How To Draw A Motor Circle Diagram: Simple Guide

The circle diagram of an induction motor might seem hard to understand. But it is really just a simple picture. This picture tells you all about your motor. It is a tool that uses a drawing. It helps you see how the motor will work. You don’t have to do hard math. This article will show you how to draw this helpful diagram. We will go step by step. You will learn what facts you need. You will also learn how to use these facts to see how well your motor works. Reading this will save you time. It will make a tricky subject easy to learn.

What is a Circle Diagram of an Induction Motor?

A circle diagram of an induction motor is a picture. It shows how an induction motor acts or works. You can think of it like a map for your motor. This one diagram can tell you many things. It can tell you about the motor’s current, power factor, efficiency, and torque. It shows this for any amount of load. It all begins with the motor’s equivalent circuit. As the motor’s slip changes, the tip of the line showing the stator current draws a path. This path makes a circle.

This picture, the circle diagram is a graphical drawing that shows the motor’s whole story. It shows how the motor uses power from the power line. You can see every detail, from no load at all to a full load. It is a very strong tool for engineers and people who fix machines. It helps them look at the performance of a machine. They can do this without needing to run a lot of different tests.

Why Should You Learn the Construction of the Circle Diagram?

Learning how to make the construction of the circle diagram is very helpful. You can use simple shapes and lines instead of hard math problems. This makes it simple to see how the motor works. You can find the most output power or the most torque the motor can make. This information is very important. It helps you use the motor in the correct way.

This diagram helps you check the performance of a machine. You can quickly find the motor’s efficiency at a specific load. You can also see the power factor and how it changes. This helps make sure the motor is running well and not wasting power. It gives you a full picture of all the operating conditions of the motor. And it’s all on one piece of paper.

What Information Do You Require to Start?

To draw the circle diagram, you first require some facts. You do not need to run the motor under every kind of load. Instead, you just need to do two simple tests. You also need to know the resistance of the stator winding.

Here is what you will need:

No-Load Test: This test is done when the motor is running with no load on its shaft. It helps you find out the no-load current and the no-load losses.
Blocked Rotor Test: This test is done when the motor’s rotor is held still so it cannot spin. It is like the short-circuit test on a transformer. It gives you facts about the motor in a high slip condition.
Stator Resistance: You need to measure the resistance of the stator windings for each phase. This is a simple measurement you can do with a tool called an ohmmeter.

These three sets of facts are all you need to build the whole diagram.

How Do You Obtain the No-Load Test Result?

To obtain the findings from the no-load test, you run the induction motor at its planned voltage and frequency. You do this with no load. The motor will run at a speed that is very close to its synchronous speed. While the motor is running, you have to measure three things.

You will measure:

The line voltage (Vnl)
The no-load current (I₀)
The total input power (W₀)

With this data, you can figure out the no-load power factor. You can find this by using the formula for power. The power you measure here is used to feed the constant loss in the machine. This includes things like friction, air resistance, and core loss. This gives us the first important point we need to begin drawing our diagram.

How Do You Obtain the Blocked Rotor Test Result?

The second test is the blocked rotor test. For this test, you stop the rotor from spinning. You hold it still. The rotor is not moving, so this is like a condition of the biggest possible slip. We do not use the full voltage. Using full voltage would pull a very large current and could hurt the motor. Instead, we use a smaller voltage. We slowly turn up this small voltage until the normal full-load current is flowing in the stator.

During this test, you will measure:

The applied line voltage (Vbr)
The stator current (Ibr)
The total input power (Wbr)

The power you measure here is mostly the copper loss in the wires because the motor is not spinning. This blocked rotor test gives us the second important point for our circle diagram. The data must be adjusted for the motor’s normal working voltage.

What is the Step-by-Step Construction of the Circle Diagram?

Now you have the data, so you can start the construction of the circle diagram. Follow these easy steps to draw the circle.

