Definition, rotation principle, and power generation principle of new energy vehicle drive motors

If you want to work in new energy vehicle related fields and have a deep understanding of new energy vehicles (electric vehicles), you cannot avoid the three major new energy components - drive motor, electric drive controller and battery.

From the perspective of vehicle cost ratio, the three-electric system accounts for almost 50% of the material cost of the vehicle;

From a technical perspective, these three parts are the core technologies that distinguish them from traditional fuel vehicles, and are also the direct factors that affect the performance of vehicle products.

In this series of tweets, we will systematically sort out the drive motor, one of the three major components, covering the definition and working principle of the motor, as well as the basic structure, components, protection principles, basic performance and other aspects of the motor.

We hope that through our systematic analysis, we can help readers working in the new energy vehicle industry understand this part that best represents the difference between new energy vehicles and fuel vehicles.

01

Definition of drive motor

Electric machinery, commonly known as "motor", refers to an electromagnetic device that converts or transmits electrical energy based on the law of electromagnetic induction.

The motor is represented by the letter M (D in the old standard) in the circuit. Its main function is to generate driving torque as a power source for electrical appliances or various machines. The generator is represented by the letter G in the circuit. Its main function is to convert mechanical energy into electrical energy.

02

The rotation principle of the motor

The principle of the motor is very simple. Simply put, it is a device that uses electrical energy to generate a rotating magnetic field on the coil and drives the rotor to rotate. Anyone who has studied the law of electromagnetic induction knows that a coil with electricity will be forced to rotate in a magnetic field. This is the basic principle of the motor, which is the knowledge of junior high school physics.

To facilitate the subsequent explanation of the motor principles, let us review the basic laws/rules regarding current, magnetic field and force.

As mentioned before, the power generation of the motor essentially relies on electromagnetic induction.

The diagram on the left shows that current flows according to Fleming's right-hand rule, also known as the generator rule or the right-hand rule.

The direction of the current generated when a conductor moves in a magnetic field can be found.

The three fingers of the right hand are perpendicular to each other. The direction of the thumb is the direction of movement of the wire, the index finger is the direction of the magnetic field, and the middle finger is the direction of the current.

The movement of the wire in the magnetic flux generates an electromotive force in the wire and a current flows.

The middle diagram is Faraday's law.

Faraday's law describes the relationship between the amount of electricity passing through the electrode and the weight of the electrode reactants, also known as the law of electrolysis.

It is divided into two sub-laws, namely Faraday's first law and Faraday's second law.

The first law states that the mass of the substance that undergoes chemical changes at the electrode interface is proportional to the amount of electricity supplied.

The second law states that when electricity is passed through a circuit of several electrolytic cells connected in series, when the charges of the elementary particles taken are the same, the amount of substance reacting at each electrode is the same, and the mass of the precipitated substance is proportional to its molar mass.

The figure on the right shows Lenz's law.

Lenz's law states that the induced current has such a direction that the magnetic field of the induced current always opposes the change in magnetic flux that causes the induced current. It can also be stated as follows: the effect of the induced current always opposes the cause of the induced current.

When the magnet (magnetic flux) moves closer to or further away from the coil, the current flows in different directions.

We will explain the principle of power generation on this basis.

03

Motor power generation principle

Assume that a coil with an area of ​​S (= l × h) rotates at an angular velocity of ω in a uniform magnetic field.

At this time, assuming that the parallel direction of the coil surface (yellow line in the middle figure) and the perpendicular line (black dotted line) relative to the magnetic flux density direction form an angle θ (=ωt), the magnetic flux Φ penetrating the coil is expressed by the following formula:

In addition, the induced electromotive force E generated in the coil by electromagnetic induction is as follows:

When the parallel direction of the coil surface is perpendicular to the magnetic flux direction, the electromotive force becomes zero, while the absolute value of the electromotive force is maximum when it is horizontal.

In this way, the motor has the function of generating electricity. What I am saying here is that the motor has a rotating motion and a function of generating electricity, but it does not mean that the motor should be used for generating electricity. If you want to generate electricity, you usually use a generator that is optimized for generating electricity.

04

Motor structure

The motor is mainly composed of two parts, the fixed stator part and the rotating rotor part, and other parts.

1. Stator (stationary part)

The stator core is an important part of the motor magnetic circuit, and the stator winding is placed on it;

Structure: The stator core is generally punched and laminated from 0.35~0.5 mm thick silicon steel sheets with an insulating layer on the surface. Evenly distributed slots are punched on the inner circle of the core to embed the stator winding.

The stator core slot types are as follows:

Semi-closed slot: The efficiency and power factor of the motor are higher, but the winding wire embedding and insulation are more difficult. Generally used in small low-voltage motors. Semi-open slot: It can embed shaped windings, generally used in large and medium-sized low-voltage motors. The so-called shaped winding means that the winding can be insulated before being placed in the slot.

Open slot: used to embed molded windings, convenient insulation method, mainly used in high-voltage motors.

The essence of the stator winding is the coil, the circuit part of the motor, connected to the power supply, used to generate a rotating magnetic field;

The structure of the stator winding: It is composed of three windings with exactly the same structure that are spaced 120° apart in space and arranged symmetrically. The coils of these windings are embedded in the slots of the stator according to a certain rule.

The main insulation items of the stator winding are the following three: (ensuring reliable insulation between the conductive parts of the winding and the iron core, as well as reliable insulation between the windings themselves).

1) Insulation to ground: insulation between the stator winding as a whole and the stator core.

2) Phase-to-phase insulation: insulation between stator windings of each phase.

3) Inter-turn insulation: insulation between turns of each phase stator winding.

2. Rotor and rotor winding (moving part)

【Rotor】

Rotor: The function of the rotor is to serve as part of the motor's magnetic circuit and to place the rotor windings in the core slots.

Structure: The material used is the same as the stator, which is made of 0.5 mm thick silicon steel sheets punched and laminated. The outer circle of the silicon steel sheet is punched with evenly distributed holes to accommodate the rotor winding. The rotor core is usually punched with the inner circle of the silicon steel sheet after the stator core is punched out. Generally, the rotor core of a small asynchronous motor is directly pressed on the shaft, while the rotor core of a large and medium-sized asynchronous motor (rotor diameter is above 300~400 mm) is pressed on the shaft with the help of a rotor bracket.

【Rotor winding】

Function: Cutting the stator rotating magnetic field generates induced electromotive force and current, and forms electromagnetic torque to make the motor rotate.

Structure: divided into squirrel cage rotor and wound rotor.

1) Squirrel Cage Rotor: The rotor winding consists of multiple bars inserted into the rotor slots and two annular end rings. If the rotor core is removed, the entire winding looks like a squirrel cage, so it is called a cage winding. Small cage motors use cast aluminum rotor windings, and motors above 100KW use copper bars and copper end rings welded together.

2) Wound rotor: The wound rotor winding is similar to the stator winding, and is also a symmetrical three-phase winding. It is generally connected in a star shape, with the three output ends connected to the three collector rings of the rotating shaft, and then connected to the external circuit through brushes.

Features: The structure is more complex, so the application of wound-rotor motors is not as extensive as that of squirrel-cage motors. However, additional resistors and other components are connected in series in the rotor winding circuit through collector rings and brushes to improve the starting, braking and speed regulation performance of asynchronous motors. Therefore, they are used in equipment that requires smooth speed regulation within a certain range, such as cranes, elevators, air compressors, etc.

05

Other parts of the motor

In addition to the two most critical core components, the stator and rotor windings, the motor is also composed of several other important parts, including fans, junction boxes, bases, bearings, end covers and other parts.