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Efficiency studies on transformers

Efficiency studies on transformers

Introduction: (Initial Observation)

When energy is converted from one form to the other, part of that will be lost, usually in the form of heat. Transformers are no exception. In a transformer, alternative electric current enters a coil of wire and converts to electromagnetic waves. The electromagnetic waves will then induce electricity in a second coil of wire. Today many of electric and electronic devices have transformers.

Transformers convert the voltage to the one acceptable by each device. You may have noticed that transformers get hot. That is the part of energy that is being wasted in the form of heat. How much of the energy is being lost during transformation? How efficient the transformers are?

In this project you will study and measure the efficiency of some transformers. You will also study the factors affecting the efficiency of transformers. The results of your experiments may be used in designing more efficient transformers.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

DANGER: The experiments in this project require using AC current such as electricity supplied to homes. If you don’t have previous experience in working with AC current, get help. Do not perform your experiments when you are alone or while small children are around. Wear rubber gloves and do not touch live wires.

Information Gathering:

Find out about transformers and how they work. Read books, magazines or ask professionals who might know in order to learn about the factors that may affect the efficiency of transformers. Select one of such factors to be studied for your experiment. Keep track of where you got your information from.

In your study also look for methods of determining the efficiency of transformers.

Following are some helpful information:

High Voltage Reduces Power Losses

In order for the electrical power distribution network to function, voltages must be stepped up before power is transmitted great distances over power lines. One major problem is that power is lost between the power plant and the consumers because currents use some of the power to heat the transmission lines. The power transmitted along the line is equal to the voltage times the current. The higher the voltage the lower the current that must flow within the transmission lines to deliver the same power. Lower currents produce much less heating and much less power loss. Of course, the high voltages (needed to drive the low currents) must be stepped back down before power is supplied to our homes. Transformers are the critical elements that step up and down the voltages at each end of the line.

What is a Transformer?

A transformer is just a piece of iron with a pair of wires coiled around it – one with many more turns in the coil than the other. The coils of wire are not physically connected. The iron core may be immersed in an insulating oil bath which does not conduct electricity well.

How Does It Work?

The basic physical process underlying the operation of transformers is electromagnetic induction. If a conductor, such as a copper wire, is sitting in a magnetic field that is changing, a current will flow in the conductor. This current will not be steady but will also be changing. Alternatively, if a changing current is present, it will produce a changing magnetic field.

The typical current flowing in our homes and in power distribution networks changes direction 60 times every second. Around every wire, through which this current flows, a magnetic field is produced.

A transformer works only with AC (alternating current) circuits. The changing AC current enters the primary coil of wire in the transformer. A magnetic field is produced that is concentrated in the iron core of the transformer. A secondary coil of wires (also conductors) is wrapped around the iron core, not physically touching the first set of wires. The changing magnetic field produced by the first coil is experienced by the second coil and current begins to flow in these wires as well. The second coil has many more turns of wire and offers a higher resistance to the current flow than the first coil. The greater resistance means that a larger voltage drop (than is present across the first coil of wire) is produced from one end of the coil to the other. Therefore a low voltage enters the transformer and a high voltage exits, or vice versa.

How Power Moves Through the Transformer

Transformer is a passive device — it cannot add power. Power is equal to the product of the voltage and the current. If the voltage increases the current drops. A high voltage and low current exits the transformer carrying almost the same amount of power along the transmission lines that the initial low voltage and high current did. Most transformers operate at high efficiency, under normal conditions, transmitting about 99% of the power that enters them. (About 1% of the power is lost in heating the transformer.)

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to identify a method for testing the efficiency and use it to measure the efficiency of some transformers. We also intend to study the factors affecting the efficiency of transformers.

Select one of the following questions for your study.

  1. How does the type of core material affect the efficiency of transformers?
  2. How does the number of windings in the primary coil affect the efficiency of a transformer?
  3. How is the efficiency of transformers affected by the amount of load?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other. Following are three variable definitions for the three sample questions suggested above.

