Week-6 Challenge: EV Drivetrain
Week 6 Challenge
EV Drivetrain
AIM:
To study the types of power converter circuits employed in electric and hybrid electric vehicles.
Objective:
Types of power converter circuits are employed in electric and hybrid electric vehicles.
To calculate the steady-state condition of electric vehicles.
Author’s perspective about the given blog.
1. Types of power converter circuits used in electric and hybrid electric vehicle
There are four main categories of power converters circuits which are used in electric and hybrid electric vehicles.
AC to DC converter
This type of converter takes an AC power supply and converts it to a DC one. The power generated from a power plant is transmitted to our homes at a voltage of 230V at a frequency of 50Hz, which we can use to power electrical appliances. But our appliance operates on a DC supply, not an AC one and so we need a converter for this application. Examples of AC to DC converter are phone charger, laptop charger, etc., An AC power supply has an alternating waveform which repeatedly reverses direction. An AC to DC converter will prevent the current from changing directions so that it only flows unidirectionally. In the circuit of an AC to DC converter, there is a step-down transformer, a diode, a capacitor, and a low dropout voltage regulator.
The step-down transformer takes the high-level 230V from the main supply and steps it down to a lower level. Example 12 or 13V for our use, but it is still in AC form. Then, a diode or group of diodes is implemented to restrict the current to flow in just one direction. In an AC waveform, there will be positive and negative peaks at each half cycle like a sine wave. The diode will remove the negative parts of the waveform so that there are positive peaks only. This is called rectifying and this part of the circuit is referred to as a rectifier. This can be a half-wave, full-wave, or a bridge rectifier. Then, the rectified output voltage waveform is smoothened by a filter capacitor. Finally, the LDO regulator will regulate the output for our use.
In the case of EVs or HEVs, the battery pack which operates with a DC power supply is charged at home or at a charging station both of which are supplied power in AC form from the electric grid. So, the chargers of these vehicles are an example of an AC to DC power converter.
DC to AC converter
This type of converter takes a DC supply and converts it to an AC supply. The power which is supplied from a battery or capacitor can have its form changed to the AC type for use in homes or systems, which have an AC load. For instance, the power generated by solar panels is stored in batteries in the DC form and this can power a house, which uses an AC power supply that we can utilize this power from the mains. This type of converter is referred to us an inverter and is widely used in homes and in EVs/HEVs.
The DC supply is changed to an AC waveform using an electronics switch, which is basically a set of semiconductor transistors, e.g., IGBTs, MOSFETs. These switches are controlled by a controller to either be in an open or closed position to direct the flow of current in a circuit. The controller sends signals to the switches to open/close and even decide the frequency for the same. By doing this operation, the DC current mimics an AC current and can be fed to the load which requires it.
In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer which consists of a primary and secondary winding. The current from the battery is first supplied to the primary winding where the electronic switches will repeatedly reverse the direction of the current to create an AC wave form. This current will return back the way it came from. The AC current in the primary winding will induce an AC current in the secondary winding which then supplies this current to an AC load, e.g., the AC traction motor.
DC to DC Converter
This type of converter is similar to a transformer in that it can change the voltage from its current level to another level. It can take an input supply of voltage and increase it or do the exact opposite and decrease its similar to a step-up or a step-down transformer. This kind of converter is called a “chopper”. The purpose of using this type of converter is to provide the necessary level of power to a particular component and for the safety of the appliance and the user. If a component supplied with too little or too much voltage, it won’t operate due to lack of power or it will get damaged, so it all depends on the load requirement.
It works by taking the DC supply and passing it through a switching element which converts the waveform to a square wave which is actually AC in nature. Then this AC is passed through a filter which will turn it back into a DC supply for use in a load. In EVs, HEVs, and conventional vehicles, this type of converter is used for various purposes. E.g., in the case of stepping down the voltage, the car radio may require a stepped-down voltage of 12V to operate and would not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of a car may require 500V to operate, but the vehicle may need more voltage than the battery pack can provide.
