Question

distinction between active and real power?

distinction between complex and apparent power?

distinction between effective and rms value of a voltage or current?

what are three applications of transformer?

Answer 1

A ans)

The required power supply to an electric circuit depends on the

• active power - real electrical resistance power consumption in circuit
• reactive power - imaginary inductive and capacitive power consumption in circuit

The required power supply is called the apparent power and is a complex value that can be expressed in a Pythagorean triangle relationship as indicated in the figure below.

Apparent Power - S

The apparent power is the power supplied to the electric circuit - typical from a power supplier to the grid - to cover the real and reactive power consumption in the loads.

Apparent power can be calculated as

S = (Q² + P²)^1/2     (1)

where

S = apparent power supply to the circuit (volt ampere, VA)

Q = reactive power consumption in load (volt ampere reactive, VAR)

P = active power consumption in load (watts, W)

Apparent power is measured in volt-amperes (VA) - the AC system voltage multiplied with flowing current. Apparent power is a complex value and the vector sum of the active and reactive power as indicated in the figure above.

Single Phase Current

S = U I        (2a)

where

U = electric potential (V)

I = current (A)

Three Phase Current

S = 3^1/2 U I  

   = 1.732 U I (2b)

Active Power - P

Active - or real or true - power do the actual work in the load. Active power is measured in watts (W) and is the power consumed by electrical resistance.

Single Phase Current

P = U I cos φ

   = U I PF (3a)

where

φ = phase angle between the electrical potential (voltage) and the current

PF = cos φ

    = Power Factor

Three Phase Current

P = 3^1/2U I cos φ

= 1.732 U I PF (3b)

Direct Current

P = U I                         (3c)

Reactive Power - Q

Reactive power is the imaginary or complex power in a capacitive or inductive load. Reactive power represents an energy exchange between the power source and the reactive loads where no net power is gained or lost. The net average reactive power is zero. Reactive power is stored in and discharged by inductive motors, transformers, solenoids and capacitors.

Reactive power should be minimized because it increases the overall current flowing in an electric circuit without providing any work to the load. Increased reactive currents only provides unrecoverable power loss due to power line resistance.

Increased reactive and apparent power will devrease the power factor - PF.

Reactive inductive power is measured in volt-amperes reactive (VAR).

Single Phase Current

Q = U I sin φ

   = U I PF                    (4a)

where

φ = phase angle

Three Phase Current

Q = 3^1/2 U I sin φ

   = 1.732 UI PF             

B ans)

C and)

The term RMS, ONLY refers to time-varying sinusoidal voltages, currents or complex waveforms were the magnitude of the waveform changes over time and is not used in DC circuit analysis or calculations were the magnitude is always constant. When used to compare the equivalent RMS voltage value of an alternating sinusoidal waveform that supplies the same electrical power to a given load as an equivalent DC circuit, the RMS value is called the “effective value” and is generally presented as: V_eff or I_eff.

In other words, the effective value is an equivalent DC value which tells you how many volts or amps of DC that a time-varying sinusoidal waveform is equal to in terms of its ability to produce the same power.

For example, the domestic mains supply in the United Kingdom is 240Vac. This value is assumed to indicate an effective value of “240 Volts rms”. This means then that the sinusoidal rms voltage from the wall sockets of a UK home is capable of producing the same average positive power as 240 volts of steady DC voltage as shown below.

RMS for voltage

D Ans)

Application of transformer

• The transformer used for impedance matching.
• The transformer used for isolate two circuits electrically.
• It is used to increase or decrease the alternating voltages in electric power applications.
• The transformer used in voltmeter, ammeters, protective relay etc.

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