How Does a Start-Run Capacitor Work?

Understanding the vital role of start and run capacitors in electric motors.

When it comes to electric motors, few components are as essential (or as misunderstood) as the start-run capacitor. Found in everything from air conditioners and refrigeration units to industrial pumps and fans, capacitors help motors start, run smoothly, and deliver consistent torque.

But what exactly does a capacitor do in a motor, and what’s the difference between a start capacitor and a run capacitor? 

This guide explains how they work, why they’re important, and how to identify when one might need replacement.

What Is a Capacitor in an Electric Motor?

A capacitor, sometimes called a condenser in motor systems, is an electrical component that stores and releases energy. Think of it like a small rechargeable battery that provides a quick burst of power when needed.

In single-phase electric motors, capacitors play a critical role in generating a rotating magnetic field that enables the motor to start and maintain operation. Because single-phase motors can’t start on their own, the capacitor provides the initial "push" to start the motor.

Without a motor capacitor, your fan, compressor, or pump may hum, vibrate, or fail to start entirely.

The Difference Between Start Capacitors and Run Capacitors

Although they serve similar purposes, start capacitors and run capacitors perform distinct functions during the motor’s operation. 

Some systems use one or the other, while more advanced capacitor-start, capacitor-run motors use both.

Let’s look at how each one works.

1. Start Capacitor – Providing the Initial Boost

A start capacitor delivers a surge of electrical energy to help the motor begin turning. It is active only during startup, typically for a second or two, and disconnects once the motor reaches approximately 70–80% of its full speed.

The capacitor increases the phase difference between the current in the start winding and the main winding, creating a rotating magnetic field strong enough to overcome inertia.

In short:

• It gives the motor the torque it needs to start.

• It operates for a brief moment during startup.

• It’s typically rated for higher microfarads (µF) than run capacitors.

Example: A motor start capacitor might be used in a compressor motor, refrigeration unit, or pump that requires high torque to begin rotation.

Explore Run Capacitors

2. Run Capacitor – Maintaining Efficient Operation

A run capacitor remains connected to the circuit after the motor starts. Its role is to improve running efficiency, power factor, and performance stability.

It smooths current through the motor’s windings, reducing vibration and noise. The motor run capacitor ensures that the current and voltage remain properly phased, preventing overheating and energy waste.

In short:

• It stays in the circuit continuously while the motor runs.

• It improves efficiency and performance.

• It typically has a lower µF rating but a higher voltage tolerance.

Example: A motor run capacitor might be used in air conditioners, ceiling fans, or blower motors that operate continuously for long periods.

Explore Start Capacitors

What Is a Start-Run (Dual) Capacitor?

Many modern systems use a start-run capacitor, also known as a dual capacitor or a capacitor-start, run motor capacitor. This single unit combines the start and run functions in a single housing and includes three terminals instead of two.

Inside, it contains two capacitors:

• A start capacitor section that engages during startup.

• A run capacitor section that stays connected for ongoing operation.

This design saves space, reduces wiring complexity, and simplifies maintenance — especially in HVAC systems, refrigeration compressors, and air-conditioning blower motors.

How a Start-Run Capacitor Works in a Motor Circuit

Here’s a step-by-step breakdown:

1. Power is applied: When the motor is switched on, current flows to both the start and run windings.

2. Start boost: The start capacitor creates a phase shift, delivering extra torque to overcome mechanical resistance.

3. Run phase: Once the motor reaches speed, a centrifugal switch or relay disconnects the start capacitor.

4. Continuous operation: The run capacitor remains in the circuit to maintain smooth, efficient operation and stable torque output.

This sequence enables a capacitor-start, capacitor-run motor to deliver both high startup torque and efficient continuous performance.

Why Capacitors Are Essential in Electric Motors

Capacitors might be small, but their impact is massive. 

Here are a few reasons they’re so important:

• Efficient starting: Motors reach operating speed quickly without stalling.

• Reduced power consumption: The capacitor balances current flow, improving power factor.

• Lower heat generation: Keeps windings cooler, extending motor lifespan.

• Smooth operation: Minimises vibration, noise, and wear on components.

In industrial applications, such as pumps, fans, compressors, and ventilation systems, using the right motor capacitor can make a noticeable difference in performance and reliability.

Common Signs of a Failing Motor Capacitor

When a motor start-run capacitor begins to fail, you’ll usually notice symptoms long before complete breakdown. 

Here are some of the most common warning signs:

• The motor hums but doesn’t start.

• The system struggles to reach full speed.

• There’s reduced airflow in fans or blowers.

• The capacitor shows bulging, leaking, or burnt marks.

• Fuses or circuit breakers trip during startup.

A faulty motor run capacitor can cause overheating and higher operating costs, while a failed start capacitor may prevent the motor from starting.

How to Test a Motor Capacitor

If you suspect a capacitor issue, it’s important to test it safely. Always disconnect power and discharge the capacitor before testing. Using a multimeter set to capacitance mode, compare the reading to the µF value printed on the capacitor’s label.

If the reading is significantly below the rated value, or if the capacitor shows visible damage, it’s time to replace it.

Choosing the Right Capacitor for Your Motor

When replacing a capacitor, always match both the microfarad rating (µF) and the voltage rating. Using the wrong size can lead to poor performance or motor damage.

Tips for Selecting the Correct Capacitor:

• Check your motor’s nameplate or manufacturer's manual.

• Match the µF rating as closely as possible.

• Choose a voltage rating equal to or higher than the original.

• Always replace start capacitors and run capacitors with the correct type — never interchange them.

BCB Sales & Service carries a wide variety of industrial and commercial-grade capacitors designed for starting electric motors, HVAC systems, and machinery applications.

Expert Support and Capacitor Replacement Services

Unsure which capacitor your system needs? Our experienced technicians can help identify the correct motor start-run capacitor for your application. Whether it’s a capacitor-start, capacitor-run motor in a workshop fan or a motor-run, start-capacitor in an evaporative cooler, we’ll find the right solution quickly.

We also provide motor repairs, rewinding, and testing for clients across Melbourne and regional Australia, ensuring your system returns to peak efficiency.

Learn More About Motor Modifications ›

Discuss Your Needs With a Start and Run Capacitor Today

The start-run capacitor may be small, but it’s vital to the reliable operation of almost every single-phase electric motor. It allows motors to start efficiently, run smoothly, and consume less power — all while protecting your equipment from unnecessary wear and overheating.

At BCB Sales & Service, we stock the complete range of motor capacitors, including start, run, dual, and industrial-grade capacitors — each tested for quality and designed for long-term use in Australian conditions.

For expert help with motor selection, capacitor replacement, or repairs, get in touch today.