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What is the Difference Between an Inner Rotor PM Motor and an Outer Rotor PM Motor?

2024-02-27 15:05:21 Volcano Motor Read

What is the Difference Between an Inner Rotor Permanent Magnet Motor and an Outer Rotor Permanent Magnet Motor

Permanent magnet synchronous motors can be divided into outer rotor structure and inner rotor structure according to the position of the rotor in the motor. The difference is whether it rotates inside or outside. The inner rotor structure is an inner rotor structure, and the outer rotor structure is an outer rotor structure. Usually the rotating part is the part embedded with magnetic steel, which is the rotor; the non-rotating part is the coil part, which is the stator.


Let’s talk about the difference between the inner rotor and the outer rotor of the permanent magnet motor.


The Difference Between Inner Rotor and Outer Rotor

In practical applications, the inner rotor and the outer rotor have the following differences:


1. Difference in rotation position.

The inner rotor, that is, the transmission part of the motor is inside, the casing is static, and the inside rotates; the outer rotor is internally static, and the outer casing rotates. Generally speaking, the embedded magnet steel (permanent magnet) part will rotate, but the coil part does not rotate. According to the permanent magnet inlay process and position, the outer rotor and the inner rotor can also be distinguished. 

 Difference in rotation position

2. The size of the permanent magnet.

The permanent magnet of the outer rotor is small in size and axial size, and the system operation stability is good. The inner rotor permanent magnet has a large volume, correspondingly larger axial size, high center of gravity, and poor operating stability.


3. Speed. 

Taking the same brushless DC motor as an example, the speed of the inner rotor motor is significantly higher than that of the outer rotor motor.


4. Application.

The inner rotor is often used in power machinery such as motors, generators, gas turbines, and compressors, while the outer rotor is usually installed on the impeller to take on the role of heat dissipation and cooling.


The characteristic of the outer rotor motor is that the stator is fixed in the middle position of the shaft, and the rotor rotates around the periphery of the stator. It also has a radial air gap flux structure. Compared with the inner rotor structure, the rotor and stator have changed positions.


Figure 1 is a plan view of an external rotor generator. The stator inside the motor is called the inner stator, and the rotor outside the motor is called the outer rotor.

External rotor generator plan view.jpg

External rotor generator plan view


Direct Drive External Rotor Permanent Magnet Wind Turbine

The following introduces the composition and structure of the direct-drive external rotor permanent magnet wind turbine. Its structure is introduced through a permanent magnet external rotor generator model. The left picture in Figure 3 is a diagram of the inner stator core. The stator core is made of silicon steel sheets with good magnetic conductivity. , there are many slots on the outer circumference of the stator core, and the windings of the generator are embedded in the slots. The windings are distributed according to the three-phase rule.


Generally, the outer rotor of a large direct-drive wind turbine has 30 to 40 pairs of magnetic poles, and the number of stator slots is about 180 to 240. In order to clearly show the construction of the inner stator core, this model has much fewer coil slots than an actual direct drive generator.

Direct drive generator inner stator core and windings.jpg 

Direct drive generator inner stator core and windings


The stator core is installed on the stator bracket. One end of the stator bracket has a flange installed on the nacelle base. The other end is the outer rotor shaft, which is also the main shaft of the wind turbine. The main shaft has to bear the weight and wind force of the entire wind wheel and outer rotor. The main shaft The flange must have high strength.

Direct drive generator internal stator structure.jpg 

Direct drive generator internal stator structure



Figure 4 is a cross-sectional view of the outer rotor structure, showing its structure in two directions. The outer rotor is like a bucket set outside the stator. It is made of iron material with good magnetic conductivity. Magnetic poles made of permanent magnets are fixed on the inner circumference of the "bucket". The "bucket" is the magnetic yoke of the rotor. This structure One of the advantages is that the magnetic poles are easy to fix and will not fall off due to centrifugal force. The outer rotor yoke is fixed on the rotor sleeve.

Direct drive generator internal stator structure.jpg 

Direct drive generator outer rotor structure

Install the outer rotor on the main shaft of the generator to form an outer rotor generator. The figure below shows its structure from two directions. The outer rotor bushing not only fixes the outer rotor, but also installs the entire wind wheel, which bears a large load, so it is installed on the main shaft of the generator through two large bearings.

Direct drive generator outer rotor structure 2.jpg 


Advantages and Disadvantages of Inner Rotor and Outer Rotor

The advantages and disadvantages of the inner rotor and the outer rotor are: The advantages of the inner rotor include lightweight fan blades, easy production and assembly of the motor, lower cost, etc. However, the motor heats up quickly, consumes a lot of power, has unstable voltage, easily damages the motor, and has a short lifespan. question. The advantage of the outer rotor is that it can be made into a fully enclosed structure and can run quickly after startup. It has low power consumption, high speed, high efficiency, and long service life. However, it has poor sealing, large rotor inertia, high noise, and high requirements for dynamic balance.

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