OBJECT:- EV Drivetrain

AIM:-

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

2. 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 is2'. 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 V-s

Chopper Switching frequency= 400 Hz

The vehicle characteristics are given by the equation

TTR=24.7+0.0051w2

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

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

ANSWER-1

4-Different Power Converters

There are four power converters but basically three power converters are employed in ELECTRIC VEHICLE discussed bellow.

Power electronic converters are used everywhere in normal daily routines at home, commercial workplaces or in an industrial environment.

Due to the high power handling with higher efficiencies, these converters become an integral part of industrial electric drives, high electric power supplies, electric traction systems and automobile control equipments.

There are different types of power electronic converters used for performing different functions (such as inversion, rectification, etc.) which are rated from a few milliwatts to a few thousand watts. Let us look at these converters in detail.

• Introduction to Power Electronic Converters

Power electronic technology deals with processing and controlling the flow of electrical energy in order to supply voltages and currents in a form that optimally suited for end user’s requirements.

A power electronic converter uses power electronic components such as SCRs, TRIACs, IGBTs, etc. to control and convert the electric power. The main aim of the converter is to produce conditioning power with respect to a certain application.

The block diagram of a power electronic converter is shown in figure above. It consist of an electrical energy source, power electronic circuit, a control circuit and an electric load. This converter changes one form of electrical energy to other form of electrical energy.

The power electronic circuit consists of both power part and control part. Power part transfers the energy from source to load and it consists of power electronic switches (SCR or TRIAC), transformers, electric choke, capacitors, fuses and sometimes resistors.

The control circuit or block regulates the elements in the power part of the converter. This block is built with a complex low power electronic circuit that consists of either analog or digital circuit assembly.

Power electronic converters perform various basic power conversion functions. This converter is a single power conversion stage that can perform any of the functions in AC and DC power conversion systems.

epending on the type of function performed, power electronic converters are categorized into following types.

• AC to DC = Rectifier: It converts AC to unipolar (DC) current
• DC to AC = Inverter: It converts DC to AC of desired frequency and voltage
• DC to DC = Chopper: It converts constant to variable DC or variable DC to constant DC
• AC to AC = Cycloconverter, Matrix converter: It converts AC of desired frequency and/or desired voltage magnitude from a line AC supply.

These types of power electronic converters may be found in a wide variety of applications such as switch mode power supplies (SMPS), electrical machine control, energy storage systems, lighting drives, active power filters, power generation and distribution, renewable energy conversion, flexible AC transmission and embedded technology.

Let us go in detail about each converter.

• AC to DC Converters or Rectifiers

An AC to DC converter is also called a rectifier, which converts AC supply from main lines to DC supply for the load. The block diagram of an AC to DC converter is shown in figure below.

The essential components in this rectifier include transformer, switching unit, filter and a control block.

Here, the transformer adjusts the primary AC source supply to the input of rectifier stage. Usually it is a step-down transformer that reduces the supply voltage to a circuit operating range.

The rectifier converts the low voltage AC supply into DC supply.

It comprises diode and/or thyristors based on type of rectifier. The output of the rectifier is of pulsed DC and hence it is filtered using filter circuit, which is usually made with a capacitor or a choke.

The control block controls the firing angle of thyristors in case of phase controlled rectifiers. Since the diode is not a controllable device, control block is not needed in case of diode rectifiers.

Rectifiers are majorly classified into two types

• Uncontrolled diode rectifiers
• Controlled rectifiers

• DC to AC Converters or Inverters

  These converters are connected between DC source of fixed input, and variable AC load. Most commonly, these DC to AC converters are called as inverters. An inverter is a static device that converts fixed DC supply voltage to variable AC voltage.

Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency.

This conversion from DC to AC along with variable supply is produced by varying the triggering angle to the thyristors. Most of the thyristors used in inverters are employed with forced commutation technique.

These can be single phase or three phase inverter depending on the supply voltage. These converters are mainly divided into two groups. One is PWM based inverters and other multilevel inverters.

Further, these are classified voltage source inverter and current source inverter. Each type is subdivided into different types such as PWM, SVPWM, etc. Multilevel inverters are more popular in industrial applications.

• DC to DC Converters

  Many DC operated applications need different levels of DC voltage from a fixed DC source.

