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Motors are essential power sources used in a wide range of products, including industrial equipment, home appliances, and electric mobility. In recent years, as demand for energy savings has increased, it has become increasingly important to select the most suitable high-efficiency motor according to the performance requirements and application of each system.

One technology drawing attention is the IPM motor (Interior Permanent Magnet Motor). IPM motors offer advantages such as high efficiency and high-power density, and they are increasingly being adopted in a broad range of fields, including electric vehicles (EVs) and industrial equipment.

In this column, we explain IPM motors in two parts. In Part 1, we introduce the basic operating principles and features of IPM motors, as well as how they differ from other motor types.

In Part 2, we explain key points to consider when selecting an IPM motor.

1. What Is an IPM Motor?

An IPM motor has a structure in which permanent magnets are embedded inside the rotor. By using permanent magnets, it can operate with high efficiency. For this reason, it is widely used in applications such as EVs, industrial servo motors, and household appliances.

As shown in Figure 1, there are many types of motors, and IPM motors are classified as synchronous motors. Induction motors, which are classified as asynchronous motors, differ in both rotation principles and control methods.

　　　　　　　　　　　　　　Figure 1. Motor classification

2. Structural Features of IPM Motors

A key feature of IPM motors is that the permanent magnets are positioned inside the rotor.

In contrast, SPM motors—also permanent-magnet motors, but with magnets placed on the rotor surface—have a simpler structure and are therefore comparatively easier to design, control, and manufacture.

Compared with SPM motors, IPM motors have magnets embedded inside the rotor, which increases the difficulty of design, control, and manufacturing. However, because the magnets are less affected by centrifugal force, IPM motors are well suited to high-speed operation (see Figure 2).　　　

Figure 2. Comparison of rotor structures for permanent-magnet motors

3. How Torque Is Generated in IPM Motors

A key feature of IPM motors is that they can generate both “magnet torque” produced by permanent magnets and “reluctance torque” produced by utilizing iron (see Supplement 1).

By combining these two types of torque, the ranges of torque and efficiency characteristics can be widely expanded. As a result, IPM motors can more readily achieve high torque, high efficiency and high-power density compared with other motor types. This makes it possible to select motor characteristics suited to a variety of applications. In addition, appropriate current control by the motor controller can maximize IPM motor performance. 

This provides several benefits:

• Reduced power consumption

• More compact equipment

• Reduced heat generation

These are some of the advantages that can be expected.

Supplement 1: What Is Reluctance Torque?

Reluctance torque is torque generated by the tendency of magnetic flux lines to follow a path with low magnetic resistance (reluctance) in other words, a path that is easier for the flux to pass through.

As a familiar example, consider the phenomenon using a magnet and a nail. The figure below illustrates the concept.

　　The nail　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

　　　　aligns with the direction

　　　　of the magnetic flux lines 　　　　　　　　　　　and rotates. 

Figure 3. Image of rotation due to reluctance torque

4. Why IPM Motors Can Achieve High Efficiency

Because IPM motors use permanent magnets, they do not incur secondary copper loss caused by current flowing in the rotor, as in induction motors.

This makes it easier to maintain high efficiency over a wide operating range, from low speed to high speed. In addition to reducing power consumption, it also helps reduce heat generation and cooling load, contributing to improved energy efficiency of the overall system.

5. Summary

By optimizing design of permanent magnets, IPM motors can more easily maintain high efficiency across a wide operating range. For these reasons, adoption has been increasing in recent years across various fields, including industrial equipment and electric mobility. On the other hand, to fully leverage IPM motor performance, it is important to select an appropriate motor according to the application and operating conditions.