Understanding EV Drive Motor Technology: A Comprehensive Guide to Structure, Principles, and Key Advancements

The drive motor system is one of the three core systems of electric vehicles and is the main drive system for the vehicle. Its characteristics determine the main performance indicators of the vehicle and directly affect the vehicle's power, economy and user driving experience.

1.?Characteristics and Classification of Drive Motors

The drive motor is the power core of electric?vehicles. It not only has to drive the vehicle forward, but also generates electricity when the vehicle brakes or coasts, and has the function of recovering energy. Due to the compact structure, high speed and endurance requirements of electric?vehicles, the basic requirements for drive motor design include: wide speed range, high density and light weight, high efficiency, energy recovery, high reliability and safety, and continuous cost reduction.

There are many classifications based on the working principle and structure of the motor, and they are divided according to the type of driving current of the motor. The main motor classifications are shown in the figure below.

The drive motors commonly used in pure electric vehicles are DC motors, asynchronous motors, permanent magnet synchronous motors and switched reluctance motors. The performance comparison of the four commonly used drive motors in electric vehicles is shown in the table below.

Performance comparison table of common drive motors.

2.?Drive Motor Structure and Working Principle

(1)?Three-Phase Current Synchronous Motor?

Synchronous motors can be used as generators to provide electrical energy for electronic appliances and charge batteries, and are mostly used in new energy hybrid vehicles.

Working principle: A three-phase current is applied to the stator winding to generate a corresponding rotating magnetic field, and the magnetic poles of the rotor rotate accordingly with the direction of the rotating magnetic field. The speed of the rotor rotation is the same as the speed of the rotating magnetic field.

(2)?Three-Phase Current Asynchronous Motor

Three-phase current asynchronous motors can be used as motors or generators.

Working principle: The magnetic field induction between the rotor and the stator rotating magnetic field generates the rotor magnetic field. The rotor uses the induced current generated by the stator rotating magnetic field. Usually, asynchronous motors are also called induction motors.

(3) Permanent magnet synchronous motor

A permanent magnet synchronous motor replaces the rotor excitation winding of a separately excited synchronous motor with a permanent magnet, inserts the magnet into the rotor, and forms synchronously rotating magnetic poles.

Working principle: Starting and running are formed by the interaction of the magnetic fields generated by the stator winding, rotor cage winding and permanent magnet.

More here: PMSM Motor Drive System Features and Assembly

(4) Switched reluctance motor

The switched reluctance motor has a simple structure, superior performance, high reliability, and covers various high and low speed drive speed control systems with a power range of 10W~15MW.

Working principle: By controlling the amplitude and width of the current pulse added to the motor winding and its relative position with the rotor (conduction angle, cut-off angle), the size and direction of the motor torque are controlled.

3.?Classification of Drive Modes

New energy pure electric vehicles use motors to convert the electrical energy stored in high-voltage batteries into mechanical energy, which is then transmitted to the wheels by the transmission mechanism to generate propulsion and drive the vehicle. Motors have the advantages of relatively small size and light weight, and have diversified drive modes. Currently, pure electric vehicles use drive modes of: centralized drive and independent wheel drive.?

(1)?Centralized Drive

Centralized drive means that the whole vehicle uses one or two power sources (hybrid vehicles), which are concentrated in a reasonable position of the vehicle, and the speed is reduced and the torque is increased through the transmission and reducer (or only through the reducer), and finally the drive torque is roughly evenly distributed to the left and right drive half shafts through the differential. It can be front-wheel drive, rear-wheel drive or four-wheel drive. There are currently three basic typical structures for the centralized drive system layout of pure electric vehicles: traditional drive mode, motor-drive axle combined drive mode, and motor-drive axle integrated drive mode.

(2)?Independent Wheel Drive

The independent wheel drive system of electric vehicles uses several independently controlled motors to drive the wheels of the vehicle separately, and there is no mechanical transmission link between the wheels. The motor and the wheel can be connected by shaft, or the motor can be embedded in the wheel to become a wheel motor. There are currently two basic typical structures of independent wheel drive layout: dual drive mode and hub motor drive mode.

4.?Key Technologies of Drive Motors

The key technologies of drive motors mainly include motor design and optimization, control algorithms and systems, sensor technology, and the development of power electronic devices. This article describes the key technologies of drive motor development from the perspectives of hairpin?motors, oil cooling technology, and all-in-one electric drive systems.

(1)?Hairpin Motors

Compared with traditional round wire motors, the bare copper slot fill rate of hairpin motors can be increased by 20%-30%, the total copper consumption is reduced by 21%, and the efficiency is increased by about 1%. The increase in slot fill rate is equivalent to the motor being able to output higher power and torque under the condition of the same volume; or under the condition of the same power, the outer diameter and volume of the motor can be reduced, thereby reducing the weight of the motor. Therefore, the hairpin?winding motor has a higher power density, which can enable the permanent magnet motor to continue to develop in the direction of miniaturization.

(2) Oil Cooling Technology

Three commonly used motor cooling systems: air cooling, liquid cooling, evaporative cooling and additional heat path enhanced cooling system.

Air cooling system: With the advantages of low cost, high reliability and easy installation, it is widely used in the field of low-power motor cooling.

Liquid cooling system: It has high heat dissipation power and is suitable for heat dissipation occasions with high motor heat generation and high heat flux density. However, the liquid cooling system requires additional circulating liquid circuit and sealing system, which increases the cost and complexity of the motor system.

Evaporative cooling system: It is mainly used in the cooling system of large-capacity megawatt-level generator sets, and uses gas-liquid phase change cycle to achieve efficient cooling of the motor.

The winding ends of permanent magnet synchronous motors generate a lot of heat. In water cooling, the coolant cannot directly contact the windings. In oil cooling, the cooling oil can directly contact the windings, which has higher cooling efficiency and more prominent advantages. The cooling oil of oil cooling technology can directly contact the heating parts of the motor, and the heat dissipation efficiency is much higher than that of traditional water cooling systems. The oil medium has the advantages of good insulation, high dielectric constant, low freezing point and high boiling point. BYD DMI's drive motor uses direct injection rotor oil cooling technology, which can increase the motor power density by 32%.

5.?All-in-one Electric Drive System

Electric?vehicles have higher and higher requirements for driving range, power density, and energy utilization efficiency. Electric drive systems are rapidly developing in the direction of integration, miniaturization, and lightweight. The all-in-one electric drive systems that have been released include three-in-one, four-in-one, six-in-one, seven-in-one, and even eight-in-one, among which the three-in-one electric drive system is the most common. The all-in-one electric drive system will integrate components such as motors, reducers, and controllers, and share parts such as housings and wiring harnesses to achieve integration, cost reduction, and lightweight.

In summary, the gaps between the flat copper wires of the hairpin?motor are large, and the cooling oil is easy to penetrate, which promotes the application of direct oil cooling technology. At the same time, the cooling oil has good insulation and can be reused in multiple scenarios, which accelerates the integration of the vehicle thermal management system and will promote the implementation and popularization of the all-in-one electric drive system assembly.

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