Capacitor Sizing for AC Motors
AC motors need the right capacitor for stable torque. A run capacitor helps a single phase motor create a shifted auxiliary winding current. A correction capacitor improves power factor when the motor draws lagging reactive power. Both jobs use capacitance, yet the purpose is different.
Why Size Matters
A small capacitor may leave poor starting torque. It can also keep current high during normal work. A large capacitor can overheat windings, raise voltage on the auxiliary circuit, or trip protection. Correct sizing reduces wasted current. It also supports smoother running and better service life.
Important Input Values
Voltage and frequency are central values. The same capacitor gives different reactive current at different frequencies. Motor power shows the useful output. Efficiency converts that output into estimated input power. Power factor describes the phase angle between voltage and current. Three phase banks also need the connection type. Delta and star connections do not use the same per phase capacitance.
Run and Start Capacitors
Run capacitors are normally rated for continuous duty. They stay in the circuit while the motor operates. Start capacitors are used briefly. They provide higher phase shift and stronger starting torque. This calculator estimates a start value from a chosen multiplier. Always compare the result with the motor nameplate and manufacturer chart.
Power Factor Correction
Power factor correction uses the difference between two reactive power values. The present angle comes from the existing power factor. The target angle comes from the desired power factor. The capacitor bank supplies the difference in kVAR. The result is then converted into microfarads by using voltage and frequency.
Practical Notes
Use motor rated voltage, not only line nameplate voltage. Use measured current when available. Keep the target power factor realistic. A value near 0.95 is common for planning. Avoid over correction, especially on lightly loaded motors. Select capacitors with proper AC voltage rating, temperature rating, and discharge safety. Use a qualified electrician for final wiring. This tool gives planning values, not a replacement for local code, testing, or motor maker guidance. Record each calculation result before buying parts. Keep notes about load, duty cycle, ambient heat, and enclosure space. These details help future maintenance decisions and safer replacements.