Discussion on electric vehicle technology based on in-wheel motor drive

With the successive promulgation of relevant policies and standards for energy conservation and emission reduction, traditional power vehicles are gradually transitioning to new energy vehicles. The latter is obviously relatively simpler than the former in terms of mechanical and electrical structure. The traditional engine is replaced by the system integration of the electric motor and the battery. However, there is another biggest problem that plagues electric vehicle developers. In addition to the corresponding simplification of the gearbox structure, the transmission system is still very complicated. At present, if the in-wheel motor technology can be fully promoted, it will be able to replace the existing transmission system of automobiles.

With the successive promulgation of relevant policies and standards for energy conservation and emission reduction, traditional power vehicles are gradually transitioning to new energy vehicles. The latter is obviously relatively simpler than the former in terms of mechanical and electrical structure. The traditional engine is replaced by the system integration of the electric motor and the battery. However, there is another biggest problem that plagues electric vehicle developers. In addition to the corresponding simplification of the gearbox structure, the transmission system is still very complicated. At present, if the in-wheel motor technology can be fully promoted, it will be able to replace the existing transmission system of automobiles.

1. Application background

As we all know, batteries, motors, and Electronic control are the three essential core components for new energy vehicles. Current new energy vehicles all use a motor drive system to convert electrical energy into mechanical energy to provide power for the vehicle. Therefore, the drive motor is also one of the core technologies of new energy vehicles.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 1 The main system architecture of new energy vehicles

At present, centralized motor drive is the main driving form of electric vehicle power. Although its advantages are obvious, that is, the layout of the transmission system and the control system is relatively simple, but there are also some problems. Because new energy vehicles driven by such motors have mechanical transmission components such as transmissions, clutches, and transmission shafts, the chassis structure is more complicated. The resulting impact is that the seating space is very small, and the transmission system transmits power through mechanical components. Will cause energy loss, resulting in low energy utilization.

In addition, this kind of transmission system will produce a lot of noise during the driving of the new energy vehicle, and the comfort of the occupants cannot be guaranteed. Foreign experts and scholars have carried out technical research on in-wheel motor drive in the early years, thereby optimizing the structural compactness and energy utilization efficiency of the motor drive in the chassis of new energy vehicles; and relevant domestic colleges and units have researched on in-wheel motor drive technology. Still shallow. At present, the in-wheel motor drive technology has been applied to some new energy vehicles and has made good progress.

Second, the concept of hub motor

The origin of the in-wheel motor technology can be traced back to the first year of the 20th century. At that time, Ferdinand Porsche developed an electric car equipped with in-wheel motors for the front wheels before the PORSCHE automobile company was founded. In the 1970s, the use of in-wheel motor technology on mining trucks achieved good results. In addition, Japanese car companies have carried out relatively early research on passenger wheel hub motor technology, and basically occupy a leading position. International auto giants such as Toyota and General Motors have also set foot in this technology. At the same time, domestic manufacturers of independent brands that develop in-wheel motor technology have gradually emerged.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 2 In-wheel motor vehicles in history

The hub motor, in layman’s terms, is to directly merge the metal hub and the drive device into a whole drive motor. In other words, it is to combine the drive motor and the transmission brake device into the wheel hub. It is commonly called “electric wheel”, also called a wheel. Type motor (wheel motor). It contains bearings, stators and rotors, small inverters, etc.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 3 The internal structure of the in-wheel motor (Protean Drive TM)

Third, the wheel hub motor drive mode

(1) Reduced drive
This drive method uses a high-speed internal rotor motor, and is equipped with a reducer with a fixed transmission ratio. The power density is relatively high. The motor speed can reach up to 10k r/min.

Advantages: It has high specific power and efficiency, small size and light weight; after the deceleration structure increases the torque, the output torque is greater and the climbing performance is good; it can ensure that the car obtains a large stable torque when running at low speed.

Disadvantages: It is difficult to achieve lubrication, the gears of the planetary gear reduction structure wear quickly, the service life is relatively short, it is not easy to dissipate heat, and the noise is relatively large.

(2) Direct drive
This driving method adopts a low-speed outer rotor motor. The outer rotor of the motor is directly mechanically connected to the hub. The speed of the motor is generally around 1.5K r/min. There is no deceleration structure, and the speed of the wheel is consistent with the speed of the motor.

