Author: Wu Qingguo? The article was first published on the WeChat public account of "Electric New Horizons"
1. Description?
Volkswagen is committed to the development of electric transportation systems. A new generation of ID.?family electric vehicles will be launched in 2020. There will be different classes of zero-emission vehicles with ranges comparable to today's gasoline vehicles. ID.?CROZZ, ?ID.?VIZZION and ?ID.?BUZZ have been made public. The first ID. model to hit the market in 2020 will be the ID., an affordable, four-door, fully connected compact car (Figure 1). The Volkswagen Group plans to launch 27 MEB cars globally by 2022. These include electric models from the Audi, Seat, Skoda, Volkswagen and Volkswagen Commercial Vehicles brands.
Figure 1?The?ID.?family:?(from left)?the?ID.,?ID.?CROZZ,?ID.?VIZZION?and?ID.?BUZZ
The ID. will mark the debut of the world’s first model based on the Modular Electrification Kit (MEB), a technology platform developed specifically for fully electric vehicles (Figure 2). The components of the electric drive system and the battery pack are precisely system interconnected. The high-voltage battery is located centrally between the axles. It is scalable to accommodate different battery types and comes with integrated liquid cooling. Therefore, it is relatively easy to integrate into the various power outputs of the ID. model. Depending on the battery size and ID. model, the range can reach approximately 330 kilometers to more than 550 kilometers. An AC charger with charging power up to 11kW is integrated in the vehicle. DC charging of up to 125kW can be achieved using CCS (Combined Charging System) devices. Basically, two electric drive systems can be installed on the platform, driving one or two axes through the retractable parts of the MEB.
Figure 2? MEB model platform
ID.’s zero-emission drive system is mainly composed of a motor combined with the rear axle, including a power inverter and a single-speed transmission, installed on High-voltage battery under the car and auxiliary components located at the front of the car to save space. The compact drive system consists of an electric motor, power inverter and single-speed transmission. With a driving range similar to that of today's gasoline vehicles and the same price as diesel vehicles, ID. also has the potential to promote the development of environmentally friendly electric transportation and start a new era of electric drive systems.
2. Introduction to high-voltage battery system?
The key factor in determining the voltage range is the high-voltage battery. It is integrated into the underbody between the front and rear axles, which saves space and provides a more spacious interior, while ensuring an optimal front-to-rear 50%:50% weight distribution and a low center of gravity. Advantages. High-voltage batteries are the most important cost factor for electric vehicles. During its development and design process, in addition to technical standards such as long-range battery capacity and power density, excellent driving performance and fast charging capabilities, economic factors such as cost and service life must also be considered.
For MEB, Volkswagen has developed a high-performance lithium-ion high-voltage battery, which ensures high practicality and long service life under certain operating habits and working temperature conditions. It provides an electronic driver with repeatable high power output over a wide temperature band and charge range. In short charging times, high levels of continuous current capacity, charging power up to 125kW. Scalable battery capacity ensures that different ID. model families are available with driving ranges ranging from 330 kilometers to more than 550 kilometers (according to WLTP).
High-voltage batteries are composed of parallel and series-connected modules, which in turn are composed of individual battery cells. Due to the modular design, the number of cells in the high-voltage battery can be varied. This enables high-voltage batteries with different energy contents and scaling to be adapted to different vehicle concepts and customer requirements. Powerful thermal management with a direct cooling system ensures that the high-voltage battery can still operate within its optimal temperature range of 25 to 35°C even under high load or low temperature conditions. Current, voltage and temperature are monitored via the unit module controller and the main control unit.
3. Charging technology?
Figure 3? Charging options for MEB vehicles
In addition to mileage, charging issues are also crucial to the daily practicality of electric vehicles . Customers have clear requirements for charging technology: charging times are as short as possible and charging options are sufficient.
Volkswagen assumes that most ID. drivers only charge their electric vehicles once a week, which means that 50% of charging activity may take place at home. Therefore, vehicles based on the MEB will be equipped as standard with a type 2 charging connection, which can be charged via an AC connection, either via a standard household socket at 2.3 kW or in an 11 kW wall cabinet. AC charging from the wall box at night provides sufficient power to recharge the battery. Since the battery can only be charged with DC power, an 11kW charger is integrated into the vehicle to convert AC power from an outlet, wall cabinet or AC charging station into DC power to charge the high-voltage battery.
