Ac Theory Level 3 Lesson 1 (2024)

FAQs

What is the power factor for a purely capacitive circuit is always percent? ›

In a purely capacitive circuit, the current leads the voltage by 90° and the power factor is zero leading.

Current lags voltage by 90° in a pure inductive circuit. Things get even more interesting when we plot the power for this circuit: In a pure inductive circuit, instantaneous power may be positive or negative.

The ideal power factor is unity, or one (1.0) which means that all the energy supplied by the source is consumed by the load. Anything less than one means that extra power is required to achieve the actual task at hand. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.

Voltage lags current by 90° in a pure capacitive circuit. As you might have guessed, the same unusual power wave that we saw with the simple inductor circuit is present in the simple capacitor circuit, too: In a pure capacitive circuit, the instantaneous power may be positive or negative.

PF = kW / kVA

A steel stamping operation runs at 100 kW (Working Power) and the Apparent Power meter records 125 kVA. To find the PF, divide 100 kW by 125 kVA to yield a PF of 80%. This means that only 80% of the incoming current does useful work and 20% is wasted through heating up the conductors.

Therefore, the power factor of pure inductor and pure capacitor circuit is zero.

In circuits with primarily capacitive loads, current leads the voltage. This is true because current must first flow to the two plates of the capacitor, where charge is stored. Only after charge accumulates at the plates of a capacitor is a voltage difference established.

The voltage-current phase relationship in a capacitive circuit is exactly opposite to that in an inductive circuit. The current of a capacitor leads the voltage across the capacitor by 90 degrees.

The phase difference between current and voltage in a pure inductive circuit is: 90°

You can improve power factor by using power factor correction (PFC) devices, such as capacitors, reactors, or filters. These devices are connected in parallel or series with your loads and compensate for the reactive power in the circuit. They can be fixed or variable, depending on the type and variation of your loads.

How to correct power factor? ›

You can improve power factor by adding power factor correction capacitors to your plant distribution system. When apparent power (kVA) is greater than working power (kW), the utility must supply the excess reactive current plus the working current . Power capacitors act as reactive current generators (see Figure 6) .

A capacitor helps to improve the power factor by relieving the supply line of the reactive power. The capacitor achieves this by storing the magnetic reversal energy. Figure 8. Improvement in power factor when the capacitor is added to the circuit.

Remember, if the wave is shifted to the left then it is leading and positive, while a shift to the right is lagging or delayed in time, and thus negative.

In the case of an AC source, we have an alternating voltage which continuously charges and then discharges the capacitor. While charging the capacitor the voltage across the plates of the capacitor rises and the charge also builds up, and when the voltage across the plates decreases the charge will also decrease.

The most Common AC waveform is the sine wave, which derives its name from the fact that the current or voltage varies with the sine of the elapsed time. Other common AC waveforms are the square wave, the ramp, the saw-tooth wave, and the triangular wave.

The net power in a purely capacitive circuit is zero, as it was in the purely inductive circuit.

Power Factor values typically range from 0.80 to 0.98 and are often expressed as a percentage (80% to 98%). For Electric Choice customers, excess Reactive Power resulting from operations below 80% Power Factor will be charged $3.50 for each excess kVAR.

It is defined as the cosine of angle between the voltage and current. In case of DC circuit the power factor is zero because both inductive and capacitive reactance are zero because of zero frequency. But in case of AC we have frequency so the power factor exits in AC networks.

For the purely resistive circuit, the power factor is 1 (perfect), because the reactive power equals zero. Here, the power triangle would look like a horizontal line, because the opposite (reactive power) side would have zero length.