The secrets behind F1’s MGU tech

Last month we looked at batteries and acknowledged there are many different types of devices which fit that generic term. This month we take a similar look at one of the other major components of a hybrid system: the motor or, as it’s known on a vehicle, the motor generator unit or MGU.

The motor is the device that converts electrical energy to mechanical energy. It’s a reversable process and so, if driven, a motor can also turn mechanical energy into electrical energy as it acts as a generator, hence the name MGU. Motors consist of many parts but the primary ones are the rotor in the centre, which turns and provides motion, and the stator which surrounds
 it and is non-moving.

We know batteries store electrical energy as direct current. The poles of a battery are always either positive or negative, so one might think that the logical type of motor to use with a battery is a direct current or DC motor. For years this was the case – and in fact for much of the 20th century the UK had the highest number of electric vehicles of any country in the world, due to the large number of electric urban delivery vehicles which delivered milk to households every morning. The majority of these used lead acid batteries connected together to give 60 volts and drive a DC motor.

The DC motor, while simple to control, required a lot of maintenance since the current was delivered to the rotor via sprung-loaded carbon brushes which 
needed frequent replacement.

All modern electric vehicles use alternating current (AC) motors. But even in this generic name lie many different configurations. The simplest form of AC motor is the induction motor. In an induction motor the rotor is an arrangement of conducting bars. The stator has windings of copper wire through which an alternating current passes, creating a magnetic force.

The current is switched at high frequency using solid-state switches known as IGBTs. This switching action causes the magnetic field to rotate and hence the induced current in the rotor produces a rotating force or torque. This type of motor is used on the front axle of the Tesla Model S and has the advantage of not needing the rare-earth magnets found in other motors, but its efficiency doesn’t match that of the more common type of motor.

Most vehicles, including Tesla on the rear of the Model S, and all round in most other vehicles, use a motor known as a permanent magnet synchronous motor (PMSM). This type of motor is also used in all motorsport applications, including the kinetic and heat MGUs on current Formula 1 hybrid power units.

This Renault illustration details how fundamental magnetic motors are to the hybrid power units

Photo by: Renault

These motors have better efficiency than induction motors but need to have magnets embedded in the rotor. The magnets are extremely powerful and are commonly made from neodymium which is known as a rare-earth material. This term is somewhat misleading since the various types of material used for rare-earth magnets are relatively abundant in the earth’s crust – but are mixed with other elements such as copper and zinc, which makes them challenging to mine and even more difficult to refine, meaning there are few sources of supply. By far the largest source is China which produces nearly four times as much as the USA which is the next largest producer.

The magnets need to be embedded near the surface of the rotor and this too produces a challenge since the rotor spins at a very high speed, 125,000rpm in the case of the MGU-H, such that the centrifugal forces are constantly trying to throw the magnets off. They are generally retained by a carbon fibre sleeve which encases the rotor.

The permanent magnet synchronous motors also require a more complex control and need to have the rotor cooled in order to keep the magnets at a temperature where they maintain their magnetic properties. However, the fact is that in this type of motor there is much less energy loss than in an induction motor. This may amount to a 2% difference in efficiency but is enough to make the PMSM almost universal in vehicles.

The inverter changes the direct current the battery stores to the alternating current that the motor requires. In the context of a vehicle it also needs to act as a speed controller for the motor

The control of the motor is essentially the control of the current in the coils of the stator to produce a rotating magnetic field. This interacts with the magnetic field of the permanent magnets on the rotor to produce torque. This is done by the inverter which, in an F1 hybrid system, is a silicon carbide switching device which is faster and more efficient than the older pure silicon devices.

The inverter changes the direct current the battery stores to the alternating current that the motor requires. In the context of a vehicle, it also needs to act as a speed controller for the motor and does this by altering the frequency of the alternating current output.

This changes the rotational speed of the magnetic field in the stator and hence its interaction speed with the magnets on the rotor. The rotor therefore spins at the speed or frequency of the current in the stator which is why this type of motor is called synchronous. The inverter can also change the amplitude of the alternating current, thereby controlling the torque of the motor.

Motors can also be radial flux or axial flux. Radial-flux motors are more compact in diameter, but axial-flux motors which are pancake shaped are gaining popularity in many vehicles since they’re ideal for either hub mounting or mounting between the engine and gearbox.

So, just as is the case with batteries, we’ve seen that the generic description of an electric motor covers a multitude of different designs and fundamental principles, each with its advantages and disadvantages. Ultimately, as with all things in Formula 1, the most efficient wins through.

Efficiency is crucial in determining the potency of an MGU in F1

Photo by: Zak Mauger / Motorsport Images