Phasor

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An example of series RLC circuit and respective phasor diagram for a specific o Wykres wektorowy by Zureks.svg
An example of series RLC circuit and respective phasor diagram for a specific ω

In physics and engineering, a phasor (a portmanteau of phase vector [1] [2] ), is a complex number representing a sinusoidal function whose amplitude (A), angular frequency (ω), and initial phase (θ) are time-invariant. It is related to a more general concept called analytic representation, [3] which decomposes a sinusoid into the product of a complex constant and a factor that encapsulates the frequency and time dependence. The complex constant, which encapsulates amplitude and phase dependence, is known as phasor, complex amplitude, [4] [5] and (in older texts) sinor [6] or even complexor. [6]

Physics Study of the fundamental properties of matter and energy

Physics is the natural science that studies matter, its motion, and behavior through space and time, and that studies the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.

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Engineering is the application of knowledge in the form of science, mathematics, and empirical evidence, to the innovation, design, construction, operation and maintenance of structures, machines, materials, software, devices, systems, processes, and organizations. The discipline of engineering encompasses a broad range of more specialized fields of engineering, each with a more specific emphasis on particular areas of applied mathematics, applied science, and types of application. See glossary of engineering.

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Contents

A common situation in electrical networks is the existence of multiple sinusoids all with the same frequency, but different amplitudes and phases. The only difference in their analytic representations is the complex amplitude (phasor). A linear combination of such functions can be factored into the product of a linear combination of phasors (known as phasor arithmetic) and the time/frequency dependent factor that they all have in common.

The origin of the term phasor rightfully suggests that a (diagrammatic) calculus somewhat similar to that possible for vectors is possible for phasors as well. [6] An important additional feature of the phasor transform is that differentiation and integration of sinusoidal signals (having constant amplitude, period and phase) corresponds to simple algebraic operations on the phasors; the phasor transform thus allows the analysis (calculation) of the AC steady state of RLC circuits by solving simple algebraic equations (albeit with complex coefficients) in the phasor domain instead of solving differential equations (with real coefficients) in the time domain. [7] [8] The originator of the phasor transform was Charles Proteus Steinmetz working at General Electric in the late 19th century. [9] [10]

Euclidean vector geometric object that has magnitude (or length) and direction

In mathematics, physics, and engineering, a Euclidean vector is a geometric object that has magnitude and direction. Vectors can be added to other vectors according to vector algebra. A Euclidean vector is frequently represented by a line segment with a definite direction, or graphically as an arrow, connecting an initial pointA with a terminal pointB, and denoted by

Derivative Operation in calculus

The derivative of a function of a real variable measures the sensitivity to change of the function value with respect to a change in its argument. Derivatives are a fundamental tool of calculus. For example, the derivative of the position of a moving object with respect to time is the object's velocity: this measures how quickly the position of the object changes when time advances.

Integral operation in calculus

In mathematics, an integral assigns numbers to functions in a way that can describe displacement, area, volume, and other concepts that arise by combining infinitesimal data. Integration is one of the two main operations of calculus, with its inverse operation, differentiation, being the other. Given a function f of a real variable x and an interval [a, b] of the real line, the definite integral

Glossing over some mathematical details, the phasor transform can also be seen as a particular case of the Laplace transform, which additionally can be used to (simultaneously) derive the transient response of an RLC circuit. [8] [10] However, the Laplace transform is mathematically more difficult to apply and the effort may be unjustified if only steady state analysis is required. [10]

In mathematics, the Laplace transform is an integral transform named after its inventor Pierre-Simon Laplace. It takes a function of a real variable t to a function of a complex variable s. The transform has many applications in science and engineering.

Transient response the response of a system to a change from an equilibrium state

In electrical engineering and mechanical engineering, a transient response is the response of a system to a change from an equilibrium or a steady state. The transient response is not necessarily tied to abrupt events but to any event that affects the equilibrium of the system. The impulse response and step response are transient responses to a specific input.

Fig 2. When function
A
[?]
e
i
(
o
t
+
th
)
{\displaystyle \scriptstyle A\cdot e^{i(\omega t+\theta )}}
is depicted in the complex plane, the vector formed by its imaginary and real parts rotates around the origin. Its magnitude is A, and it completes one cycle every 2p/o seconds. th is the angle it forms with the real axis at t = n*2p/o, for integer values of n. Unfasor.gif
Fig 2. When function is depicted in the complex plane, the vector formed by its imaginary and real parts rotates around the origin. Its magnitude is A, and it completes one cycle every 2π/ω seconds. θ is the angle it forms with the real axis at t = n•2π/ω, for integer values of n.

