AUTHORS

DR. MERTEN JUNGis Project Leader for the Powertrain

of the ActiveE at BMW Group in Munich (Germany).

DR.-ING. JÖRG MERWERTH is responsible for the Eletro-magnetic

Motor Design of the ActiveE at BMW Peugeot Citroën Electrification

in Munich (Germany).

HENDRIK UEBERLEis Responsible Engineer for the

Electrical Storage System of the ActiveE at BMW Group

in Munich (Germany).

FRANK VOGELis Department Leader

Engine Electrics at BMW Group in Munich (Germany).

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BACKGROUND

Due to the increasing significance of electromobility for the automobile indus-try, the BMW Group has made the strate-gic decision to develop the components of the electric drivetrain inside the com-pany. Following the fleet test with the Mini E on the basis of a purchased drive-train, the completely newly developed drivetrain will be tested by customers in the BMW ActiveE. The experience from

both fleet tests will then flow in to the BMW i3, creating the first BMW mass production electric vehicle, 1.

In contrast to the “purpose-built” BMW i3 obtainable as of 2013, which is based on a so-called Life Drive archi-tecture with CFRP cabin weight-opti-mised for the electric drive, the ActiveE is still a “conversion” vehicle; in this case, conversion of the BMW 1 Series Coupé which is optimised for conven-tional drives.

DEVELOPMENT OBJECTIVES

The following development objectives for the BMW ActiveE were derived from the experience of more than 15 million driven kilometres in the Mini E fleet test [1]: : retaining all four seats and a luggage

compartment capacity of 200 l : range of 205 km according to NEDC

and 160 km in customer operation : consumption of 0.16 kWh/km accord-

ing to NEDC (including charging)

THE ELECTRIC DRIVE OF THE BMW ACTIVE EFollowing up on its Mini E, BMW is now presenting a further car with an

electric powertrain, the BMW ActiveE. its electric motor develops a power

output of 125 kW and 250 nm of torque. The high-voltage battery with

a net energy content of 28 kWh and the drive electronics have both been

systematically developed in-house by BMW. in spite of its single-speed

transmission, the ActiveE features very good powertrain dynamics due to

the use of a special permanently energised synchronous motor.

1 The BMW Group’s path towards a mass production electric vehicle

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: optimised temperature management of the high-voltage accumulator

: acceleration from 0 to 100 km/h in 9 s : top speed of 145 km/h : these objectives had to be broken

down to the components of the electric drivetrain, resulting in the following major requirements:

: efficiency of the drivetrain of at least 85 %

: net energy content of the high-voltage accumulator of at least 28 kWh

: liquid cooling of the high-voltage accumulator

: electric drive power of 125 kW and 250 Nm torque

: maximum speed of the electrical machine of 12.000 rpm.

In order to implement driving character-istics that are typical of BMW, a balanced weight distribution and rear-wheel drive were also required.

THE ELECTRIC DRIVE WITH ITS COMPONENTS

Alongside achieving the functional objectives, the great challenge lay in the geometric integration of the drivetrain into the base car, as the BMW 1 Series Coupé is conceived purely for conven-tional drives. In order to retain all four seats, the high-voltage accumulator was split into three individual accumulators and the electrical machine with flanged-

on drive electronics and gearbox were integrated in the rear axle, 2. This required in particular far-reaching inter-ventions in the design of the body plat-form and rear axle. The focus of this article, however, is on the components of the electric drivetrain; these will be described in detail below. Details of the vehicle integration and driving charac-teristics can be found in [2].

LITHIUM-ION HIGH-VOLTAGE ACCUMULATOR

The high-voltage battery system provides the energy for the electric drive. The design envelope created by elimination of the combustion engine drive in the 1 Series Coupé (tank, tunnel and bulkhead area) required three accumulator units of different sizes and shapes in order to integrate the required 28 kWh net energy content into the vehicle.

The three-way split of the accumulator required for the ActiveE as a “conversion” car is counterproductive from the point of view of weight, as each individual par-tial accumulator has to have its own elec-trical system/electronics and its own housing. The weight distribution of high-voltage battery unit on both axles and lowering the vehicle centre of gravity by fitting a high-voltage battery unit in the transmission tunnel have a positive influ-ence on the dynamic driving properties.

