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• How to write a lab report

How To Write A Lab Report | Step-by-Step Guide & Examples

Published on May 20, 2021 by Pritha Bhandari . Revised on July 23, 2023.

A lab report conveys the aim, methods, results, and conclusions of a scientific experiment. The main purpose of a lab report is to demonstrate your understanding of the scientific method by performing and evaluating a hands-on lab experiment. This type of assignment is usually shorter than a research paper .

Lab reports are commonly used in science, technology, engineering, and mathematics (STEM) fields. This article focuses on how to structure and write a lab report.

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Table of contents

Structuring a lab report, introduction, other interesting articles, frequently asked questions about lab reports.

The sections of a lab report can vary between scientific fields and course requirements, but they usually contain the purpose, methods, and findings of a lab experiment .

Each section of a lab report has its own purpose.

• Title: expresses the topic of your study
• Abstract : summarizes your research aims, methods, results, and conclusions
• Introduction: establishes the context needed to understand the topic
• Method: describes the materials and procedures used in the experiment
• Results: reports all descriptive and inferential statistical analyses
• Discussion: interprets and evaluates results and identifies limitations
• Conclusion: sums up the main findings of your experiment
• References: list of all sources cited using a specific style (e.g. APA )
• Appendices : contains lengthy materials, procedures, tables or figures

Although most lab reports contain these sections, some sections can be omitted or combined with others. For example, some lab reports contain a brief section on research aims instead of an introduction, and a separate conclusion is not always required.

If you’re not sure, it’s best to check your lab report requirements with your instructor.

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Your title provides the first impression of your lab report – effective titles communicate the topic and/or the findings of your study in specific terms.

Create a title that directly conveys the main focus or purpose of your study. It doesn’t need to be creative or thought-provoking, but it should be informative.

• The effects of varying nitrogen levels on tomato plant height.
• Testing the universality of the McGurk effect.
• Comparing the viscosity of common liquids found in kitchens.

An abstract condenses a lab report into a brief overview of about 150–300 words. It should provide readers with a compact version of the research aims, the methods and materials used, the main results, and the final conclusion.

Think of it as a way of giving readers a preview of your full lab report. Write the abstract last, in the past tense, after you’ve drafted all the other sections of your report, so you’ll be able to succinctly summarize each section.

To write a lab report abstract, use these guiding questions:

• What is the wider context of your study?
• What research question were you trying to answer?
• How did you perform the experiment?
• What did your results show?
• How did you interpret your results?
• What is the importance of your findings?

Nitrogen is a necessary nutrient for high quality plants. Tomatoes, one of the most consumed fruits worldwide, rely on nitrogen for healthy leaves and stems to grow fruit. This experiment tested whether nitrogen levels affected tomato plant height in a controlled setting. It was expected that higher levels of nitrogen fertilizer would yield taller tomato plants.

Levels of nitrogen fertilizer were varied between three groups of tomato plants. The control group did not receive any nitrogen fertilizer, while one experimental group received low levels of nitrogen fertilizer, and a second experimental group received high levels of nitrogen fertilizer. All plants were grown from seeds, and heights were measured 50 days into the experiment.

The effects of nitrogen levels on plant height were tested between groups using an ANOVA. The plants with the highest level of nitrogen fertilizer were the tallest, while the plants with low levels of nitrogen exceeded the control group plants in height. In line with expectations and previous findings, the effects of nitrogen levels on plant height were statistically significant. This study strengthens the importance of nitrogen for tomato plants.

Your lab report introduction should set the scene for your experiment. One way to write your introduction is with a funnel (an inverted triangle) structure:

• Start with the broad, general research topic
• Narrow your topic down your specific study focus
• End with a clear research question

Begin by providing background information on your research topic and explaining why it’s important in a broad real-world or theoretical context. Describe relevant previous research on your topic and note how your study may confirm it or expand it, or fill a gap in the research field.

This lab experiment builds on previous research from Haque, Paul, and Sarker (2011), who demonstrated that tomato plant yield increased at higher levels of nitrogen. However, the present research focuses on plant height as a growth indicator and uses a lab-controlled setting instead.

