Electric regeneration operation during braking and deceleration

Introduced a few years ago on conventional diesel locomotives, regenerative braking is now becoming increasingly important as hybrid and electric vehicles become more democratic.

So let's take a look at the fundamental aspects of this technique, which therefore is about getting electricity from motion (or rather kinetic energy / inertial force).

The basic principle

Whether it is a thermal imager, a hybrid or an electric vehicle, energy recovery is now everywhere.

In the case of thermal imaging machines, the goal is to unload the engine by switching off the alternator as often as possible, the role of which is to recharge the lead-acid battery. Thus, freeing the engine from the alternator limitation means fuel savings and power generation will be generated as much as possible when the vehicle is on the engine brake, when kinetic energy can be used rather than engine power (when slowing down or going down a long slope without acceleration).

For hybrid and electric vehicles, it will be the same, but this time the goal will be to recharge the lithium battery, which is calibrated at a much larger size.

Using kinetic energy by generating current?

The principle is widely known and democratized, but I must get back to it quickly. When I cross a coil of conductive material (copper is best) with a magnet, it generates a current in this famous coil. This is what we're going to do here, use the movement of the wheels of a running car to animate the magnet and therefore generate electricity that will be recovered in the batteries (i.e. the battery). But if it sounds elementary, you will see that there are a few more subtleties to be aware of.

Regeneration during braking / deceleration of hybrid and electric vehicles

These cars are equipped with electric motors to propel them, so it is wise to use the latter's reversibility, namely that the engine tows if it receives juice and that it delivers energy if it is driven mechanically by an external force (here a car started with spinning wheels).

So now let's look a little more specifically (but remain schematic) what this gives, with a few situations.

1) Motor mode

Let's start with the classic use of an electric motor, so we circulate the current in a coil located next to the magnet. This circulation of current in the electrical wire will induce an electromagnetic field around the coil, which then acts on the magnet (and therefore makes it move). By cleverly designing this thing (wrapped in a coil with a rotating magnet inside), it is possible to obtain an electric motor that rotates the axle as long as current is applied to it.

It is the "power controller" / "power electronics" that is responsible for routing and controlling the flow of electricity (it chooses the transfer to the battery, the motor at a certain voltage, etc.), so it is critical. role, since it is it that allows the engine to be in the "engine" or "generator" mode.

Here I have developed a synthetic and simplified circuit of this device with a single-phase motor to make it easier to understand (a three-phase works on the same principle, but three coils can complicate things in vain, and visually it is therefore easier in a single-phase).

The battery runs on direct current, but the electric motor does not, so an inverter and a rectifier are needed. Power electric is a device for distributing and dosing current.

2) Generator / energy recovery mode

Therefore, in generator mode, we will do the opposite process, that is, send the current coming from the coil to the battery.

But back to the specific case, my car accelerated to 100 km / h thanks to a heat engine (oil consumption) or an electric engine (battery consumption). So, I have acquired kinetic energy associated with this 100 km / h, and I want to convert this energy into electricity ...

So for that I will stop sending current from the battery to the electric motor, the logic I want to slow down (hence the opposite will make me speed up). Instead, the power electronics will reverse the direction of energy flows, i.e., direct all the electricity produced by the engine to the batteries.

Indeed, the simple fact that the wheels make the magnet spin causes electricity to be generated in the coil. And this electricity induced in the coil will again generate a magnetic field, which will then slow down the magnet and no longer accelerate it like when it is done by applying electricity to the coil (therefore thanks to the battery) ...

It is this braking that is associated with energy recovery and therefore allows the vehicle to be decelerated while recovering electricity. But there are some problems.

If I want to recuperate energy while continuing to move at a stabilized speed (i.e. hybrid), I will use a heat engine to propel the car and an electric motor as a generator (thanks to the movements of the engine).

And if I don't want the motor to have too many brakes (due to the generator), I send the current to the generator / motor).

When you brake, the computer distributes the force between the regenerative brake and conventional disc brakes, this is called "combined braking". Difficulty and therefore elimination of the sudden and other phenomenon that may interfere with driving (when done poorly, the braking feel can be improved).

A problem with the battery and its capacity.

The first problem is that the battery cannot absorb all the energy transferred to it, it has a charge limit that prevents too much juice from being injected at the same time. And with a full battery, the problem is the same, it doesn't eat anything!

Unfortunately, when the battery absorbs electricity, electrical resistance occurs, and this is when braking is most severe. Thus, the more we “pump” the generated electricity (and, therefore, by increasing the electrical resistance), the stronger the engine braking will be. Conversely, the more you feel the engine braking, the more it will mean that your batteries are charging (or rather, the engine is generating a lot of current).

But, as I just said, batteries have an absorption limit, and therefore it is undesirable to do sudden and prolonged braking to recharge the battery. The latter will not be able to appropriate it, and the surplus will be thrown into the trash ...