3 shows the three high-voltage battery units and their position in the car.

In order to meet the high requirements for density of electromagnetic energy and cycle resistance, lithium-ion cells were selected as the ActiveE high-voltage accumulators; these are supplied by SB LiMotive. The high-voltage accumulator contains a total of 192 cells with 40 Ah each, whereby two cells are switched in parallel (forming cell pairs) and there are 96 cell pairs in series.

The cells are integrated in a total of 25 compact modules, designed and manu-factured by BMW. To achieve the best possible utilisation of the available space, these cell modules each consist of six, eight and/or ten individual cells. A complex contacting system ensures the interconnection of the cells. The connec-tion of the cell modules to the complete vehicle required the additional new development of the following electrical/ electronic components: : accumulator management electronics

control unit, SME : switching box (S box) with switching

relay, sensors and overcurrent protec-tion device

: Cell Supervision Circuits, CSC.The three SME control units work in a master-slave mode, whereby the SME in the tunnel accumulator is the master control unit. The battery management implemented on these control units

2 The integration of the electric drivetrain

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includes sequence control (for example wake-up, switching of the relays), safety functions, algorithms with state recogni-tion of the high-voltage accumulator (charge condition, age level, current per-formance capability) and diagnosis func-tions. In an interplay with functions in the car, it ensures reliable operation. In the event of disruptive infl uences, a deg-radation concept maintains operation as far as possible. The high-voltage accu-mulator is disconnected from the high-voltage vehicle electrical system in the event of safety-critical states. Battery management also regulates cooling of the high-voltage accumulator.

The relays integrated in the switching box disconnect the cell module from the vehicle wiring system when de-ener-gised. Each switching box contains a high-voltage fuse to safeguard against short circuits.

The Cell Supervision Circuits (CSC) monitor the individual cell pair voltages and the temperature. Another task of CSC is to ensure the cells have the same symmetrical state of charge. The CSC and SME are connected within the high-voltage battery system across a local CAN bus.

Derived from the requirements of practical driving, the high-voltage accu-mulator has been equipped with a water-glycol cooling. The temperature of the battery system increases due to the power dissipation during operation and due to the external application of heat resulting from high outdoor tempera-tures. The cooling system is connected via a heat exchanger (chiller) to the coolant circuit of the vehicle and via a switching valve to the heating circuit. This enables not only cooling but also heating of the accumulator, so that full power output and energy can be pro-vided even in wintery conditions. The conditioning of the accumulator (and also of the interior) can be set in advance using a timer function in the car and also remote-controlled using a Smart-phone application.

ELECTRICAL MACHINE

Alongside the effi ciency, it was above all the characteristics of the machine in the fi eld weakening range that were decisive for the selection of the engine concept. As the BMW ActiveE has a single-speed gearbox with a fi xed gear ratio, the elec-1 Bildunterschrift

3 The three high-voltage accumulator units in the ActiveE

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trical machine must be able to deliver a power output that remains as constant as possible over a broad speed range.

This requirement means that neither an asynchronous machine (ASM) nor a permanently excited synchronous machine with surface-mounted magnets (PSM) can be used. An electrically excited machine would exhibit very good characteristics in the field weakening range, but this machine type has a very high specific gravity.

In order to meet all the requirements, a special permanently excited synchro-nous machine with embedded magnets was chosen, 4. The rotor design consist-ing of several layers of magnets requires a reluctance difference between the d axis and q axis that is as large as possi-ble. This machine type is often referred to as a “hybrid synchronous machine”.

This expresses the fact that the torque is composed of both the interaction of the rotor field with the stator current and the reluctance torque that arises.

As the d inductance is always less than the q inductance with this design (Ld < Lq), a negative d current always leads to a positive reluctance torque. The necessary weakening current therefore raises the torque in the field weakening range. This enables implementation of a wider rotational-speed range with only moderate power loss.

