Phasor Diagrams in PE Power Exam

Understanding the intricate relationships between voltages, currents, and complex impedances is paramount in power engineering. Enter phasor diagrams—the visual key that unlocks the mysteries of AC circuits and simplifies the intricate design between electrical quantities. 

Phasor diagrams are a powerful tool for power engineers, enabling them to graphically represent and comprehend the behavior of alternating current systems. Enriched with vectors, angles, and magnitudes, these diagrams provide a concise and intuitive snapshot of the complex association between voltages, currents, and impedances. 

By utilizing the principles of trigonometry, phasor diagrams in PE power exam preparation unlock the phase relationships, resolve power factors, and uncover the hidden depths of complex power systems. 

Our detailed guide on phasor diagrams in the PE Power exam will discuss the crucial mathematical underpinnings of phasor diagrams and showcase their inevitable role in the analysis and optimization of power systems. Let’s dive deep into the details.

Fundamentals of Phasor Diagrams

Circuit phasor diagrams are graphical representations that help us understand the relationship between multiple sinusoidal quantities, such as voltages or currents, with the same frequency.

Phasor diagrams in PE Power exam are intended to prepare aspiring power engineers to analyze and solve problems in AC (alternating current) circuits.

Simply, a phasor is a complex number that represents a sinusoidal quantity. It has two components: magnitude and phase. 

  • The magnitude represents the sinusoid’s amplitude
  • The phase represents the angular position of the sinusoid at a particular point in time.

Mathematical Interpretation

In mathematics, a phasor can be represented using Euler’s formula:

A⋅e^(jθ)

Here

  • A is the magnitude or amplitude of the phasor
  • θ is the phase angle.
  • The complex unit e^(jθ) represents a sinusoidal function with a phase angle θ.

To visualize the phasors in a diagram, we use a complex plane. The real axis represents the real part of the phasor, while the imaginary axis represents the imaginary part. The reference axis, usually the horizontal axis, is called the reference phasor.

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Mathematical Operations on Phasors

Addition/Subtraction

To add or subtract phasors, we simply add or subtract their corresponding complex numbers.

Consider two phasors:

V1 = 10∠30° V

V2 = 5∠-45° V

a) To find the sum of V1 and V2, we simply add their corresponding components:

Phasor Diagram

V_sum = V1 + V2

= 10∠30° V + 5∠-45° V

To add phasors, we can convert them to rectangular form (using the magnitude and angle) and add their real and imaginary parts separately. Then, we convert the result back to polar form.

V_sum = (10∠30° V) + (5∠-45° V)

= (10 cos 30° + 5 cos -45°) + j(10 sin 30° + 5 sin -45°)

= (8.66 + 3.54j) V

Converting back to polar form

V_sum = √[(8.66)^2 + (3.54)^2]∠arctan(3.54/8.66)

= 9.35∠21.8° V

Therefore, the sum of V1 and V2 is 9.35∠21.8° V.

b) To find the difference between V1 and V2, we subtract their corresponding components:

corresponding components

V_diff = V1 – V2

= 10∠30° V – 5∠-45° V

Using the same process as above

V_diff = (10∠30° V) – (5∠-45° V)

= (10 cos 30° – 5 cos -45°) + j(10 sin 30° – 5 sin -45°)

= (12.07 + 10.54j) V

Converting back to polar form

V_diff = √[(12.07)^2 + (10.54)^2]∠arctan(10.54/12.07)

= 15.86∠45.4° V

Therefore, the difference between V1 and V2 is 15.86∠45.4° V.

Multiplication

Multiplying phasors is equivalent to multiplying their magnitudes and adding their phase angles.

Consider two phasors:

I1 = 3∠60° A

I2 = 4∠30° A

To find the product of I1 and I2, we simply multiply their magnitudes and add their phase angles.

I_product = I1 * I2

= 3∠60° A * 4∠30° A

Multiplying the magnitudes

I_product = 3 * 4∠(60° + 30°) A

= 12∠90° A

Therefore, the product of I1 and I2 is 12∠90° A.