Advantages: Because there is no deceleration mechanism, the structure of the entire drive wheel is more compact, the axial size is also smaller than the previous drive type, and the transmission efficiency is higher.

Disadvantages: Large current is required when starting, headwind or climbing and other situations that need to carry large torque, it is easy to damage the battery and permanent magnets, the motor efficiency peak area is small, and the efficiency drops quickly after the load current exceeds a certain value.

4. Status quo at home and abroad

(1) Mitsubishi of Japan
Mitsubishi’s MIEV technology started in 2006 and is applied to its MIEV prototypes. At present, the prototype has been developed to the third generation. The most representative ones are the Colt EV and the four-wheel drive sports car (Lancer Evolution MIEV). Among them, Mitsubishi’s hub motor technology is provided by Japan’s Toyo Electric. The hub motor has the following characteristics: the Inverter adopts the BOOST boost scheme, and each motor is controlled by one inverter; the motor uses a permanent magnet synchronous motor and a hub The one-piece solution retains the original brake and damping system; Toyo Electric’s solution also has a cooling problem, uses natural cooling, and has not been promoted in batches.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 4 System diagram

(2) Michelin in France
Michelin has developed a dynamic damping in-wheel motor system. The system adds a set of shock absorbers between the electric motor and the wheels, thereby improving the driving comfort and active safety of the vehicle. The company’s newly announced new generation in-wheel motor system features the following: lightweight and compact structure, and reduced system mass; unique structure of the suspension device, the motor suspension device is composed of linear guide blocks, coil springs, and shock absorbers , The buffer stop is formed between the axle and the motor. The linear guide block controls the up and down movement of the motor. The spiral spring supports the weight of the motor. The shock absorber is used for shock absorption. The reliability of the motor is improved, and the sealing technology of the motor is applied. As well as component coupling technology, the in-wheel motor has higher reliability in the special environment of dust and rain.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 5 Wheel hub motor drive system (Siemens / Michelin)

(3) Protean-E
The Protean-E hub motor adopts a distributed motor solution, that is, the integrated motor includes 8 small permanent magnet motors with shared buses, the ring capacitor rotates inside the motor, and the inverter is also divided into 8 groups of modules and fixed on the hub. Protean -E’s motor system uses natural cooling for heat dissipation.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 6 Protean-E motor assembly drawing

(4) Tianjin FAW
Adopting the hybrid scheme of front cabin centralized drive and rear wheel hub motor drive; the outer rotor is connected to the rim after the brake is installed; the original front wheel drive problem: the principle of avoidance is adopted, and the space is small; the nominal 7.5k W hub motor (actual Rated 5k W), the maximum speed can reach 90 kilometers, and the start-up is slow due to the small torque.

Five, motor control principle

The principle of DC brushless control, the controller reads the Hall signal to determine the sector where the motor rotor is located, and determines the switching logic of the inverter bridge arm. The square wave control is essentially a relatively simple six-step commutation operation. At any time, there is a phase stator winding in forward conduction, that is, the phase current flows out in the forward direction; the second phase stator winding is reversed conduction, that is, the phase current is reversed. Inflow; the third group is not energized. The electromagnetic torque comes from the magnetic field generated by the stator windings, which attracts the rotor magnetic field to continuously rotate. If the reluctance torque (surface-mounted permanent magnet synchronous motor) is ignored, the quadrature-axis magnetic field generated by the stator windings produces all the electromagnetic torque; on the contrary, when this is fixed When the magnetic field of the rotor coincides, that is, the interaction of the direct-axis magnetic field of the stator on the rotor magnet, the electromagnetic torque produced is zero. Therefore, it is necessary to constantly change the position of the stator magnetic field to drive the continuous rotation of the rotor magnet, and control the stator magnetic field to always lead the rotor magnetic field by a certain angle, thus forming the permanent magnet magnetic field always catching up with the winding composite magnetic field. The controller detects the sector where the rotor’s magnetic field is located, and then controls the winding to generate a magnetic field that points to the next sector. To control the rotor to make one revolution, it only needs to change the stator winding six times. However, because the number of pole pairs of in-wheel motors is usually not the same, every completion of a energization cycle means that the rotor has only rotated one electrical angle, and the mechanical angle of the rotor has not been achieved. Therefore, the commutation cycle required for the rotor to rotate one mechanical angle is The number is the same as the number of pole pairs.

