Category: FuroSystems

How do electric motors in ebikes work?

To continue our educational series on the physical principles behind electric bicycles, we would now like to talk about the brushless DC motors that equip your bikes. Without those, there would be no sleek and smooth ebikes like the FX and the SIERRA.

First of all, an electric motor is an electrical to mechanical or electro-mechanical energy convertor. It converts electrical energy to mechanical movement (rotational or translational). They were first invented by William Sturgeon in 1832, 44 years before Nikolas August Otto created the first practical petrol engine. Our society went the petrol direction because of the wide availability and the greater energy density of fossil fuels as described in our previous article.

first practical electric dc motor

Today, there exists multiple types of electric motors: brushed, brushless, uncommutated, permanent magnet stators and wound stators. Most of these exploit the forces created by magnetic fields and possess some kind of internal mechanism.

Let’s look into the simplest DC motor possible as shown in the picture above. It is made of a stator and a rotor. The stator does not move and is equipped with magnets. The rotor is a coil or armature placed within the stator through which a current passes. This current generates an electromagnetic field aligned with the center of the coil. The direction and intensity (or magnitude) of this current determines the direction and intensity of the magnetic field produced by the coil. The following picture represents this phenomenon, with I the current. This is also called an electromagnet as it creates a magnetic field when electricity passes through it.

Magnetic field produced by current running through a coil

The magnetic field created in the rotor attracts and repels the magnets placed in the stator (positive pole attracts negative pole and negative pole attracts positive pole). If everything is kept equal the rotor will consequently turn a maximum of 180 degrees before stopping. To keep this rotational motion the poles of the electromagnet that is the rotor have to be switched, guess how? … By switching the direction of the current passing through it as shown in the following animation:

DC motor rotating

The question is, how do you switch the current’s direction when your battery is always connected in the same way? Well, in brushed motors, actual conducting brushes make contact with the rotor as it spins and sequentially reverse the current going through it, in turn switching its polarity. However, the obvious problems with using brushing contacts to switch the current are that the whole thing heats up faster and therefore needs a bigger cooling system, it creates sparks, it wears out and consequently limits the speed of the motor.

This is where modern computers and electronics come in. They allowed us to get rid of those brushes to create Brushless DC (BLDC) Motors, the wonderful components that make your favorite electric bicycles possible! Essentially, a BLDC motor is a brushed DC motor turned inside out. The rotor is made with permanent magnets placed in the center of the motor while electromagnets are placed around it in the stator. A computer, using powerful transistors, then switches the polarity of these electromagnets which leads the rotor to turn. This enhances how well the energy stored in your battery is converted to movement, reduces the need for maintenance and removes the cap on speed as the only limit is now the power available in your system and the speed at which your computer can switch the electromagnets.

Brushless DC motor in electric bicycles

More specifically, the computer in your ebike is called a controller and extracts current from your battery in order to switch the electromagnets in the stator of your motor.

Hopefully, this is all kind of clear, if you have any questions, do not hesitate to ask in the comments!

How do Lithium-ion electric bicycle batteries work?

As we just made the preorders for the the bigger battery version of the SIERRA, the SIERRA MAX available, today appeared as a great opportunity to thoroughly explain the fundamental principles and physics behind Lithium-ion batteries and why they currently are the state-of-the-art in electric bikes.When looking at batteries, you basically want to maximise stored energy while minimising volume, weight, and price. As a society, we have been primarily using fossil fuels for personal transportation. This is because this type of of power source is really good at storing a lot of energy in a small volume and with a low weight while remaining affordable. Now that these chemicals are slowly but surely running out, and we witness how impairing they are to our sustainable development, cleaner options such as rechargeable electric batteries are progressively becoming the norm.

There are many types of those: Nickel-Cadmium, Lead-acid, Silver-Zinc, Lithium-Ion, etc. The following graph produced by NASA shows that Lithium-Ion chemistry has the highest energy density and is consequently the best choice to store electrical energy in small volumes and low weights.

