Week-6 Challenge: EV Drivetrain

1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?

The Function of a Converter

More properly called a voltage converter, this electrical device actually changes the voltage (either AC or DC) of an electrical power source.2 There are two types of voltage converters: step up converters (which increases voltage) and step down converters (which decreases voltage). The most common use of a converter is to take a relatively low voltage source and step-it-up to high voltage for heavy-duty work in a high power consumption load, but they can also be used in reverse to reduce voltage for a light load source.

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. 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 charger, 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 EV/HEVs.

The DC supply is changed to an AC waveform using an electronic switch, which is a set of semiconductor transistors e.g. IGBTs, and 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 them to open/close, and even decides the frequency for the same. By doing this operation, the DC mimics an AC 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 that 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 the way it came from. The AC in the primary winding will induce an AC 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 that is actually AC in nature. Then this AC is passed through a filter which will 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 2 main types of DC to DC converters which are isolated and non-isolated types. The non-isolated type can be subdivided into 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 frequency can be changed or maintained depending on the application. This converter has many applications in our daily life e.g. lamp dimming, and motor speed control.

2. An Electric Vehicle's powertrain 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,

Motor and Controller Parameters:

Rated Armature voltage= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

The vehicle speed-torque characteristics are given by the below equation

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

Solution:

Given,

Battery pack Vbat = 72V;

Ia = 200A;

Ra = 0.5ohm;

Motor constant = 0.7;

chopper switching frequency = 400HZ;

Duty cycle = 70%;

Vehicle output voltage (Vo) is given

Vo = Vbat*duty cycle

Vo = 72*0.70

Vo = 50.4 V.

We know equation for torque

To = (Vo-w*Kphi)*Kphi/Ra

Where,

To = vehicle torque

Vo = vehicle voltage

w = steady state speed of the vehicle (rps)

Kphi = motor constant

Ra = Armature resistance

Now , substitute all values in equation,

To = (50.4-0.7w) 0.7 /0.5

To = 50.4*0.7/0.5 - 0.7^2w/0.5

= 70.56-0.98w

Tv = 24.7+0.0051w^2

70.56-0.98w = 24.7+0.0051w^2

24.7+0.0051w^2 = 70.56-0.98w

24.7-70.56 = 0.0051w^2+0.98w

-45.85+0.0051w^2+0.98 = 0

0.0051w^2+0.98-45.85

simplify the above quadratic equation

we get, w = 38.40 ; w = - 234.40

we should take positive values so will take w = 38.40

EV steady state speed w = 38.40rps

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.

ω">ωω= V/Kϕ">ϕϕ -Ra/Kϕ">ϕϕ^2 .T

mathematical model

K_phi subsystem

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Answer:

According to the blog ‘Induction vs DC Brushless motors by Wally Rippel, Tesla’, the writer states the changes in the world years ago and the latest technological trends that are happening. He then explains about the DC Brushless motors and Induction motors.

DC Brushless Machines:

According to him, the rotor has two or more permanent magnets that generate a DC magnetic field. This magnetic field will enter the stator core which interacts with the currents flowing in the windings to produce an interaction between rotor and stator. It needs a proper flow of current into the rotor and stator if it’s not connected properly the current will not generate and the torque decreases. During the Electric and mechanical conversion when the rotor is rotating, the magnitude and polarity of the stator current will continuously vary such that the torque remains constant and optimal efficiency is achieved.

Induction Motor:

The 3-phase induction motor was invented by Nikola Tesla sometime before 1889. The stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of distributed windings that are inserted within the stator core. The essential difference between the two machines is with the rotor.

Unlike the DC brushless rotor, the induction rotor has no magnets. They have just stacked steel laminations with buried peripheral conductors that form a shorted structure. Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. The frequency of this magnetic field as seen by the rotor is equal to the difference between the applied electrical frequency and the rotational frequency of the rotor itself. An induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. The currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence there’s a speed difference. Finally, these currents interact with the original magnetic field to produce forces.

When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset. The motor has the ability to start under load. No inverter is needed. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success. In contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose phase is maintained in step with the angular position of the rotor.

Differences:

He then explained that there were only a few brushless drives in the past but now most of the Hybrid Electric Vehicles use it. There’s a difference between brushless and inverter controls is the control. The Dc brushless requires a position sensor but an induction motor requires only a speed sensor.

Less rotor heat is generated in DC brushless drive and Rotor cooling is easier and peak point efficiency is generally higher.

DC brushless drive operates at a power factor of one, whereas the induction drive power factor is 85%.

With the increase in the size of the DC Brushless machine, magnetic losses increase and the part-load efficiency decreases, whereas the induction drives, as the machine size increases, losses don’t grow. During high-performance tasks, induction drives are suitable.

Finally, we can know that the writer says that the DC brushless will dominate the Hybrid Electric Vehicle Markets but the Induction drive will maintain dominance in the High performance in the pure electric vehicles. As of now, there is no winner out of the two as each has its own pros and cons and excels at its own forte.