Next, go into detail on the theoretical basis for your study and describe any directly relevant laws or equations that you’ll be using. State your main research aims and expectations by outlining your hypotheses .

Based on the importance of nitrogen for tomato plants, the primary hypothesis was that the plants with the high levels of nitrogen would grow the tallest. The secondary hypothesis was that plants with low levels of nitrogen would grow taller than plants with no nitrogen.

Your introduction doesn’t need to be long, but you may need to organize it into a few paragraphs or with subheadings such as “Research Context” or “Research Aims.”

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A lab report Method section details the steps you took to gather and analyze data. Give enough detail so that others can follow or evaluate your procedures. Write this section in the past tense. If you need to include any long lists of procedural steps or materials, place them in the Appendices section but refer to them in the text here.

You should describe your experimental design, your subjects, materials, and specific procedures used for data collection and analysis.

Experimental design

Briefly note whether your experiment is a within-subjects  or between-subjects design, and describe how your sample units were assigned to conditions if relevant.

A between-subjects design with three groups of tomato plants was used. The control group did not receive any nitrogen fertilizer. The first experimental group received a low level of nitrogen fertilizer, while the second experimental group received a high level of nitrogen fertilizer.

Describe human subjects in terms of demographic characteristics, and animal or plant subjects in terms of genetic background. Note the total number of subjects as well as the number of subjects per condition or per group. You should also state how you recruited subjects for your study.

List the equipment or materials you used to gather data and state the model names for any specialized equipment.

List of materials

35 Tomato seeds

15 plant pots (15 cm tall)

Light lamps (50,000 lux)

Nitrogen fertilizer

Measuring tape

Describe your experimental settings and conditions in detail. You can provide labelled diagrams or images of the exact set-up necessary for experimental equipment. State how extraneous variables were controlled through restriction or by fixing them at a certain level (e.g., keeping the lab at room temperature).

Light levels were fixed throughout the experiment, and the plants were exposed to 12 hours of light a day. Temperature was restricted to between 23 and 25℃. The pH and carbon levels of the soil were also held constant throughout the experiment as these variables could influence plant height. The plants were grown in rooms free of insects or other pests, and they were spaced out adequately.

Your experimental procedure should describe the exact steps you took to gather data in chronological order. You’ll need to provide enough information so that someone else can replicate your procedure, but you should also be concise. Place detailed information in the appendices where appropriate.

In a lab experiment, you’ll often closely follow a lab manual to gather data. Some instructors will allow you to simply reference the manual and state whether you changed any steps based on practical considerations. Other instructors may want you to rewrite the lab manual procedures as complete sentences in coherent paragraphs, while noting any changes to the steps that you applied in practice.

If you’re performing extensive data analysis, be sure to state your planned analysis methods as well. This includes the types of tests you’ll perform and any programs or software you’ll use for calculations (if relevant).

First, tomato seeds were sown in wooden flats containing soil about 2 cm below the surface. Each seed was kept 3-5 cm apart. The flats were covered to keep the soil moist until germination. The seedlings were removed and transplanted to pots 8 days later, with a maximum of 2 plants to a pot. Each pot was watered once a day to keep the soil moist.

The nitrogen fertilizer treatment was applied to the plant pots 12 days after transplantation. The control group received no treatment, while the first experimental group received a low concentration, and the second experimental group received a high concentration. There were 5 pots in each group, and each plant pot was labelled to indicate the group the plants belonged to.

50 days after the start of the experiment, plant height was measured for all plants. A measuring tape was used to record the length of the plant from ground level to the top of the tallest leaf.

In your results section, you should report the results of any statistical analysis procedures that you undertook. You should clearly state how the results of statistical tests support or refute your initial hypotheses.

The main results to report include:

• any descriptive statistics
• statistical test results
• the significance of the test results
• estimates of standard error or confidence intervals

The mean heights of the plants in the control group, low nitrogen group, and high nitrogen groups were 20.3, 25.1, and 29.6 cm respectively. A one-way ANOVA was applied to calculate the effect of nitrogen fertilizer level on plant height. The results demonstrated statistically significant ( p = .03) height differences between groups.

