Week-6 Challenge: EV Drivetrain

Aim: Challenge on EV Drivetrain

Objectives:

1. To understand the types of power converter circuits are employed in an electric and hybrid electric vehicle.
2. Duty ratio and power electronics on the vehicle control.
3. To explain briefly about the author's perspective on induction versus dc brushless motors by Wally Rippel, tesla.

Introduction:

Electric Vehicle Powertrain:

Fig- Block diagram of Electric vehicle powertrain

The above figure powertrain block consists of a DC-DC converter and inverter (DC-AC ). The DC-DC converter is a fixed DC input voltage into variable DC voltage or vice versa. The DC output voltage is controlled by varying duty cycle.

This electrical device actually changes the voltage (either AC or DC) of an electrical power source. There are two types of voltage converters: step-up converters (boost converter) and step down converters (buck converter). The most common use of a converter is to a take 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.

In EV, the main source of electric energy from the battery to drive the motor. To circulate this energy continuously to auxiliaries, a boost converter is used to increase the voltage with a low current. The main features of Boost converter are:-

• Continuous input current eliminates input filter.
• Pulsed output current increases output voltage ripple.
• Output voltage is always greater than the input voltage.

Hybrid Electric Vehicle Powertrain:

Fig- Block diagram of Hybrid Electric vehicle powertrain

The above figure powertrain block consists of the DC-DC converter, AC-DC converter (Rectifier) and inverter (DC-AC ). The DC-DC converter is a fixed DC input voltage into variable DC voltage or vice versa. In HEV, the main source of electric energy from the engine generator to drive the motor, with assistance from the battery. To circulate this energy continuously to auxiliaries, buck converter and boost converter are used to vary the voltage with respect to the current.

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

Power 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 the 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.

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 equipment.

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.

Power Electronic Converter:

• The block diagram of a power electronic converter is shown in the figure above. It consists of an electrical energy source, power electronic circuit, a control circuit and an electric load. This converter changes one form of electrical energy to another form of electrical energy.
• The power electronic circuit consists of both the power part and control part. The 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 analogue 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.

Depending on the type of function performed, power electronic converters are categorized into the 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 the 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.

1. 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 the figure below.

• The essential components in this rectifier include a transformer, switching unit, filter and a control block.
• Here, the transformer adjusts the primary AC source supply to the input of the 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 a DC supply.
• It comprises a diode and/or thyristors based on the type of rectifier. The output of the rectifier is of pulsed DC and hence it is filtered using a filter circuit, which is usually made with a capacitor or a choke.
• The control block controls the firing angle of thyristors in the case of phase-controlled rectifiers. Since the diode is not a controllable device, a control block is not needed in the case of diode rectifiers.

2. DC to AC Converters or Inverters:

These converters are connected between the DC source of fixed input and variable AC load. Most commonly, these DC to AC converter is called an inverter. 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 the 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 as 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.
• The inverters overcome the drawbacks of PWM based inverters.

3. 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, trolleybuses, 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 a different magnitude (which can be lower or higher) than the input value. The block diagram of a DC chopper is shown in the figure below.

• The chopper circuit is connected between the 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 the required DC voltage to the load.
• The output voltage is controlled by adjusting the ON time of the thyristor (or switch) which in turn changes the width of the DC voltage pulse at the output. This method of switching is called 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.

Non-isolated converters:

The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio (less than 4:1). Moreover, when there is no problem with the output and input having no dielectric isolation. There are five main types of the converter in this non-isolated group, usually called the buck, boost, buck-boost, and Cuk and charge-pump converters. The buck converter is used for voltage step-down, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up. The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power applications.

(a) Step-Down (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 the 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 freewheeling diode that allows the load current to flow through it when the 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 the turn ON/OFF periods of a thyristor. When the thyristor is turned ON, the output voltage is the 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.

(b) Step-Up (Boost) Converter:

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

(c) Buck-Boost Converter:

This chopper can be used both in step-down and step-up modes by continuously adjusting its duty cycle. The configuration of the buck-boost converter is shown in the figure below that consists of only one switching device, i.e., one thyristor along with an inductor and diode, the additional capacitor is connected in parallel with this circuit.

• When the thyristor is turned ON, the supply current flows to the inductor through the thyristor and induces the voltage in an inductor.
• When the thyristor is OFF, the current in the inductor tends to decrease with the induced emf reversing polarity. The output voltage of this converter remains constant as the capacitor is connected across the load.
• By varying the value of duty ratio to a certain value, the output voltage is lower than the input voltage, typically in the range 0 ≥ k > 0.5, thus a buck converter.
• And the output is higher than the input voltage if the duty ratio is in the range of 0.5 > K ≥ 1, thus acts as a boost converter.

Isolated converters:

Usually, in this type of converters, a high-frequency transformer is used. In the applications where the output needs to be completely isolated from the input, an isolated converter is necessary. There are many types of converters in this group such as Half-Bridge, Full-Bridge, Fly-back, Forward, and Push-Pull DC/DC converters (Garcia et al., 2005), (Cacciato et al., 2004). All of these converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high. Some of the types are explained below,

AC-to-AC Converters:

AC/AC converters connect an AC source to AC loads by controlling the amount of power supplied to the load. This converter converts the AC voltage at one level to the other by varying its magnitude as well as the frequency of the supply voltage.

These are used in different types of applications including uninterrupted power supplies, high power AC to AC transmission, adjustable speed drives, renewable energy conversion systems and aircraft converter systems.

