Understanding EV motors

Introduction

Electric vehicles (EVs) are currently on the tip of everybody’s tongue within and without the auto industry. Almost every big car manufacturer has already introduced its own vision about them or has started selling these revolutionary cars, following the footsteps of Tesla’s success. And I think we can safely say now that EVs are the future of passenger - and even utility - vehicles, considering their lack of emissions  compared to conventional internal combustion (IC) engine cars, plus significant fuel cost savings. That’s why understanding their operating principles has now become a necessity.

In this article we’re going to understand how motion is produced by an electric motor, how maintenance is performed on EVs, and how their lifecycle differs from conventional IC cars.

From electricity to motion

The only new aspect about an electric car is that you won’t find the internal combustion engine, the clutch assembly, the tailpipe nor the fuel tank. You won’t even find the manual transmission if it’s not a converted car. Instead you will find an electric motor, a controller and rechargeable batteries: these components are the ones that will create motion.

Batteries

Unlike in IC engine cars, the battery represents an electric car’s heart. In fact, it is the organ that will “pump” the power to the electric motor through the controller, allowing the vehicle to work. Like every other battery, it is composed of two electrodes, a negative (overabundance of electrons) and a positive one (deficit of electrons), that generate electricity once connected to each other, allowing the flow of electrons from the negative (anode)  to the positive (cathode) electrode. This overabundance or deficit of electrons is caused by chemical reactions that differ from some types of battery to others. The most famous types used in the automotive industry are lithium-ion, lead-acid and nickel metal hydride batteries.

Lithium-ion batteries

These rechargeable batteries came into commercial use in 1991. Their basic operating principle consists of forcing the circulation of electrons by creating a difference of potential (what actually causes the electrons to move) between the two electrodes, which are immersed in an ion-conducting liquid called electrolyte. When the battery powers a device, the electrons accumulated in the anode are released through an external circuit to reach the cathode: this phase is called the discharge phase. Conversely, when the battery is charging, the energy transmitted by the charger causes the electrons to return from the positive electrode to the negative one. In an electric car, the battery is made up of 48 modules, every module is then made up of 4 cells where the circulation of electrons takes place. Each cell produces approximately 3.7 volts.

The main reason for its huge success lies mainly in the storage density (the ratio between the storage capacity offered by the battery and its weight) provided by lithium-ion technology: it’s normally in the range of 300-500 Wh/kg, about ten times that of a lead-acid battery. Lithium-ion batteries today represent the best compromise between capacity, volume and weight in the electric mobility sector. It offers high power, ease of recharging and good durability. Incidentally, they’re also the type of batteries most commonly used in smartphones.

Lead-acid batteries

Lead-acid batteries were the first commercialised rechargeable batteries, invented in 1859 by the French physicist Gaston Plante. This type of battery is widely used in IC engine cars, in order to provide the functions of starting, igniting and appliances. Each cell of the battery produces 2.1 volts and each module contains 6 cells. Lead-acid batteries have a density in the range of 30-40 Wh/kg, so low that they would add a tremendous amount of weight if they were to be used for EVs (10x as much!).

They are less durable than the other two types, especially when deep-cycled (when batteries are discharged almost completely before recharging). They only provide around 200-300 charge/discharge cycles before they need to be changed, compared to 300-500 charge/discharge cycles for lithium-ion batteries.

Nickel-metal hydride batteries

These batteries came into commercial use in the late 1980s. They are mostly only used in hybrid cars because of their low density compared to lithium-ion batteries (80 Wh/kg): if you use them for your future or current EV then you should charge your car every 20 km, which is definitely not practical for most people’s daily commute. The reason research is not being conducted to improve their efficiency is that they contain a high amount of Nickel, and its price is rising.

