AC Capacitors: Complete Guide

An AC capacitor may look small, but it plays a vital role in starting motors and keeping them running smoothly. In this article, we will explore what an AC capacitor is, how it works, its types, applications, signs of failure, and how to choose the right one.In this blog, we’ll explore what makes the Deeplift 2.5 MFD Oil Capacitor special, why oil-filled capacitors are better, and how Deeplift ensures long-lasting performance for your fans.

9/4/20252 min read

What is an AC Capacitor?

An AC capacitor is an electrical component used in alternating current (AC) systems to store and release electrical energy. It helps motors, compressors, and fans start smoothly and run efficiently. AC capacitors are designed to handle alternating current (unlike DC electrolytic capacitors).

In simple terms:

It stores charge temporarily.
Provides a phase shift for single-phase motors.
Ensures smooth operation and improves efficiency.

How Does an AC Capacitor Work?

Most home appliances like fans, refrigerators, and air conditioners use single-phase induction motors.
These motors cannot start on their own.
The AC capacitor creates a phase difference between the main and auxiliary windings of the motor.
This phase difference produces a rotating magnetic field, which helps the motor start and keeps it running.

Types of AC Capacitors

1. Start Capacitor

Used only for starting motors.
Provides a high starting torque.
High capacitance value (80µF – 300µF).
Works for a short time (disconnected after the motor starts).
Common in: Air conditioners, pumps, refrigerators.

2. Run Capacitor

Works continuously while the motor is running.
Improves efficiency and power factor.
Capacitance value (2µF – 70µF).
Rated for continuous AC voltage (370V, 400V, 440V AC).
Common in: Fans, blowers, compressors.

3. Dual Capacitor (Start + Run Capacitor in One)

Found in modern AC units.
Has three terminals (C, FAN, HERM).
Runs both the fan motor and the compressor motor with one unit.
Saves space and cost.

Applications of AC Capacitors

Ceiling Fans: Run capacitors (2.5µF – 4µF).
Air Conditioners: Dual capacitors for compressor + fan.
Refrigerators & Coolers: Start and run capacitors for motors.
Water Pumps: Start capacitors for high torque.
Industrial Motors: For efficiency and power factor correction.

Signs of a Faulty AC Capacitor

Fan or compressor not starting.
Humming noise from motor.
Motor overheats.
Fan rotates slowly or in the wrong direction.
Swollen or leaking capacitor.

Safety Note

Always discharge a capacitor before handling (it stores charge even when power is off).
Use proper insulated tools.
Replace faulty capacitors with exact specifications.

Conclusion

AC capacitors are essential for the proper functioning of fans, refrigerators, water pumps, and air conditioners. They:

Help motors start smoothly.
Keep them running at stable speed.
Improve power efficiency.
Protect motors from damage.

Without a capacitor, most single-phase motors simply won’t run.

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How to Select the Right AC Capacitor?

Capacitance Value (µF): Match with the motor rating (e.g., fan needs 2.5µF, AC compressor may need 30µF + 5µF).
Voltage Rating (V): Should be equal or higher than the system requirement (commonly 370V or 440V AC).
Type: Start capacitor, run capacitor, or dual capacitor depending on the application.
Quality: Always choose reliable brands to ensure durability.