The new BMW i3 combines two electric motors that do not produce their magnetic field in the same way. At the rear, a synchronous motor with electrical excitation. At the front, an asynchronous machine whose operation requires a slip. Behind these terms lies an engineering choice that affects efficiency, weight, and the cost of electric propulsion.
The slip in question does not concern tires nor a component that would slip in the transmission. It designates a difference in speed inside the motor, between its rotor and the magnetic field that drives it. Without this gap, the asynchronous motor cannot produce its torque by induction.
The new BMW i3 offers a concrete example of this physical peculiarity. Its 40 xDrive and 50 xDrive versions each feature two electric motors, one on each axle. But BMW did not simply install two copies of the same machine: the rear motor is synchronous, while the front is asynchronous.
Both convert electrical energy into motion. The way they achieve this differs enough for the manufacturer to pair them within its sixth generation of electric propulsion, called Gen6.
At the rear, an electromagnet follows the rotating field
To understand this pairing, two elements must be distinguished. The stator is the fixed part of the motor. The rotor is the moving part, whose rotation is transmitted to the wheels via a gearbox. The electromagnetic interactions between these two assemblies allow the motor torque to be produced.
On the i3’s rear axle, BMW uses a synchronous motor with electrical excitation, abbreviated as EESM. The stator windings, fed with three-phase alternating current, create a rotating magnetic field. The rotor has its own windings, fed with direct current, which turn it into an electromagnet.
In synchronous operation, the rotor turns at the same speed as the stator’s rotating field. The term describes this relationship between two speeds inside the machine, and not a synchronization between the front and rear wheels.
Electrical excitation makes it possible to adjust the intensity of the rotor’s magnetic field. BMW explains that it can notably reduce this field when demand is low in order to limit certain losses, then tailor the excitation to higher demands.
This architecture differs from a motor whose rotor uses permanent magnets. It adds a control parameter: the excitation current. That current must, however, be supplied and it also causes electrical losses in the windings. The challenge is therefore to find the best setting for each situation, rather than always maximizing the magnetic field.
BMW reserves this machine for the rear axle, which it presents as the i3’s main drive axle.
In the front, the rotor must stay out of sync
The front motor uses another solution. It is an asynchronous machine, also called an induction motor, designated by the acronym ASM.
Its rotor features a conductive cage. In BMW’s description, it is made of aluminum bars linked by short-circuit rings. The current needed to create the rotor’s magnetic field is not supplied by a dedicated excitation: it is induced by the stator’s rotating field.
This induction requires relative motion. When the machine operates as a motor, its rotor turns slower than the rotating field. It is this slip, called slip, that allows currents to be induced in the cage and to produce torque. If the rotor reached exactly the speed of the field, this mechanism would no longer be able to sustain torque in the ideal model.
The term can evoke a loss of grip or a mechanical fault. It describes here a normal operating condition. The induction motor generates its effort through this speed difference; the synchronous motor works with a rotor that follows the rotating field.
This does not mean that the front wheels must turn more slowly than the rear wheels. Each motor has its own electrical control and its own reduction gear ratio. The slip is measured relative to the magnetic field of the machine concerned.
Two architectures to balance the trade-offs
Why combine these two technologies? BMW highlights the rear unit’s tunability and efficiency, while the front asynchronous motor offers a compact and economically attractive solution. This distribution is part of Gen6 design already presented for the Neue Klasse models.
In both i3 versions, the front motor is rated at 123 kW, or 167 hp. The rear motor develops 195 kW in the 40 xDrive and 240 kW in the 50 xDrive. The maximum power ratings for the complete assemblies reach, respectively, 275 kW (374 hp) and 345 kW (469 hp).
The benefit of the pairing, however, is not limited to maximum power. A car alternates starts, low-load cruising, accelerations, and energy recovery. The efficiency of an electric machine varies with its rotational speed and the torque demanded. The control of the entire system must account for these changes.
The inverter plays a central role here. It converts the battery’s direct current into alternating current suited for the motors and manages their supply. For its Gen6 synchronous machine, BMW says it has redesigned the rotor, the stator, the electronics, and the cooling. The inverter notably uses silicon carbide semiconductors and integrates into the 800 V architecture.
Final efficiency therefore depends on a set of choices. The available documents do not provide efficiency maps or a comparison isolating the benefit of the front asynchronous motor alone. They describe an architecture and its objectives, without proving that it would be superior to all others in every usage condition.
Des pertes réduites de 40 % ne donnent pas 40 % d’autonomie en plus
BMW accompanies this propulsion generation with three figures: 40% less energy losses, 20% lower costs, and a 10% drop in mass, compared with a previous-generation xDrive configuration. The automaker links these gains to improvements across the propulsion system, including the electric machines, the electronics, and the thermal management.
The first figure requires careful reading. A reduction in losses does not correspond to an identical reduction in all energy consumed.
Let us take a purely pedagogical example, which does not describe a BMW measurement. A system receives 100 units of energy, converts 90 into useful mechanical energy, and loses 10. If its losses decrease by 40%, they drop from 10 to 6 units. For the same input energy, it now provides 94 useful units: its efficiency rises from 90% to 94%, a four‑point gain.
At the car’s scale, other elements come into play: air resistance, rolling resistance of the tires, mass, temperature, and the energy use of equipment. The 912 km WLTP maximum range announced for the i3 50 xDrive, equipped with a usable 108.7 kWh battery, cannot therefore be attributed to the motor pairing alone.
The BMW choice illustrates more the precision of the optimization work. The rear rotor follows its rotating field; the front must maintain a lag with its own field when producing torque. Two different operating modes, coordinated by the electronics to deliver the requested effort while minimizing energy losses.
