Is it possible to restore some of the energy that was given to a electric motor? Regenerative braking is a technique to recover partially some current back to a electric grid or a battery, while it slows down the motor. In this post, let's take a look briefly on what is this technique about.
My undergraduate thesis in the university was actually about this topic. I will leave a link to it in the Resources heading.
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What is Dynamic Braking?
Before starting, let's start by explaining that most electrical machines are characterized by being reversible. They consume electrical energy when they behave as motors or deliver it when they behave as generators. Motors are mainly composed of a stationary part (stator) and the rotating part (rotor). When the power flow is from stator to rotor, then it will work as a motor. However, when the power flow is from the rotor to the stator then it will work as a generator.
So, what is dynamic braking?
An electric motor can be stopped if it is disconnected from its power source. On the other hand, it can be stopped more quickly if the motor terminals are reconnected to a circuit to discharge its energy. However, as the motor slows down, generation decreases, as does braking effectiveness. This means that the motor cannot be safely braked by this method alone. This technique is known as dynamic braking, there being 2 types: rheostatic braking and regenerative braking. [2]
What is Rheostatic Braking?
In rheostatic braking, the electrical energy generated in the motor is dissipated as heat. The motor stator is disconnected from the power supply and connected to a resistor circuit. A part of the mechanical energy of the rotation will be converted into electrical energy, which will be discharged with these resistors. The result will be that the motor will stop sooner than if that energy were not dissipated. [2]
What is Regenerative Braking?
The operation of this system consists of returning energy to the battery when it is desired to decelerate the vehicle while it is in motion. A vehicle using a regenerative braking system based on an electric motor will be able to generate additional electrical energy from the initial charge to extend its distance traveled. In addition, it will consume less electricity when recharging the vehicle and increase the life of the friction brakes. This brake works similarly to the braking technique based on the natural compression of a combustion engine. [2]
In regenerative braking, unlike rheostatic braking, the electric motor is reconnected during braking as a generator. The power supply terminals become energy suppliers, which conduct electricity to an energy store that can be a battery or capacitors of a vehicle or back to the mains or grid. This electrical energy is stored for future use.
Traditionally, friction-based braking is still used in conjunction with regenerative braking for the following reasons: The effectiveness of regenerative braking is lower at low speed and the amount of energy to be recovered is limited to the ability of the power system to absorb it or the state of charge of the batteries or capacitors. For this reason it is necessary to have a rheostatic or friction brake to absorb the excess energy. [2]
Principles of Regenerative Braking
Some principles involved in braking and electricity generation in an electric motor are:
- The magnetic field can be generated by permanent magnets or by electromagnets. In the latter case the magnetic field can be variable.
- According to Faraday's law, when the winding wires move and cut magnetic field lines, an electromotive force is produced in the conductor.
- According to Lenz's law, an induced current in the conductor will create a magnetic field that will oppose the magnetic field that produces it. This is the force that stops the electric motor.
Requirements for Regenerative Braking?
The feature in the electric motor shouldn't just ‘exist'. There has to be a mechanism or circuit that takes the energy from the regenerative braking in the terminales of the motor to then transform it to the nominal battery voltage. Although, there are cases where no extrahardware is needed but it depends on the controller. Definitely, there is not one way to do so.
In my thesis, I opted to design a circuit capable of doing this. It was built with relays, that disconnected the motor from the controller and then connect it to a DC/DC converter and then to the battery. In many cases, no extra hardware is needed but since I didn't wanted to touch or compromise the controllers, I built a different circuit.
How much energy can be restored?
The quantity of energy available to directly proportional to the size or power rating of the motor. Instead of focusing on such parameter, we should focus instead in the efficiency of the system. As I found out in my thesis, there was a efficiency increase as the regenerative braking was more intense. The more braking happens, the more efficient it was per meter travelled. In a test, with higher differential energy potential available, the efficient rose up to 13%. On the other hand, when braking in a plain with lower differential energy potencial, the efficiency was lower, up to 7%,
Research on this topic indicates that energy savings are indeed achievable with a recovery of 8% to 25% of the vehicle's total energy use, depending on the state of charge and its control strategy. This additional energy recycling can be achieved without adding additional components. [3]
Due to the importance of this technology, many researchers have conducted such research. The safety and driving comfort of vehicles are strongly related to braking performance, due to various phenomena associated with pedal and brake operations. In regenerative braking systems (RBS), the objectives of safety, reliability and easy application are the main factors to be considered. [3].
Is Regenerative Braking available in engines from fossil fuels?
Electric vehicles have a lower energy storage capacity than fuel tanks. Currently, energy density is low and energy consumption must be optimized. Regenerative braking is a solution to extend the autonomy of a battery because braking could generate a torque generator. This is one of the most important advantages of electric motors over combustion engines, which are not reversible. Even worse, braking using a fossil fuel engine increases the consumption of energy since revolutions per minute increases.
Electric motors where Regenerative Braking is available
Take a look at some of the most popular electric motors that you need to know.
Resources
- My Thesis. Link.
- S. L. H. Electric Motor Control, 9th Edition, Delmar, Cengage Learning, 2010.
- L. Z. Z. L. R. Y. Y. W. L. Y. G. F. L. Y. y X. Z. , «A Control Strategy of Regenerative Braking. System for Intelligent Vehicle,» IET Conference Publications, vol. CP697, pp. 1-3, 2016.
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[…] Regenerative Braking, the interesting feature in electric motors […]
[…] Regenerative Braking, the interesting feature in electric motors […]
[…] models of VFD incorporate a very interesting feature: regenerative braking, which is a special method of dynamic braking. It consists in returning of the energy back the grid […]