Công cụ tính bù hệ số công suất

How this is calculated#

The tool finds the capacitor bank that lifts a load to a target power factor. It then gives the current before and after.

1. Real power. You enter kW, or kVA with a power factor. The tool finds the real power.
2. Reactive power. It finds the reactive power at the present power factor. It does the same at the target.
3. Correction required. The difference is the kvar the bank must supply.
4. Bank steps. It rounds up to whole steps of the size you enter. It then recalculates the power factor with that bank.
5. Currents. It finds the current before and after. The difference is the capacity the bank releases.
6. Bank conductors. It sizes the bank conductors at 135 % of the bank rated current. This follows AS/NZS 3000 Cl 4.15.2.3.

AS/NZS 3000 Cl 4.15.2.2 covers switching, and Cl 4.15.3.1 covers discharge.

Worked example#

• 100 kW three‑phase load at 400 V, 0.75 to 0.95, 25 kvar steps. The correction required is 55.32 kvar. The bank is 3 × 25 kvar. The power factor with the bank is 0.991. Current falls from 192.45 A to 145.59 A. The released capacity is 32.47 kVA. The bank conductors need at least 146.14 A.

One motor#

Set Correct to one motor to size a capacitor fitted at a motor and switched with it. The capacitor must not supply more than the motor's own magnetising kvar. If it does, the motor, still turning after it is switched off, can excite itself from the capacitor and raise its voltage. The tool takes the no-load current as all magnetising and keeps the capacitor to 90 % of the no-load kvar, a rule of practice. It fits the smaller of that and the kvar to reach the target at full load, then gives the line current with it from the motor tool's full load current.

• A 30 kW motor at 0.85 with 17 A no-load current can take 10.6 kvar at most. The 9.7 kvar to reach 0.95 fits, and the line current falls from 56.6 A to 50.64 A.
• An 11 kW motor at 0.8 with 7.5 A no-load current can take 4.68 kvar at most, which lifts it to 0.939 and cuts the line current to 18.79 A. Correct the rest at the board.
• A 55 kW motor at 0.75 with 30 A no-load current can take 18.71 kvar at most, which lifts it to 0.867 and cuts the line current to 101.78 A.

Limits#

The tool does not assess harmonics unless you enter the THDi. Switch each step with a device rated for capacitors. The tool does not model resonance with the supply.

See also: Power factor and reactive power, Power factor correction for a 250 kW factory and Three phase power.