Electric bike Lithium-Ion battery energy density comparison

While still 60 times less energy dense than gasoline, Lithium-ion cells are the best available alternative to store electric power safely and economically in personal vehicles.

The following diagram simplifies the discharge mechanism in a Lithium-ion battery, which is tantamount to the motor pulling power from it. Positive Lithium-ions travel across an electrolyte and a separator from an anode towards a cathode, which are two dissimilar conducting materials. The cathode then becomes more positively electrically charged than the anode. This creates a voltage between the anode and the cathode, which is essentially a driving force that pushes electrons between two points. The higher the voltage, the higher the force. This can be visualised as a waterfall. The taller the waterfall, the greater the force driving the water from its top to its bottom.

Electric bicycle Lithium-ion battery discharge mechanism

This voltage drives electrons across the electromechanical systems of your ebike. They leave the anode to go through the controller, followed by the motor and back to the cathode. As electrons move across the system, the voltage of the battery goes down. The charging process is the exact opposite. A voltage is applied across the cathode and the anode which forces electrons to move in the opposite direction and the Lithium-ions to go back towards the anode. This restores the driving voltage between the anode and the cathode for later discharge.

Electric bicycle Lithium-ion battery charge mechanism

These chemical processes are contained within cells. Today most cells, whether in laptops, cars or ebikes, are cylindrical pieces of metal. This allows to contain pressure better and increase batteries’ safety and resilience to their environment. These cells usually provide a nominal voltage of 3.6V. This means that they reach 4.2V when fully charged and progressively go down to a safe minimum of 2.5V during discharge after which a battery management system shuts your battery down to preserve its capacity and health.

Lithium-ion cells have to be connected and assembled to build a battery with a total voltage that is enough to drive its target electro-mechanical systems; the controller, motor and screen in our case. Individual cells are packed and connected in Series to increase the total battery voltage to the required value (for example, 36V). Once this value is reached, these series of cells are packed in parallel to reach the total battery capacity, the value usually quoted in Ah. An average cell packs 3.6V and 2.5Ah, hence a battery producing 10Ah at 36V most likely consists of 4 groups of ten cells connected in series, which are then connected in parallel. I know that this is tough to get your head around but the following diagrams, courtesy of Cadex, should help you understand:

Electric bicycle Lithium-ion battery series assembly

Group of cells assembled in Series to increase total voltage.
Electric bicycle Lithium-ion battery parallel assembly

Group of cells assembled in Parallel to increase total capacity once the desired voltage is reached.
Hopefully, you now have a better idea of the internal workings of your ebike or more generally your laptop, phone or car.

If you have any questions, do not hesitate to ask in the comments!

Exclusive Pre-orders on our First Production Batch Now Available

We are proud to announce that after years of development, hours of testing and tens of iterations, our innovative full carbon high performance folding and mountain electric bikes are finally in production!

Our inspection team will make sure that throughout each step of the fabrication process, from frame moulding to assembly, the highest standards of manufacturing are exerted. This is particularly important to us as our bikes are not only truly revolutionary in design and practicality but also carry our vow to provide you with only the best quality at the most competitive prices achievable by technology today! There will always be cheaper bikes, but those offering similar performance, aesthetics and build quality will also always be significantly more expansive.

We have made the FX and SIERRA available for pre-order at a significant 25% discount until they arrive in our European warehouse. This is to thank you for your trust and being one of the pioneers of our community. Once our ebikes are in stock, first backers will be delivered and pre-orders will be discontinued to allow for direct orders with shipment within 2 weeks at retail price.

We are always available to answer your questions and talk with with you. You can contact us here.

Let’s change cycling and modern transportation together!

Thank you for your trust!

You can obtain more info and access FX  pre-orders for here, and here for the SIERRA.

Eliott Wertheimer

 

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