Max Capacitor kVAR at Motor Terminals (Self-Excitation Limit)
Returns the largest power-factor capacitor that can be switched with a motor before self-excitation overvoltage.
Example
You enter
- Motor line-to-line voltage (V) 480
- No-load (magnetizing) current (A) 8
- Safety factor (default 0.90) 0.9
You get
- Magnetizing kVAR 6.65
- Max terminal capacitor kVAR 5.99
Details, formula, and sources
Magnetizing kVAR = sqrt(3) x V x no-load current / 1000 (NEMA MG-1 / IEEE 18), times a safety factor (default 0.90). The manufacturer's max-kVAR table by HP and speed governs; pair with the power-factor correction result and keep the smaller value.
magnetizing_kvar = sqrt(3) x V_ll x I_no-load / 1000; max_capacitor_kvar = safety_factor x magnetizing_kvar (default 0.90).
Maximum motor-terminal capacitor to avoid self-excitation, NEMA MG-1 and IEEE 18, by name.
A capacitor switched with a motor must stay below the motor's magnetizing (no-load) kVAR, or the coasting motor can self-excite into a damaging overvoltage. The manufacturer's max-kVAR table by HP and speed governs.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: v_ll, i_noload_a, safety_factor, magnetizing_kvar, max_capacitor_kvar
- Magnetizing kVAR the three-phase no-load reactive power sqrt(3) x V x I_no-load / 1000 is the self-excitation ceilingNEMA MG-1 / IEEE 18
- Safety margin the recommended terminal capacitor is a fraction (default 0.90) of the magnetizing kVAR; keep the smaller of this and the target-PF sizemanufacturer convention