Circle Diagram of Induction Motor

What is a Circle Diagram

A circle diagram is defined as a graphical tool that shows how an electrical machine works. It’s used for transformers, alternators, synchronous motors, and induction motors. This diagram helps us understand the machine’s performance under different conditions, making it easier to grasp than complex theories and math.

Importance of Circle Diagram

A circle diagram gives information that a regular phasor diagram doesn’t. A phasor diagram shows the relationship between current and voltage for just one circuit condition. If the condition changes, we have to redraw it. A circle diagram, however, shows multiple conditions on one plane. For induction motors, it displays details like power output, power factor, torque, slip, speed, copper loss, and efficiency in a clear, graphical form.

Test Performed to Compute Data Required for Drawing Circle Diagram

To draw a circle diagram, we perform no-load and blocked rotor tests on an induction motor. In the no-load test, the motor runs without a load, and we use two wattmeters to measure its power consumption, which includes only no-load losses. We assume the slip is zero. From this test, we calculate the no-load current and the angle between voltage and current, which is usually large because of the motor’s high inductive reactance.

In the blocked rotor test, the rotor is blocked, similar to short-circuiting the secondary of a transformer. We calculate the short circuit current and the lag angle between voltage and current from this test. We also determine the copper losses in the rotor and stator.

How to Draw Circle Diagram of Induction Motor

A circle diagram is commonly used to represent an induction motor. To draw a circle diagram like the one below:

1. The no-load current and the no load angle calculated from no load test is plotted. This is shown by the line OA, where Ɵ₀ is the no load power factor angle.
2. The short circuit current and the angle obtained from block rotor test is plotted. This is shown by the line OC and the angle is shown by Ɵ_B.
3. The right bisector of the line AC is drawn which bisects the line and it is extended to cut in the line AE which gives us the centre.
4. The stator current is calculated from the equivalent circuit of the induction motor which we get from the two tests. That current is plotted in the circle diagram according to the scale with touching origin and a point in the circle diagram which is shown by B.
5. The line AC is called the power line. By using the scale for power conversion that we have taken in the circle diagram, we can get the output power if we move vertically above the line AC to the periphery of the circle. The output power is given by the line MB.
6. The total copper loss is given by the line GM.
7. For drawing the torque line, the total copper loss should be separated to both the rotor copper loss and stator copper loss. The line DE gives the stator copper loss and the line CD gives the rotor copper loss. In this way, the point E is selected.
8. The line AD is known as torque line which gives the torque developed by induction motor.

Parts of a Circle Diagram

The parts of a circle diagram include:

• Maximum output power
• Maximum torque
• Maximum Input Power

Maximum Output Power

When the tangent to the circle is parallel to the line then output power will be maximum. That point M is obtained by drawing a perpendicular line from the center to the output line and extending it to cut at M.

Maximum Torque

When the tangent to the circle is parallel to the torque line, it gives maximum torque. This is obtained by drawing a line from the center in perpendicular to the torque line AD and extending it to cut at the circle. That point is marked as N.

Maximum Input Power

It occurs when tangent to the circle is perpendicular to the horizontal line. The point is the highest point in the circle diagram and drawn to the center and extends up to S. That point is marked as R.

Conclusion of Circle Diagram

This method uses some approximations and rounding off values to draw the circle diagram. Despite these errors, it still provides a good estimate of the results.

The main downside to a circle diagram is that although it is easy to interpret and read, it can be quite time consuming to draw. Alternative methods include using mathematical formulas or equivalent circuit models instead to find out the various performance parameters. If you’re looking to learn more about circle diagrams and other electrical engineering topics, check out our full list of basic electrical questions.