Next, post-hoc tests were performed to assess the primary and secondary hypotheses. In support of the primary hypothesis, the high nitrogen group plants were significantly taller than the low nitrogen group and the control group plants. Similarly, the results supported the secondary hypothesis: the low nitrogen plants were taller than the control group plants.

These results can be reported in the text or in tables and figures. Use text for highlighting a few key results, but present large sets of numbers in tables, or show relationships between variables with graphs.

You should also include sample calculations in the Results section for complex experiments. For each sample calculation, provide a brief description of what it does and use clear symbols. Present your raw data in the Appendices section and refer to it to highlight any outliers or trends.

The Discussion section will help demonstrate your understanding of the experimental process and your critical thinking skills.

In this section, you can:

• Interpret your results
• Compare your findings with your expectations
• Identify any sources of experimental error
• Explain any unexpected results
• Suggest possible improvements for further studies

Interpreting your results involves clarifying how your results help you answer your main research question. Report whether your results support your hypotheses.

• Did you measure what you sought out to measure?
• Were your analysis procedures appropriate for this type of data?

Compare your findings with other research and explain any key differences in findings.

• Are your results in line with those from previous studies or your classmates’ results? Why or why not?

An effective Discussion section will also highlight the strengths and limitations of a study.

• Did you have high internal validity or reliability?
• How did you establish these aspects of your study?

When describing limitations, use specific examples. For example, if random error contributed substantially to the measurements in your study, state the particular sources of error (e.g., imprecise apparatus) and explain ways to improve them.

The results support the hypothesis that nitrogen levels affect plant height, with increasing levels producing taller plants. These statistically significant results are taken together with previous research to support the importance of nitrogen as a nutrient for tomato plant growth.

However, unlike previous studies, this study focused on plant height as an indicator of plant growth in the present experiment. Importantly, plant height may not always reflect plant health or fruit yield, so measuring other indicators would have strengthened the study findings.

Another limitation of the study is the plant height measurement technique, as the measuring tape was not suitable for plants with extreme curvature. Future studies may focus on measuring plant height in different ways.

The main strengths of this study were the controls for extraneous variables, such as pH and carbon levels of the soil. All other factors that could affect plant height were tightly controlled to isolate the effects of nitrogen levels, resulting in high internal validity for this study.

Your conclusion should be the final section of your lab report. Here, you’ll summarize the findings of your experiment, with a brief overview of the strengths and limitations, and implications of your study for further research.

Some lab reports may omit a Conclusion section because it overlaps with the Discussion section, but you should check with your instructor before doing so.

If you want to know more about AI for academic writing, AI tools, or fallacies make sure to check out some of our other articles with explanations and examples or go directly to our tools!

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A lab report conveys the aim, methods, results, and conclusions of a scientific experiment . Lab reports are commonly assigned in science, technology, engineering, and mathematics (STEM) fields.

The purpose of a lab report is to demonstrate your understanding of the scientific method with a hands-on lab experiment. Course instructors will often provide you with an experimental design and procedure. Your task is to write up how you actually performed the experiment and evaluate the outcome.

In contrast, a research paper requires you to independently develop an original argument. It involves more in-depth research and interpretation of sources and data.

A lab report is usually shorter than a research paper.

The sections of a lab report can vary between scientific fields and course requirements, but it usually contains the following:

• Abstract: summarizes your research aims, methods, results, and conclusions
• References: list of all sources cited using a specific style (e.g. APA)
• Appendices: contains lengthy materials, procedures, tables or figures

The results chapter or section simply and objectively reports what you found, without speculating on why you found these results. The discussion interprets the meaning of the results, puts them in context, and explains why they matter.

In qualitative research , results and discussion are sometimes combined. But in quantitative research , it’s considered important to separate the objective results from your interpretation of them.

