How this calculator works
Energy-based sizing gives capacitance for a specified stored-energy target. It cannot predict starting torque because winding impedance, speed and phase shift also matter. The AC mode uses the current in the capacitor branch, which can differ substantially from motor line current. Start and run capacitors have different duty and construction ratings.
Formula
C = 2E/V²; E = C V²/2; V = sqrt(2E/C). AC branch: C = I/(2 pi f V).
Worked example
Storing 50 J at 100 V requires 0.01 F, or 10000 microfarads. A 0.001 F capacitor at 200 V stores 20 J.
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Frequently asked questions
Can this select a capacitor from motor horsepower alone?
No. Use the motor maker's specified capacitance and voltage rating; horsepower alone does not define the auxiliary winding.
Which voltage belongs in the equation?
Use the voltage actually across the capacitor. In the energy modes this is the charged voltage; in AC mode it is RMS voltage.
Should I choose the exact result?
Choose an appropriately rated component and check tolerance, pulse duty, temperature and service life. The calculated value is an ideal estimate.