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.

Looking at the graph, the current wave seems to have a “head start” on the voltage wave; the current “leads” the voltage, and the voltage “lags” behind the current. Voltage lags current by 90° in a pure capacitive circuit.

Power factor is an expression of energy efficiency. It is usually expressed as a percentage—and the lower the percentage, the less efficient power usage is. Power factor (PF) is the ratio of working power, measured in kilowatts (kW), to apparent power, measured in kilovolt amperes (kVA).

Therefore, it is true that 100% of the apparent power is used or consumed in a purely resistive circuit. This is not the case in circuits that have reactive elements, such as inductors and capacitors, where the current and voltage are out of phase with each other.

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.

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.

Measure the current (I) through the load. Determine the real power (P) in watts. Calculate the apparent power (S) using the formula: S = √3 * V * I. Calculate the power factor using the formula: PF = P / S.

In most cases, a power factor greater than 1 is not desirable as it can result in inefficient use of electricity and higher energy costs. However, in some industrial applications, leading power factor can be beneficial as it can improve the efficiency of certain equipment and reduce the overall energy consumption.

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) .

We thus define: Apparent power: S = V x I (kVA) Active power: P = V x Ia (kW) Reactive power: Q = V x Ir (kvar)

What is the power factor of a pure resistive and pure inductive circuit have a ratio of? ›

Hint: The power factor is $1$ (perfect) for the purely resistive circuit because the reactive power approaches zero. For the completely inductive circuit, the power factor is zero because true power approaches zero.

Purely Resistive Circuit

No presence of inductance and capacitance. The backward movement of both the alternating current and the voltage can be observed. Hence, they form a sine wave or sinusoidal waveform. The value of current and voltage reaches the maximum at the same time.

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

Assertion (A) : In purely capacitive AC circuit, average power consumed is zero. Reason (R) : The phase difference between the current and voltage in case of a purely capacitive circuit is 90∘.

Power factor is a ratio of true power to apparent power. There exist good, bad, and poor power factors with specific ranges where 1.0 to 0.95 is said to be good, 0.95 to 0.85 poor, and 0.85 and below is said to be bad. When the system achieves a power factor of 1.0, the system is approved to be perfect.

A capacitive load always has a leading power factor.