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How to Write a Lab Report – with Example/Template

April 11, 2024

Perhaps you’re in the midst of your challenging AP chemistry class in high school, or perhaps college you’re enrolled in biology , chemistry , or physics at university. At some point, you will likely be asked to write a lab report. Sometimes, your teacher or professor will give you specific instructions for how to format and write your lab report, and if so, use that. In case you’re left to your own devices, here are some guidelines you might find useful. Continue reading for the main elements of a lab report, followed by a detailed description of the more writing-heavy parts (with a lab report example/lab report template). Lastly, we’ve included an outline that can help get you started.

What is a lab report?

A lab report is an overview of your experiment. Essentially, it explains what you did in the experiment and how it went. Most lab reports end up being 5-10 pages long (graphs or other images included), though the length depends on the experiment. Here are some brief explanations of the essential parts of a lab report:

Title : The title says, in the most straightforward way possible, what you did in the experiment. Often, the title looks something like, “Effects of ____ on _____.” Sometimes, a lab report also requires a title page, which includes your name (and the names of any lab partners), your instructor’s name, and the date of the experiment.

Abstract : This is a short description of key findings of the experiment so that a potential reader could get an idea of the experiment before even beginning.

Introduction : This is comprised of one or several paragraphs summarizing the purpose of the lab. The introduction usually includes the hypothesis, as well as some background information.

Lab Report Example (Continued)

Materials : Perhaps the simplest part of your lab report, this is where you list everything needed for the completion of your experiment.

Methods : This is where you describe your experimental procedure. The section provides necessary information for someone who would want to replicate your study. In paragraph form, write out your methods in chronological order, though avoid excessive detail.

Data : Here, you should document what happened in the experiment, step-by-step. This section often includes graphs and tables with data, as well as descriptions of patterns and trends. You do not need to interpret all of the data in this section, but you can describe trends or patterns, and state which findings are interesting and/or significant.

Discussion of results : This is the overview of your findings from the experiment, with an explanation of how they pertain to your hypothesis, as well as any anomalies or errors.

Conclusion : Your conclusion will sum up the results of your experiment, as well as their significance. Sometimes, conclusions also suggest future studies.

Sources : Often in APA style , you should list all texts that helped you with your experiment. Make sure to include course readings, outside sources, and other experiments that you may have used to design your own.

How to write the abstract

The abstract is the experiment stated “in a nutshell”: the procedure, results, and a few key words. The purpose of the academic abstract is to help a potential reader get an idea of the experiment so they can decide whether to read the full paper. So, make sure your abstract is as clear and direct as possible, and under 200 words (though word count varies).

When writing an abstract for a scientific lab report, we recommend covering the following points:

• Background : Why was this experiment conducted?
• Objectives : What problem is being addressed by this experiment?
• Methods : How was the study designed and conducted?
• Results : What results were found and what do they mean?
• Conclusion : Were the results expected? Is this problem better understood now than before? If so, how?

How to write the introduction

The introduction is another summary, of sorts, so it could be easy to confuse the introduction with the abstract. While the abstract tends to be around 200 words summarizing the entire study, the introduction can be longer if necessary, covering background information on the study, what you aim to accomplish, and your hypothesis. Unlike the abstract (or the conclusion), the introduction does not need to state the results of the experiment.

Here is a possible order with which you can organize your lab report introduction:

• Intro of the intro : Plainly state what your study is doing.
• Background : Provide a brief overview of the topic being studied. This could include key terms and definitions. This should not be an extensive literature review, but rather, a window into the most relevant topics a reader would need to understand in order to understand your research.
• Importance : Now, what are the gaps in existing research? Given the background you just provided, what questions do you still have that led you to conduct this experiment? Are you clarifying conflicting results? Are you undertaking a new area of research altogether?
• Prediction: The plants placed by the window will grow faster than plants placed in the dark corner.
• Hypothesis: Basil plants placed in direct sunlight for 2 hours per day grow at a higher rate than basil plants placed in direct sunlight for 30 minutes per day.
• How you test your hypothesis : This is an opportunity to briefly state how you go about your experiment, but this is not the time to get into specific details about your methods (save this for your results section). Keep this part down to one sentence, and voila! You have your introduction.

