Electrical Power

Definition – Electric power is the rate at which electric energy is transferred by an electric circuit. The unit of power is the watt, one joule per second.Electric power is usually produced by electric generators, but can also be produced by other sources such as wind power, solar power, hydro power, etc. The value of power depends on supply voltage, current, resistance, reactance, impedance, power factor ect.

Let us understand electrical power in DC & AC system/circuits.

A – Power in DC Circuits

Power in a DC circuit is calculated by –   P =V I

Power in a DC circuit is calculated by –   P = I²R(where V = IR)

Power in a DC circuit is calculated by –   P = V²/R(where I = V/R)

Where P = Power in watts, V= Voltage in V & I = Current in Amp & R is the resistance.

(Note: In DC system, load shows only resistive behavior when DC supply is applied to the load because DC supply doesn’t change with time).

(Note – A DC load can’t be used on AC supply vice versa.)

B – Power in AC Circuits

There are two types of power used in AC system – 1) Single phase & 2) Three Phase. Normally electrical power is generated by the generating plants. Type of power depends on type of supply e.g. Single phase power needs single phase supply & three phase power needs three phase supply. 3 Phase power can be used for three phase load as well as single phase load. 3 phase power is normally used for high capacity 3 phase loads & single phase power is used for small electrical loads.

A –Power in single phase Circuit

Power in 1- phase electrical system –       P = VI CosØ or Vph IphCosØ

Power in 1- phase electrical system –       P =  Iph²ZCosØ  (where Vph = IphZ)

Power in 1- phase electrical system –       P =  Vph² / ZCosØ (Where Iph = Vph/Z)

Where P = Power in watts, Iph = phase current, Vph = phase voltage, CosØ = Power Factor& Z = Impedance.

In sigle phase supply, the value of voltage is 230V.

(Note– In ac system, load offers resitive, inductive & capacitive behaviour which depends on the type of load. The overall effect of above behaviour is called impedance. The above behaviour is because of AC supply which varies with time. While in DC system, load offers only resitive behaviour.)

B – Power in 3 phase Circuit –

Power in 3- phase electrical system –P = Г3 VL X ILXCosØ,

Power in 3- phase electrical system –       P = 3 Vph X IphXCosØ,

where VL = Line Voltage, IL = Line Current, CosØ = Power Factor,

where Vph = Phase Voltage, Iph = Phase Current, CosØ = Power Factor,

LINE & PHASE VOLTAGE /CURRENT

In 3- phase circuit there are two terms which generally used – Line voltage & phase voltage& Current. Let us understand both terms.

Line Voltage– It is a voltage which is measured between two lines such as between R&Y, Y&B & RB.

Phase Voltage – It is a voltage which is across the winding such as Vph1, Vph2& Vph3.

Line Current – It is a current which flows through the line like IR, Iy& IB

Phase Current – It is a current which flows through winding such as Iph1, Iph2& Iph3.

Now clearly we can see (through diagram) that voltage & current in delta & star connections are not same.