Choose a Scale: First, pick a good scale for the current. For example, you could say 1 cm = 5 A.
Draw Axes: Draw two lines that cross at a right angle. The flat line is the x-axis. The up-and-down line is the y-axis, which will show the voltage. The point where they meet is called the origin, O.
Plot No-Load Point: From the no-load test, you have the no-load current (I₀) and the no-load angle (Φ₀). From the origin O, draw a line. Make its length equal to I₀ (using your scale). Draw it at an angle of Φ₀ away from the up-and-down voltage line. The end of this line is point O’.
Plot Blocked Rotor Point: From the blocked rotor test, you have the current (Is) and the angle (Φs). From the origin O, draw a line. Make its length equal to Is at an angle of Φs. The end of this line is point A.
Draw the Chord: Draw a straight line that connects point O’ and point A. This line, O’A, is a chord of the circle.
Find the Center: To find the center of the circle, you must draw the perpendicular bisector of the chord O’A. A perpendicular bisector is a line that cuts another line in half at a 90-degree angle.
Draw the Base Line: Draw a flat line from point O’ that runs next to the x-axis.
Locate the Center: The point where the perpendicular bisector of the chord crosses the flat line from O’ is the center of the circle, C.
Draw the Circle: With C as the center and the length of CO’ as the radius, draw the circle. This circle should also go through point A if you did it all correctly. Your diagram of an induction motor is now taking shape.

How Do We Draw the Important Power and Torque Lines?

The circle itself shows the total current the motor can pull. But we want to know about power and torque. To do this, we must add a few more lines to our diagram. These lines help us split up the different kinds of power and loss.

First, drop a perpendicular line straight down from point A to the flat base line. Let’s name the meeting point G. The line AG shows the total power going into the machine during the blocked rotor test. This power is made up of stator copper loss and rotor copper loss. We can split these losses. At a point F on the line AG, we can divide the line. AF will show rotor copper loss, and FG will show stator copper loss.

The output line is the line that connects O’ and F. This line is very important. The second important line is the torque line. This is the same as the output line. The torque the motor creates is related to the vertical distance from any working point on the circle down to this torque line.

How Do You Find the Motor’s Maximum Power from the Diagram?

One of the greatest things about the circle diagram of an induction motor is how simply you can find the biggest values. To find the maximum output power, you need to find the point on the circle. This point should be the farthest away from the output line O’F.

To do this, draw a line that runs next to the output line O’F. This new line should just barely touch the edge of the circle. The point where this new line touches the circle is the point of maximum output power. To find out how much power this is, draw a vertical line from this point down to the output line. The length of this line, changed back using your power scale, gives you the most output power the machine can make.

Thing to Find	How to Find It on the Diagram
Maximum Output Power	The longest vertical line from the circle to the output line.
Maximum Torque	The longest vertical line from the circle to the torque line.
Maximum Input Power	The longest vertical line from the circle to the base line.

What Other Performance Results Can You Get from the Diagram?

The circle diagram is packed with helpful information. After you pick a working point P on the circle, you can find out many things about the motor’s performance at that exact load.

Input Power: Draw a perpendicular line from P straight down to the flat base line. The length of this line is the input power.
Output Power: The length of the vertical line from P down to the output line O’F is the output power.
Efficiency: The efficiency can be found very easily. It is the output power divided by the input power. You just divide the length of the output power line by the length of the input power line.
Power Factor: Draw a line from the origin O to the working point P. The angle this line makes with the up-and-down voltage line is the power factor angle. The cosine of this angle is the power factor.
Slip: The slip of the motor can be found by looking at the rotor copper loss. You can get the slip by dividing the rotor copper loss by the total rotor input power.
Current: The length of the line from the origin O to the point P tells you the size of the current the motor is pulling.

What Does the Final Result of the Diagram Show Us?

The final result is a total picture of your induction motor. The circle diagram of induction motor shows you the connection between all the important things: current, power, power factor, speed, torque, and efficiency. You can see how one thing changes when another thing changes. For example, you can see how the current and power factor move when you put more load on the motor.

This single tool, in the form of a picture, takes the place of many hard calculations. It makes the way the machine works clear and simple to understand. From just two easy tests, you can figure out the motor’s entire range of performance. It is proof of how a simple shape, a circle, can explain the detailed workings of an electrical machine. This is why the diagram of an induction motor is so important in electrical engineering.

Summary: Key Things to Remember

The circle diagram is a picture that helps you see all the performance details of an induction motor.
You only need facts from a no-load test and a blocked rotor test to draw it.
The diagram is a half-circle. Its diameter rests on a flat line.
From the diagram, you can find torque, output power, efficiency, power factor, and slip for any load you are given.
Up-and-down lines on the diagram show different kinds of power and loss.
The biggest output power is found at the point on the circle that is the greatest distance from the output line.
It is a strong tool that makes understanding a complicated machine much easier.