Resistive loads include devices such as heating elements and incandescent lighting. In a purely resistive circuit, current and voltage rise and fall at the same time. They are said to be "in phase."
All the power drawn by a resistive circuit is converted to usefulwork. This is also known as true power in a resistive circuit. Truepower is measured in watts (W), kilowatts (kW), or megawatts(MW). In a DC circuit or in a purely resistive AC circuit, truepower can easily be determined by measuring voltage and current. True power in a resistive circuit is equal to system voltage (E) times current (I).
Inductive loads include motors, transformers, and solenoids. In a purely inductive circuit, current lags behind voltage by 90°.Current and voltage are said to be "out of phase." Inductive circuits, however, have some amount of resistance. Depending on the amount of resistance and inductance, AC current will lag somewhere between a purely resistive circuit (0°) and a purely inductive circuit (90°). In a circuit where resistance and inductance are equal values, for example, current lags voltageby 45°.
Capacitive loads include power factor correction capacitors and filtering capacitors. In a purely capacitive circuit, current leads voltage by 90°. Capacitive circuits, however, have some amount of resistance. Depending on the amount of resistance and capacitance, AC current will lead voltage somewhere between a purely resistive circuit (0°) and a purely capacitive circuit (90°).In a circuit where resistance and capacitance are equal values,for example, current leads voltage by 45°.
Circuits with inductive or capacitive components are said to be reactive. Most distribution systems have various resistive and reactive circuits. The amount of resistance and reactance varies,depending on the connected loads.
Just as resistance is opposition to current flow in a resistive circuit, reactance is opposition to current flow in a reactive circuit. It should be noted, however, that where frequency has no effect on resistance, it does effect reactance. An increase in applied frequency will cause a corresponding increase in inductive reactance and a decrease in capacitive reactance.
R = E/I, Where R = resistance in Ohms, E = voltage and I = current
For inductive Reactance
XL = 2 x 3.14 x f x L , where XL is inductive reactance in ohms, f = applied freq and L = inductance in henrys
For Capacitive reactance
XC = 1 / (2 x 3.14 x f x C) where XC =capacitive reactance, f = applied freq and C = capacitance in farads
ENERGY IN REACTIVE CIRUCUITS:
Energy in a reactive circuit does not produce work. This energy is used to charge a capacitor or produce a magnetic field around the coil of an inductor. Current in an AC circuit rises to peak values (positive and negative) and diminishes to zero many times a second. During the time, current is rising to a peakvalue, energy is stored in an inductor in the form of a magnetic field or as an electrical charge in the plates of a capacitor. This energy is returned to the system when the magnetic field collapses or when the capacitor is discharged.
Power in an AC circuit is made up of three parts; true power,reactive power, and apparent power. We have already discussed true power. Reactive power is measured in volt-amps reactive(VAR). Reactive power represents the energy alternately stored and returned to the system by capacitors and/or inductors.Although reactive power does not produce useful work, it still needs to be generated and distributed to provide sufficient true power to enable electrical processes to run.
Not all power in an AC circuit is reactive. We know that reactive power does not produce work; however, when a motor rotates work is produced. Inductive loads, such as motors, have some amount of resistance. Apparent power represents a load which includes reactive power (inductance) and true power(resistance). Apparent power is the vector sum of true power,which represents a purely resistive load, and reactive power,which represents a purely reactive load. A vector diagram can be used to show this relationship. The unit of measurement for apparent power is volt amps (VA). Larger values can be stated inkilovolt amps (kVA) or megavolt amps (MVA).
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