Development of stator winding technology for new energy vehicle drive motors

1 Definition and function of drive motor stator winding

The stator winding is the power heart of the drive motor for new energy vehicles. It usually refers to a symmetrical circuit connection loop composed of multiple coils or coil groups through different winding methods. As shown in Figure 1 and Figure 2, the separate drive motor stator winding independently developed by United Electronics. Under driving conditions, when external electrical energy is connected to the input terminal of the stator winding through a high-voltage wiring harness, the stator winding can convert the input electrical energy into magnetic energy and store it in the stator-rotor air gap, and finally convert it into mechanical energy to provide driving force.

2 Development of drive motor stator winding technology

The history of the development of motor windings is the process of studying how to more conveniently embed more copper conductors into the stator core slots, thereby achieving a higher slot fill rate (slot fill rate is a key indicator of drive motor winding technology, defined in this article as the cross-sectional area of ​​the bare conductor divided by the cross-sectional area of ​​the core slot). From the development history of drive motor stator winding technology, it can be divided into the first generation of radial embedded winding technology and the second generation of axial embedded winding technology.

First generation winding technology: radial embedded winding

Radial embedded winding means that after the copper conductor is wound into shape, the winding is assembled into the core slot from the radial direction along the pole shoe opening of the stator core tooth (as shown in Figure 3 and Figure 4, which are round wire/flat wire radial embedded windings developed by United Electronics).

Since 1888, the mainstream winding technology used in industrial motors has been radial embedded winding. The initial winding technology was mainly distributed round wire radial embedded winding. In 1942, centralized round wire radial embedded winding gradually emerged. Then by 1995, centralized flat wire winding and distributed wave-wound flat wire winding were developed.

With the development of the new energy vehicle drive motor market, the winding technologies in the above industrial motor field have been applied to the drive motor field one by one. The following Table 1 shows the main performance comparison of distributed round wire windings, centralized flat wire windings, and distributed wave-wound flat wire windings in typical radial embedded windings of drive motors (all performance comparisons are typical values ​​converted to the same electromagnetic design scheme):

Table 1 Performance comparison of typical radial embedded windings

However, for radial embedded winding technology, due to the structural design limitations of the core slot pole shoe shape, it will directly affect the peak/continuous characteristics and NVH performance of the motor. In addition, manual intervention and adjustment are often required in the production process, making it difficult to achieve high-cycle (within 60s) automated production.

Second generation winding technology: axial embedded winding

Starting from 1958, with the further maturity of winding technology, the second generation of axial embedded winding technology began to enter the market. The initial axial embedded winding was also mainly used in large and medium-sized industrial motors.

Axially embedded winding refers to the process of assembling semi-formed or unformed flat copper wire conductors into the core slots along the end surface notches of the stator core from the axial direction.

The first technical branch of axially embedded winding: Hairpin winding

Since 1995, with the development of the new energy vehicle drive motor market, electric drive suppliers such as Remy, Denso, Hitachi, LG, United Electronics, and Bosch have absorbed the idea of ​​axial embedded winding in industrial motors and have successively developed "hairpin" type Hairpin flat wire windings for use in drive motors. This is the first technical branch of axial embedded windings.

The Hairpin winding process is to preform the flat wire winding into a "hairpin" structure and then assemble it into the stator core slot along the axial direction (as shown in Figure 5 and Figure 6, which are the coaxial drive motor Hairpin windings developed by United Electronics).

Since the axially embedded winding is not affected by the shape of the pole shoe, the assembly reserved space and conductor gap required for winding embedding can be greatly reduced, and its slot fill rate can reach about 70% (the highest slot fill rate of Hairpin products currently in mass production on the market is 69%). Hairpin windings have rapidly occupied the mainstream technology market with their excellent power, torque and efficiency performance.

The second technical branch of axial embedded winding: I-Pin winding

The I-Pin winding structure that appeared in large industrial motors in 1966 has also been applied to the electric drive field by electric drive suppliers such as United Electronics and Bosch. This is the second technical branch of axial embedded winding.

The idea of ​​I-Pin winding forming is to directly embed the straight flat wire conductor into the core slot along the axial direction without preforming the flat wire conductor (as shown in Figure 7 and Figure 8, which are the distributed drive motor I-pin windings developed by United Electronics).

Since the I-Pin winding does not require pre-forming and is assembled in a single slot, the reserved space for winding assembly can be further reduced, and its slot fill rate can reach about 74% (taking the I-pin products currently in mass production by United Electronics as an example), with more excellent power, torque and efficiency performance.

Table 2 shows the performance comparison of Hairpin winding and Ipin winding in typical axial embedded winding of drive motor (all performance comparisons are typical values ​​converted to the same electromagnetic design scheme):

Table 2 Performance comparison of typical axial embedded windings

Hairpin winding or I-pin winding, both have their own advantages and disadvantages in structure and process manufacturability, but their winding technologies have the same goal, which is to improve the power, torque and efficiency performance of the drive motor by increasing the slot fill rate of the winding to meet the increasingly stringent technical requirements of the electric drive market. This is also the essence of the development of drive motor winding technology.

3 Axially embedded winding platform developed by United Electronics

The axial embedded windings developed by United Electronics include I-Pin windings and Hairpin windings, among which the I-Pin winding platform has been the first to achieve mass production.