12-06-2012, 02:08 PM
Introduction of Power in AC Circuits
Power in AC Circuits.ppt (Size: 288.5 KB / Downloads: 205)
Introduction
The instantaneous power dissipated in a component is a product of the instantaneous voltage and the instantaneous current
p = vi
In a resistive circuit the voltage and current are in phase – calculation of p is straightforward
In reactive circuits, there will normally be some phase shift between v and i, and calculating the power becomes more complicated.
Active and Reactive Power
When a circuit has resistive and reactive parts, the resultant power has 2 parts:
The first is dissipated in the resistive element. This is the active power, P
The second is stored and returned by the reactive element. This is the reactive power, Q , which has units of volt amperes reactive or var
While reactive power is not dissipated it does have an effect on the system
for example, it increases the current that must be supplied and increases losses with cables
Power Factor Correction
Power factor is particularly important in high-power applications
Inductive loads have a lagging power factor
Capacitive loads have a leading power factor
Many high-power devices are inductive
a typical AC motor has a power factor of 0.9 lagging
the total load on the national grid is 0.8-0.9 lagging
this leads to major efficiencies
power companies therefore penalise industrial users who introduce a poor power factor.
The problem of poor power factor is tackled by adding additional components to bring the power factor back closer to unity
a capacitor of an appropriate size in parallel with a lagging load can ‘cancel out’ the inductive element
this is power factor correction
a capacitor can also be used in series but this is less common (since this alters the load voltage)
for examples of power factor correction see Examples 16.2 and 16.3 in the course text .
Three-Phase Systems
So far, our discussion of AC systems has been restricted to single-phase arrangement
as in conventional domestic supplies
In high-power industrial applications we often use three-phase arrangements
these have three supplies, differing in phase by 120
phases are labeled red, yellow and blue (R, Y & B).
Power Measurement
When using AC, power is determined not only by the r.m.s. values of the voltage and current, but also by the phase angle (which determines the power factor)
consequently, you cannot determine the power from independent measurements of current and voltage
In single-phase systems power is normally measured using an electrodynamic wattmeter
measures power directly using a single meter which effectively multiplies instantaneous current and voltage.