Electric Circuit Class Discussions

Week Eleven: Three-Phase Circuits

Biyernes, Marso 20, 2015 | by Unknown

Important Facts:

The conductors connected to the three points of a three-phase source or load are called lines.

The three components comprising a three-phase source or load are called phases.

Line voltage is the voltage measured between any two lines in a three-phase circuit.

Phase voltage is the voltage measured across a single component in a three-phase source or load.

Line current is the current through any one line between a three-phase source and load.

Phase current is the current through any one component comprising a three-phase source or load.

In balanced “Y” circuits, line voltage is equal to phase voltage times the square root of 3, while line current is equal to phase current.

Three-phase wye “Y” connection has three voltage sources connected to a common point.

Balanced Wye-Wye Connection

Three-Phase Connections Involving Delta Sources or Loads

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Reflection:

I have learned the different types of three phase connections and how they are produced by generators.

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Week Ten: Power Factor and Complex Power

| by Unknown

Power Factor

For a DC circuit the power is P=VI, and this relationship also holds for the instantaneous power in an AC circuit. However, the average power in an AC circuit expressed in terms of the rms voltage and current is

where

is the phase angle between the voltage and current. The additional term is called the power factor

From the phasor diagram for AC impedance, it can be seen that the power factor is R/Z. For a purely resistive AC circuit, R=Z and the power factor = 1.

Significance of Power Factor

A power factor of one or "unity power factor" is the goal of any electric utility company since if the power factor is less than one, they have to supply more current to the user for a given amount of power use. In so doing, they incur more line losses. They also must have larger capacity equipment in place than would be otherwise necessary. As a result, an industrial facility will be charged a penalty if its power factor is much different from 1.

Apparent Power

-is the combination of reactive power and true power. It is the product of a circuit's voltage and current, without reference to phase angle. It is measured in the unit of Volt-Amps (VA) and is symbolized by the capital letter S.

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Reflection:

I have learned the concept behind the power triangle and it's method for finding the unknowns. I'm also surprised that power factor has a big role especially when we are dealing with electricity consumption.

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Week Nine: AC Power Analysis

| by Unknown

Important terms and formulas to remember:

Instantaneous Power p(t)

-absorbed by an element is the product of the instantaneous voltage v(t) across the element and the instantaneous current i(t) through it.

p(t) = v(t) i(t)

v(t) = Vm cos(ωt + θv)

i(t) = Im cos(ωt + θi)

Average Power

-is the average of the instantaneous power over one period.

Maximum Average Power Transfer

-is the maximum amount of power will be dissipated by a load resistance when that load resistance is equal to the Thevenin/Norton resistance of the network supplying the power. If the load resistance is lower or higher than the Thevenin/Norton resistance of the source network, its dissipated power will be less than maximum.

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Reflection:

I think we must develop a deeper understanding about the basic theories of AC power analysis. Also, we must be able to familiarize ourselves with the formulas. I have learned that power is a significant and common quantity when you are planning to work in a electric company.

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Week Eight: Thevenin's Theorem in AC Circuits

Sabado, Pebrero 21, 2015 | by Unknown

Thevenin Equivalent Circuit

Thévenin’s theorem, as stated for sinusoidal AC circuits, is changed only to include the term impedance instead of resistance.
Any two-terminal linear ac network can be replaced with an equivalent circuit consisting of a voltage source and an impedance in series.
VTh is the Open circuit voltage between the terminals a-b.
ZTh is the impedance seen from the terminals when the independent sources are set to zero.