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Motor Capacitor Calculator

Calculate the correct start and run capacitor sizes for single-phase AC motors based on motor specifications.

Start Capacitor Purpose

Start capacitors provide the extra torque needed to start a motor. They are only in the circuit during startup (typically 1-3 seconds) and are disconnected by a centrifugal switch or relay.

Motor Specifications

Motor Power ⓘ

HP kW

Supply Voltage (V) ⓘ

Application Details

Supply Frequency (Hz) 50 Hz (Europe/Asia/Africa) 60 Hz (Americas/Japan)

Motor Application ⓘ Normal Starting Torque (fans, pumps) High Starting Torque (compressors, conveyors) Very High Torque (loaded starts)

Motor Efficiency (%) ⓘ

Recommended Start Capacitor

Capacitance

-

Voltage Rating

-

Minimum capacitor voltage

Motor Current

-

Estimated full load current

Run Capacitor Purpose

Run capacitors remain in the circuit continuously during motor operation. They improve power factor, efficiency, and torque. They are rated for continuous duty and have lower capacitance values than start capacitors.

Motor Specifications

Motor Power

HP kW

Supply Voltage (V)

Application Details

Supply Frequency (Hz) 50 Hz 60 Hz

Desired Power Factor Correction ⓘ Standard (0.85 PF) Good (0.90 PF) Excellent (0.95 PF)

Motor Efficiency (%)

Recommended Run Capacitor

Capacitance

-

Voltage Rating

-

Minimum (continuous duty)

Type

Oil-Filled

For continuous operation

Capacitor Start - Capacitor Run (CSR) Motors

CSR motors use both start and run capacitors for maximum performance. This provides high starting torque and improved running efficiency. Common in HVAC compressors, pumps, and industrial equipment.

Motor Specifications

Motor Power

HP kW

Supply Voltage (V)

Supply Frequency (Hz) 50 Hz 60 Hz

Application Details

Motor Application HVAC Compressor Pump / Water System Industrial Equipment General Purpose

Motor Efficiency (%)

Power Factor

Complete Capacitor Recommendations

Start Capacitor

-

Run Capacitor

-

Estimated Current

-

Motor Power

-

Disclaimer: This calculator provides estimated capacitor values based on common formulas and industry guidelines. Actual requirements may vary based on motor design, manufacturer specifications, and operating conditions. Always verify with motor nameplate data and consult a qualified electrician for critical applications.

Quick Reference Guide

Start Capacitors

• • 70-800 µF typical range
• • 125-330V AC rating
• • Electrolytic type
• • Short duty cycle only

Run Capacitors

• • 5-100 µF typical range
• • 370-440V AC rating
• • Oil-filled / film type
• • Continuous duty rated

Common Rule of Thumb

• • Start: 50-100 µF per HP
• • Run: 2-4 µF per HP
• • Voltage ≥ 1.5× supply
• • Match motor nameplate

Safety Tips

• • Discharge before handling
• • Match voltage ratings
• • Replace with same type
• • Check for swelling/leaks

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