The optional CCS charging port can significantly shorten charging time. It combines a Type 2 plug with two additional power contacts for DC charging (Figure 3). Via the CCS charging port, the high-voltage battery can be charged using power sources up to 125kW. It can charge 80% of its power in 30 minutes. In the long term, MEB is also ready for inductive charging, which requires neither cables nor plugs. The vehicle simply parks on a so-called charging pad and is charged via this pad.
4. MEB’s electric drive system?
Figure 4? MEB’s rear-drive system
Two new electric drive systems have been developed for MEB. The main drive is a permanent magnet synchronous motor (PSM, Figure 4) on the rear axle. It combines a power inverter (PI) and a parallel-axis reducer. The output power is 150?kw, the torque is 310?Nm, and the maximum speed is 16000?rpm. PSM is a system component with high power density and high efficiency, which can continuously provide output over a wide speed range.
According to vehicle planning, MEB front-wheel drive can provide power. The front drive is an electric drive system with an induction asynchronous motor, which can achieve four-wheel drive for the entire vehicle. Its power output is 75?kW, torque is 151?Nm, and maximum speed is 14,000?rpm. Asynchronous machines (ASM) are known for their ability to operate overloaded for short periods of time and with low resistance losses. Therefore, it is very suitable for auxiliary driving.
The following will focus on the composition, technical characteristics and performance data of the MEB permanent magnet synchronous motor (PSM) electric drive.
4.1? Working principle of PSM/ASM
Working principle of permanent magnet synchronous motor
The current in the three-phase copper winding of the stator generates rotating magnetic flux (rotating magnetic field ). The excitation magnetic field in the rotor is generated losslessly by the permanent magnets and penetrates the stator. This creates a tangential force in which the rotating fields of the rotor and stator rotate at the same speed (synchronously) (Fig. 5, left).
Working principle of asynchronous machine (ASM)
The current in the three-phase copper winding of the stator generates rotating magnetic flux (rotating magnetic field), which penetrates the rotor through the short-circuit winding. The rotor in an asynchronous motor rotates at a speed slightly lower than the rotating magnetic field of the stator (asynchronous). This creates a change in the magnetic field in the short-circuited winding, thereby producing an electric current. The resulting magnetic field produces a tangential force in the rotor, which acts as a torque on the rotor shaft (Fig. 5, right).
Figure 5? Basic structure of PSM (left) and ASM (right)
4.2? Inverter (PI)
The three-phase current of the motor is given by Supplied with a liquid-cooled power inverter (PI) mounted directly on the motor. Figure 6 shows an exploded view of the power inverter. Inside the power inverter, three IGBT power modules of the latest generation are connected to form a classic B6 power inverter. Inside the module carrier, the power module is framed by a cooling structure so that the driver board can be plugged directly into the contact pins of the power module. A shielding cover is installed between the driver board and the control board.
Figure 6? Power inverter (PI) structure
Other important components inside the PI include: DC input filter components, DC bus capacitors, three-phase bus copper bars and liquid cooling cooling unit.
PI's modular design is suitable for mass industrial production. From the power module via the module carrier to the power supply and controller modules, a modular system is created that provides a basis on which next-generation electronic drive projects can be completed with minor modifications. In addition, fully automated production of power electronics ensures consistent structural and functional quality even in large-scale production. Importing and processing sensor data to regulate motor current values ??is a highly dynamic process. The result is optimal power utilization, especially at dynamic operating points.
Some vehicle functions, such as vibration damping and slip control functions, are integrated directly into the power electronics system. Therefore, bus communication without delay can be achieved. The advantage of this design is that there are more straightforward adaptation options during development to meet the needs of specific vehicle driving behavior.
In the MEB platform, the DC/DC converter is not integrated into the PI, but is designed as a separate liquid-cooled component. DC/DC can be flexibly installed elsewhere in the vehicle and is available in two power levels, 1.8kW and 3.0kW.
