Let's talk about the choice of fixing method of motor stator in motor housing

As we all know, as the requirements for the power density of new energy drive motors continue to increase, new energy practitioners are gradually shifting their thinking on motor cooling solutions from water cooling to oil cooling.

The Molde 3 rear-wheel drive electric drive system and Nidec 150kw electric drive system, which began mass production in 2017, have not only become industry benchmarks, but have also ignited the enthusiasm of a number of manufacturers for the research and development of oil-cooled motors.

However, as benchmarks, the motor oil cooling solutions of the two are different, and the resulting stator fixing solutions are also different. Today, let's talk about the choice of fixing method of the motor stator in the motor housing.

1

Current status of stator fixing scheme for oil-cooled motors

Tesla Model 3 uses an interference fit connection between the outer circle of the stator core and the housing (hereinafter referred to as the interference fit solution), which is consistent with water-cooled motors. The same is true for Tesla Model 3's latest oil-cooled flat wire motor.

The stator fixing solution adopted by Nidec is to pass four long bolts through the through holes on the outer circle of the stator core to lock the stator axially to the casing.

Looking at the oil-cooled motor field of pure electric systems, in addition to Tesla and Nidec, we can also see:

• GM's Bolt, released in 2016, uses a bolt-on solution

• Hyundai's E-GMP electric drive released in 2020 - bolt-on solution

If we turn our attention to the hybrid system field, the motor stator is mostly fixed by bolts. Representatives in the industry include Toyota THS, Honda IMMD, GM Volt. Toyota Prius, from Gen1 to Gen4, no matter whether the enameled wire is round wire or flat wire, the stator fixing solution is firmly bolted.

After reading this, readers may have the following two questions?

1. Why do motor stators in hybrid oil cooling systems all use bolt fixing solutions?

2. Why are there both interference fit and bolt fixing options for motor stators in pure electric oil cooling systems? What are the advantages and disadvantages of these two options?

Try to find the answer through the following dimensions.

2

Mechanical reliability

Whether it is an interference fit solution or a bolt fixing solution, it is related to the positioning and fixing of the stator in the motor. Generally speaking, the coordination between the stator and the motor housing needs to meet the three basic functions of axial positioning, radial positioning, and anti-rotation.

The axial positioning of the stator in both solutions is achieved through the stepped positioning surface inside the housing, while the bolt fixing solution has several bolt holes opened on the outer circle of the stator core. Considering the assembly clearance between the bolts and the through holes and the position tolerance of the threaded holes in the housing, its radial positioning accuracy is slightly inferior to that of the interference solution.

Considering the influence of electromagnetic force, mechanical vibration shock, temperature and other factors, the matching design of the stator and the housing becomes more complicated.

The interference solution usually increases the minimum interference between the core and the shell to improve reliability. However, considering the temperature difference between the two and the different expansion coefficients of the materials, the accuracy of the simulation design is challenged. At the same time, it is necessary to balance the local stress and the mechanical properties of the material under the maximum interference.

Interference design is a common fit, but it leads to some additional considerations in oil-cooled motor design. For example, will the cooling oil enter between the housing and the core to reduce the friction coefficient between the two? Will the oil flow through the oil channel on the stator core yoke lead to a decrease in the reliability of the interference connection?

Back to the bolt fixing scheme, the principle is to use the friction generated by tightening the bolts to provide the torque required to prevent the stator from rotating under various conditions. In theory, reliability can also be improved by increasing the bolt specifications and tightening torque.

However, it should be noted that the torque attenuation of the bolt locking method is inevitable, whether after tightening or during the use of the product. The resulting decrease in axial clamping force will bring the risk of stator slippage. The stator core is made of countless "soft" silicon steel sheets stacked together, and the problem of bolt torque attenuation caused by this soft connection needs to be treated with caution. Therefore, the selection of bolts, anti-loosening solutions, and the design and verification of tightening processes are more important.

3

cool down

In order to increase the heat dissipation capacity of the core, the interference solution (refer to Tesla) chooses to realize the characteristics of the cooling oil channel by increasing the radial size of the core, which will inevitably increase a thermal resistance. The bolt solution can actively cool the specific parts of the core and winding through the design of auxiliary parts such as oil pipes (Toyota) and oil pans (Nidec), and because a gap is allowed between the core and the shell, the heat on the core surface will be directly taken away by the cooling oil.

In addition, in order to allow the cooling oil to smoothly pass through the entire motor oil channel system and even be ejected at a certain flow rate, the interference solution will inevitably produce a large pressure drop from oil inlet to oil outlet, which puts higher requirements on the oil pump capacity. The motor oil circuit of the bolt solution is relatively "smooth" and has less burden on the entire system.

Of course, the benefits of the interference solution can also be seen. Since the iron core oil channels are spread all over the stator surface, the cooling of the windings by the entire iron core, including the matching oil ring, is relatively uniform.

4

NVH performance

First, let’s look at the NVH performance of the motor.

Motor vibration noise can be divided into three categories: mechanical noise, electromagnetic noise, and aerodynamic noise. Among them, the noise related to the stator is mainly electromagnetic noise.

In simple terms, part of it comes from electromagnetic force (due to the magnetomotive force generated by the winding inside the stator core, the inner surface of the stator core is subjected to radial electromagnetic force distributed along the circumferential direction), and the other part comes from the torque pulsation produced by the coupling of the stator and rotor magnetic fields, which is called tangential electromagnetic force. In actual noise situations, the former often dominates.

Stator electromagnetic force and mode

The biggest difference between the two fixing schemes is the different stator noise transmission paths. The stator of the interference fit scheme is matched with the shell through the outer surface of the core, and the electromagnetic force will be directly transmitted to the entire shell, and finally form a response by being suspended on the whole vehicle. The matching part of the stator and the shell of the bolt scheme is the axial positioning surface on the shell, and the circumferential gap between the two cuts off the radial transmission path of the electromagnetic force, which undoubtedly plays a better role in suppressing the electromagnetic noise of the whole machine.

Secondly, due to the existence of oil holes in the outer ring of the core, the radial stiffness of the stator of the interference fit scheme is relatively poor, which brings higher difficulty to NVH design.

In the working state, the stator will cause continuous changes in size due to the magnetostrictive effect (mentioned later) in the alternating sinusoidal magnetic field. As can be seen from the figure below, as the compressive stress value along the magnetization direction increases, the magnetostriction value increases synchronously (where λ=△l/l). Although its magnitude is generally only 10-6~10-5, its impact on motor noise should not be ignored.

The above situation will be even worse in the interference scheme, because it is not difficult to find through calculation that the radial stress on the iron core in the interference scheme is basically greater than the axial stress on the iron core in the bolt scheme, and the winding magnetic field is mainly distributed in the plane direction of the punching sheet, rather than the thickness direction.

Magnetostriction of non-oriented electrical steel sheets under external stress

Similarly, the selection and design of bolts in the bolt fixing scheme also have a great impact on NVH. If the axial preload provided by the bolts is insufficient, the electromagnetic force generated by the stator will overcome the static friction and eventually transmit the noise to the housing through the bolts.

5

Motor output performance

Here we need to introduce the "magnetoelastic coupling effect": a phenomenon of the relationship between the magnetic properties and mechanical properties of magnetic materials, including magnetostrictive effect and inverse magnetostrictive effect (also called magnetoelastic effect).

The former is defined as the relative deformation of an object along the direction of magnetic lines of force when it is subjected to an external magnetic field; the latter is defined as the change in the magnetic properties of a ferromagnetic material under the action of mechanical stress, which is related to the magnitude of the stress and the angle between the magnetic field and the stress direction.