Variables for question 1:

  1. The type of core material (nothing, aluminum, copper, iron, graphite, …) is the independent variable (also known as manipulated variable).
  2. The efficiency of transformer is the dependent variable (also known as responding variable)
  3. The transformer size, physical specification, voltage and load are the controlled variables.

Variables for question 2:

  1. The number of windings in the primary coil is the independent variable.
  2. The efficiency of transformer is the dependent variable
  3. Controlled variables are the core size and material, input voltage, load, the secondary coil windings and all other environmental and physical specification of transformer.

Variable for question 3:

  1. The amount of load is the independent variable
  2. The efficiency of transformer is the dependent variable
  3. Controlled variables are transformer and input voltage

What is load? Load is the device that consumes electricity. A 100 Watt light bulb for example is a 100 watts load.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

Following are some sample hypothesis for the three proposed questions:

  1. Iron core can result a higher efficiency. My hypothesis is based on my gathered information and the fact that almost all transformers use iron cores.
  2. More windings in primary creates a stronger magnetic field, so it will make the transformer more efficient.
  3. The efficiency of transformers decline by increasing the load. My hypothesis is based on my observation that the transformers get very hot if the amount of load exceeds their recommended limit.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


The efficiency of a transformer is the ratio of the output energy to the input energy. The unit of energy is Watt-hour. For example a 100 watts light bulb consumes 100 watt-hour electrical energy in one hour. In 2 hours, the same light bulb consumes 200 watt- hour.

Watt is the rate of consumption or production of energy. Think of that as the measure of the power of electricity.

watt-hour can also be converted to other units of energy. 1 watt-hour = 860 calories or 1 watt-hour = 3600 joules.

To calculate the efficiency, we can divide the output power (watts) by input power.

Power also can be calculated as a product of voltage and current.

W = V * I or Watts = Volts x Amps


Setup a transformer and a load with ammeters already installed in primary and secondary circuits as shown in the following diagram. For your first experiment try to use a real transformer that you may purchase from a hardware store. For your future experiments you may build your own transformers.

Load is any device that consumes electricity. A light bulb (desk lamp for example) is a perfect choice because it stays on.

Connect the AC power and make sure that the ammeters show a current. It does not matter if you see any light in the light bulb!.

Record the Input current. That is shown in the ammeter in the input side or primary circuit of the transformer.

Record the output current. That is shown in the ammeter in the load side or secondary circuit of the transformer.

Record the input voltage. That is the voltage between X and Y while the circuit is connected.

Record the output voltage. That is the voltage between W and Z while the circuit is connected.

Record the results in a table like this:

Primary Secondary Load Resistance
Current (Amps) 194 Ohms
Voltage (Volts)
Watts (Volts x Amps)

Efficiency at a certain load = Secondary watts : Primary watts

Questions and answers:

The above diagram shows that I need two ammeters. What if I only have one multi-meter and want to use that for both circuits as well as measuring the voltage?

You can do that; however, you will need to repeat your measuring a few times.

First set your multi-meter to measure the current. Place it in the primary circuit and plug your circuit to the AC power. Read and record the current. Then unplug the power for safety.

Second, while your multi-meter is still set to measure the current, place it in the secondary circuit. Plug your primary to the AC source and record the current in the secondary. hen unplug the power for safety.

Third, set your meter to measure AC voltage. Measure and record the primary and secondary voltages.

My volt-meter, multi-meter or ammeter is adjustable to different ranges. How do I know what range to select?

Start from the highest range and move down. In this way you will not overload your meter.

New … I am doing this test on a transformer that I have made. The primary wire is getting very hot, the current goes very high an fuse blows. What am I doing wrong.

Either increase the number of loops in primary coil or use a lower voltage AC input. Increasing the number of loops will increase the inductive resistance of the coil and that will reduce the current. For example if your primary has 100 turns of wire, increase it to 1000 turns. That will reduce the current to 1/10th.

