Week-6 Challenge: EV Drivetrain

Q1) Which types of power converter circuits are employed in electric and hybrid electric vehicle?

There are 4 main categories of power converter circuits that are used in electric and hybrid electric vehicles. They are listed and elaborated upon below:

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 our electrical appliances. But our applianced operate on a DC supply, not an AC one and so we need a converter for this application. Examples of Ac to DC converters are phone chargers, laptop chargers, etc. An AC power supply has an alternating waveform that repeatedly reverses direction. An AC to DC converter will prevent the current from changing direction 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 (LDO) regulator.

The step-down transformer takes the high-level 230V from the mains supply and steps it down to a lower level e.g. 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(s) 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 for these vehicles are an example of the AC to DC power converter.

DC to AC converter

This type of converter takes a DC supply and converts it to an AC one. 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 that uses an AC power supply so that we can utilize this power from the mains. This type of converter is referred to as an 'inverter' and is widely used in homes and in EV/HEVs.

The DC supply is changed to an AC waveform using an electronic 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 tell it to open/close, and even decides 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 waveform. 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 it similar to a step up or 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 is 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 in to 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 2 main types of DC to DC converters which are isolated and non-isolated type. The non-isolated type can be subdivided to 5 types:

• Buck - to step down the voltage
• Boost - to step up the voltage
• Buck/Boost - this can do both steps 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 subdivided into 2 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 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/or freqency 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. There are 4 main types of AC to AC converters:

• Cycloconverters

This type is also known as a frequency changer as it will change the frequency of an input AC waveform and output an AC waveform with another frequency but it can also change the magnitude of the power. This type of converter does not have any DC linkages and avoids any conversion between AC and DC to prevent losses and save costs. The input wave's frequency is changed by changing the firing angle applied to a thyristor. By switching the positive and negative limb thyistors, we can get an output waveform that has a stepped up/down frequency compared to the input.

• AC to AC convert with DC Link

As the name suggests, this type includes a DC link along with a rectifier and an inverter. The input AC waveform is first rectified by the rectifier to remove the negative portions of the AC wave and output only the positive portions. This creates a DC waveform which is then stored in the DC link and is then converted back to an AC waveform using the inverter.

• Matrix Converter

This type converts from AC to AC directly without using a DC linkage to reduce costs and losses that would otherwise be incurred with a DC link storage element. It is used mainly to increase the reliability and efficiency of a system. Matrix converters use bidirectional switches (IGBTs) to conduct current and block voltages of both polarities.

• Hybrid Matrix Converter

This type is very similar to the matrix converter in that they combine the AC//DC/AC design with the matrix design. It uses unidirectional switches and 2 converter stages, without the DC link to reduce the size and weight of the converter.

Q2) An Electric Vehicle's Drivetrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'. The other parameters of the electric vehicle is given below,

m=1000bkg, C_d =0.2, A=2 m², C₀=0.009, C₁=1,

ρ=1.1614 kg/m³

Radius of the wheel= 11 inches

Motor and Controller Parameters:

Rated Armature voltage= 72 V

Rated armature current= 400 A

R_a= 0.5Ω, L_a= 8 mH, KΦ= 0.7 V-s

Chopper Switching frequency= 400 Hz

The vehicle characteristics are given by the equation

T_TR=24.7+0.0051w²

What is EV steady-state speed if the duty cycle is 70%?

ANS:

Firstly, using the given information, we can determine the rolling resistance and drag forces.

The rolling resistance formula is given by: Frr=μrr⋅mg

where, μrr

is the coefficient of rolling resistance and mg is the weight.

The drag force formula is given by: Fd=12⋅Cd⋅ρA⋅v2

where, Cd

is the coefficient of drag

ρ

is the air density in ,kgm3

A is the frontal cross sectional area in m2

V is the velocity in m/s

Using these 2 formulas, the rolling resistance and drag forces were found to be:

Frr=88.29

Fd

= 0.23228v2

Since we do not know the velocity, we can only write an expression for the drag force in terms of velocity.

Next, we can equate the equations of tractive force using the summation of forces on the right hand side and the motor torque, gear ratio and the wheel radius on the left hand side.

Fte=T⋅Gr

We know the summation of forces, so we can place that on the RHS. In the given data, gear ratio is 1, and the wheel radius is 11 inches which is equal to 0.2794m. We do not know the torque, so we have to write the LHS in terms of T.

T⋅10.2794=0.23228v2+88.29

Since we used the wheel radius for the LHS, we also know that velocity can be written as a product of angular speed and radius.

v=r⋅ω

T⋅10.2794=0.23228⋅(0.27942⋅ω2)+88.29

Simplifying this, gives us an expression for the torque, T shown below:

T=0.0051ω2+24.67

Next, we know the torque-speed relation for a motor. We are provided the armature resistance, voltage and kphi values. We also know the duty cycle is 70%.