How to write a discussion section

Here, we’re skipping ahead to the next writing-heavy section, which will directly follow the numeric data of your experiment. The discussion includes any calculations and interpretations based on this data. In other words, it says, “Now that we have the data, why should we care?”  This section asks, how does this data sit in relation to the hypothesis? Does it prove your hypothesis or disprove it? The discussion is also a good place to mention any mistakes that were made during the experiment, and ways you would improve the experiment if you were to repeat it. Like the other written sections, it should be as concise as possible.

Here is a list of points to cover in your lab report discussion:

• Weaker statement: These findings prove that basil plants grow more quickly in the sunlight.
• Stronger statement: These findings support the hypothesis that basil plants placed in direct sunlight grow at a higher rate than basil plants given less direct sunlight.
• Factors influencing results : This is also an opportunity to mention any anomalies, errors, or inconsistencies in your data. Perhaps when you tested the first round of basil plants, the days were sunnier than the others. Perhaps one of the basil pots broke mid-experiment so it needed to be replanted, which affected your results. If you were to repeat the study, how would you change it so that the results were more consistent?
• Implications : How do your results contribute to existing research? Here, refer back to the gaps in research that you mentioned in your introduction. Do these results fill these gaps as you hoped?
• Questions for future research : Based on this, how might your results contribute to future research? What are the next steps, or the next experiments on this topic? Make sure this does not become too broad—keep it to the scope of this project.

How to write a lab report conclusion

This is your opportunity to briefly remind the reader of your findings and finish strong. Your conclusion should be especially concise (avoid going into detail on findings or introducing new information).

Here are elements to include as you write your conclusion, in about 1-2 sentences each:

• Restate your goals : What was the main question of your experiment? Refer back to your introduction—similar language is okay.
• Restate your methods : In a sentence or so, how did you go about your experiment?
• Key findings : Briefly summarize your main results, but avoid going into detail.
• Limitations : What about your experiment was less-than-ideal, and how could you improve upon the experiment in future studies?
• Significance and future research : Why is your research important? What are the logical next-steps for studying this topic?

Template for beginning your lab report

Here is a compiled outline from the bullet points in these sections above, with some examples based on the (overly-simplistic) basil growth experiment. Hopefully this will be useful as you begin your lab report.

1) Title (ex: Effects of Sunlight on Basil Plant Growth )

2) Abstract (approx. 200 words)

• Background ( This experiment looks at… )
• Objectives ( It aims to contribute to research on…)
• Methods ( It does so through a process of…. )
• Results (Findings supported the hypothesis that… )
• Conclusion (These results contribute to a wider understanding about…)

3) Introduction (approx. 1-2 paragraphs)

• Intro ( This experiment looks at… )
• Background ( Past studies on basil plant growth and sunlight have found…)
• Importance ( This experiment will contribute to these past studies by…)
• Hypothesis ( Basil plants placed in direct sunlight for 2 hours per day grow at a higher rate than basil plants placed in direct sunlight for 30 minutes per day.)
• How you will test your hypothesis ( This hypothesis will be tested by a process of…)

4) Materials (list form) (ex: pots, soil, seeds, tables/stands, water, light source )

5) Methods (approx. 1-2 paragraphs) (ex: 10 basil plants were measured throughout a span of…)

6) Data (brief description and figures) (ex: These charts demonstrate a pattern that the basil plants placed in direct sunlight…)

7) Discussion (approx. 2-3 paragraphs)

• Support or reject hypothesis ( These findings support the hypothesis that basil plants placed in direct sunlight grow at a higher rate than basil plants given less direct sunlight.)
• Factors that influenced your results ( Outside factors that could have altered the results include…)
• Implications ( These results contribute to current research on basil plant growth and sunlight because…)
• Questions for further research ( Next steps for this research could include…)
• Restate your goals ( In summary, the goal of this experiment was to measure…)
• Restate your methods ( This hypothesis was tested by…)
• Key findings ( The findings supported the hypothesis because…)
• Limitations ( Although, certain elements were overlooked, including…)
• Significance and future research ( This experiment presents possibilities of future research contributions, such as…)
• Sources (approx. 1 page, usually in APA style)

Final thoughts – Lab Report Example

Hopefully, these descriptions have helped as you write your next lab report. Remember that different instructors may have different preferences for structure and format, so make sure to double-check when you receive your assignment. All in all, make sure to keep your scientific lab report concise, focused, honest, and organized. Good luck!

