Abstract: With the successive introduction of relevant policies and standards for energy conservation and emission reduction, traditional fuel vehicles are gradually transitioning to new energy vehicles. The latter are significantly simpler in mechanical and electrical structure than the former, replacing the traditional engine by integrating the motor and battery system. However, one major problem still plagues electric vehicle developers: although the gearbox structure has been simplified, the transmission system remains very complex. Currently, if in-wheel motor technology can be fully implemented, it could replace the existing transmission system in automobiles.
I. Application Background
As we all know, batteries, motors, and electronic controls are the three essential core components of new energy vehicles. Current new energy vehicles all use electric motor drive systems to convert electrical energy into mechanical energy to power the vehicle; therefore, the drive motor is also one of the core technologies of new energy vehicles.
Figure 1. Main system architecture of new energy vehicles
Currently, centralized electric motor drive is the main power source for electric vehicles. While its advantages are obvious—namely, the relatively simple arrangement of the transmission and control systems—it also presents some problems. Because these electric motor-driven new energy vehicles have mechanical transmission components such as gearboxes, clutches, and drive shafts, the chassis structure becomes more complex. This results in very limited passenger space, and the transmission system also causes energy loss as it transmits power through mechanical components, leading to low energy efficiency.
Furthermore, this type of transmission system generates significant noise during the operation of new energy vehicles, compromising passenger comfort. Foreign experts and scholars began researching in-wheel motor drive technology early on, optimizing the structural compactness and energy efficiency of motor drives in new energy vehicle chassis. However, research on in-wheel motor drive technology by relevant domestic universities and institutions is still in its early stages. Currently, in-wheel motor drive technology has been applied in some new energy vehicles and has achieved considerable progress.
II. Concept of Hub Motor
The origins of in-wheel motor technology can be traced back to the beginning of the 20th century, when Ferdinand Porsche developed an electric vehicle equipped with in-wheel motors even before founding Porsche. In the 1970s, in-wheel motor technology was successfully applied to mining vehicles. Furthermore, Japanese automakers began research into in-wheel motor technology for passenger vehicles relatively early and have largely taken a leading position. International automotive giants such as Toyota and General Motors have also ventured into this technology. Meanwhile, domestic Chinese brands have gradually emerged that are developing in-wheel motor technology.
Figure 2 Historical in-wheel motor automobiles
A hub motor, simply put, is a drive motor that integrates a metal hub and a drive unit into a single unit. In other words, it combines the drive motor and transmission/braking system into the hub, commonly known as an "electric wheel" or a wheel motor. Internally, it includes bearings, a stator and rotor, and a small inverter.
Figure 3 Internal structure of the hub motor (ProteanDriveTM)
III. Hub Motor Drive Method
Reduction drive
This drive system uses a high-speed internal rotor motor and is equipped with a reducer with a fixed transmission ratio, resulting in relatively high power density. The motor's maximum speed can reach 10 kr/min.
Advantages : It has high power-to-weight ratio and efficiency, small size and light weight; the torque amplification structure makes the output torque greater and the climbing performance better; it can ensure that the car can obtain a large and smooth torque when running at low speed.
Disadvantages : Lubrication is difficult to achieve; the gears in the planetary gear reduction structure wear out quickly, resulting in a relatively shorter service life; heat dissipation is not easy; and noise is relatively loud.
direct drive
This drive system uses a low-speed external rotor motor. The external rotor of the motor is directly mechanically connected to the wheel hub. The motor speed is generally around 1.5 Kr/min. There is no reduction gear structure, and the wheel speed is the same as the motor speed.
Advantages : Because there is no reduction gear, the structure of the entire drive wheel is more compact, the axial dimension is smaller than the previous drive type, and the transmission efficiency is higher.
Disadvantages : It requires a large current when starting, going against the wind, or climbing uphill, which can easily damage the battery and permanent magnet. The peak efficiency range of the motor is small, and the efficiency drops rapidly after the load current exceeds a certain value.
Current situation at home and abroad
(1) Japan Mitsubishi
Mitsubishi's MIEV technology began in 2006 and was applied to its MIEV prototype. The prototype has now reached its third generation. Representative examples include the Colt EV and the four-wheel-drive sports car (Lancer Evolution MIEV). Mitsubishi's in-wheel motor technology is supplied by Toyo Electric Corporation of Japan. This in-wheel motor has the following characteristics: the inverter uses a BOOST boost converter, and each motor is controlled by a separate inverter; the motor uses a permanent magnet synchronous motor integrated with the wheel hub, retaining the original brakes and damping system; Toyo Electric's solution also suffers from cooling issues, relying on natural cooling and has not been mass-produced.
Figure 4 System Schematic Diagram
(2) Michelin of France
Michelin has developed a dynamic damping hub motor system. This system adds a damping device between the motor and the wheel, thereby improving vehicle ride comfort and active safety. The company's latest generation of hub motor systems features: lightweight and compact design, reducing system weight; a uniquely constructed suspension system consisting of linear guide blocks, coil springs, shock absorbers, and buffer blocks, located between the axle and the motor. The linear guide blocks control the motor's vertical movement, the coil springs support the motor's weight, and the shock absorbers dampen vibrations; and improved motor reliability, achieved through advanced sealing and component coupling technologies, resulting in higher reliability even in dusty and rainy environments.
