Regenerative Braking

Inertia, Force, and Mass

Everything has inertia; if it has a mass, it has inertia. A hybrid reclaims energy through the fundamentals of physics. Do you remember any high school or college physics?

You apply a force to move an object. The equation for this is:

F=ma
“F” being the force, “m” being the mass, and
“a” being the acceleration

The faster you want an object to accelerate, the more force you have to apply.

Let’s just look at the electric motor for now. Energy from the battery (Watts) is applied to the coil windings in the motor. These windings then produce a magnetic force on the rotor of the motor, which produces torque on the output shaft. This torque is then applied to the wheels of the car via a coupling of gears and shafts. When the wheel turns, it applies a force to the ground, which due to friction between the wheel and the ground causes the vehicle to move along the surface. This is like if you were in a boat at a dock, and you grabbed the dock and pushed with your arm. The force you are generating is moving the boat relative to the location of the dock. The more force you apply, the fast you get the boat to move.

Friction in Hybrids

There is friction everywhere in the hybrid system. There is electrical friction between the atoms and electrons moving in the wires between the battery and the motor and through the motor itself. There is magnetic friction in the metal laminations that make up the magnetic circuit of the motor, as well as in the magnets again on the atomic level. Then, there is mechanical friction between every moving part, such as the bearings, seals, gears, chains, and so on. The by-product of friction is heat. Take your hands and rub them together and your palms get warm. The faster you do it, the faster they heat up. Also, the harder they are pressed together, the faster they will heat. Friction is energy lost to heat. When all of these losses are added up, that is what determines the efficiency of the vehicle.

Frictional Losses in Conventional Cars

A standard car generates torque to move the wheels to drive the vehicle down the road. During this time, it is generating friction and losses. When you apply standard brakes, it is just another friction device that has specially designed material to handle the heat from friction, which is applied to the drums and rotors that stop the wheel from turning. The friction between the wheel and the ground stops the vehicle. This standard vehicle has frictional losses to move the vehicle—and uses the fundamental behind frictional losses to stop the vehicle. So it’s a lose-lose situation.

Reclaiming Energy in a Hybrid

On a hybrid that has regenerative brakes, you can reclaim some of this energy that would normally be lost due to braking. Using the vehicle’s inertia is the key. What is inertia? It is basically what makes something difficult to start moving and what makes something hard to stop moving. Let’s review the boat at the dock. If you begin to push, you have to accelerate the mass of the boat and you. The heavier the boat, the more force it takes to get the boat to move. Also, you notice that the heavier the boat, the longer you have to apply this force to get the boat to move. The same thing happens when you try to stop the boat. Once the boat is moving, you find it takes a similar amount of force to stop the boat and also it takes about the same amount of time to make it stop.

This is all about inertia—the amount of energy that is required to change the direction and speed of the boat.

Transferring Torque Back to the Motor

This inertia is the fundamental property of physics that is used to reclaim energy from the vehicle. Instead of using 100% of the foundation brakes of the vehicle, which are the friction brakes, we now let the linkages back to the motor such as the drive shafts, chains, and gears transfer the torque from the wheels back into the motor shaft. One of the unique things about most electric motors is that electrical energy can be transferred into mechanical energy and also mechanical energy can be transferred back into electrical energy. In both cases, this can be done very efficiently.

Thus, through the technology of the motor and motor controller, the force at the wheels becomes torque on the electric motor shaft. The magnets on the shaft of the motor (called the rotor—the moving part of the motor) move past the electric coils on the stator (the stationary part of the motor), passing the magnetic fields of the magnets through the coils, producing electricity. This electricity becomes electrical energy, which is pumped back to the battery. This, in turn, charges the hybrid battery pack. This is where the comment “regeneration” or “reclaiming energy” comes from.

That is the basics of how regeneration works. How much energy you can reclaim depends on a lot of factors. There are different regeneration theories and designs, which fall into two groups: one being called parallel regen and the other called series regen, which are different from the parallel and series hybrids. These regen groups strictly are design topologies for braking systems. It also matters how many wheels you are using to reclaim energy. Most vehicles to date are front wheel drive so you can only reclaim energy from the front wheels. The back wheels still waste energy to standard friction brakes unless they are somehow connected back to the electric motor. The other factor is battery state of charge and how hard you can drive that energy back into the battery.