4.3?PSM rear axle drive
MEB rear drive motor is a three-phase permanent magnet synchronous motor (PSM) with four pairs of poles in the rotor and a maximum speed of 16000?rpm. It consists of a power inverter, four parts of the housing (motor housing, motor rear end cover, reducer front housing, reducer rear housing, see Figure 4), stator, rotor, rotary transformer with temperature sensor, single gear It is composed of reducer and other main modules. The electric drive assembly is produced at the Volkswagen plant in Kassel. The rotor and stator are supplied by the Volkswagen Salzgitter plant.
The stator contains bus windings for three-phase connection. The permanent magnets in the rotor are made of neodymium alloy and are embedded in the laminations. The stator and rotor are mounted in a cast housing and the stator is liquid cooled. Two deep groove ball bearings are installed at both ends of the rotor shaft.
A resolver rotor is installed at the rear end of the motor shaft. The low-voltage terminal block includes the winding temperature sensor and resolver signal, and is preferably closed by the motor cover. The resolver and temperature low voltage signals are finally connected to the controller. The reducer reduces speed and increases torque. The front housing of the reducer and the front end cover of the motor are integrated to reduce weight and size (see Figure 4).
4.3.1 Stator structure
Figure 7? PSM stator
The stator is mainly composed of laminations and three-phase hairpin wire windings (Figure 7). The lamination stack consists of individual, welded, layered, individually coated metal plate laminations with an outer diameter of 220mm. The laminations have high magnetic permeability, are 0.27mm thick, and are coated with an electrically insulating layer on both sides. The stator is divided into four sections, each offset by 90 degrees during assembly. This reduces the effect of metal grain orientation on the uniformity of the rotating magnetic field.
The winding is inserted into the stator slot, the three-phase ends are welded (Figure 8), and the three-phase copper bars are automatically connected. The end winding of the stator structure contains a contact device for a temperature sensor. The stator is also impregnated with resin to increase insulation, improve heat conduction and strengthen the windings. The stator goes through an automatic test program and is automatically press-fitted to the motor housing.
Figure 8? Stator coil assembly
4.3.2 Rotor structure
Figure 9? Exploded view of the rotor
The rotor consists of It consists of a shaft, a lamination embedded with a V-shaped permanent magnet, a pressure plate and a rotor. The rotor is divided into four sections. The rotor end faces are compressed with pressure plates and connected together by four tensioning screws that pass through the laminations (Fig. 9). Fully automatic pressing of laminations and automatic pressing of rotor shaft to complete assembly.
The rotor permanent magnet adopts "V+1" slope arrangement. They are protected by an expanded magnetic coating. The purpose is to improve the NVH performance of the motor. Laminations are die-cut from sheet metal of the same material.
The rotor shaft is designed as a hollow shaft and is welded from two parts. It is connected to the input shaft of the transmission via a longitudinal internal spline. The entire motor shaft and reducer input shaft are supported by three bearings, and the bearings are low-friction deep groove ball bearings. Reduce mechanical losses.
When installing the rotor shaft and laminations, the lamination assembly needs to be heated. This also causes thermal activation of the permanent magnets and expansion of the magnetic coating, requiring the permanent magnets to be fixed.
4.3.3? Resolver with temperature sensor
Fig. 10? PSM? Components on the b-side bearing shield
In order to provide the correct access to the stator winding For three-phase alternating current, it is necessary to detect the correct position of the rotor. This task is accomplished by spin change. It consists of the rotor on the rotor shaft and the stator fixed on the rear bearing shield of the motor (Figure 10).
A dedicated fixed point is designed in a hairpin on the stator winding, where a temperature sensor for measuring the winding temperature is installed.
The signals from the resolver and the temperature sensor are transmitted to the PI via the signal plug and then evaluated.
The power inverter is bolted to the motor housing.
The three busbars for the stator phase windings are integral parts of the PI and are fixed to the contact bridges of the stator after the stator is fixed to the motor housing.
Both the A-end and B-end covers contain special collision elements inside. In the event of a rear-end collision, this element can isolate the drive device from the body frame, thereby preventing the high-voltage battery from short-circuiting.