There are two main types of DC to DC converters which are isolated and non-isolated type. The non-isolated type can be sub-divided into 5types: buck-to step down the voltage; boost-to step up the voltage; buck/boost-this can do both step-up and step down operations, but is mainly used to reverse polarity; cuk-similar to the buck/boost type; charge pump-to step up/step down the voltage for low power sources. The isolated type can be sub-divided into two types: Flyback converter-similar to the buck-boost converter except it uses a transformer instead of an inductor to store energy; forward converter-this type will use a transformer to send energy between the input and the output in a single step
AC to AC converter
This type of converter takes an input AC waveform with a particular voltage and frequency and outputs a different AC waveform with another voltage and frequency. The output voltage and frequency can be changed or maintained depending on the application. This converter has many applications in our daily life. E.g., lamp dimming, motor speed control.
- An electric vehicle’s drivetrain with 72V battery pack is shown in the diagram below. The duty ratio for acceleration operation is d1 and for the braking operation, the duty is d2. The other parameters of the electric vehicle is given below
Motor and controller parameters
Rated armature voltage,`Va = 72V`
Rated armature current, `Ir = 400A`
`Ra = 0.5Ω, KΦ = 0.7V-s`
Chopper switching frequency = 400Hz
The vehicle characteristics are given by the equation
`T TR =24.7+0.0051.Ꞷ^2`
The derived formula for motor speed characteristics
`T=V.Kϕ/R_a - (Kϕ)^2/R_a .Ꞷ`
Substituting the values
`T=(54.4*0.7)/0.5- (0.7)^2/0.5.Ꞷ`
`T=70.56-0.98.Ꞷ`
Now comparing both the motor characteristics equations
`0.0051.Ꞷ^2+0.98.Ꞷ-45.89=0`
Solving the above quadratic equation we get values
Ꞷ = 38.92 rad/s and -231.07 rad/s
Therefore, Ꞷ = 38.92 rad/s
3.Refer to the blog below topic:
“Induction versus DC brushless motors by Wally Rippel, tesla”, explain in brief about author’s perspective.
This blog focuses on the difference between the two most commonly used motor drives in the automobile industry; the DC brushless motor drive and the induction motor drive. The author gives his detailed view of the construction of both motor drives. A brief explanation about the blog is followed by,
DC brushless
Rotor heat generated will be much less than that of the induction motor. And generated heat can be cooled easily.
Rotor construction is enabled with permanent magnets.
Stator construction in both DC brushless motor and induction motor is distributed windings inside stator core.
As the name suggests DC brushless motor drive uses DC type supply voltage.
Inverter needed to vary stator currents to generate rotating magnetic field and it’s easier to control.
Brushless DC motor produces no starting torque when directly connected to fixed frequency utility power.
Speed can be controlled and varied by the inverter, which cannot be achieved in an induction motor.
DC brushless motor drive has high efficiency when compared with an induction motor, and can operate at a unity power factor
The magnetic field in the DC brushless motor cannot be varied. Magnetic losses happen with an increase in the size of the machine.
Because of the usage of permanent magnets, the motor drive is more costly.
Induction motor
Due to the eddy current, the motor drive generates more heat. When compared DC brushless motor generates less heat.
Induction motor drive doesn’t employ any usage of magnets, it is constructed with stacked steel laminations.
Stator construction is the same as DC brushless motor, which is distributed windings inside the stator core.
Supply voltage in an induction motor is AC three phase-type, whereas DC brushless motor uses DC type.
No inverter is required to control, it directly runs on three-phase power.
Induction motor drive can generate starting torque from the onset when connected to fixed frequency supply.
Speed variation cannot be achieved in this motor drive, while DC brushless motor drive could achieve speed control. Induction motor drives at a constant speed.
Efficiency is much lower than that of DC brushed motor drive.
Magnetic field achieved in an induction motor can be varied, while DC brushed motor uses permanent magnets. And there are no magnetic losses.
Induction motor drive is much cheaper, because of no usage of magnets.
Conclusions:
Various types of power converter circuits in power electronics and their applications are studied, and also the steady-state speed for an electric drivetrain is calculated. And “induction and DC brushless motor” blog by Wally Rippel’s is discussed.