With the iX3, BMW introduced the fifth generation of Bavarian electric motors. These are externally excited synchronous motors (FSM), also known as “current-excited synchronous motors” (SSM). The engines belong in the context of BMW’s modular system of highly integrated e-axles, which combine the electric motor, the transmission and the power electronics in a compact manner.
The housing is manufactured in the Landshut plant using complex casting technology. The high level of integration reduces flange surfaces, integrates the media routing of power, data and liquids and simplifies assembly. The goals are minimized error potentials both in production and in operation at the customer’s site while at the same time being cost-effective. The performance classes of the drives result on the one hand from the scaling of the active parts (rotor/stator) in terms of diameter and length and on the other hand from the interaction of battery power, converter and software. Based on two stator diameter variants (170 and 220 mm), there are currently five power classes with a range from 140 to 360 kW.
Why a current excited motor?
The most important thing about electric motors in a nutshell: The converter uses pulse width modulation to modulate the voltage and frequency of the three-phase current that flows through the stator. The resulting dynamic magnetic field turns the rotor. With a “synchronous motor”, the magnetic field and rotor rotate at the same speed, i.e. “synchronously”. In order for the rotor to turn, it must be magnetic. In the case of a permanently excited rotor, permanent magnets are used to build up the magnetic field, mostly made of neodymium-iron-boron. In the case of the current-controlled rotor, it is an electromagnet that must therefore be supplied with current. In the case of the BMW engine, this happens via maintenance-free slip rings.
In pre-development, BMW worked intensively on which technology the next generation of engines should use. In the years 2010/2011, the price of neodymium increased massively, 95 percent of which comes from China. One requirement was therefore to eliminate this dependency. Another aspect was the popularity of all-wheel drives among BMW customers. Motors that run freely without current (i.e. asynchronous motors and SSM) have advantages here. “We have considered everything,” says Uwe Deuke, project manager for the 5th generation electric drive system at BMW.
Target dynamics and efficiency
For example, the favorable costs and the overload capacity spoke in favor of the asynchronous motor. However, the performance characteristics and the lower efficiency ultimately spoke against it, because BMW wanted to achieve high dynamics combined with the best possible customer range. A cleverly designed external excitation soon turned out to be the best solution. It combined efficiency, high power density and good cost structures.
(Image: BMW)
Efficiency and its map sinks
Actually, the most efficient motors are permanent magnet synchronous motors (PSM). In fact, there are definitely advantages for SSM in real operation. The most common design for an e-drive is a high-speed e-motor on a single-speed gearbox. These engines still have to run efficiently in the five-digit speed ranges because they are found on the Autobahn. The BMW engine, for example, revs up to a rated speed of around 17,000 rpm. In the upper speed ranges, however, efficiency usually drops, which is why many electric cars are limited to lower top speeds than before or (rarely) come with two-speed gearboxes.
In order to get the best possible efficiency and performance, the converter controls the current according to a map of the speed and load, in which there is a lot of experience, just like in the control software. With the SSM, however, there is an additional control dimension, namely the excitation current. The control of the excitation current allows the rotor to be magnetized either in terms of efficiency or performance in all load ranges.
This results in efficiency advantages for SSM, especially when driving on country roads or motorways. However, it is also possible to switch off the field winding. This helps with the aforementioned dynamically switched all-wheel drives as well as with towing the vehicle on the axle. Functional reliability is also increased: permanent magnets always induce currents when rotating, so that safety systems in the converter of a PSM drive must ensure that e.g. B. can be rolled out safely from high speeds in the event of a sudden failure on the freeway.