For more reading on coursework success, check out the following articles:

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Experiment (5e Background)

From d&d wiki.

• 1.1 Purpose
• 1.2 Feature: Scars of Conception
• 1.3 Alternate Feature: Success
• 1.4 Suggested Characteristics

Experiment [ edit ]

You are the result of a twisted combination of experimentation and the manipulation of life. Your master manipulated you from his own hand to serve his curiosity and desire to push the godlike powers of trial and error to the limit. Perhaps you were created by your master, or perhaps you were kidnapped and "modified". You have since fled your confines or have been released, and found your place in the outside world, adapting to it or surviving in spite of it. You could have spent any number of years trapped with your creator. Did he do malicious tests on you while you were chained to a wall, was he obsessive and caring about his pet, or did he just seek perfection? Were you caged or trapped in an atrium for the amusement of a jeering crowd? Was there just one magician or scientist behind you, or was it a team of curious voyeurs filled with the urge to find out the limits of our natural world? Did you escape or were you set free? Were there more of you?

Skill Proficiencies: Choose Two. Deception, Persuasion, Medicine, Perception or Survival.

Tool Proficiencies: Choose One: Herbalism Kit, Disguise Kit, Thieves' Tools, Alchemist's Supplies.

Languages: Draconic, Infernal, or Primordial. One of your choice.

Equipment: A backpack, a (ragged) set of common or traveler's clothes, a sack, a crowbar or small knife, a lantern, two flasks of oil, (incomplete?) lab notes on the experiment, a blanket, 50 feet of hempen or silk rope, and a belt pouch containing 15 GP.

Purpose [ edit ]

Why were you created?

Feature: Scars of Conception [ edit ]

You have some physical deformity that marks you out as a victim of arcane or physiological anatomization. This abnormality is the result of a signature operation that identifies the group or individual that worked on you. This can be recognized by other victims of your master; and by scientists, wizards, and students familiar with your master's work. Deformities might include a hunched back, digitigrade legs, cranial vivisection scars, protruding bone or metal, grafted animal parts, elemental veins visible through your skin, or any other dramatic, visibly distinctive disfiguration.

In addition, you are familiar with the layout of laboratories in the most general sense, and know the function (if not the names) of laboratory equipment such as athanors, alembics, pressure chambers, electric baths, blood transfusion machines, or exotic arcane surgery tools.

Alternate Feature: Success [ edit ]

The experiment conducted on you turned out to be a success! You are either one of the first successful subjects, or a later recipient of the perfected process, and were released into the world to carry out your purpose. Work with your DM to determine what your purpose is, and whether or not you are still subservient to your creator. (Likely purposes include being a super-soldier, begetting superior progeny, fulfilling some function in a ritual/machine/prophecy, etc.)

As a consequence of the experiment, you have control over your cosmetic quality, and can control its appearance or apparence as part of how you communicate. (For example, a character with sharp teeth could bare their fangs to threaten, a character with bioluminescence could give off colored flashes to express emotion, a hybrid could change their attire to emphasize the traits of one race to appear very different, etc.) Anyone familiar with the experiment will notice you right away though- you're famous in a small circle of researchers.

Suggested Characteristics [ edit ]

Your tortured experience has left scars, both psychological and physical.

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Experiment 3: Phase Sequence Measurements

To understand the phase sequence of a three phase supply and study methods to measure the phase sequence of a given power supply.

Read the Experiment through. Analyze the circuit in Figure 6 for a capacitance of 50 µF and a few values of R (R = |X c |, R = |X c |/2 and R = 2|X c |) to determine which gives you the largest difference in the magnitude of Vbn in the figure for the two different phase sequences, abc and acb. You will use the values of R (R = |X c |, R = |X c |/2 and R = 2|X c |) and C = 50 µF in Fig. 6 of method 3.

