How to Use the Single-Phase Motor Capacitor Calculator
A single-phase motor cannot produce a rotating field on its own, so a capacitor is wired in series with the auxiliary winding to shift the phase. When the nameplate has worn off or a swollen capacitor is all that is left, this tool estimates a candidate value from horsepower, supply voltage, frequency and efficiency.
The method works from reactive power. Input power is rated output divided by efficiency, the capacitor is assumed to supply 30 percent of that as reactive VAR, and capacitance follows from C = Qc × 10⁶ ÷ (2πf·V²). The 30 percent figure was back-calculated from real permanent split capacitor motors: a 1/2 HP unit on 230 V at 60 Hz lands near 8 μF, which matches what is actually sold. The same formula also shows why a 50 Hz motor needs more capacitance for identical output.
Start capacitors run 4 to 8 times the run value to lift starting torque, but they are short duty electrolytic units that fail quickly if left in circuit, so never substitute one for a run capacitor. Voltage across the capacitor reaches 1.3 to 1.5 times line voltage, which is why the recommended rating is well above the supply. Treat the result as a reference only, give priority to any nameplate or existing marking, and isolate the supply and discharge the capacitor before service, with a qualified electrician confirming the work.
Frequently Asked Questions
The nameplate, or the marking on the capacitor that was already fitted. Manufacturers choose the value from the actual winding design and test data, so treat this calculation as a starting point for motors whose nameplate is unreadable or whose capacitor is missing.
A run capacitor stays in circuit continuously, so it is a small film type rated for continuous duty. A start capacitor is much larger to boost starting torque but is an electrolytic unit rated only for a few seconds, switched out by a centrifugal switch or relay.
No. Voltage across a capacitor in series with the auxiliary winding reaches 1.3 to 1.5 times line voltage in normal running. That is why 230 V motors use 370 VAC film capacitors, and an under-rated unit will swell or burst within months.