This kind of control mainly realizes the speed control of the motor. By reading the position signal of the Hall sensor, the rotor position is judged. At the same time, the motor speed controller performs closed-loop control of the motor speed. Since the voltage is proportional to the speed, the speed control can be achieved by controlling the output phase voltage. This control method changes the armature magnetic field through a simple six-step commutation, leading the rotor to rotate, and only two-phase windings are turned on at any time. The specific control process is shown in the figure:

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 7 Block diagram of square wave control logic

The square wave control adopts Hall element as the position sensor. The three Halls are respectively placed at the positions of 0°, 120° and 240° in electrical angles, as shown in Figure 8, the electrical angle of 360° is divided into 6 sectors. The controller detects the sector where the rotor is located, and controls the armature magnetic field to direct the rotor to the next sector.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 8 Schematic diagram of sectors

Sixth, the phenomenon of motor drive leakage
The reasons are roughly as follows:

Motor drive leakage phenomenon

main
Classification

Moisture of the winding reduces the insulation resistance

Long-term overload operation of the motor

Metal foreign objects invade the inside of the winding and damage the insulation

When rewinding the stator winding, the insulation damage and the iron core are damaged

Winding end touches the end cover base

Insulation burns caused by friction of stator and rotor

Lead wire insulation damage and collision with shell

Harmful gas corrosion

Figure 9 Leakage phenomenon of the motor

Seven, current detection scheme

At present, the use of Hall Current Sensor or Current Transformer to feedback and detect the DC bus current on the power converter has many limitations. Because the current through the main switching device is generally relatively large, the rated parameters of the Hall device or current transformer used must also be large. At this time, the solution is large in size and high in cost. In addition, it is inconvenient to achieve high power density of the power converter.

This article introduces a novel solution-a current detection circuit based on semiconductor devices, which can directly lay out the circuit on the control PCB of the power converter. It is not only low cost, small in size, easy to install, but also has good performance. The power converters are solidified together to form an application-specific integrated circuit (ASIC).

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 10 Current detection circuit based on MOSFET

The working principle of the circuit (as shown in Figure 10):
The driving signal of the low-bridge MOSFET is L0.

(1) When L0 is 0V, the Q of the low-bridge MOSFET tube is off, and the V1 signal point at the right end of D2 is the diode’s tube voltage drop of 0.5V. At this time, the positive input terminal of U1 is 0.5V, and the negative input terminal The voltage is 10V. At this time, the output of U1 is low and the output of U2 is also low. LM339 is an open-collector output mode. There is also the problem of the conduction voltage drop V2, so the signal VO1 is subtracted from the signal VO2 to eliminate LM339 The detection error caused by the conduction voltage drop plays a role in eliminating the input error.

(2) When L0 is 12V, the low-bridge MOSFET Q is in the on state, and the signal at point V1 at the right end of D2 is 12V. At this time, the positive input terminal of U1 is 12V, the negative input terminal voltage is 10V, and the output of U1 is In the high-impedance state, the voltage of VO1 is the voltage drop on the internal resistance of Q plus the voltage drop of the fast recovery diode D1. At the same time, the output of U2 is also in the high-impedance state, and the voltage of VO2 is the voltage drop of the diode D3. The signal VO1 is subtracted from the signal VO2 by the subtraction circuit composed of the operational amplifier TLC2274, and the voltage drop on the internal resistance of Q can be obtained.

Discussion on electric vehicle technology based on in-wheel motor drive

Figure 11 Waveform diagram corresponding to each point

The waveform analysis of the pressure drop of the switch tube and the relevant points of the current detection circuit is shown in Figure 3. In the 120° pairwise conduction mode, in the electric or braking state, there is always one lower bridge arm in working state, so the sum of the conduction voltage drops of the three lower bridge arms is approximately equal to the average current of the motor windings.

T1 and T3 are the turn-on moments, T2 is the turn-off moment of the MOSFET, DV1 is the tube pressure drop of D3 when it is turned on, and V2 is the turn-on pressure drop. In this detection circuit, U3 functions as an overcurrent protection. When the voltage of VO1 is greater than 1.5V, the output of U3 is low level (overcurrent signal).

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