• Phase sequence detection box (in lab)
• 3-phase Variac (in lab)
• Capacitor Box
• Resistive Load Cart or Variable Resistor/Rheostat
• Coax cable (BNC to BNC – Check out of stockroom (SR))
• Power lab box with cables and Fluke meter (SR)

Background:

Given a 3-phase voltage source on the three wires a , b , and, c .  If the voltage waveform of wire a , is the one numbered  1  as shown in Fig. 1, which waveform represents the voltage of wire b ?  If that waveform is the one numbered  2  in Fig. 1, then the voltage sequence is abc .  This is a clockwise rotation or positive sequence with waveform  1  our “reference” voltage source for phase angle (0o), then waveform  2  will have a phase angle of -120o (120o lagging, or 240o leading) and waveform  3  an angle of -240o (or 120o leading). If on the other hand, we have the representation of Fig. 2, then the sequence is acb with a counter-clockwise rotation or negative sequence. Now, waveform 2 will be leading 120o ahead of 1 instead of lagging, and 3 will be another 120o ahead of 2. You will examine several ways that the phase sequence can be determined.

The direction of rotation of polyphase induction and synchronous motors depends on the phase sequence of the applied voltages. Also, the two wattmeters in the two wattmeter method of measuring three-phase power interchange their readings when subjected to a reversal of phase sequence, even though the system is balanced. Magnitudes of the various currents and component voltages in balanced systems are not affected by a reversal of phase sequence.

If the phase sequence of the applied voltages is reversed in an unbalanced system, certain branch currents change in magnitudes as well as in time phase, although the total watts and vars generated remain the same.

In practice it is desirable, and sometimes necessary, to know the phase sequence of a three- phase power system. For example, when paralleling 2 three-phase transformers, if the wrong sequence is assumed the result could be catastrophic.  The phase sequence also determines the direction induction motors will turn.

There are many possible ways to determine the sequence.   A wattmeter can be used to determine the phase sequence.  A 3-phase inductive load can be connected and a wattmeter is used such that I a is passed through the current coil of the wattmeter, then the reading of the wattmeter will be proportional to either cos( 30 + phi) or cos(30 – phi) depending on whether V12 or V13 is applied to the voltage coil. Other methods, discussed below, depend on unbalanced polyphase circuit phenomena.

One method of determining phase sequence is based on the direction of rotation of induction motors. This is called Rotating type. A three phase supply is connected to the same number of coils producing a rotating magnetic field, and this rotating magnetic field produces eddy EMF in the rotatable aluminum disc.

This eddy EMF produces eddy current on the aluminum disc, due to the interaction of the eddy currents with the rotating magnetic field a torque is produced which causes the aluminum disc to rotate. The clockwise direction rotation of the disc indicates the sequence as a b c , and the anti-clockwise rotation of the disc indicates the change in phase sequence ( a c b ).

Another method uses the oscilloscope as in the circuit of Fig. 3.

Generally, any unbalanced set of load impedances may be employed as a voltage phase sequence checker. The effects produced by change in phase sequence can be determined theoretically, and when an effect peculiar to one sequence is noted, that effect can be used to designate the phase sequence of the system.

A common type of circuit for checking phase sequence in three-phase systems is the unbalanced circuit arrangements shown below

If lamp a is brighter than lamp b the phase sequence of the line to line voltages is ab, bc, ca.  If lamp b is brighter than lamp a, the phase sequence is ab, ca, bc.

The circuit in Fig. 5 (taken from the web, but the source does not exist anymore) uses a capacitor instead of the inductor of Fig. 4.

If lamp S is brighter than lamp T the phase sequence of the phase voltages is RST . If Lamp T is brighter than lamp S , the phase sequence is RTS .

Another voltage sequence checker may be made using the circuits shown in Fig. 5. The current taken by the voltmeter should be negligible compared with the current through X and R.

You are to make measurements on each of the three methods described above to determine the phase sequence and to allow checking of the result by calculations. Generally you will want to know all of the voltages and currents in each of the branches of the circuitry for methods 2 and 3.

Verify the phase sequence at your bench using the circuit of Fig. 3.

