The energy efficiency of an electric car motor

Published by on 03.09.2021 - 4 min

The purpose of a car motor is to convert fuel or electricity into movement energy. To measure the energy inherently lost as part of this conversion, we talk about “efficiency”. This measure is therefore of paramount importance in determining the energy consumption and range of an electric vehicle. Let’s take a closer look.

What is the efficiency of an electric motor?

The notion of efficiency, when it comes to a motor, refers to the ratio between useful energy and the total energy consumed, and can be expressed as a percentage. For an electric vehicle, energy efficiency is estimated at 90%, meaning that 10% of the electricity consumed by the electric motor has not been used in the propulsion of the vehicle — not a very large amount. In addition to this high level of efficiency, it should be noted that Renault electric and hybrid vehicles also benefit from regenerative braking which recharges the battery during certain driving phases, moving their efficiency even closer to 100%.

Using the efficiency percentage, we are able to measure the range of an electric vehicle and its electricity consumption more precisely. The higher the efficiency, the fewer “injected” kilowatts of energy are needed for a charge in order to cover the desired number of kilometers.

How to calculate the efficiency of an electric motor  

Mathematically, the efficiency of an electric motor is calculated by dividing the amount of useful energy by the amount of energy initially consumed.

In practice, automobile manufacturers and firms specializing in mechanical performance estimate the efficiency of vehicles (electric and hybrid) by taking several factors into consideration, so as to obtain as accurate a figure as possible. The measurement process first requires testing the charging conditions and battery efficiency to find out how much energy is being injected. The electric car, assessed on its average speed and energy recovery, is then driven in everyday situations and along typical routes, to calculate the number of kilometers traveled on a single charge.

The ratio between the two values obtained shows why, in the automobile industry, specialists believe that the efficiency of an electric motor is three to four times higher than that of a gasoline or diesel combustion engine of equal power.

motor exhibition

What’s the difference between synchronous and asynchronous motors?

To propel an electric or hybrid (in electric mode) vehicle and to provide electric power to the motor, current delivered by the battery is transformed into an electromagnetic field which turns the rotor, generating traction.

The way an electric motor works differs depending on the type of technology used to produce this electromagnetic field, which is why we talk about “synchronous” and “asynchronous” to describe the most frequently used types of motor in the industry.

The synchronous electric motor works with a static part, the stator, and a rotating part, the rotor. Electricity from the battery passes through the stator where it is transformed into an electromagnetic field. The rotor — which contains a copper coil or permanent magnet — follows this magnetic field and begins to spin on itself at a speed proportional to the frequency of the electric current. This technology is the most common in the automobile industry.

In an asynchronous motor, the rotor and stator do not function proportionally. The stator “pulls” the rotor into rotation, but with a slight lag: the speed of the rotating magnetic field is always greater than that of the rotor.

Synchronous electric motors: different technology

Used particularly in the automobile industry, synchronous electric motors can use two different technologies: magnet rotors and wound rotors.

The former incorporates so-called “permanent” magnets near the rotor. Coils within the stator activate its sensitivity to magnetic force, generating rotation and consequently the vehicle’s traction. This technology has the advantage of a certain level of power density and excellent energy efficiency at low speeds. This is why it is used in light city vehicles such as Dacia Spring or as an electric motor in Renault’s E-Tech and E-Tech Plug-in hybrid motors.

In a vehicle with a wound rotor, a copper coil replaces the permanent magnet. This machinery also renders very high energy efficiency up to high speeds of over 100 km/hour, offering an increase in autonomy of several dozen kilometers. This technology can be found in the motors of Renault ZOE and Twingo Electric.

Motor charging

The efficiency of an asynchronous electric motor

The physical properties of the asynchronous motor are why it has slightly lower energy efficiency than its synchronous counterpart. “Slip” describes the gap in speed between the rotor and the stator, explaining the difference between the two motors. “Asynchronous” motors therefore offer energy efficiency of between 75-80%, compared to 90% for a “synchronous” motor.

The improvement of motor energy efficiency

Manufacturers and industrial groups are working from multiple angles to improve the efficiency of motors for electric cars. Part sizes, the use of quality materials and further increases in air flow all help reduce the energy losses caused by friction between parts and the transformation of some energy into heat.

Behind the wheel, applying eco-driving behavior to take advantage of energy recovery helps reduce unnecessary kilowatt consumption, bringing the motor closer to its maximum energy efficiency level.

ZOE 2017 technical cutting

Maximum efficiency 

The maximum efficiency given for an electric or hybrid motor refers to the ideal scenario of using energy from the batteries to achieve optimal motor speed. The actual efficiency of a motor is always lower than this value, and depends on the conditions (weather, traffic, driving style) in which the car is operating.

The efficiency of a hybrid motor

In a hybrid motor, the presence of an electrified section tends to increase the car’s maximum efficiency, whether it is a plug-in hybrid or not. This is partly due to the ability to recharge the batteries during deceleration, therefore recovering immediately “useful” energy.

The efficiency of a hydrogen motor

A hydrogen-powered car is propelled by a synchronous electric motor with an energy efficiency comparable to that of the traditional electric vehicle battery. The production of hydrogen by the fuel cell generally decreases the vehicle’s efficiency by a small amount, with some of the energy converted into water vapor.

Knowing the efficiency of an electric motor enables you to understand energy expenditure and vehicle range data.


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