*Always check the units and convert them between rectangular and polar forms as needed. Practice similar problems to become familiar with phasor addition/subtraction and multiplication techniques.

Relationship Between Phasors and Sinusoidal Waveforms

A phasor diagram allows us to visualize these sinusoidal quantities by representing them as vectors in a complex plane. The vector’s length represents the phasor’s magnitude, and the phase is represented by the angle the vector makes with a reference axis.

Phasors and sinusoidal waveforms are closely related in the context of AC circuits. Phasors provide a mathematical representation of sinusoidal waveforms, allowing us to simplify and analyze complex AC circuit behaviors. 

Let’s discuss the trigonometric relations and mathematical formulas that connect phasors and sinusoidal waveforms.

Sinusoidal Waveform

Sinusoidal Waveform

A sinusoidal waveform represents a periodic oscillation or variation that follows a sine or cosine function. It can be described using the following mathematical equation:

v(t) = Vm * sin(ωt + φ)

Where:

  • v(t) represents the instantaneous value of the waveform at time t.
  • Vm is the amplitude or peak value of the waveform.
  • ω is the angular frequency, equal to 2πf, where f is the frequency in hertz.
  • φ is the phase angle, which determines the position of the waveform relative to a reference.

Phasor Representation

A phasor represents a sinusoidal waveform using a complex number. It has a magnitude (amplitude) and a phase angle. The phasor representation of a sinusoidal waveform can be expressed as

V = Vm * e^(jφ)

Where:

  • V is the phasor representing the sinusoidal waveform.
  • Vm is the magnitude or amplitude of the waveform.
  • e^(jφ) is the complex exponential term derived from Euler’s formula.

Trigonometric Relations

Several trigonometric relations connect phasors and sinusoidal waveforms. Let’s start with some crucial ties.

  • Magnitude – The magnitude or amplitude of a phasor is equal to the peak value of the corresponding sinusoidal waveform. Vm = |V|.
  • Phase Angle – The phase angle of a phasor represents the angular displacement between the phasor and a reference axis. It corresponds to the phase angle in the sinusoidal waveform equation. φ = arg(V).
  • Real and Imaginary Components – A phasor’s real and imaginary components relate to the sinusoidal waveform’s instantaneous values. The real part corresponds to the cosine term, and the imaginary part corresponds to the sine term. The real component is given by Re(V) = Vm * cos(φ), and the imaginary component is given by Im(V) = Vm * sin(φ).

To convert between a phasor and its corresponding sinusoidal waveform, we can use the following approaches:

Phasor to Sinusoidal

Given a phasor V = Vm * e^(jφ), the corresponding sinusoidal waveform is v(t) = Re(V) * cos(ωt) – Im(V) * sin(ωt).

Sinusoidal to Phasor

Given a sinusoidal waveform v(t) = Vm * sin(ωt + φ), the corresponding phasor is V = Vm * e^(jφ).

*These trigonometric relations and mathematical formulas allow us to analyze and manipulate sinusoidal waveforms using phasors, simplifying calculations, and circuit analysis in AC circuits.

Phasor Representation in Different Forms

Here’s a tabular representation that explains phasors and their various forms:

Phasor NotationTrigonometric FormPolar FormEuler’s Form
A∠θA cos(θ) + jA sin(θ)A∠θA e^(jθ)

In the table above, A represents the magnitude or amplitude of the phasor, and θ represents the phase angle.

  • Phasor Notation – This is the general representation of a phasor using the magnitude A and the phase angle θ.
  • Trigonometric Form – As discussed above, it represents a phasor as a combination of its real and imaginary components using trigonometric functions. The real part is A cos(θ), and the imaginary part is A sin(θ).
  • Polar Form – It represents a phasor using its magnitude A and phase angle θ. The magnitude A is written before the angle θ using the polar coordinate system.
  • Euler’s Form – It represents a phasor using Euler’s formula, which connects complex numbers, exponential functions, and trigonometric functions. The formula A e^(jθ) combines the magnitude A and the phase angle θ using the complex unit e^(jθ).

These different forms of phasor representation are interchangeable and provide alternative ways to express the same phasor. Each form has its advantages in different situations.

