Enter the motor nameplate and no-load current to see how many kvar to connect at the terminals without self-excitation risk, how much current drops and what to reset the overload to.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Without and with the chosen capacitor, from 25% to 125% of rated power. A fixed capacitor helps most at light load, where the motor power factor is worst.
Motor kvar at no load and full load, the amount needed for the target power factor, the self-excitation limit, the table value and the chosen size.
Input power, power factor and line current.
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It means connecting a capacitor right at a motor terminals so it supplies the reactive power the motor uses to magnetize. The supply delivers less current, cables and transformer are relieved and any reactive charges drop. It suits large motors that run many hours a day.
It depends on the present and target power factor: Qc = P ÷ η × (tan φ1 − tan φ2). In the example, a 40 hp motor at 94.1% efficiency and 0.85 PF needs about 9.2 kvar to reach 0.95; the calculator picks the standard 10 kvar unit, below the 10.8 kvar self-excitation limit.
When the motor is switched off with the capacitor still at its terminals, the capacitor can keep it magnetized and the coasting motor becomes a generator, with overvoltage. Keep capacitor kvar at or below 90% of no-load reactive power, √3 × V × no-load current; NEC 460.7 says it must not exceed the value that raises no-load power factor to unity. With 15 A no-load at 460 V that is 12.0 kvar and a 10.8 kvar limit.
On the manufacturer data sheet, or measure it with a clamp meter with the motor running uncoupled. It is usually 25% to 50% of full-load current. Without it, the calculator uses the typical kvar table by size and poles, which you can replace with the manufacturer table.
Yes, if the capacitor is on the load side of the overload: NEC 460.9 requires the setting to be based on the improved power factor. The new setting is FLA × PF before ÷ PF after; in the example, about 42 A instead of 47 A. With the capacitor on the line side, the setting stays.
No. The motor still uses the same real power (kW). What drops is current in the supply, cable and transformer losses and any utility power factor or kVA demand charges.
On motors fed by variable frequency drives, soft starters without bypass, reversing or plugging duty, or fast reclosing. Correct at the main switchboard with an automatic bank instead; see the plant power factor calculator.
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