Surface Permanent Magnet Motors Vs Interior ...

The differences between surface permanent magnet motors and interior permanent magnet motors are the topics that we&#;ll discuss in this article. 

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Permanent magnet motors (PM) are a type of AC motor that uses magnets built into or mounted on the motor rotor surface to generate a magnetic field. In electric vehicles, permanent magnet synchronous motors, known for their high torque density and efficiency, are commonly used. The magnets used in these motors are neodymium iron boron magnets, also known as super magnets or powerful magnets.

These magnets are characterized by an extremely concentrated magnetic field, with high magnetic field intensity per square centimeter, giving them strong attractive forces. Despite their small size, the high magnetic field density helps enhance the strength and efficiency of the motor.

Image Source: Stanfordmagnets

Due to the high efficiency and magnetic field density of permanent magnet motors, motors of the same performance can be one-third the size of traditional motors. Moreover, the high efficiency minimizes energy consumption in electric vehicles. Importantly, the magnetic life of supermagnets can be as long as around 400 years, ensuring sustained efficiency and reliability over time.

Permanent magnet motors can be divided into two types: Interior Permanent Magnet Motors (IPM) and Surface Permanent Magnet Motors (SPM). Both types generate magnetic flux by fixing permanent magnets on or inside the rotor. Surface permanent magnet motors attach magnets to the external surface of the rotor, while interior permanent magnet motors embed magnets inside the rotor.

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The Influence of Hybrid and Electric Vehicles

The major advantage of interior permanent magnet motors lies in their high-speed performance, making them particularly advantageous in the applications of vehicles. On the other hand, the power-speed curve of surface permanent magnet motors roughly forms a hyperbolic shape, gradually rising to a quasi-constant power region within a narrow speed range and then decreasing.

For decades, surface permanent magnet motors have dominated the permanent magnet motor market. However, the rise of hybrid and electric vehicles in recent years has driven an increased demand for interior permanent magnet motors. Interior permanent magnet motors exhibit constant power over a wider speed range and offer good solutions for applications like traction and auxiliary motors.

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In vehicle applications, interior permanent magnet motors have distinct advantages over surface permanent magnet motors. The design of interior permanent magnet motors allows better control of magnetic path magnetization. They can provide a larger range or more consistent torque, and through a technique, designers can change their performance by applying current. The technique mainly involves adjusting the stator magnetic field to partially counteract the influence of the permanent magnet.

Surface Permanent Magnet Motors

Surface permanent magnet motors fix magnets to the rotor&#;s surface, and their mechanical strength is lower, limiting the motor&#;s maximum safe mechanical speed. Additionally, regardless of the rotor&#;s position, the measured inductance values at the rotor terminals remain consistent. So, surface permanent magnet motor design largely relies on the magnetic torque component to generate torque.

Image Source: Audi

Interior Permanent Magnet Motors

Interior permanent magnet motors embed magnets inside the rotor, offering better mechanical performance and suitability for high-speed operation. These motors also have a relatively higher Lq/Ld ratio. Because of this, interior permanent magnet motors can generate torque by utilizing both magnetic torque and reluctance torque, adapting to various electric vehicle requirements.

Want more information on Permanent Magnet Synchronous Ac Motor? Feel free to contact us.

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The Vision of IPM and SPM

For high-speed applications like traction motors, interior permanent magnet motors are the optimal choice, using fewer magnet materials and achieving a certain degree of torque. In addition to magnetic torque, these motors use reluctance torque to achieve high torque, responding to the rotation of high-speed motors through vector control.

Simultaneously, by better controlling the magnetization of the magnetic path, interior permanent magnet motors can operate efficiently over a wide speed range. Mechanical reliability is also improved, as magnets do not detach due to centrifugal force, and the rotor is more robust. Under the same power, interior permanent magnet motors can save 30% of energy consumption.

Learn the basics of back EMF, how to measure it, and the advantages and disadvantages of different motor types.