  Some of these applications include subway cars, DC traction systems, control of large DC motors, battery operated vehicles, trolley buses, etc. They require variable DC to produce variable speed, so a power conversion device is needed.

  A DC chopper is a static device that converts a fixed input DC voltage to variable DC output or a fixed DC output of different magnitude (which can be lower or higher) than input value. The block diagram of a DC chopper is shown in figure below.

The chopper circuit is connected between DC input source and DC load. This chopper consists of power electronic switching devices such as thyristors which are connected in such a way that they produce required DC voltage to the load.

The output voltage is controlled by adjusting ON time of the thyristor (or switch) which turn changes the width of DC voltage pulse at the output. This method of switching is called as pulse width modulation (PWM) control.

The output of the chopper can be less or greater than the input and also it can be fixed or variable. These can be unidirectional or bidirectional devices based on the application it is intended for.

DC choppers are mainly used in DC drives, i.e., electric vehicles and hybrid electric vehicles.

DC choppers are classified into three basic types based on input and output voltage levels and are discussed below.

• Step-up Chopper or Boost converter

  In this chopper, the output voltage is always greater than input voltage. The configuration of a boost converter is shown in figure below.

  Here also a switch is used, which is connected in parallel with the load. This switch is a thyristor or an SCR.

  As similar to the buck converter, a diode is placed in series with the load that allows the load current to flow when the thyristor is turned OFF.

When the thyristor is turned ON, the diode is reverse-biased and hence it isolates the load circuit from the source. So the inductor charges to the maximum input voltage source.

When the thyristor is turned OFF, the load gets the voltage from input as well as from inductor. So the voltage appearing across the converter output will be more than the input.

Here the output voltage is equal to (1/ 1 – d) times the input voltage, where d is the duty ratio (TON / T). By varying this duty ratio, the output voltage will be varied till the load gets desired voltage.

• Step-down Chopper or Buck converter

  A step-down chopper produces an average output voltage lower than the input DC voltage. The circuit for this converter is shown in figure below.

  Here the switching component is a thyristor that switches the input voltage to the load when it is triggered at particular instants.

  A diode acts as a free wheeling diode that allows the load current to flow through it when thyristor is turned OFF. If this diode is absent, a high induced EMF in inductance may cause damage to the switching device.

The average output voltage of the converter is varied by controlling turn ON/OFF periods of thyristor. When thyristor is turned ON, the output voltage is same as the input voltage and if it is turned OFF, the output voltage is zero.

The output voltage is equal to (TON / T) Vin. So, by controlling the duty ratio K = (TON / T), the output voltage will be increased.

• Buck–boost converter

The buck–boost converter is a type of DC to DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a transformer.

• Flyback converter

The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a Plasma Lamp or a Voltage Multiplier the rectifying Diode of the boost converter is left out and the device is called a Flyback Transformer.

• Forward converter

The forward converter is a DC/DC converter that uses a transfarmer to increase or decrease the output voltage (depending on the transformer ratio) and provide galvanic isolation for the load. With multiple output windings, it is possible to provide both higher and lower voltage outputs simultaneously.

While it looks superficially like a flyback converter, it operates in a fundamentally different way, and is generally more energy efficient. A flyback converter stores energy in the magnetic field in the inductor air gap during the time the converter switching element (transistor) is conducting. When the switch turns off, the stored magnetic field collapses and the energy is transferred to the output of the flyback converter as electric current. The flyback converter can be viewed as two inductors sharing a common core with opposite polarity windings.

In contrast, the forward converter (which is based on a transformer with same-polarity windings, higher magnetizing inductance, and no air gap) does not store energy during the conduction time of the switching element — transformers cannot store a significant amount of energy, unlike inductors. Instead, energy is passed directly to the output of the forward converter by transformer action during the switch conduction phase.

While the output voltage of a flyback converter is theoretically infinite, the maximum output voltage of the forward converter is constrained by the transformer turns ratio :

where D is the PULSE WIDTH MODULATOR duty cycle.

The forward converter is typically used in off-line supplies to provide an intermediate power output level of 100–200 watts.

• AC to AC Converter

These types of power electronic converters are used to convert the alternating current waveforms of specific magnitude and specific frequency into alternating current waveforms with different magnitude and different frequency, as the signal remains AC, hence this converter is termed as AC to AC converter. To operate a few devices and machines we require some specific voltage with specific frequency which can be obtained using AC to AC converters. By regulating AC power using the AC to AC converter we can regulate the speed of induction motors. There are different types of AC to AC converters which are classified based on different criteria.