Sample Experiment Notes and Data:

I found this transformer in a broken overhead projector. I connected it’s primary coil to a cable with a plug. This makes it easy for me to plug and unplug my circuit during the experiment. First I measured the output voltage or the voltage induced in the secondary coil, it was 12 volts.

I connected the output to a small light bulb and started the measurements.

The resistance of the light bulb was 194 Ohms. I measured it before connecting it to the circuit. Since I only had one multi-meter, I had to unplug and plug back the cable a few times and each time my multi-meter was in a different part of circuit and measured a different value.

Following are the results:

Primary Secondary Load Resistance
Current (Amps) 0.22 Amps 0.03 Amps 194 Ohms
Voltage (Volts) 110 Volts AC 12 Volts
Watts (Volts x Amps) 24.2 Watts 0.36 Watts

The efficiency happened to be very low. I also noticed that the transformer consumes electricity even if it is not connected to any load.

In the second test, I increased the load by selecting something that has a lower resistance. I found some heating element that is Nichrome (Nickel Chromium alloy) or Tungsten. The resistance of this piece was 0.5 Ohms.

Since the heating element could get very hot, I placed it in a china plate.

Following are the results:

Primary Secondary Load Resistance
Current (Amps) 0.93 Amps 11.7 Volts 0.5 Ohms
Voltage (Volts) 110 Volts 8.5 Amps
Watts (Volts x Amps) 102 Watts 99.45 Watts

Here the efficiency happened to be very high.

Other Experiments:

Depending on the question that you choose for your project, you will need to perform additional experiments to test your hypothesis. Following information will help you design your additional experiments.

Your first question was:

How does the type of core material affect the efficiency of transformers?

Procedure: For this project you need to build your own transformers. Before you choose this experiment, make sure that you can find three identical size rods from different metals. Iron, Aluminum and Copper are the most commonly available metals that you may choose to use.

Without identical size rods, you can not proceed with this experiment.

This section is updated on 2/18/04:………………………..

The diameter of the rods must be 1/2″ or more. The length may be 3 to 5 inches. It does not matter if the cross section of the rod is round or square.

Turn 1000 loops of magnet wire (insulated copper wire) AWG 23 as the primary coil and 500 loops of wire as the secondary coil. Primary and secondary coil do not have to be wind apart. You can wind the primary, cover it with paper and wind the secondary over that.

(AWG stands for American Wire Gauge. Higher gauges have lower thickness)

Wrap some insulating tape over the wires for extra protection.

When you have 3 identical transformers with 3 different core material, connect the primary to a 10 Volt AC input and use a 5 volt light bulb as the load. Repeat your measurements of voltage and current across the primary and secondary for all three samples. Calculate the efficiency for each transformer and report the results.

End of update ……………………………………………………

Your second question was:

How does the number of windings in the primary coil affect the efficiency of a transformer?

Procedure: Build three transformers with Iron as the core metal with three different number of wire loops in primary. For example you may try 200, 300 and 400 turns of wire in your primary coil, while the secondary coil remains the same (100 loops).

Repeat measurements and calculations with the above transformers and repeat the results.

Your third question was:

How is the efficiency of transformers affected by the amount of load?

Procedure: Repeat the main experiment with different loads. Note that a lower resistance provides a higher load (more current and more watts).

Note that different lengths of heating elements have different amounts of resistance, so you can use one heating element for all of your experiments.

Materials and Equipment:

List of material can be extracted from the experiment section and depends on your final experiment design.

  • At least one multi-meter
  • One transformer
  • different loads

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


You will need to multiply voltage by current in each circuit to calculate the power in watts. You will then divide the watts of secondary circuit by the watts of primary circuit to calculate the efficiency of transformer.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Following are some web based references. You will need to visit a local library and find some additional references in the electricity or physics section.