Active Power
The power drawn by the load to do work is called the True power or Active power or real power. When electrical energy is fed to load the electrical energy gets converted into other forms of energy like thermal,mechnaical or chemical. Thus, the power actually consumed by electrical load is called the active power. A heater rated 220 volts, 400 watts consumes 400 watts when 220 volts is applied to its resistance element.The 400 watts power consumed by heater is the real power or active power. The active power is measured in kilo watt(kW) or MW.
To calculate active power, the current flowing in phase with applied voltage is calculated.
The product of the voltage and the current in phase with the voltage gives the real power or active power.
Reactive Power
The power which flows from the source to load and load to source is called the reactive energy. The reactive energy flows in both the direction. The reactive power is measured in kilo volts ampere reactive(kVAR) or MVAR.
The inductive load cause reactance current and thus the current lags with the applied voltage. The capacitive load cause reactance to applied voltage and thus the current leads the applied volatge. The phase displacement between the voltage and current always exist if the load is capacitive or reactive type.
The impedance offered by the capacitive and inductive loads cause power flow back and forth from source to load and load to source. In a purle inductive circuit the current lags the voltage by 90 electrical degree. In a purely capacitive circuit the curruent leads the voltage by 90 electrical degree.
The active power in case of purely inductive and capacitive circuit is VICosΦ= VI Cos90=0. The reactive power in case of purely inductive and capacitive circuit VISinΦ= VI Sin90=VI.
If the load is neither purley resistive nor purely reactive, the current drawn by the load has two components of the current.
Active Component of the Current :
The current which is in phase with the applied voltage is called active component of the current. The active or real power consuption of the load depends on the active current component of the circuit.
Reactive Component of the Current :
The current which is 90 degree out of phase with the applied voltage is called the reactive comonent of current or wattless current. The reactive component of current contribute to the reactive power.
The load whether drawing the active or reactive current, the total current of the system will increase. Therefore, the electrical system capacity is expressed in the apparent power kVA or MVA. The system need to handle both active and reactive current and therefore system is designed taking care of apparent power.
Let an electrical inductive load draws current I and phase displacement between voltage and current is Φ.
The active, reactive and apparent power drawn by inductive load can be calculated as follow.
Active component of the current in phase with the voltage is ICosΦ and the reactive component of the current component out of phase with voltage is ISinΦ.
Active Power of Single phase Load
Active power(P)
= Voltage x Current in phase with the voltage
= V x ICosΦ
= V I CosΦ
Active Power of Three Phase Load
Active power (P)
= Voltage x Current in phase with the voltage
= √3 Vx ICosΦ
= √3 V I CosΦ
Reactive Power of Single phase Load
Reactive power(Q)
Q = Voltage x Current out of phase with the voltage
= V x ISinΦ
= V I SinΦ
Reactive Power of Three phase Load
Reactive power (Q)
Q =Voltage x Current out of phase with the voltage
= √3V x ISinΦ
= √3 V I SinΦ
The apparent power is the vector sum of the active and the reactive power.
For single phase supply system apparent power drawn cam be expressed by the following mathematical expression.
For three phase load the apparent power is;
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