Start Capacitors For Electric Motors

Find high-quality start capacitors for electric motors, pumps, and HVAC systems at BCB Sales & Service, Australia’s trusted source for motor components and replacements.

When an electric motor won’t start, struggles to reach speed, or hums without spinning, the most common culprit is the start capacitor. This small yet powerful device gives motors the initial boost they need to overcome inertia and begin rotating.

At BCB Sales & Service, we stock a full range of motor start, pump, and replacement capacitors suitable for single-phase induction motors, split-phase systems, and capacitor-start, run motors used in industrial, commercial, and domestic applications.

Browse our range of start and run capacitor options available online across Australia today.

What Is a Start Capacitor?

A start capacitor (sometimes referred to as a motor starting capacitor) is an electrolytic capacitor designed to provide an initial surge of electrical energy, helping an electric motor start turning.

When you power on a single-phase induction motor, it doesn’t naturally produce a rotating magnetic field. To address this, a start winding is connected in series with the start capacitor, creating a phase shift between the windings. This phase difference generates the initial torque required to start the rotor.

Once the motor reaches about 70–80% of its rated speed, a centrifugal switch or relay disconnects the start capacitor from the circuit, allowing the motor to run on its own with help from the run capacitor (in dual-capacitor designs).

In short, the start capacitor provides the torque to get the motor moving; without it, the motor may hum, vibrate, or stall.

How a Start Capacitor Works in a Motor

To understand the function of a motor start capacitor, it helps to know a little about the motor’s inner workings.

1. Initial Power-Up

When power is applied, the start capacitor stores and releases energy almost instantly, supplying an extra burst of current to the motor’s start winding.

2. Creating a Rotating Field

The capacitor alters the phase relationship between the current in the main and auxiliary windings, creating the rotating magnetic field required to initiate motion.

3. Starting Torque

This field delivers high starting torque, especially for pump motors, compressors, and fans that require strong initial movement to overcome load.

4. Switching Off Automatically

Once the motor reaches operational speed, the capacitor is disconnected to prevent overheating or energy waste. In systems that require continuous efficiency, a motor run capacitor is used to stabilise current and maintain the power factor.

Explore Run Capacitors ›

Start Capacitors vs. Run Capacitors

Although both types are referred to as motor capacitors, their functions differ significantly.

Many capacitor-start and capacitor-run motors use both, delivering high startup torque while maintaining stable performance under continuous load.

Applications of Start Capacitors

Start capacitors are used across a wide range of electrical and mechanical systems, including:

• Pump motors: Delivering strong starting torque to overcome static water pressure.

• Air conditioners: Assisting the compressor motor and blower fan during startup.

• Refrigeration units: Ensuring smooth start cycles for condenser fan motors.

• HVAC systems: Powering fans and capacitor start-run motors for consistent airflow.

• Industrial machinery: Supporting conveyor drives, mixers, and ventilators.

Whether you’re servicing a split-phase induction motor or a capacitor-start, run motor, the right start capacitor ensures reliability and extends equipment life.

Choosing the Right Motor Start Capacitor

When replacing or upgrading a capacitor, it’s important to match both the capacitance (µF) and voltage rating (V) to your motor’s specifications.

Here’s what to look for:

• Capacitance value: Measured in microfarads (µF). A mismatch can cause the motor to underperform or fail to start.

• Voltage rating: The capacitor’s voltage must be equal to or higher than the motor’s rating (e.g. 110v, 250V, 330V).

• Size and mounting: Ensure physical compatibility with the existing bracket or enclosure.

• Application type: Check whether it’s for a pump, compressor, or blower.

If you’re unsure, contact our technical team at BCB Sales & Service, and we’ll help you identify the correct motor start-run or pump capacitor for your application.

Why Choose BCB Sales & Service For Your Starting Capacitor

For over 30 years, BCB Sales & Service has supplied quality electric motor capacitors and replacement components to tradespeople, technicians, and maintenance professionals across Australia.

What Sets Us Apart:

• Extensive range: Capacitors for single-phase, capacitor start-run, and industrial-grade motors.

• Proven brands: Reliable manufacturers with strict quality assurance.

• Local stock: Fast shipping Australia-wide.

• Expert advice: Speak directly to motor specialists.

• Competitive pricing: Affordable solutions without compromising performance.

We understand how critical it is to restore functionality quickly, which is why our capacitors are tested for voltage tolerance, consistency, and longevity before reaching your hands.

Installation & Safety Tips

Replacing a motor starting capacitor is straightforward, but it requires care:

1. Turn off the power and unplug the motor or disconnect from the mains supply.

2. Discharge the old capacitor using a resistor to prevent electric shock.

3. Take note of wiring polarity before removal.

4. Avoid short circuits by ensuring wires are properly insulated.

5. Test with a multimeter to verify correct capacitance.

6. Use protective gloves and eyewear.

If your system still fails to start after replacement, the issue may lie in the starting winding, centrifugal switch, or overall motor wiring. In that case, a technician can assist with motor repairs or modifications.

Learn more about Motor Modifications ›
Explore Equipment Repairs ›

Contact BCB Sales & Service to Learn More About Our Range of Capacitor-Start Motor Options

For expert support, reach out to the BCB team today. Contact us by calling (03) 9546 3833, emailing sales@bcbsales.com.au, or submitting an online enquiry. Find the right start, run, or motor capacitor for your application — and keep your systems running efficiently year-round.

Common Questions About Start Capacitors

1. What happens if a start capacitor fails?

If the start capacitor in your single-phase motor fails, the motor won’t develop enough starting torque. You may hear a low hum, but the shaft won’t spin. Over time, this can cause overheating and permanent damage to the starting winding.

2. Can I replace a start capacitor with a higher µF rating?

Slight variations (within ±10%) are acceptable, but exceeding the manufacturer’s recommended range can lead to short circuits or premature failure. Always check the motor plate or manual for specifications.

3. What’s the difference between a capacitor start and a capacitor run motor?

A capacitor start motor uses the capacitor only during startup. A capacitor-start, run motor includes a second run capacitor that remains connected to improve power factor and efficiency.

4. How do I know if my pump capacitor needs replacing?

If your pump motor struggles to start, trips the circuit breaker, or makes unusual noises, the pump capacitor or start capacitor has likely degraded. Testing with a capacitance meter will confirm the issue.

5. Where can I buy capacitors near me?

BCB Sales & Service supplies capacitors online and in-store across Australia. Whether you need replacement capacitors for pumps, air conditioners, or split-phase motors, we’ll help you find the right model fast.