In Part 1 of this FAQ, we looked at brushed DC and brushless DC (BLDC) motors, with the latter having some similarities to a permanent-magnet synchronous motor (PMSM). The simplified PMSM diagram in Figure 1 resembles the diagram presented in Part 1 for the BLDC motor minus the electronic commutation circuit. Subtle yet significant differences between the two, however, manifest themselves in the BLDC motor having a sinusoidal back electromotive force (EMF) while the PMSM has a trapezoidal back EMF.

What&#;s back EMF, and how can I measure it?\

Back EMF (VBEMF) is the voltage generated by a rotating motor in its dual role as a generator and is proportional to speed. Figure 2a shows a simplified model showing VBEMF, series resistance (RS), and series inductance (LS). To measure VBEMF, disconnect the motor from any input power source, hook up your oscilloscope to the motor terminals, and spin the motor at a known speed. You can directly observe the VBEMF on the oscilloscope (Figure 2b) because with no current flowing, RS and LS won&#;t contribute any voltage drop.

Often, you want to measure back EMF during motor operation to obtain speed and position information. In a typical three-phase BLDC motor electronic-commutation scheme, at any given time, only two phases carry current, and a motor controller can measure back EMF on the third, unenergized phase. Alternatively, you can measure series current I and calculate the back EMF:

For more, see &#;Lenz&#;s Law and Back EMF&#; in Motion Control Tips.

What are the disadvantages of PMSM or BLDC motors?

Regarding DC motors, sometimes it&#;s convenient to vary the field strength, which you cannot do with permanent magnets. Given constant field strength, a conventional DC motor operates in a constant-torque mode up to the speed at which the motor&#;s back EMF approaches the input voltage. At this point, reducing the field current reduces the back EMF, and the motor can operate at higher speed but at lower torque, establishing a constant-horsepower mode. This mode can be useful in applications such as machine tools, where a tool can operate with constant cutting power even as material densities change.

For any permanent-magnet motor, permanent-magnet supply-chain issues are becoming critical, as highlighted in this U.S. Department of Energy report. Consequently, research is ongoing to find alternatives. Options have long existed, including the brushed DC motor discussed in part 1, albeit with drawbacks. Another alternative is the wound-rotor synchronous motor (WRSM). Figure 3 shows brushes and slip rings carrying DC current to and from a WRSM&#;s rotor windings. The slip rings and brushes lack the arcing and flashover risk of the brushed DC motor commutator, yet they remain subject to mechanical wear.

Another option is the AC induction motor. In Figure 4 left, the rotor consists of a conductive loop surrounding a laminated iron core. When the shaft rotates, the stator magnetic field induces AC currents in the loop, causing magnetic-polarity reversals in the rotor core, as shown in the oblique view on the right. The rotor tries to keep up with the stator&#;s rotating magnetic fields, but it never succeeds. Were it to do so, the conductive loop would no longer be cutting through the stator&#;s magnetic field, and its current would drop to zero. Consequently, the motor operates at a &#;slip&#;&#; a few percent shy of synchronous speed.

Instead of one conductive loop, a typical three-phase induction motor has multiple conductors arranged in a configuration resembling a squirrel cage, and it&#;s often called a squirrel-cage motor. With no commutator or slip rings, this motor has long been a reliable workhorse. It&#;s optimal when running at full speed and load in applications such as periodically replenishing a water tower because the motor is driving a constant-speed pump. Its efficiency takes a hit when operating at varying speeds and loads&#;driving a variable-speed pump that maintains constant water pressure despite varying demand, for example, or serving as a traction motor in an electric vehicle.

What else should I know about motors?

In Part 3 of this series, I&#;ll describe an electric motor that has no permanent magnets, no commutator, no brushes, no slip rings, and no squirrel cage. We&#;ll also look at the role of modern drive electronics in making this motor practical and in boosting the performance and efficiency of other motor types. Finally, we will look at power and efficiency measurements. Meanwhile, EEWorld has just produced a series of presentations on motor-drive design, which you can view on demand here.

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