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Anatomy of all key technologies of hub motors (illustrated)

2019-08-05

Reprinted from:Wangcai Electric and Electronic Control

Hub motor technology, also known as wheel-mounted motor technology, its greatest feature is that the power, transmission and braking devices are integrated into the hub, so the mechanical part of the electric vehicle is greatly simplified.
Hub motor technology is not a new thing, as early as 1900, Porsche first made the front wheel equipped with hub motor electric vehicle, in the 1970s, this technology was applied in mine transport vehicles and other fields. For the hub motor used in passenger cars, Japanese manufacturers carried out early research and development of this technology, and are currently in a leading position. International automobile giants, including GM and Toyota, are also involved in the technology. At present, there are also independent brand automobile manufacturers began to develop this technology, in the 2011 Shanghai Auto Show exhibition Ruiqi X1 extended range electric vehicle on the use of hub motor technology.
In this paper, the wheel motor technology is further elaborated by means of a simple and easy-to-understand diagram.

The hub motor drive system is divided into two structural types according to the rotor type of the motor: the inner rotor type and the outer rotor type. The outer rotor type uses a low-speed external rotor motor, the maximum speed of the motor is 1000-1500r/min, no deceleration device, the wheel speed is the same as the motor, while the inner rotor type uses a high-speed internal rotor motor, equipped with a fixed transmission ratio of the decelerator, in order to obtain a higher power density, the motor speed can be as high as 10000r / min.
With the advent of more compact planetary gear reducers, the inner rotor hub motors are more competitive in power density than low-speed external rotors.
Types of hub motors

There are brush motors and brushless motors, because the efficiency is too low, the car has brush motor is phased out.
Brush motor and brushless motor, because the efficiency is too low, the vehicle brush motor is phased out.

There are sensors and sensorless, and some electric bicycles have to step on them because there are no sensors in them. It directly measures the back EMF of the motor and knows the position of the rotor. Sensors must be used to know the relative position of rotor and stator before starting.

with gears and no gears, motors that reduce the starting current to prevent demagnetization of magnetic steel must use a deceleration gear to improve the starting efficiency. After the magnetic steel material is improved, it does not have to be geared.

Construction of hub motors

 

The principle of the hub motor

Sensors must be used to know the relative position of rotor and stator before starting brushless motor. The induction motor directly measures the back EMF of the motor and knows the position of the rotor, and the power tube is driven by the controller to change the phase.

Although the memory can record the relative position of stator and rotor, the waveform of reverse EMF of motor winding can not be understood for very slow rotating system. When the motor reaches a certain speed, because the wave peak and valley are limited by inertia, they all represent a certain angle, so the motor is turned off when braking. Therefore, the use of magnetic sensors hub motor is the mainstream.

The principle diagram of hub motor the red magnetic steel rotor is in the dead angle position, which depends on the winding above the blue magnetic steel rotor to turn on the electricity and get out of the dead angle. Figure 2 shows that the motor has no dead angle, as long as you know the position of the rotor, you know how to drive the power tube.

 

Figure 1 Figure 2 shows the motor looks like a straight-line motor rolled up, winding power is like using food to lure donkeys (magnetic steel) to keep running, but always maintain a distance, it is more powerful, relatively heavy, simple structure, low noise.
Magnetic manual gear clutch high-speed brushless hub motor Use three large, thin 2-mode steel gears to slow down to get the power you need. When taxiing is required, the shaft, piston and pull hook of the shaft of the shaft clutch drive are pulled by the eccentric clutch handle, so that the outer rotor end of the motor gear is displaced and the motor gear is separated from the transmission gear.
Do not use the motor magnetic reset when taxiing to achieve gear manual meshing, its clutch mechanism is simple, eliminating the ability to exceed the clutch.

Heat dissipation, in order to prevent the motor from transferring heat to the tire, there must be a certain distance between the two, some using steel wire to isolate.