So the average voltage is given by the product of the battery voltage and the duty cycle:

Vo=V⋅dc

Vo=72⋅0.7

The average voltage is 50.4V.

We know the torque-speed relation is given by the equation:

Tm=Vo⋅(k⋅ϕ)Ra−ω(k⋅ϕ)2Ra

Tm=50.4⋅0.70.5−ω⋅(0.7)2Ra

Simplifying, will give us an expression for torque, T in terms of angular speed which is shown below:

Tm=70.56−0.98ω

Now we have 2 expressions for the torque that are both in terms of the angular speed. The first expression is the torque of the vehicle, whereas the second one is for the motor.

Equating the 2 expresssions, we will get a quadratic equation which can be solved to find the angular speed.

70.56−0.98ω=0.0051ω2+24.67

0.0051ω2+0.98ω−45.89=0

We can solve this quadratic equation using the quadratic formula:

x=−b±√b2−4ac2a

Entering all the values into the formula:

x=−0.98±√0.982−4(0.0051)(−45.89)2⋅0.0051

Simplifying this, we get:

x=−0.98±1.3770.0102

So we will have 2 solutions for x i.e. the angular speed.

ω=38.92rads

or ω=−231.08rads

Assuming the forward direction is the positive and that the vehicle is in steady speed moving in the forward direction, we will accept the first solution and reject the second solution.

Q3) Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

ANS:

The author writes this blog to entail the differences between the two most commonly used motor drives in the automobile industry: the DC brushless motor drive and the induction motor drive. Rippel goes in to detail about the differences in construction, working, and cost between the two drive types. However, with what he has written, he has not arrived at the conclusion that one type is better than the other. In the last lines of the opening paragraph, Rippel writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine. It is in this same regard that he ends the blog stating that there is still 'no winner' when it comes to motor drive selection either. In spite of both motor drive types having similarities, they also have their benefits and drawbacks.

For DC Brushless drives, we are told that their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations) It also uses an inverter for the current control, meaning that it cannot run on a DC power supply as the current direction has to be changed repeatedly in to maintain a constant torque.

For induction drives, there rotor has no magnets, but instead stacked steel laminations with buried peripheral conductors which form a 'shorted structure'. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor.

Induction motors can be used directly with power from the mains and does not require an inverter to start the drive and generate the starting torque. It has the ability to start under the mains load applied. However, DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter to do this.

Induction motor drives cannot run on a DC power supply and are solely dependent on an AC power supply. Also, when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. In order to rectify this, an inverter can be used to operate the induction drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no longer an issue - it can be varied.

DC Brushless drives use permanent magnets, and cannot change the strength of their magnetic fields. It would be desirable if they could, because when the maximum torque is needed by the rotor, the magnetic field strength should also be at a maximum level. This allows for the inverter and motor currents to be at a minimum, thereby reducing the power loss by heat dissipation. Similarly, when the torque required is minimum, the magnetic field strength should also be minimum to reduce hysteresis and eddy current losses. The permanent magnets used are also quite expensive costing $50 per kilogram and their magnetic fields are difficult to control if ferromagnetic material gets close enough to the field.

On the other hand, induction drives do not have permanent magnets and so they can vary their magnetic field strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and maximum efficiency as the magnetic field strength can be changed according to the requirement by the rotor. These losses do not increase if the size of the induction drive also increases but as a DC brushless drive gets bigger, its losses increase and their efficiency decreases. So induction drives have a cost advantage compared to DC brushless drives

Although, DC Brushless drives generate less heat at the rotors. So rotors more easily and have a higher peak point efficiency than induction drives. In addition, the DC brushless motor can operate at unity power factor i.e. 1, whereas induction drives can operate up to a power factor of approximately 85%.

In conclusion, Rippel has elaborated the different aspects to compare the induction and DC brushless motor drives and has detailed the advantages and disadvantages of both types. As mentioned earlier, at the end of the day it depends on the preference of the user. If a user is seeking a high performance with comparatively lesser costs, then the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high power factor and easier control that a user seeks, then the DC brushless drive would be the smarter choice. Both types have their place in the automobile industry and will retain their relevance in their respective applications. Rippel believes that due to the increasing performance of vehicles, one day both may be used side by side under the hood of a vehicle in order to get the best of both drives. This would result in a new kind of hybrid that combines the characteristics of an ordinary vehicle e.g. sedan with that of a sports car.