4.3.4? Cooling and Heating Electronic Drives
The electric drive system is liquid cooled. The flow of coolant into the electronic drive is first run through the power inverter, since the semiconductors dictate the maximum permissible coolant temperature. After flowing through the PI, the coolant enters the cooling water jacket of the motor housing through the sealing plug element. The heat is mainly generated by the resistive losses of the stator copper windings and reaches the cooling water jacket in the casing through the winding insulation layer and laminations. The cooling medium enters the stator through optimized circumferential cooling channels and enters the vehicle's external cooling circuit through the cooling connection hose at the end of the cooling water channel (Fig. 11).
Figure 11? Coolant flows through PI and stator
4.3.5? Electronic driver technical parameters *The weight is the total weight of PI, motor and reducer
The compact MEB electronic drive provides Volkswagen's ID. car family with an outstanding driving performance. The parallel-axis MEB rear drive axle, permanent magnet synchronous motor integrated with PI and single-speed reducer provides a peak power of 150?kW and a maximum torque of 310?Nm. The maximum speed of the motor is 16000?rpm (Figure 12).
As a four-wheel drive auxiliary drive, the coaxial MEB front axle drive is an asynchronous motor integrating PI and single-speed reducer. It provides a peak power of 75kW and a maximum torque of 151Nm. The maximum speed of this motor is 14000?rpm.
Figure 12? PSM efficiency map
The design of the electronic driver is based on a detailed evaluation of the energy conversion in the motor characteristic map for different driving cycles. When designing the magnetic circuit, we paid special attention to the operating points of the urban driving cycle to ensure that the electronic drive operates efficiently in these conditions. In a large number of real-life working conditions, the efficiency is far higher than 90% (see Figure 12 and Figure 13).
Figure 13: PSM full load diagram
4.3.6 Comparison of MEB rear drive axle and e-Golf? drive axle *The weight is the total of PI, motor and reducer Re
Technical data comparing the new MEB rear axle drive with the current electric drive axle in the e-Golf illustrates its development progress. Peak power can be increased by 50% to 150?kW, and torque can be increased by 7% to 310?Nm. Despite the increase in power and torque, the weight of the MEB rear axle drive has been reduced by 18% to 90kg. This gives the MEB rear axle drive a power-to-weight ratio of 1667?W/kg, a significant improvement of 82% compared to the e-Golf’s electric drive axle.
4.4? Single-speed gearbox
Figure 14? MEB rear drive axle single-speed gearbox
The single-speed reducer is a two-stage gear reduction mechanism. To reduce the motor speed and increase the torque output (Figure 14).
MEB specially performs NVH acoustic optimization on the reducer gears. The motor shaft and reducer input shaft are supported by 3 bearings to reduce friction. Lubricants are maintenance-free for life. A targeted lubrication design has been carried out, and the dry oil sump concept has been adopted to reduce oil churning losses and improve efficiency. In addition, the preloaded tapered bearings were changed to floating column bearings.
The reducer is designed with different speed ratios to meet different power needs. The overall speed ratio when the ID was first used was 11.5:1, and the top speed was 160km/h. At the same time, MEB will cancel the parking lock mechanism of the drive train, and will use wheel-end EPB to realize the parking function under slope conditions.
5. Summary?
The power system of Volkswagen MEB is part of a modular building tool kit, and its components can form a variety of different electronic power system configurations to configure various specifications of electric vehicles.
MEB’s parallel-axis rear axle drive system consists of a highly efficient permanent magnet synchronous motor, a friction-optimized single-speed transmission and a highly compact power inverter fastened to the motor. Combined with a high-voltage lithium-ion battery, the electronic drive of the Volkswagen ID model has a maximum torque of 310?Nm and a maximum power of 150?kW. For four-wheel drive applications, an additional coaxial electric transaxle is available for the front axle.
It is composed of an innovative asynchronous motor, paired with a low-friction single-speed reducer, and an integrated controller.
The MEB for electrified powertrains represents the systematic continuation of Volkswagen’s modular approach to new vehicles. Due to the high volume of system development, development and component costs can be significantly reduced. This is a necessary prerequisite to reduce vehicle costs and thereby increase the market penetration of electric vehicles. With a driving range similar to that of current gasoline vehicles and the same price as diesel vehicles, ID. also has the potential to promote the development of environmentally friendly electric transportation and start a new era of electric drive systems.
References
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