factor correction and efficiency. Unlike starting capacitors, they remain in the circuit while the motor is running.

Running-capacitor sizing

EXAMPLE CALCULATION FOR STARTING AND RUNNING CAPACITOR

Tips for Capacitor Sizing

✅ Refer to the Motor Nameplate: Always check manufacturer specifications for recommended capacitance values.
✅ Use Quality Capacitors: Choose capacitors with a high tolerance rating to ensure stable performance.
✅ Monitor Performance: After installation, observe motor operation to detect signs of under- or over-capacitance, such as excessive heat or poor startup performance.

Summary

🔹 Starting capacitors provide an initial boost and are temporary, while running capacitors stabilize performance and remain connected.
🔹 Use 30 to 50 µF per kW as a starting guideline for starting capacitors.
🔹 Running capacitor sizing depends on motor current, voltage, and frequency.
🔹 Voltage rating should be at least 30% higher than the supply voltage.
🔹 Consult motor specifications and perform calculations to ensure optimal motor performance and longevity.

By following these guidelines, you can confidently size capacitors for electric motors, ensuring efficiency and reliability. Finally I hope you get some knowledge how to correctly size both starting capacitors and running capacitors, covering key calculations and factors that influence selection.

🛠 Try Our Online Capacitor Sizing Calculator! 🚀

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brian k

11/03/2012 at 3:21 AM

hi couldnt make out the formula for single phase [start] caps, blurry.any way to get a clear copy? thanks brian

parvindra saini

06/09/2012 at 7:50 AM

pls.
see clear copy of single phase motor capacitor sizzing &with example

Roelof van Heerden

31/12/2012 at 10:30 AM

To whom it may concerin
Thank you for valuable information. It is a long time since I last worked with these motors and I forgot to work out the size of the capacitor for staring a S/P motor. Your infog in this article helped me a lot and I hope to recover my motor which was out of order for about a year now.
Happy new year!
Roelof van Heerden

Robert

11/02/2013 at 4:02 AM

Hey, I found your post very valuable. But I got a question regarding the run capacitor. You aproximate sizing of the capacitor is in the range of 4-40 mF and at the motors I have 1.1-2.2 kW the run capacitor is sized between 35-70 uF. The difference is more than 100 times, is there a calculation error?

merrick

14/02/2013 at 4:35 PM

can u send me on CLEAR copy of capacitance motor formulla!

tanx
mg.

shahid mahmood

01/10/2014 at 5:37 AM

Dear sir I want to know one information we have one air-condition inside that there is a contactor voltage 110 and 50 hertz.They fix on the coil 0ne capacitor and a resistance. I want to know what is the function of this and how to calculate the capacitor and resistance

J Roux

05/03/2015 at 11:01 AM

GOOD

John

31/03/2015 at 8:45 PM

Please send me a clear copy of this formula? Cannot make out the mathematical operations that are a part of the formula.
Thank you.

Steve

13/05/2015 at 12:14 PM

What clear copy?

Steven

14/05/2015 at 9:18 PM

Can you please send a clear copy of the formula? I can’t make out the signs in the formula
or the exponent. Can’t tell if they are + ,-,* they all look the same blob.
A friend has an Airfoxx carpet dryer 1HP. The cap got fried along with most of the wires. Can replace wires but cannot read anything on the Cap as is too badly damaged. The manufacturer has apparently gone out of business. It has three speeds and a 4 pole motor. I am assuming it was a start capacitor as it was plastic I was told most start caps are plastic and running are metal. Any help would be appreciated.
Thank you.
P.S.
The manufacturer plate only gives amps 8,9 and 11 amps and 1HP. Says 4 pole motor.
It runs on 120volt ac.