The types of AC to AC converters are:

a) AC-to-AC Voltage Converters:

• These converters control the RMS value of output voltage at a constant frequency. The common application of these converters includes starting of AC motors and controlling power to heaters.
• A single-phase AC to AC voltage converter consists of a pair of anti-parallel thyristors along with a control circuit as shown in the figure below.
• The other names of this controller are single-phase full-wave converter and AC voltage controller.

• During the positive half cycle of the input signal, thyristor-1 is forward biased and it starts conducting when the triggering is applied. Thus the power flows from source to load.
• In the negative half cycle of the input, thyristor-2 is forward biased and starts conducting when it is triggered, while thyristor-1 is turned OFF by natural commutation.
• By varying the triggering or conduction angle of each thyristor during each half-cycle, the magnitude of voltage that appeared across the load is controlled.
• The other popular form of AC voltage controller is the use of TRIAC in place of two anti-parallel thyristors. The figure below shows a TRIAC based AC controller along with a triggering control circuit.

• Here DIAC controls the positive and negative triggering to the TRIAC so that the average output voltage to the load is controlled.

b) AC-to-AC Frequency Converters:

These converters are mainly used for varying the frequency of the input source to the desired level of the load. An AC to AC frequency converter changes the frequency of input voltage/current of the load compared to the frequency of the source.

Some of these converters may control the magnitude of voltage besides the frequency control. These are mainly used for adjusting the speed of AC drives and also for induction heating.

Q2. An Electric Vehicle's powertrain with a 72V battery pack is 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 are 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 the duty cycle is 70%?

Solution:

Parameters of the electric vehicle given are –

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

`T_v = 24.7 + (0.0051)omega^2` (1)

The duty cycle is given as 70%.

The average voltage `V_0` is given by-

`V_0 = "Supplied Voltage" times "Duty Cycle"`

`V_0 = 72 times 70/100`

`therefore V_0 = 50.4 V`

The DC motor torque-speed equation is given by-

`T_m = (V_0 times Kphi)/R_a - ((Kphi)^2 times omega)/R_a` (2)

Substituting the given values in the above equation (2) we get,

`T_m = (50.4 times 0.7)/0.5 - ((0.7)^2 times omega)/0.5`

`therefore` `T_m = 70.56 - 0.98omega` (3)

At the steady-state speed of Electric Vehicle, the Torque of both vehicle and motor will be the same:

i.e `T_v = T_m`

`therefore` Equating equations (1) & (3)

`24.7 + (0.0051)omega^2 = 70.56 -0.98omega`

By equating equations (1) & (3) we get a quadratic equation,

`0.0051 omega^2 + 0.98omega – 45.86 = 0`

Solving quadratic equation using the formula-

`omega = (-b pm sqrt(b^2 - 4ac) )/(2a)` (4)

where,

a = 0.0051

b = 0.98

c = 45.86

Substituting the above values in equation (4)

`omega = (-0.98 pm sqrt(0.98^2 - 4 times 0.0051 times (-45.86)) )/(2 times 0.0051)`

Solving the above equation for both (+) and (-) we get two different values of `omega` that are:

`omega = 38.91 "Rad"/sec`

`omega = -231.07 "Rad"/sec`

A positive `omega` value indicates the forward direction.

`therefore` EV steady state speed is `omega = 38.91 "Rad"/sec`.

Q3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

About Author:-

Wally Rippel is a long-time proponent of electric vehicles. 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.

Brushless DC(BLDC) drives:

• In BLDC, the rotor includes two or more permanent magnets that generate a DC magnetic field. In turn, this magnetic field enters the stator core 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.
• Brushless DC motor produces no starting torque when directly connected to fixed frequency utility power and they need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor.
• BLDC works on fixed frequency utility power and difficult to control.
• 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.
• In brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable.
• DC brushless drives require an absolute position sensor.
• In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced.
• With DC brushless machine as the size grows, losses increase proportionately and part load efficiency drops.
• Not favoured where high performance is desired.

GIF.- Brushless DC Drive

Induction Motor drives:

• 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.
• Induction motors are directly compatible with conventional utility power and easy to control is the main reason for their success.
• 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.
• In contrast, induction machines have no magnets, and B fields are adjustable.
• Induction drives require only a speed sensor.
• induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.
• The induction machine when operated with a smart inverter has an advantage over a DC brushless machine – magnetic and conduction losses can be traded such that efficiency is optimized.
• With induction, as machine size grows, losses do not necessarily grow.
• Induction drives may be the favoured approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

Similarities of Induction And DC brushless drives:

• 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.
• Moreover, with digital controllers, the only control differences are with the control code.

Cost-effectiveness :

Permanent magnets are expensive something like $50/kilogram and these rotors are also difficult to handle due to large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines and also due to field weakening capabilities of induction machines inverter ratings and costs appear to be lower especially for high-performance drives since spinning induction machines produces little or no voltage when de-excited and they are easier to protect.

Induction machines are more difficult to control. the control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than brushless DC motor. This means added development costs but little or no recurring costs.

Conclusion:

1. From the author’s point of view, it is clear that ‘There is no one best engine type, rather there are different types of engines to suit personal requirements’ such as price and performance and this is also true for electric vehicle drives.
2. While developing hybrid or pure electric vehicle we need high performance and low developing cost we need to select an induction motor.
3. In the future hybrid electric vehicles also will be developed to wrap a lot of performance characteristics like pure EVs and hence author concludes with the vision stating ‘we will see induction and DC brushless working together at the same time in the coming golden era of hybrid and electric vehicles'.