The problem is related to the progressiveness of regenerative braking

Some would like to use regenerative braking as their primary and therefore definitely dispense with disc brakes, which are energetically poor. But, unfortunately, the very principle of operation of the electric motor prevents access to this function.

Indeed, the braking is stronger when there is a difference in speed between the rotor and the stator. Thus, the more you decelerate, the less powerful the braking will be. Basically, you cannot immobilize the car through this process, you must have additional normal brakes to help stop the car.

With two coupled axles (here E-Tense / HYbrid4 PSA hybridization), each with an electric motor, the energy recovery during braking can be doubled. Of course, this will also depend on the bottleneck on the side of the battery ... If the latter doesn't have a lot of appetite, it doesn't make much sense to have two generators. We can also mention the Q7 e-Tron, whose four wheels are connected to an electric motor thanks to the Quattro, but in this case only one electric motor is installed on the four wheels, not two as in the diagram (so we only have one generator)

3) The battery is saturated or the circuit is overheated

As we said, when the battery is fully charged, or it draws too much power in too short a time (the battery cannot charge at too high a speed), we have two solutions to avoid damaging the device:

• The first solution is simple, I cut out everything ... Using a switch (controlled by the power electronics), I cut the electrical circuit, thereby making it open (I repeat the exact term). This way the current no longer flows and I no longer have electricity in the coils and therefore I no longer have magnetic fields. As a result, regenerative braking no longer works and the vehicle coasts. As if I no longer have a generator, and therefore I no longer have electromagnetic friction that slows down my moving masses.
• The second solution is to direct the current with which we no longer know what to do to the resistors. These resistors are designed for this, and to be honest, they are quite simple ... Their role is really to absorb current and dissipate this energy as heat, thanks therefore to the Joule effect. This device is used on trucks as auxiliary brakes in addition to conventional discs / calipers. Therefore, instead of charging the battery, we send current into a kind of "electric trash cans" that dissipate the latter in the form of heat. Note that this is better than disc braking because at the same braking rate the rheostat brake heats up less (a name given to electromagnetic braking, which dissipates its energy in resistors).

Here we cut the circuit and everything loses its electromagnetic properties (it's as if I were twisting a piece of wood in a plastic coil, the effect is gone)

Here we use a rheostat brake which

4) modulation of regenerative braking force

Fittingly, electric vehicles now have paddles to adjust the force of the return. But how can you make regenerative braking more or less powerful? And how to make it so that it is not too powerful, so that the driving is bearable?

Well, if in regenerative mode 0 (no regenerative braking) I just need to disconnect the circuit in order to modulate the regenerative braking, another solution will need to be found.

And among them, we can then return some of the current to the coil. Because if the production of juice by rotating the magnet in the coil causes resistance, I would have much less (resistance) if, on the other hand, I injected the juice into the coil myself. The more I inject, the less brakes I will have, and even worse, if I inject too much, I end up accelerating (and there, the engine becomes the engine, not the generator).

Therefore, it is the fraction of the current re-injected into the coil that will make the regenerative braking more or less powerful.

To return to freewheel mode, we can even find another solution besides disconnecting the circuit, namely, send current (exactly what is needed) in order to have the feeling that we are in freewheeling mode ... A bit like when we stay in the middle of the pedal on the thermal for parking at a steady pace.

Here we are sending some electricity into the winding to reduce the "engine brake" of the electric motor (it is not actually an engine brake, if we want to be precise). We can even get a freewheel effect if we send enough electricity to stabilize the speed.

All comments and reactions

Dernier comment posted:

Reggan (Date: 2021, 07:15:01)

Hi,

A few days ago, I had a meeting at a Kia dealership about the scheduled maintenance of my 48000 Soul EV 2020 km. Ã ?? my big surprise, I was advised to replace all front brakes (discs and pads) because they were finished !!

I told the service manager that this was not possible because I made the most of the recuperative brakes from the beginning. His answer: an electric car's brakes wear out even faster than a regular car !!

This is really funny. Reading your explanation of how regenerative brakes work, I received confirmation that the car is slowing down using a process other than standard brakes.

Il I. 1 reaction (s) to this comment:

• Administrator SITE ADMINISTRATOR (2021-07-15 08:09:43): Being a dealer and saying that an electric car wears out brakes faster is still the limit.

  Because if the excessive severity of this type of vehicle should logically lead to faster wear, regeneration reverses the trend.

  Now, perhaps recovery level 3 uses the brakes in parallel to artificially increase the engine brake (thus using the magnetic force of the engine and brakes). In this case, you can understand why the brakes wear out faster. And with frequent use of regeneration, this will cause long pads on discs with unpleasant heat from wear and tear (when we learn to drive, we are told that the pressure on the brakes must be strong, but short to limit heating).

  It would be nice if you saw the wear and tear of these elements with your own eyes to see if the dealership is tempted to make illegal numbers (unlikely, but it's true that “here we can doubt it”).

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