Ideal Sinusoidal AC Capacitor Worksheet
Infer capacitance from voltage and current measured or specified for the capacitor itself at one sinusoidal frequency. This worksheet does not size a motor start or run capacitor from motor power,line current or supply voltage.
Document one capacitor operating point
Use the fundamental-frequency RMS voltage directly across the capacitor and the corresponding RMS current through that capacitor. They must describe the same operating state and frequency component.
Inferred capacitance C
Capacitive reactance |XC|
Angular frequency ω
Ideal reactive-power magnitude
Input voltage component
Input current component
Equations implemented
| Mennyiség | Equation | Hatókör |
|---|---|---|
| angular frequency | ω=2πf | f in hertz;ω in rad/s. |
| ideal capacitive reactance magnitude | |XC|=1/(ωC)=VC/enC | Same-frequency sinusoidal RMS terminal quantities. |
| inferred capacitance | C=IC/(2πfVC) | C in farads;multiply by10⁶ for µF. |
| ideal reactive-power magnitude | |QC|=VCIC=ωCVC² | Magnitude in var for the ideal single-frequency component;no loss estimate. |
The code uses the entered terminal quantities directly. It does not infer capacitor current from motor output power,efficiency or power factor and contains no hidden selection multiplier.
Authoritative scope
The IEC Electropedia defines an ideal capacitor as a linear capacitive two-terminal element and capacitance as charge divided by terminal voltage. IEC’s reactance entry gives the capacitive term −1/(ωC),and its sinusoidal-condition and phasor entries establish the same-frequency RMS basis used here.
IEC60252-1:2010+A1:2013,Edition2.1 is listed valid with a2026 stability date. Its public scope covers specified motor capacitors connected to asynchronous-motor windings,up to100Hz and rated voltages through660V,and addresses performance,testing,rating,safety,installation and operation. It explicitly notes that operation above rated voltage reduces life expectancy.
IEC60252-2:2010+A1:2013,Edition2.1 separately covers motor start capacitors for asynchronous motors at mains frequency,including metallized paper/plastic-film constructions and electrolytic capacitors within its scope. Both Part1 and Part2 Edition3 projects are under development with forecast publication in2028;the current consolidated editions remain valid at the2026-07-12 access date.
The public IEC cards do not provide a universal motor-capacitance selection equation,voltage multiplier,start/run ratio,duty cycle or discharge procedure. Exact selection,rating,marking,installation and safety clauses remain NEEDS_LICENSED_SOURCE and must be applied with the motor and capacitor manufacturers’ controlled data.
What was corrected
The former calculator estimated motor line current from output power,efficiency and power factor,assumed capacitor-branch current was60% of that value,treated supply voltage as capacitor voltage,multiplied the result by3.5 for a start capacitor and proposed a voltage rating at1.5 times supply rounded to50V. None of those hidden multipliers was sourced or generally valid.
It also published unsourced30–50µF/HP and100–150µF/HP rules,universal start/run ranges and the statement that start capacitors are always electrolytic,although IEC60252-2 also covers film/paper constructions. Defaults,presets,prefix parsing and local history could silently preserve an unsuitable result. All of those paths are removed.
Motor-capacitor decision boundary
- Use the exact replacement capacitance,tolerance,rated voltage,duty class,temperature category,protective class and switching arrangement specified by the motor/equipment manufacturer.
- Capacitor terminal voltage can exceed or otherwise differ from supply voltage because it is set by the winding/capacitor circuit. Do not derive a rating from supply voltage alone.
- A start capacitor is selected together with starting winding impedance,required torque,switching threshold and permitted duty. A fixed multiple of a run capacitor is not a design method.
- A measured value from this worksheet can support diagnosis only after instrument bandwidth,waveform,harmonics,temperature and capacitor tolerance are documented;it is not an authorization to energize or replace a component.