For example, Euler’s form is applicable when dealing with complex arithmetic operations, while the trigonometric form helps visualize a phasor’s real and imaginary components.

*It’s important to note that these forms are all equivalent representations of a phasor and can be converted into one another using mathematical operations. Understanding these forms and their conversions is crucial in phasor analysis and solving problems in AC circuits.

Let’s convert the phasor V = 5∠30° V into its trigonometric, polar, and Euler forms.

Given: V = 5∠30° V

Trigonometric Form

To convert the phasor into its trigonometric form, we use the trigonometric functions:

V_trig = V cos(θ) + jV sin(θ)

V_trig = 5 cos(30°) + j(5 sin(30°))

= 4.33 + j2.5 V

Therefore, the trigonometric form of the phasor V is V_trig = 4.33 + j2.5 V.

Polar Form

To convert the phasor into its polar form, we simply express the magnitude and phase angle:

V_polar = A∠θ

V_polar = 5∠30° V

Therefore, the polar form of the phasor V is V_polar = 5∠30° V.

Euler’s Form

To convert the phasor into its Euler’s form, we use Euler’s formula:

V_euler = A e^(jθ)

V_euler = 5 e^(j30°)

To express the exponential term in rectangular form, we use Euler’s identity: e^(jθ) = cos(θ) + j sin(θ).

V_euler = 5 (cos(30°) + j sin(30°))

= 4.33 + j2.5 V

Therefore, the Euler’s form of the phasor V is V_euler = 4.33 + j2.5 V.

*By converting between different phasor forms, we can represent the same phasor in different mathematical representations convenient for different calculations and analyses. Practice converting between these forms to become comfortable with phasor notation and its conversions.

Phasor Diagrams – Use Cases

Phasor diagrams in the PE Power exam to help analyze AC circuits. In AC circuits, voltages and currents can have different magnitudes and phases. By representing these quantities as phasors in a diagram, we can quickly determine their relationships, calculate circuit parameters, and analyze circuit behavior.

Phasor diagrams can help in various ways, such as:

  • Determining the total impedance in an AC circuit by vectorially summing the individual impedance phasors.
  • Analyzing the power flow in AC circuits by examining the relationships between voltage, current, and phase angles.
  • Solving resonance problems, where an AC circuit’s frequency matches a component’s natural frequency.
  • Understanding the behavior of AC circuits with reactive components, such as inductors and capacitors, by examining the phase differences between voltage and current.

Phasor diagrams in the PE power exam provide a powerful tool for simplifying the analysis of AC circuits, especially when dealing with complex quantities and multiple sinusoidal signals.

They enable power engineers to gain insights into circuit behavior, determine optimal operating conditions, and design efficient electrical systems.

*It’s important to note that while circuit phasor diagrams are valuable for understanding AC circuits, they are based on simplifying assumptions and represent a snapshot of the sinusoidal behavior at a particular instant in time. More advanced mathematical techniques, such as Fourier analysis, are required for a comprehensive analysis of time-varying signals.

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Power and Power Factor in Phasor Diagrams

Phasor diagrams help analyze power and power factor in AC circuits. The power factor measures how effectively a course utilizes the power delivered to it. It indicates the phase relationship between the voltage and current in an AC circuit.

Phasor diagrams help us visualize and calculate power factors by examining the angle between the voltage and current phasors.

To illustrate the process, let’s consider a simple example: in an AC circuit, the voltage phasor V is 100∠0° V, and the current phasor I is 80∠-30° A. Let’s determine the power factor of the circuit.

Step 1 – Represent the Phasors

  • Draw the Phasors in the Phasor diagram.
  • Draw a vector representing the voltage phasor V with a length of 100 units and an angle of 0° counterclockwise from the reference axis (horizontal).
  • Draw a vector representing the current phasor I with a length of 80 units and an angle of -30° counterclockwise from the reference axis.

Step 2 – Angle Measurement

Measure the angle between the voltage and current phasors in the phasor diagram.

In our example, the angle between the voltage phasor V and the current phasor I is 30°.