Advantages of Hub Motor
Omit a lot of transmission components, make it simple for vehicle structure.
The application of hub motor can greatly simplify the structure of the vehicle, the traditional clutch, gearbox, transmission shaft will cease to exist. This also means saving more space.
For traditional vehicles, clutch, transmission, transmission shaft, differential and even distributor are essential, and these components not only weigh not only, which makes the structure of the vehicle more complex, but also needs regular maintenance and failure rate. But the hub motor solves this problem very well. In addition to the simpler structure, the vehicle driven by hub motor can obtain better space utilization rate, and the transmission efficiency is also much higher. A variety of complex driving methods can be realized, because each tire is driven separately, so it is very easy to realize the four-drive form. The application of hub motor technology can even realize both sides. The wheel is reversed to achieve the purpose of in situ steering. In addition, for some special vehicles, such as the “centipede car” with too many wheels, hub motor is also a very good solution.
Because the hub motor has the characteristic of independent drive of a single wheel, it can be easily realized in the form of front drive, rear drive or four-wheel drive, and it is very easy to realize the full-time four-drive on the vehicle driven by the hub motor. At the same time, the hub motor can realize the differential steering of similar crawler vehicle through the different rotational speed or even reversal of the left and right wheels, which can greatly reduce the turning radius of the vehicle, and can almost realize the in situ steering under special circumstances (but the wear and tear of the steering mechanism and tire of the vehicle is large at this time), which is very valuable for special vehicles.
It is convenient to adopt a variety of new energy vehicle technology, many new energy models are driven by electricity, so hub motor drive has been put into great use. Whether it is pure electric, fuel cell electric vehicle or extended range electric vehicle, hub motor can be used as the main driving force. Even for hybrid models, hub motor can be used as the starting power or rapid acceleration power, which can be described as a multi-purpose machine. At the same time, many technologies of new energy vehicles, such as braking energy recovery (that is, regenerated braking), can also be easily realized on hub motor driven models.

Shortcomings of Hub Motor
Increasing the reed quality and the inertia of the wheels, the control of the vehicle has an impact on the wheel motor to be installed in the wheel circle, which makes the vehicle reed quality increase is not conducive to control. In terms of wheel motors produced by Protean, the mass of a single motor is 30kg.
For ordinary civilian vehicles, it is often used to make hanging parts with relatively lightweight materials such as aluminum alloys to reduce the quality of the reeds and improve the response speed of suspension. However, the hub motor just significantly increased the quality of the reed, but also increased the rotation of the wheel inertia, which is not conducive to the vehicle’s handling performance. But that’s not the biggest flaw, given that electric vehiclemodels are mostly limited to the pursuit of power performance.
The electric braking performance is limited, the operation of the brake system needs to consume a lot of electrical energy Electric vortex braking capacity is not high, in heavy vehicles need to work together with mechanical braking system. For electric vehicles, achieving higher braking effects requires higher energy consumption, which affects the range to some extent.
Nowadays, many traditional power commercial vehicles are equipped with auxiliary deceleration equipment based on the principle of Eddy current braking (that is, resistance braking), such as the electric retarder used by many trucks. Due to the relationship of energy, electric braking is also the first choice for electric vehicles, but for vehicles driven by hub motor, because the electric braking capacity of hub motor system is small and can not meet the braking performance requirements of the whole vehicle, additional mechanical braking system is needed, but for ordinary electric passenger cars, without the vacuum pump driven by traditional internal combustion engine, electric vacuum pump is needed to provide braking power. But that means there’s more energy consumption, even if it’s regeneration. Braking can recover some energy. If we want to ensure the efficiency of braking system, the energy consumed by braking system is also one of the important factors that affect the mileage of electric vehicle.
In addition, the wheel motor work environment is harsh, facing water, dust and other aspects of the impact, in the sealing also has higher requirements, at the same time in the design also need solely for the hub motor to consider the heat problem.

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