Definition

Euler's formula indicates that sinusoids can be represented mathematically as the sum of two complex-valued functions:

Eulers formula Expression of the complex exponential in terms of sine and cosine

Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex exponential function. Euler's formula states that for any real number x:

Complex number Element of a number system in which –1 has a square root

A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers, and i is a solution of the equation x2 = −1. Because no real number satisfies this equation, i is called an imaginary number. For the complex number a + bi, a is called the real part, and b is called the imaginary part. Despite the historical nomenclature "imaginary", complex numbers are regarded in the mathematical sciences as just as "real" as the real numbers, and are fundamental in many aspects of the scientific description of the natural world.

   [lower-alpha 1]

or as the real part of one of the functions:

The function is called the analytic representation of . Figure 2 depicts it as a rotating vector in a complex plane. It is sometimes convenient to refer to the entire function as a phasor, [11] as we do in the next section. But the term phasor usually implies just the static vector . An even more compact representation of a phasor is the angle notation: . See also vector notation.

Angle notation or phasor notation is a notation used in electronics.  can represent either the vector   or the complex number  , with , both of which have magnitudes of 1. A vector whose polar coordinates are magnitude and angle is written   To convert between polar and rectangular forms, see Converting between polar and Cartesian coordinates.

Vector notation mathematical notation for working with vectors

Vector notation is a commonly used mathematical notation for working with mathematical vectors, which may be geometric vectors or members of vector spaces.

Phasor arithmetic

Multiplication by a constant (scalar)

Multiplication of the phasor   by a complex constant,   , produces another phasor. That means its only effect is to change the amplitude and phase of the underlying sinusoid:

In electronics,   would represent an impedance, which is independent of time. In particular it is not the shorthand notation for another phasor. Multiplying a phasor current by an impedance produces a phasor voltage. But the product of two phasors (or squaring a phasor) would represent the product of two sinusoids, which is a non-linear operation that produces new frequency components. Phasor notation can only represent systems with one frequency, such as a linear system stimulated by a sinusoid.

Electrical impedance intensive physical property

Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. The term complex impedance may be used interchangeably.

Differentiation and integration

The time derivative or integral of a phasor produces another phasor. [lower-alpha 2] For example:

Therefore, in phasor representation, the time derivative of a sinusoid becomes just multiplication by the constant .

Similarly, integrating a phasor corresponds to multiplication by . The time-dependent factor, , is unaffected.

When we solve a linear differential equation with phasor arithmetic, we are merely factoring out of all terms of the equation, and reinserting it into the answer. For example, consider the following differential equation for the voltage across the capacitor in an RC circuit:

When the voltage source in this circuit is sinusoidal:

we may substitute

where phasor , and phasor is the unknown quantity to be determined.

In the phasor shorthand notation, the differential equation reduces to

  [lower-alpha 3]

Solving for the phasor capacitor voltage gives

As we have seen, the factor multiplying represents differences of the amplitude and phase of   relative to   and .

In polar coordinate form, it is

Therefore

Addition

The sum of phasors as addition of rotating vectors Sumafasores.gif
The sum of phasors as addition of rotating vectors

The sum of multiple phasors produces another phasor. That is because the sum of sinusoids with the same frequency is also a sinusoid with that frequency:

where

and, if we take , then :
  • if , then , with the sign function;
  • if , then ;
  • if , then .

or, via the law of cosines on the complex plane (or the trigonometric identity for angle differences):

where .

A key point is that A3 and θ3 do not depend on ω or t, which is what makes phasor notation possible. The time and frequency dependence can be suppressed and re-inserted into the outcome as long as the only operations used in between are ones that produce another phasor. In angle notation, the operation shown above is written

Another way to view addition is that two vectors with coordinates [ A1 cos(ωt + θ1), A1 sin(ωt + θ1) ] and [ A2 cos(ωt + θ2), A2 sin(ωt + θ2) ] are added vectorially to produce a resultant vector with coordinates [ A3 cos(ωt + θ3), A3 sin(ωt + θ3) ]. (see animation)

Phasor diagram of three waves in perfect destructive interference Destructive interference.png
Phasor diagram of three waves in perfect destructive interference