Controller

The controller is the car’s brain - it manages all of its parameters thanks to signals received from different sensors in the vehicle. It controls the rate of charge using information from the battery, and it adjusts the speed in the motor’s inverter when you press the potentiometer (acceleration pedal). It also manages energy recovery (regenerative braking), sending the electricity generated by the motor through the use of dynamos to the battery.

Inverter

The inverter’s role is to take the power from the batteries, convert it from DC (Direct Current) to AC (Alternating Current), regulate the energy flow and deliver it to the motor. It has a direct impact on road performance, driving range and reliability, and it controls the rotational speed of the wheels: it’s what keeps the motor from delivering full power instantaneously, a situation that is undesirable since it would cause a lot of stress to the mechanical components of the car.

Motor

The AC motor converts the electrical energy received from the inverter into mechanical energy that will cause the rotational movement of the wheels, using electromagnetic induction. Its main parts are:

The stator: a stationary ring of 3 pairs of coils arranged around the outside that - when powered by the alternative current from the controller - becomes an electromagnet.

The rotor: composed of a central shaft and either central permanent magnets (which are light but expensive) or a central coil (which is heavy but cheap). The central coil is powered by a direct current and consequently becomes an electromagnet.

The coils of the stator are powered one after another, thus producing a rotating magnetic field. According to Faraday’s law, this field induces an electric current inside the rotor, this new current in the rotor produces its own magnetic field which, according to Lenz’s law, tries to oppose what caused it by rotating, causing the rotation of the shaft. Finally, the shaft’s movement is what drives the wheels.

Transmission

The electric motor has a very high operating range (up to 16000 rpm) and a high torque for lower speeds, so the gearbox’s main functionality becomes only to switch from “drive” to “reverse” and there is no longer need for reduction gears.

New Technologies

Another reason why electric cars are a very good investment is that thanks to the high visibility that they have gotten as of late, new technologies are constantly being researched and implemented into EVs. For instance, there is a fairly new technology called Regenerative Braking. New IC engine cars’ braking systems are composed of  metal discs fixed on the wheels and a pedal brake that, when pressed, generates high pressure of hydraulic fluid that squeezes the brake pads into the discs, creating friction that slows the car down. Because of friction, heat is generated in the brake disc, and that can cause many problems if not properly cooled.

The idea behind regenerative braking inside electric cars is that in addition to the hydraulic braking system, a new system is implemented: it takes this kinetic energy and converts it into electricity that will be stocked in the battery, positively impacting the EV’s battery life. The energy conversion is performed by the motor: this means that when driving normally, the motor functions like we have seen in the last section, but when you press the brake pedal or you stop accelerating, it stops supplying power to the vehicle, captures the kinetic energy coming from the brakes and starts turning in reverse. Since the transmission is still on “drive”, the wheels won’t start turning backwards and the motor will convert the energy into electricity that will be stocked in the battery.

The good news about regenerative braking doesn't stop there: not only it will generate power for your vehicle enhancing battery life, but it will also prolong your brakes’ life, by saving them from working too hard.

Day-to-day maintenance

Maintenance of EVs is considerably easier and less expensive than that of internal-combustion cars. In fact, it is estimated that an EV owner can save up to 30% of maintenance costs compared to a car with the same category and same mileage of a conventional IC vehicle. There are fewer mechanical pieces to change (less failure points), you won’t have to worry about checking the oil level, the engine’s coolant level or changing the spark plugs. You will only have to change the batteries because of their loss of performance after a certain number of charging/discharging cycles, which is equivalent to 7 to 10 years of life. Even the brake pads will have to be changed less often thanks to the regenerative braking.

Conclusion

One of the European Commission’s main goals to reduce toxic emissions is to halve the use of conventional IC cars in the urban transport sector by 2030 and to phase them out completely by 2050. For now only electric cars are performant enough to substitute them, thanks to their low maintenance expenses and their contribution in saving the environment. So I advise to think twice before buying a new car, and check my next article that compares conventional IC cars to EVs in terms of costs, efficiency and emissions.