• Connect your three phases and neutral from the Variac to the phase-sequence detector.
• Connect the output of the phase sequence detector (BNC) to the oscilloscope.
• Set the scope to trigger on the AC line.
• Adjust the Variac to 20 V LN .
• You should be able to see a waveform similar to Fig. 3 on the scope by adjusting the potentiometers to different levels.
• Save the waveform for this phase sequence and for other possibilities by swapping any two of the wires at a time. Make sure you turn off power every time you swap the wires.
• Arrange to set up a circuit like the one of Figures 5 to determine the impedance of each part of the circuit. (Note that the resistance of a lamp measured by an ohm meter is significantly different than the resistance while operating.  This is because of the change in resistivity with temperature.) Remember, you will have to measure and record the voltages and currents across the three load elements (the lamps and the reactive element) in the following steps for use in your calculations.
• Apply 208 V LL from the 3-phase Variac to your circuit without capacitor. Which lamp is brightest?
• Apply 5 different values of capacitance to the circuit. Record and measure the voltages and currents across the elements in each step. Turn off the power to circuit.
• Swap any two of the power wires of your circuit. Apply power and repeat step (3).
• Arrange to set up the circuits in Figure 6 with capacitor.
• Connect circuit using R = |Xc| .
• Apply 208 VLL from the 3-phase Variac to your circuit.
• Record and measure V an , V bn , V cn , I ac , and the powers (S, Q, and P) flowing into your circuit between terminals A-n and C-n .
• Turn off the power and reverse phases A and C . Measure V an , V bn , V cn , I ac , and the powers (S, Q, and P) for this phase sequence into terminals A-n and C-n .
• Repeat steps 3 to 5 with new values of R = |Xc| /2 and R = 2|Xc| in circuit of Figure 6.
• Assume both bulbs have resistances equal to the average of their in-circuit operating resistance. Do the following for either the circuit of Fig. 4 or of figure 5. Call the current entering the terminals ABC (towards the C (or L) and the lamps) IA, IB, IC. Write KVL to give three equations for the voltages, VAB, VBC, and VCA in terms of the three currents. Since these voltages are known and assumed balanced, you have three equations in three unknowns. Using KCL at the node labelled n one can easily reduce the number of unknowns to two and use only two of the KVL equations. Some may find this approach easier to follow. A third approach is to use the superposition principle to find the voltage at the central node and from it the voltages across each element and the individual currents. Obviously, a third approach is to simulate the circuit in multi-sim. You may choose any method to solve for the expected currents, voltages and powers into each light bulb for the phase sequence you assume to confirm how this circuit works (see report section). Ask your instructor if you need more help. You may find useful hints in the following Matlab-like statement:

V_rs= (-j/Xc)Ir - Rs*Is V_tr = (j/Xc)Ir +Rt*It V_st = -Rt*It + Rs*Is V_st = 1; V_tr = a^2; V_rs = a; V = [a ;a^2;1]; Z = [-j/Xc  -Rs  0; j/Xc  0  Rt;  0  Rs –Rt]; Function[Ir, Is, It] = sequence(a,Xc,Rs,Rt)

• The circuits of figure 6 are significantly easier to solve. Once you define a phase sequence you can write down VA, VB, and VC. Then calculate VAC, and IAC. From this you can calculate the voltage at the node labelled n and hence Vbn for each of the two possible phase sequences.

Your report should include:

• An explanation of how method 1 works.
• Show and indicate the phase sequence of the waveforms saved
• Explain how the circuit of figure 3 works and how it allows you to determine the phase sequence.
• Phasor diagrams for the two circuits that you used (method 2 and 3) for at least one sequence.
• Why you cannot determine the phase sequence in method 2 without capacitor? 
• Your calculated values for the powers dissipated in each light bulb in the circuit used for method 2 for one of the phase sequences.
• The expected V bn for your circuit of figure 6 for each of the phase sequences as well as the Power and VARS consumed.
• How did the power flow and VARS for the two phase sequences for the circuit of figure 6 compare?  Explain your observation on Power flow and VARS.
• In addition to this analysis, you should include the usual elements, abstract, procedure, data, analysis and conclusions.

Bibliography

1-  http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_10.html under Design Science License.

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