In physics, this sort of addition occurs when sinusoids interfere with each other, constructively or destructively. The static vector concept provides useful insight into questions like this: "What phase difference would be required between three identical sinusoids for perfect cancellation?" In this case, simply imagine taking three vectors of equal length and placing them head to tail such that the last head matches up with the first tail. Clearly, the shape which satisfies these conditions is an equilateral triangle, so the angle between each phasor to the next is 120° (3π2 radians), or one third of a wavelength λ3. So the phase difference between each wave must also be 120°, as is the case in three-phase power

In other words, what this shows is that

In the example of three waves, the phase difference between the first and the last wave was 240 degrees, while for two waves destructive interference happens at 180 degrees. In the limit of many waves, the phasors must form a circle for destructive interference, so that the first phasor is nearly parallel with the last. This means that for many sources, destructive interference happens when the first and last wave differ by 360 degrees, a full wavelength . This is why in single slit diffraction, the minima occur when light from the far edge travels a full wavelength further than the light from the near edge.

As the single vector rotates in an anti-clockwise direction, its tip at point A will rotate one complete revolution of 360° or 2π radians representing one complete cycle. If the length of its moving tip is transferred at different angular intervals in time to a graph as shown above, a sinusoidal waveform would be drawn starting at the left with zero time. Each position along the horizontal axis indicates the time that has elapsed since zero time, t = 0. When the vector is horizontal the tip of the vector represents the angles at 0°, 180°, and at 360°.

Likewise, when the tip of the vector is vertical it represents the positive peak value, ( +Amax ) at 90° or π2 and the negative peak value, ( Amax ) at 270° or 3π2. Then the time axis of the waveform represents the angle either in degrees or radians through which the phasor has moved. So we can say that a phasor represent a scaled voltage or current value of a rotating vector which is “frozen” at some point in time, ( t ) and in our example above, this is at an angle of 30°.

Sometimes when we are analysing alternating waveforms we may need to know the position of the phasor, representing the alternating quantity at some particular instant in time especially when we want to compare two different waveforms on the same axis. For example, voltage and current. We have assumed in the waveform above that the waveform starts at time t = 0 with a corresponding phase angle in either degrees or radians.

But if a second waveform starts to the left or to the right of this zero point, or if we want to represent in phasor notation the relationship between the two waveforms, then we will need to take into account this phase difference, Φ of the waveform. Consider the diagram below from the previous Phase Difference tutorial.

Applications

Circuit laws

With phasors, the techniques for solving DC circuits can be applied to solve AC circuits. A list of the basic laws is given below.

Given this we can apply the techniques of analysis of resistive circuits with phasors to analyze single frequency AC circuits containing resistors, capacitors, and inductors. Multiple frequency linear AC circuits and AC circuits with different waveforms can be analyzed to find voltages and currents by transforming all waveforms to sine wave components with magnitude and phase then analyzing each frequency separately, as allowed by the superposition theorem.

Power engineering

In analysis of three phase AC power systems, usually a set of phasors is defined as the three complex cube roots of unity, graphically represented as unit magnitudes at angles of 0, 120 and 240 degrees. By treating polyphase AC circuit quantities as phasors, balanced circuits can be simplified and unbalanced circuits can be treated as an algebraic combination of symmetrical components. This approach greatly simplifies the work required in electrical calculations of voltage drop, power flow, and short-circuit currents. In the context of power systems analysis, the phase angle is often given in degrees, and the magnitude in rms value rather than the peak amplitude of the sinusoid.

The technique of synchrophasors uses digital instruments to measure the phasors representing transmission system voltages at widespread points in a transmission network. Differences among the phasors indicate power flow and system stability.

Telecommunications: analog modulations

The rotating frame picture using phasor can be a powerful tool to understand analog modulations such as amplitude modulation (and its variants [12] ) and frequency modulation.

, where the term in brackets is viewed as a rotating vector in the complex plane.

The phasor has length , rotates anti-clockwise at a rate of revolutions per second, and at time makes an angle of with respect to the positive real axis.

The waveform can then be viewed as a projection of this vector onto the real axis.

See also

Footnotes

    • i is the Imaginary unit ().
    • In electrical engineering texts, the imaginary unit is often symbolized by j.
    • The frequency of the wave, in Hz, is given by .
  1. This results from , which means that the complex exponential is the eigenfunction of the derivative operation.
  2. Proof

     

     

     

     

    (Eq.1)

    Since this must hold for all , specifically: , it follows that

     

     

     

     

    (Eq.2)

    It is also readily seen that
    Substituting these into   Eq.1 and   Eq.2 , multiplying   Eq.2 by   and adding both equations gives

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Further reading