How does an electric car motor work?

Published by on 10.11.2019 - 3 min

Forget cylinders, pistons and exhaust fumes: an electric car’s motor is built around a set of parts designed to convert electricity into mechanical energy through the creation of a magnetic field.

An electric motor functions via a physical process developed at the end of the 19th century. This consists of using a current to create, at the fixed part of the machine (the stator), a magnetic field whose displacement sets a rotating part (the rotor) in motion.

These days electric motors can be found in numerous everyday objects. Most of them feature direct current motors with quite basic functioning. The motor is connected directly to its power source (disposable or rechargeable battery) and its rotation speed depends directly on the intensity of the current used at the input. While easy to produce, these motors don’t meet the power, reliability or size requirements of an electric car.

For this we must turn to alternate current motors, which requires the use of a conversion circuit to transform the direct current supplied by the battery.

Synchronous or asynchronous motors

There exist two types of alternate current motors in the automobile industry. An asynchronous motor relies on an electric-powered stator to generate a rotating magnetic field. This then pulls the rotor into an endless chase, as if it were trying to catch up with the magnetic field without ever succeeding. By functioning in such a way, there is a discrepancy between the rotor’s speed and that of the magnetic field. This phenomenon, called a motor slip, has an impact on the motor’s performance.

In a synchronous motor, the rotor acts as an electromagnet, actively participating in the creation of the magnetic field. Its rotation speed is thus directly proportional to the frequency of the current that powers the motor, without any losses related to motor slip.

Whether synchronous or asynchronous, an electric machine works in a reverse manner, meaning it can convert mechanical energy into electricity during deceleration: this is the principle of regenerative braking, which derives from the alternator.

Wound rotor or permanent magnet

SCHEMA CAISSE, MOTEUR, TRAIN AVANT ET ARRIERE ZOE

Some synchronous motors use a permanent magnet for the rotor. This has the advantage of operating without a power supply but requires the use of metals or alloys such as neodymium or dysprosium. The problem is that their prices are very volatile due to China’s quasi monopoly on these renowned “rare earths”. An alternative solution, used by Renault for ZOE, involves building an electromagnet from a copper coil. This necessitates a more complex industrial process but makes it possible to avoid supply problems, all while maintaining an excellent ratio between motor weight and delivered torque.

From motor to electric powertrain

In a car, the electric machine comprised of the rotor and the stator is part of a larger unit, the electric powertrain. Also within this unit, the Power Electronic Controller (PEC) brings together all the power electronics responsible for controlling the motor’s power supply and the charging of the battery. Lastly, it includes the gear motor, the part responsible for adjusting the torque and the speed of rotation transmitted by the motor to the wheels. Together, these three elements make the electric car silent, reliable, less expensive and fun to drive!

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