Step 3 – Calculate the Power Factor

The power factor (PF) can be calculated as the cosine of the angle between the voltage and current phasors.

PF = cos(angle)

In our example, PF = cos(30°) = 0.866

Step 4 – Interpret the Power Factor

Interpret the power factor in terms of its value.

The power factor is a dimensionless value ranging from 0 to 1. A power factor 1 indicates a purely resistive circuit with purely real power. A power factor of less than 1 indicates a circuit with reactive components, such as inductors or capacitors, contributing to the overall power.

In our example, the power factor is 0.866, indicating a circuit with reactive components.

Phasor diagrams help us determine the power factor by visually analyzing the angle between voltage and current phasors. By calculating the cosine of this angle, we can find the power factor of the circuit.

Calculating the power factor provides valuable information about the circuit’s efficiency and the presence of reactive elements.

*For clarity and accuracy, remember to label the phasor diagram’s phasors, angles, and power factor values.

Phasor Diagrams in AC Circuit Analysis

Phasor diagrams for single-phase circuits or three-phase circuits are a valuable tool for representing impedance and admittance in AC circuits. Impedance and admittance are complex quantities that describe the relationship between voltage and current in a circuit.

Phasor diagrams allow us to visualize and calculate impedance and admittance by examining the magnitudes and angles of the corresponding phasors.

To illustrate the process, let’s consider another simple example: in an AC circuit, the voltage phasor V is 120∠0° V, and the current phasor I is 5∠60° A. Determine the impedance and admittance of the circuit.

Step 1: Represent the Phasors

  • Draw the phasors in the phasor diagram.
  • Draw a vector representing the voltage phasor V with a length of 120 units and an angle of 0° counterclockwise from the reference axis (horizontal).
  • Draw a vector representing the current phasor I with a length of 5 units and an angle of 60° counterclockwise from the reference axis.

Step 2 – Measure the Magnitudes

Measure the magnitudes of the voltage and current phasors in the phasor diagram.

In our example, the magnitude of the voltage phasor V is 120 units, and the magnitude of the current phasor I is 5 units.

Step 3 – Measure the Angle

Measure the angle between the voltage and current phasors in the phasor diagram.

In our example, the angle between the voltage phasor V and the current phasor I is 60°.

Step 4 – Calculate Impedance

Impedance (Z) is the complex ratio of voltage to current and is calculated using Ohm’s Law for AC circuits.

Z = V/I

In our example, Z = 120∠0° V / 5∠60° A

Z = (120/5)∠(0°-60°) Ω

Z = 24∠(-60°) Ω

The impedance of the circuit is 24∠(-60°) Ω.

Step 5 – Calculate Admittance

Admittance (Y) is the reciprocal of impedance and represents the ease with which current flows through a circuit.

Y = 1/Z

In our example, Y = 1 / (24∠(-60°) Ω)

Y = 1 / 24∠60° S

Y = (1/24)∠(-60°) S

The admittance of the circuit is (1/24)∠(-60°) S.

Phasor diagrams help us visualize and calculate impedance and admittance by analyzing the magnitudes and angles of the corresponding phasors. Impedance represents the complex ratio of voltage to current, while admittance represents the reciprocal of impedance.

These quantities provide insights into the circuit’s resistance and reactance and help us understand how the circuit responds to AC signals.

*For clarity and accuracy, label the phasor diagram’s phasors, magnitudes, angles, and units.

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Conclusion

As we conclude our guide on phasor diagrams in PE Power exam, it becomes evident that these graphical representations play a pivotal role in shaping the careers of power engineers.

With the ability to analyze complex AC circuits, understand power factor relationships, and determine impedance and admittance, mastering phasor diagrams in the PE Power exam is necessary for any aspiring power engineer.

To learn how to use phasor representation in 3-phase circuits, read our detailed guide on three-phase circuits in the PE Power exam.

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Don’t miss the opportunity to advance your power engineering career. Visit Study for FE today and explore their comprehensive PE exam preparation courses. With their expert guidance and practical approach, you’ll gain the confidence and proficiency necessary to tackle the challenges posed by phasor diagrams and other critical concepts in power engineering.

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wasim-smal

Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.