Formula Used
Motor input power: Input power = output power ÷ efficiency.
Single phase current: I = P ÷ (V × PF).
Three phase current: I = P ÷ (√3 × V × PF).
Run capacitor estimate: C = Icap ÷ (2πfV). The answer is converted to microfarads.
Start capacitor estimate: Cstart = Crun × starting multiplier.
Correction capacitor: Q = P × (tan cos⁻¹ PF1 − tan cos⁻¹ PF2).
Capacitive reactance: Xc = 1 ÷ (2πfC).
Stored energy: E = 0.5 × C × V².
How to Use This Calculator
- Enter motor power and choose the correct unit.
- Enter supply voltage and frequency.
- Select single phase or three phase operation.
- Choose run, start, or correction sizing.
- Enter efficiency and present power factor.
- Use nameplate current when it is available.
- Add a margin only when your design standard allows it.
- Press calculate, then review standard capacitor values.
Example Data Table
| Motor |
Voltage |
Frequency |
Type |
Typical Input |
Use Case |
| 0.5 HP pump |
230 V |
50 Hz |
Run |
65% capacitive current |
Continuous operation estimate |
| 1.5 HP compressor |
230 V |
60 Hz |
Start |
3 times run value |
Short duty starting torque |
| 5 kW fan motor |
400 V |
50 Hz |
Correction |
0.78 to 0.95 PF |
Feeder reactive load reduction |
Choosing a Motor Capacitor
A motor capacitor helps a single phase motor build torque. It also supports smoother running when the winding design needs a phase shift. A correction capacitor has a different job. It reduces reactive demand seen by the supply. This calculator separates those use cases.
Why Size Matters
A small capacitor may not start the motor. It can also leave the motor hot, slow, or noisy. A large capacitor can raise winding current. It may shorten motor life. It can also stress switches and relays. Good sizing starts with the motor nameplate. Voltage, frequency, current, efficiency, and power factor all matter.
Run, Start, and Correction Uses
A run capacitor stays in circuit during normal work. It is usually oil filled and rated for continuous duty. A start capacitor works for a short time only. It gives high starting torque, then a switch or relay removes it. A correction capacitor is placed across the supply. It improves power factor for the feeder, not the internal motor winding angle.
What the Results Mean
The tool estimates input power from output power and efficiency. It can also use nameplate current when it is available. For run sizing, it converts selected capacitive current into microfarads. For start sizing, it applies a starting multiplier. For correction sizing, it estimates reactive power reduction from the present and target power factors.
Practical Selection Tips
Always choose a capacitor voltage rating above the circuit voltage. Match the duty type to the job. Use motor run capacitors for continuous operation. Use start capacitors only with approved starting circuits. Pick the nearest standard value after checking the motor maker data. When two standard values are close, choose the safer value recommended by the service manual.
Safety Note
Capacitors can hold charge after power is removed. Discharge them with a proper resistor. Do not short the terminals with a screwdriver. Confirm wiring before energizing the motor. Use this calculator for planning and comparison. Final selection should follow the motor nameplate and local electrical rules.
Limits of Any Estimate
Motor designs vary by brand and load. Ambient heat also changes life. Treat the answer as a strong starting point. Confirm the final part with tested service data when possible.
FAQs
1. What does this calculator estimate?
It estimates motor run capacitors, start capacitors, and correction capacitors. It uses voltage, frequency, power, efficiency, power factor, and current data.
2. Can I use the result as the final capacitor size?
Use it as a planning value only. Final selection should follow the motor nameplate, maker data, duty rating, and local electrical rules.
3. What is a run capacitor?
A run capacitor stays connected during normal motor operation. It helps create a phase shift and supports smoother running in many single phase motors.
4. What is a start capacitor?
A start capacitor gives extra starting torque for a short time. It must be removed by a relay, switch, or approved starting device.
5. What is power factor correction?
Power factor correction reduces reactive demand seen by the supply. It does not replace the internal run capacitor required by a motor design.
6. Why is voltage rating important?
The capacitor voltage rating must exceed the circuit voltage. A low rating can cause overheating, swelling, failure, or unsafe operation.
7. Should I use nameplate current?
Yes, when available. Nameplate current usually gives a better practical estimate than calculated current because it reflects the actual motor design.
8. Why are standard values shown?
Capacitors are sold in standard microfarad values. The calculator shows nearby options so you can compare real parts more easily.