ANSWER-2

Given data:

Mass (m) = 1000kg

Drag Coefficient(Cd) = 0.2

Frontal Area = 2 Sq-m

Coefficient of Rolling Resistance(C_o) = 0.009

Gear Ratio(C1) = 1

Dencity`(rho)` = 1.1614 kg/cub-mtr

Radius of the Wheel(r) = 11 inch = 0.28 m

Rated Armature Voltage(v) = 72V

Rated Armature Current = 400A

Armature Resistance(Ra) = 0.5Ω

Armature Inductance(La) = 8mh

Motor Constant(K`psi`) = 0.7V-s

Chopper Switching Frequency = 400Hz

Duty Cycle = 70%

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

Formula Used-

Find out the Rolling Resistance Force-

`F_(rr) = mu_(rr.)m.g`

`F_(rr) = 0.009times1000times9.81`

`F_(rr)= 88.29`

Find out the Drag Force-

`F_(ad) = 1/2rhoAV^2C_d`

`F_(ad) = 0.5times1.16142timesV^2times0.2`

`F_(ad) = 0.232timesV^2`

`F_(ad)` = `0.232timesomega^2times(0.28)^2`

`F_(ad) = 0.01821timesomega^2`

Total Tractive Force-

`F_(te) = F_(rr)+F_(ad)`

`Fte = 88.29+0.01821timesomega^2.................(Eq-1)`

We know that,

`F_(te) = (G.T)/r`

`(1timesT)/0.28=88.29+(0.01821timesomega^2)`

`T __TR = 24.72+0.0051timesomega^2.................(Eq-2)`

Speed-Torque Equation For DC Motor-

`omega = V/(kpsi)-(T.R)/(kpsi)^2`

In the question, given that the Duty Cycle is 70%

Therefore we need find out the Voltage at 70% Duty Cycle

`V = 72times0.7 = 50.4V`

`T__m=(50.4times0.7)/0.5-(omegatimes0.7^2)/0.5`

`T__m = 70.56-omegatimes0.98.................(Eq-3)`

At the Steady State Condition of EV

`T__TR = T__m`

Now substituting the (Eq-2) & (Eq-3)

`24.72+0.0051timesomega^2=70.56-omegatimes0.98`

`0.0051timesomega^2+omegatimes0.98-45.84=0`

Omega value will alwyas be positive therefore

ω = 38.90 rad/sec.

ANSWER-3

WALLAY RIPPLE is a long-time Proponent of electric vehicle. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed The Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.

A Closer Look
So what are these two technologies? How do they work? What differentiates them? And what do they have in common? Let’s start with DC brushless drives.

With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an Inverter. Without it, brushless motors are useless motors.

Let’s move on to induction motor drives. A forerunner of the 3-Phase Induction motor was invented by Nikola Tesla sometime before 1889. Curiously, 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 – 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. In turn, 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. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

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. (Were an inverter needed, Tesla’s invention would have been useless until sometime in the 1960s.) 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.

While 3-phase induction motors have great utility, they also have some severe limitations. They cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequency. Hence, when used with utility power, they are constant speed machines. Finally, when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.

Add an inverter (without any feedback control) and it becomes possible to power an induction machine from a battery or other DC source; variable speed also becomes possible simply by adjusting the inverter frequency. Still, torque performance is low compared with DC machines. Add some feedback loops such that the inverter produces the exact frequency that the motor “desires,” and the induction motor is now capable of competing with DC and DC brushless for vehicle applications.

Brushless or Induction?

Back in the 1990s all of the electric vehicles except one were powered by DC brushless drives. Today, all the hybrids are powered by DC brushless drives, with no exceptions. The only notable uses of induction drives have been the General Motors EV-1; the AC Propulsion vehicles, including the tzero; and the Tesla Roadster.

Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. (DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these differences are relatively small.)

One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.

Still No Winner
My conclusion is that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics. The question is what will happen as hybrids become more electrically intensive and as their performance levels increase? The fact that so much of the hardware is common for both drives could mean that we will see induction and DC brushless live and work side by side during the coming golden era of hybrid and electric vehicles.