#How electric cars work?

26 / July 2021
How electric cars work?

Seletron Performance

How electric cars work.

In recent years, slowly at first, then more and more evidently, electric cars have increasingly appeared on the automotive scene. Some of them are not too interesting, but others have noteworthy features like Tesla and the acceleration values of some of its models, for example. These cars with electric traction and accelerations on par with a sports car can even generate accelerations so fast that they bother drivers not used to the kick of even a traditional sports car.

Let's get to the purpose of this article, which is to shed some light on how electric motors work and offer some technical considerations without going too much into the specifics. We want to create an informative article that is not too vague but which can be understood by most 4-wheel enthusiasts.

A short premise: let's debunk the myth of poor performance.

Many people tend to assume that electric cars have poor performance compared to powerful gasoline engines, like those that power sports cars and supercars. We would like to remind these people of the power of trains, especially the latest models. Very fast trains with a weight of over 450 tons are driven at 300Km/h by electric motors with over 13,000 horsepower!

The power limits of electric motors.

In principle, simplifying the issue and not considering very technical aspects related to friction, dielectric strength, torsional resistance, etc., we can trivialize the discussion by saying that the main power limit of an electric motor is caused by the possibility of thermal dissipation.

This is to say that an electric motor small enough to fit under the hood of a standard sedan or SUV could potentially have incredible performance when supplied by a potent power source. Feeding it with high voltage (V) and current (A), you could get something that could out-accelerate the highest performing internal combustion engine-powered Dragster.

Obviously, we are omitting aspects related to traction and other factors; the comparison serves to highlight that the main limitation of the power of an electric motor comes from the need to dissipate a lot of heat generated by the Joule effect of the conductors and elements that form it. For short periods of time, the ability of an electric motor to deliver torque and power is truly remarkable. Remember that the electric motor can already deliver its maximum torque at a speed of 0 rpm.

Yes, it is true; its maximum torque is already available from standstill. The electric motor does not need a minimum rotation speed to operate. It has no fluid-dynamic cycles with single active phases (combustion) nor moving pistons. It is a reversible rotating machine that can start from standstill already with its maximum thrust!

The second real limitation of electric cars.

The second factor that limits the performance (and even more the autonomy) of electric cars is the reserve of electricity, i.e., the storage battery that provides power to the electric motor to convert electricity into traction. In fact, the trains mentioned earlier do not run on batteries but through connection to a medium voltage power line and relatively high current. Therefore, there is no problem accumulating electrical energy because the generation is delocalized with respect to the point of use (position of the train) and is potentially almost limitless. For cars that must be able to circulate freely on the road, the energy is stored in special rechargeable batteries, which, as already mentioned, is a decisive aspect in the absolute performance and even more for the autonomy of the car, i.e., the ability to travel several kilometers before the batteries need to be recharged.

How an electric engine works.

The electric motor has a different architecture depending on the type of power supply; those powered by alternating voltage are generally those that require less maintenance and are relatively simpler than those running on direct voltage. Let's analyze a small basic electric motor mounted on remote-controlled model sports cars. Normally we have the body of the motor with cylindrical sections within which ceramic or neodymium magnets are fixed. These magnets are used to generate the fixed magnetic field within which the most important element of the electric motor rotates, the rotor.

This component includes the various copper wire coils and the manifold (if present, we will come back to this point), which serves to create the various commutations during rotation, providing voltage (therefore current) to the various electromagnetic coils that give rise to the torque (driving torque). If we imagine an idle engine and supply it with an electric voltage that will travel to the copper wire coils through the "brushes" (conductive elements running along the manifold), this will generate a magnetic force in reaction to the fixed magnetic field imposed by the permanent magnets and relative thrust that will cause rotation.

This type of engine's main problem is the wear and tear of the brushes and of the collector. The wear and tear is caused by mechanical factors (friction due to rubbing) and by electrical factors (micro arcs formed due to the extra inverse voltages for continuous switching of inductive loads). Another undesirable aspect is the thermal dissipation; the current that circulates inside the copper conductors causes a power dissipation caused by the product of the value of the electrical resistance of the cables themselves for the square of the value of the current flowing through them.

