Have you guys ever wondered if we could charge our EV and hybrid car batteries while driving? Yes, we can! Yes, you heard it right. In this topic, we are going to look at what is and how regenerative brakes work and how they help us to charge our batteries.
What is Regenerative Braking?
Regenerative braking helps to replenish the
battery to charge up to 5% to 10% by converting the kinetic energy into
electrical energy and also helps to slow down the vehicle gradually without
hitting the brake pedals.
What is energy?
Energy
states that it can neither be created nor be destroyed, although, it may be transformed from one form to another. While applying pedal
braking we are converting the Kinetic energy into heat energy. But in regenerative
braking, we are converting the kinetic energy into electrical energy to charge
the high-voltage battery or capacitors.
How Motor and Generator work in EV:
Motor:
An electric motor is a device
that converts electrical energy into mechanical energy to provide motion. In EV
the electric motor shafts are connected with the vehicle wheels to provide
motion.
Generator:
An electric generator is a device that
converts mechanical energy into electrical energy, which helps to charge the
battery. In this regenerative braking system with the help of vehicle wheel
rotation, the motor acts as a generator to charge the battery or capacitor.
What is synchronous speed?
When a three-phase power supply is given to
the stator of the induction motor a rotating magnetic field (RMF) is created
around the stator, this RMF speed is known as the synchronous speed. The
synchronous speed will be varied based on the number of poles (p) in the motor
and input frequency (f). The formula to find the synchronous speed,
f – Supply frequency
in Hz
P – Number of poles
What is a slip?
Calculating the speed difference between the stator RMF (synchronous
speed) and the rotor speed is known as slip. Types of slips
·
Positive
Slip
·
Zero
Slip
· Negative Slip
Positive Slip:
When the rotor rotates below the synchronous speed is known as positive slip. Let us consider the stator and rotor magnetic field in the motor as a point, like below mentioned image
Speed difference: Ns – Nr = 1500 – 1300 = 200
Slip in the motor: (Ns – Nr/Ns)*100 = (200/1500)*100 = 13.3%
By deriving the value using the formula, we came to know that it is a
positive slip (acts as the motor)
Zero Slip:
When the rotor rotates equally to the synchronous speed, the slip will become zero. But the rotor in the induction motor never attains the synchronous speed, therefore is no zero slip in the induction motor.
Speed difference: Ns – Nr = 1500 – 1500 = 0
Slip in the motor: (Ns – Nr/Ns)*100 = (1500/1500)*100 = 0%
Zero Slip will never happen in an induction motor
Negative Slip:
When the rotor rotates above the synchronous speed is known as negative slip. In this condition, the motor acts as a generator.
Speed difference: Ns – Nr = 1300 – 1500 = -200
Slip in the motor: (Ns – Nr/Ns)*100 = (-200/1500)*100 = -13%
By driving the value, we came to know that it is a negative slip.
When the Motor acts as a Generator:
From the above discussion, we came to know about the positive and negative slip, when the rotor rotates below the synchronous speed and acts as a motor. But in another condition when we apply any external force like a prime mover to rotate the rotor above the synchronous speed, the slip will become negative slip. At this time the motor acts as a generator and we can find the voltage in the motor terminals.
How regenerative braking works:
Let us take the induction motor
for regenerative braking from the above study we came to know that in an induction
motor whenever the rotor rotates above the synchronous speed the slip will
become negative, and the motor acts as a generator. Let us consider the rotor
is rotating at 6500 rpm, the synchronous speed is 7500 rpm (in stator) and the supply
frequency is 250 Hz. Whenever we take our foot from the accelerator pedal the
motor controller supplies very minimal power like 250 Hz to 80 Hz, then synchronous
speed will decrease to 3000 rpm (let's consider) and the rotor speed will
decrease gradually from 6500 rpm to 0 rpm. In the period of 6500 rpm to 3000
rpm of the rotor speed, the slip will become negative, so the motor acts as a
generator in this period. When the rotor attains below the 3000 rpm the
generator again acts as a motor. In this condition, the slip will become
positive. Simply the acts of a motor and generator are based on the speed of
the rotor and the synchronous speed.
Regenerative braking is still efficient:
Efficiency of the regenerative braking is based on kinetic energy, the more kinetic energy is there in a vehicle the more regenerative braking will happen. Based on the mass of a vehicle defines the kinetic energy, when we compare it with the bike and car, the car has more mass than a bike and it will have more kinetic energy. On the one hand, regenerative braking is more effective in trains but on another side it is not more efficient in motorcycles. The efficiency of the regenerative braking is based on where we are using it.
Advantages of Regenerative Braking:
Ø 1. Brake pads and rotor are used less, because most
of the braking works are done by RBS.
Ø 2. Regular service for brake pads will become less,
this helps the driver to save maintenance cost.
Ø 3. Spending time in the charging station for
charging the vehicle will be minimal.
Ø 4. By using RBS will increase the traveling time
and can cover the extra distance.
Ø 5. Applying RBS features in vehicles like trains can
optimistically use the RBS.
Disadvantages of Regenerative Braking:
Ø 1. RBS is not more efficient while driving on
highway roads.
Ø 2. Driver feels different when the vehicle applies RBS and needs some practice to get used to RBS.
Ø 3. Using regenerative braking in a fully charged
battery is not advisable because it can damage the battery.
Ø 4. Battery lifetime will be doubted because of irregular
loads given by the RBS.
Ø 5. Re-gaining energy by RBS is too minimal when
compared with energy discharged by the vehicle.
Ø 6. RBS is an additional unwanted feature in
vehicles like scooters and bicycles.