At the same time, this geometry ena-bles the use of a large number of pole pairs (in this case six pole pairs), which means the stator and rotor yoke can be kept narrow and weight is saved; conse-quently, a power output of 125 kW is achieved with only approximately 50 kg. The torque and power characteristics

achieved in the interplay with the drive electronics described in the next section are shown schematically in 5.

DRIVE ELECTRONICS

The key element for optimal electrical energy management in vehicles with electric drives is the drive electronics. It serves both as an inverter when sup-plying the electrical machine with power and as a voltage transformer in the interplay between the high-voltage accumulator and the 12-V vehicle elec-trical system. For all tasks, the drive electronics must combine with efficient software to regulate the required flows of current variably, efficiently, and in line with the needs of each situation when the vehicle is driven and when it is coasting.

4 Hybrid synchronous machine and its sectional drawing (in principle)

5 Measured efficiency and torque/power output characteristics (schematic view)

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With its suitability for switching fre-quencies of at least 20 kHz, the inverter of the BMW ActiveE has been specially developed to ensure it will be able to activate future multi-pole electrical machines. One of the challenges of the drive electronics here is the increasing power dissipation as the switching fre-quency rises, with simultaneous imple-mentation of large amounts of energy in a small design envelope (total volume approximately 15 l, 6). The high density of electromagnetic energy requires an innovative cooling concept for the power semiconductors and intermediate circuit capacitor. Here, alongside the high level of heat dissipation, an abso-lutely leak-tight connection of the com-ponents to be liquid-cooled must be ensured over the temperature cycles and service life.

Optimising the inverter efficiency to > 95 % made it possible to significantly reduce the overhead for cooling while at the same time increasing the electrical range of the vehicle. Over and above this, an intelligent degradation concept is the key to optimal capacity utilisation of the IGBT module that is used, while at the same time achieving the required service life.

Modern electrical machine control sys-tems require that highly precise phase current sensors with high resolution and accuracy are used in the inverter. At the same time, these must have a broad meas-uring range of up to 800 A. The higher switching frequencies for activation of the electrical machine also raise the require-ments for computing performance of the deployed 32-bit microcontrollers (Infineon TriCore 1797). The deployment of special-ised integrated controller modules and the associated load relief for the main pro-

cessors created a powerful computer sys-tem that also meets the requirements for future electrical machines.

Yet another challenge results from the specified design envelopes in conjunc-tion with the high voltages and currents. Suitable bus bars are used here within the drive electronics to ensure the sup-ply of electrical energy from and to the high-voltage accumulator, the three-phase connection of electrical machine, and the supply to high-voltage consumer units. Moreover, suitable intervals safely avoid any possible flashovers. The neces-sary deployment of faster and low-toler-ance shutdown functions for component protection of the inverter in the event of excess currents and overvoltages increased the integration requirements for the inverter and/or IGBT module to optimise the response times.

A DC-DC converter with a maximum output of 2.8 kW has been integrated in the drive electronics to supply the 12-V vehicle electrical system. In the same way as for the inverter, the efficiency has been optimised significantly. This objec-tive was achieved in the DC-DC con-verter by using a resonant switching topology and active rectification. The DC-DC converter is two-stage and its broad high-voltage input range enables application in a wide variety of electrifi-cation concepts.

SUMMARY

The consistent in-house development of electric drive components of the BMW ActiveE with the accompanying pro-found understanding down to the lowest detail level was the decisive factor for success for target achievement of the overall system.

REFERENCES[1] Schmidt, G.: Electromobility at the BMW Group – first experience with the future today. 2. German Electromobility Congress, Bonn, 2010[2] Jung, M.; Kessler, F.; Müller, P.; Wahl, S.: vehicle integration and driving characteristics of the BMW ActiveE. Scheduled in: ATZ 114 (2012), no. 10

6 Layout of the drive electronics (including integrated charging unit for follow-on projects)THANKS

The industrialisation of Li+ high-voltage accu-

mulators has been promoted and subsidised by

the Federal Ministry for Economics and Tech-

nology based on a decision in the German

parliament.

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