Challenge

Week-6 Challenge: EV Drivetrain

SSoumik Dey6 Jan 2021

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

Solution :

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.

AC to DC = Rectifier: It converts AC to unipolar (DC) current

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

Uncontrolled diode rectifiers

single phase half wave rectifier
single phase centre tapped half wave rectifier
single phase full wave bridge rectifier.

other rectifiers are as follows :

Three-phase Half-wave diode rectifier

Three-phase full-wave diode bridge rectifier

Single phase half-wave rectifier

Single phase full wave mid-point rectifier

Single phase full wave bridge rectifier

Three-phase half-wave converter

Three-phase full wave converter

DC to AC = Inverter: It converts DC to AC of desired frequency and voltage

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.

The inverters overcome the drawbacks of PWM based inverters.

DC to DC = Chopper: It converts constant to variable DC or variable DC to constant DC

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.

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

Step-down Chopper or Buck converter

Step-up Chopper or Boost converter

Buck/Boost converter

AC to AC = Cycloconverter, Matrix converter: It converts AC of desired frequency and/or desired voltage magnitude from a line AC supply.

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/AC voltage converter consists of a pair of anti-parallel thyristors along with a control circuit as shown in figure below.

The other names of this controller are single phase full wave converter and AC voltage controller.

During 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 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 angel of each thyristor during each half-cycle, the magnitude of voltage 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 TRIAC based AC controller along with triggering control circuit.

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

Cyclo Converters.

and

Matrix converters.

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'.

Solution :

Motor and Controller Parameters:

Rated Armature voltage= 72Volts.

Rated armature current= 400 A.

Ra= 0.5Ω.

KΦ= 0.7 Volt/second.

Chopper Switching frequency= 400Hz.

Governing Equation :

The equation for torque in DC Motor is :

T = (km *`phi`* Es)/Ra - ((km *`phi`)^2 * `omega`) /Ra

After putting the values of different motor and controller parameters we get :

T = (0.7 *50.4) / 0.5 - (0.7*0.7)*`omega`/0.5

T = 70.56 -0.98`omega^2`................. (equation 1)

Now comparing the (equation 1) and the governing equation we get :

Two values of `omega`:

`omega^2` = 38.91

`omega` 1 = 6.237 radians

`omega` 2 = -231.07 radians

Conclusion :

The negative values of the angular velocity shows that torque is transferred to the vehicle externally.

And the positive angular velocity suggests that the torque reaches the wheels of the vehicle.

3 . Induction Versus DC brushless motors by Wally Rippel, Tesla

Induction vs Brushless Motor

Wally Rippel, Principal Power Electronics Engineer

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

Here in the article he is describing about the rapid change of technology that the we as a generation has seen in the field of power electronics and other mechanical and electrical fields of engineering.

His View on Engines :

He is saying in the odious world of gas powered vehicles, engines are not all alike.
There are flat-heads, Hemis, straight, opposed, and V configurations. And on and on.
Only the one best engine type would be in production, there was no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance.
This is also true for electric vehicle drives.

His View on Batteries :

During his time there were lead acid batteries, DC brush motors, and contactor controllers.
Today, none of these remain, lead has been replaced by lithium and DC by either DC brushless or induction.
Contactors, meanwhile, have given way to modulating inverters.
He is contemplating that new technology will come and the old ones will wither away as they will loose their relevance.
His guess, however, is that we will see both induction and brushless machines “duke it out” for many years to come.
Each will have its loyal proponents and religious detractors.

Brushless Machine :

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.

Advantages :

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.
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.

Induction Motor :

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.
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.

Advantages :

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.

Limitation :

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
Still, torque performance is low compared with DC machines.

Comparison :

BRUSHLESS MOTOR	INDUCTION MOTOR
DC brushless drives require an absolute position sensor.	While induction drives require only a speed sensor.
Much less rotor heat is generated with the DC brushless drive.	More rotor heat is generated.
Rotor cooling is easier and peak point efficiency is generally higher for this drive.	Rotor cooling is much tough in these type of motors, it takes more time fro this drive.
The DC brushless drive can also operate at unity power factor.	The power factor for the induction drive is about 85 percent.
The peak point energy efficiency for a DC brushless drive will typically less.	The peak point energy efficiency for a induction motor drive is more.
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.	Induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency).
Induction drives may be the favored approach where low-performance is desired; peak efficiency will be more.	Induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless,
DC brushless motors are less costlier.	The induction motors are much costlier in operation.
DC motor may not be operated as a smart inverter.	Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine.
Brushless motors are more easier to control. The control laws are more relaxed and easier to understand. Achieving stability over the entire torque-speed range and over temperature is more convenient with DC brushless.	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 with DC brushless.