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Three Phase Theory

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The Basics

A three phase system produces three sine waves that are spaced 120 electrical/ mechanical degrees apart.
Each of these waves are called a phase.
Systems that contain two or more phases are called polyphase systems. The most common and a widely used in North America is three phase.


In the two pole alternator each phase has to windings placed 180 degrees apart.
The voltage induced depends on the rate of charge of flux surrounding the armature conductor.


Phases

Phase sequence is the order in which the three phases reach their positive peak.
These two sequences are A-B-C or C-B-A.
This can be reversed in two ways:
1) changing two of the line leads
2) changing the direction of the prime mover.

The color coding for the three face systems are red, black and blue. CEC 4-036.

A high leg (or red leg) is phase A.

Phasors are rotating vectors used to represent quantities that vary with time.

AC voltage and current in a three phase system are sinusoidal and cannot add their values as a total sum. They have to be added vectorally.

Phasor length represents RMS or effective value, but sometimes can represent peak. Peak = RMS x 1.414
RMS = Peak x 0.707

In series circuits the current is the same everywhere, so the current phasor is usually drawn on the reference phasor.

In parallel circuits the voltage is the same so the voltage phaser each usually drawn on a reference phasor.



Phasors are often labeled with double subscript.
" E" represents EMF or voltage rise.
" V" represents a voltage drop.

Kirchoff 's law still applies. Reversing the order of the subscripts is the same thing as changing the direction of the phasor by 180 degrees.


Delta and Wye Configurations

For calculating open fuse, open winding, or center tapped, is the following

Wye
open fuse 50%
open winding 50%
center tapped 67%

Delta
open fuse 50%
open winding 67%



Power Factor Corrections
A 480 V 3 phase system has a PF (Power Factor) of .72. The true power is 48164 W. Correct to .94



Filters

Filters-are used to either pass( offer little opposition to) or reject different frequencies. Also known as filters circuits.

Types are as follows:
Low pass-pass low frequencies of reject high
High pass-pass high frequencies and reject
Low Band pass-pass a certain range of frequency and reject lower and higher. Bandwidth is determined by Q of the conductor.
Band rejection-( notch filters) low and high frequency either side of the resonant value.

Complex filters
T-two series and one parallel
P I-two parallel and one series

Some filter circuits must be capable of tuning over a broad band of frequencies.



Harmonics

Harmonics-are distortions to the voltage and current waveforms from their normal sinusodal shape.

Devices that cause harmonics are devices that convert AC to DC, if current flow is in short bursts, personal computers, battery chargers, office machines, anything that uses D.C. for operation, SCR heating and lighting controls, adjustable speed motor drives, electrical and medical test equipment.

The problems that harmonics cause are -overheating neutrals, transformers and motors -inaccurate measurement of current and voltage -reduces ride through . .

Triplen harmonics-are the odd multiples of the third harmonic. Zero value. See below. Nonlinear loads give distortion and current acts different than Volts.