Figure 5. Hub motor drive system (Siemens/Michelin)
Protean-E
The Protean-E hub motor adopts a distributed motor solution, which includes eight small permanent magnet motors sharing a common bus in the integrated motor. The toroidal capacitor rotates inside the motor, and the inverter is also divided into eight modules fixed on the hub. The Protean-E motor system uses natural cooling for heat dissipation.
Figure 6. Protean-E motor assembly diagram
(4) Tianjin FAW
It adopts a hybrid scheme of centralized drive in the front compartment and hub motor drive in the rear wheels; the outer rotor is connected to the wheel rim after the brake is installed; the original front-wheel drive has the following problems: the space is small when adopting the avoidance principle; the hub motor with a nominal power of 7.5kW (actual rated power of 5kW) can reach a maximum speed of 90 km/h, but the starting speed is slow due to the low torque.
V. Motor Control Principle
The principle of DC brushless control is that the controller reads Hall signals to determine the sector where the motor rotor is located and decides the switching logic of the inverter bridge arms. Square wave control is essentially a relatively simple six-step commutation operation. At any given time, one phase of the stator winding is forward-conducting (i.e., the phase current flows out in the forward direction); the second phase of the stator winding is reverse-conducting (i.e., the phase current flows in the reverse direction); and the third phase is de-energized. The electromagnetic torque originates from the magnetic field generated by the stator winding attracting the rotor magnetic field to rotate continuously. If reluctance torque is ignored (in surface-mounted permanent magnet synchronous motors), the quadrature-axis magnetic field generated by the stator winding produces all the electromagnetic torque. Conversely, when the stator and rotor magnetic fields coincide (i.e., the interaction between the stator's direct-axis magnetic field and the rotor magnets), the resulting electromagnetic torque is zero. Therefore, it is necessary to continuously change the position of the stator magnetic field to drive the rotor magnets to rotate continuously, ensuring that the stator magnetic field always leads the rotor magnetic field.
The field is at a certain angle, thus forming a permanent magnet magnetic field that always chases the magnetic field formed by the windings. The controller detects the sector where the rotor magnetic field is located, and then controls the windings to generate a magnetic field pointing to the next sector. Controlling the rotor to rotate one revolution only requires changing the stator windings six times. However, since the number of pole pairs in a hub motor is usually not zero, each energizing cycle means that the rotor has only rotated one electrical angle, not one mechanical angle. Therefore, the number of commutation cycles required for the rotor to rotate one mechanical angle is the same as the number of pole pairs.
This control primarily achieves motor speed control. By reading the position signal from the Hall sensor, the rotor position is determined, and the motor speed controller performs closed-loop control of the motor speed. Since voltage is proportional to speed, the control output phase voltage achieves speed control. This control method uses a simple six-step commutation process to change the armature magnetic field, leading the rotor to rotate. At any given time, only two phase windings are conducting. The specific control flow is shown in the figure:
Figure 7. Square wave control logic block diagram
The square wave control uses Hall effect sensors as position sensors. Three Hall effect sensors are positioned at electrical angles of 0°, 120°, and 240°, as shown in Figure 8, dividing the 360° electrical angle into six sectors. The controller detects the sector where the rotor is located and controls the armature magnetic field to guide the rotor to the next sector.
Figure 8. Schematic diagram of sector
VI. Motor drive leakage phenomenon
The reasons can be roughly categorized as follows:
Figure 9. Leakage phenomenon of motor.
VII. Current Detection Scheme
Currently, using Hall effect sensors or current transformers to detect the DC bus current in power converters has several limitations. Because the current through the main switching devices is generally relatively large , the rated parameters of the Hall effect sensors or current transformers used must also be large, resulting in a large size and high cost. Furthermore, it is not conducive to achieving high power density in power converters.
This article introduces a novel solution—a current detection circuit based on semiconductor devices . This circuit can be directly arranged on the control PCB of the power converter . It is not only low in cost , small in size , and easy to install , but also has good performance . It can also be integrated with the power converter to form an application-specific integrated circuit (ASIC).
Figure 10. MOSFET-based current sensing circuit
Circuit working principle (as shown in Figure 10):
When the drive signal L0 is 0V , the Q of the lower bridge MOSFET is off . The signal point V1 on the right end of D2 is the diode voltage drop of 5V . At this time, the positive input of U1 is 0.5V and the negative input voltage is 10V. At this time, the output of U1 is low level and the output of U2 is also low level.
The LM339 uses an open collector output mode and also suffers from the on-state voltage drop V2 problem . Therefore, subtracting VO2 from the signal VO1 eliminates the detection error.
(2) When L0 is 12V , the lower bridge MOSFET Q is in the on state , and the signal at point V1 on the right end of D2 is 12V . At this time , the positive input terminal of U1 is 12V and the negative input terminal voltage is 10V.
When U1 outputs in a high-impedance state , the voltage across VO1 is the voltage drop across the internal resistance of Q plus the voltage drop across the fast recovery diode D1. Simultaneously, the output of U2 is also in a high-impedance state , and the voltage across VO2 is the voltage drop across diode D3. By obtaining the difference between signals VO1 and VO2 through the operational amplifier , the voltage drop across the internal resistance of Q can be calculated.
Figure 11 shows the waveforms corresponding to each point.
Figure 11 shows the waveform analysis of the switching transistor voltage drop and the relevant points of the current detection circuit. Using a 120-degree two-by-two conduction method , in both motoring and braking states , one lower bridge arm is always in operation . Therefore, the sum of the conduction voltage drops of the three lower bridge arms is approximately equal to the average current of the motor windings.