Even the small electric motors used in dynamic modeling have problems related to the temperature of the motor itself. A few minutes of play provided by the accumulators' battery capacity already brings the motor to high temperatures. If you continue to use it ( if you have other charged batteries available), you could easily damage the electric motor.

Brushless electric engines.

More efficient electric motors (used on electric road cars) do without the commutation system formed by brushes and a rotating collector. Some wear and reliability problems are therefore eliminated; the downside is the need to use sophisticated electronic control systems for the motor. The brushless electric motor normally has an inverted architecture compared to brush motors. The rotor is made of simple permanent magnets, while various copper coils (at least 3 on traction motors) are fixed on the stator, which is driven by electronic devices called inverters through not 2 but 3 wires (or more).

Electic motor inverters.

Inverters are electronic control units that transform the DC voltage coming from the batteries into a voltage that varies in amplitude (actually in time, via PWM) and with a rotating phase that varies with the speed of the motor. To do this, there are digital microcontrollers that generate sine waves similar to those found in the normal three-phase 400V power line. The difference is that inverters can fine-tune this rotation phase, determining precisely at what speed the brushless synchronous electric motor must rotate.

Another difference with respect to the line provided (in Italy) by ENEL is that the same source of generation of electrical voltage (alternators) creates a sinusoidal trend of the voltage value (fixed as a frequency), while the inverters simulate the sinusoidal trend by turning on and off the passage of the current by varying the opening and closing times of solid state switches (PWM system). Solid state systems are created by semiconductors (transistors, Mosfet, HexFET, IGBT) able to switch high voltages and currents quickly and without wear to be provided to the copper coils of the motor.

Torque and speed modulation on electric cars.

When you dictate the power of an electric car through the accelerator pedal, you communicate the position of the pedal itself to the motor control unit (inverter). The control unit interprets the power request of the driver and starts to generate a three-phase rotating voltage starting from a frequency close to zero Hertz, progressively rising (together with the rotation of the motor, since it is a synchronous type), while adjusting the value of the duty-cycle supplied to the PWM circuit that will control the solid-state terminals (those that perform the heavy work of voltage and current switching).

Very small values of motor torque will result from very low duty-cycle values (e.g., a duty-cycle of 5% means that the electric motor coils will be powered on for 5% of the cycle time and off for 95% of the cycle time, while a duty-cycle of 75% means that the electric motor coils will be powered on for 75% of the cycle time and off for 25%). The working frequency of the inverter will follow the rotation frequency of the motor; the percentage of load given by the duty cycle will instead be set by the driver of the electric car according to the pressure on the accelerator pedal. A duty cycle of 100% corresponds to the maximum torque output of the electric motor.

To summarize: what limits the performance of an electric car the most? What limits the range of an electric car?

As we have seen, pure performance (although itself limited by the current capacity of the batteries) is limited by the engine’s cooling abilities through thermal dissipation, while the range depends exclusively on the ability of the storage batteries to store electrical energy. The main challenge for the future is related to the possibility of creating batteries with higher and higher capacities and to the possibility of recharging them easily and quickly.

For the sake of completeness, it should be remembered that the potential danger (or better still, the potential damage) is greater the more energy is stored in the vehicle, whether of chemical origin (petrol, diesel, other fuel) or stored in the form of electrical charge, as is the case with electric cars. It is, in fact, intuitive that a vehicle that stores 300 liters of fuel is potentially more dangerous than a vehicle that stores several dozen liters of fuel in case of problems arising from failures/accidents. The same applies to electric accumulators capable of providing little or a lot of electrical energy. Also, in this aspect, technological development is directed towards increasingly safer management of this energy.

Electric car tuning.

So far, we have talked about how an electric car works, how an electric motor is made, how it is controlled by special electronics. We have also talked about the performance of electric cars and their limits in terms of autonomy and thermal dissipation. But what is changing in the world of car tuning? How will it be possible to intervene on these types of engines, and what will change on the racing scene?

To begin to answer these questions, we invite you to read our previous article in which SELETRON is one of the protagonists of these changes.... read more here >>>> E-STC Series, Super Turismo Championship for electric cars

Electric Cars.

Compare