Challenge

EV Drivetrain

MMuthavarapu Chaitanya Krishna5 Apr 2021

AIM: To Study EV Drivetrain

TASK-1: Which types of power converter circuits are employed in electric and hybrid electric vehicle?

ANSWER:

Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time. The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket.

Direct current (DC) is the one directional flow of electric charge. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. A term formerly used for this type of current was galvanic current. Direct current has many uses, from the charging of batteries to large power supplies for electronic systems, motors, and more. Very large quantities of electrical energy provided via direct-current are used in smelting of aluminum and other electrochemical processes.

Power converter circuits: 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. The circuits which are used for converting are known as Power converter circuits.

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.

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

AC to DC Converter
DC to AC Converter
DC to DC Converter
AC to AC converter

AC to DC Converter: It is commonly known as a “Rectifier”. A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. Physically, rectifiers take a number of forms, including vacuum tube diodes, wet chemical cells, mercury-arc valves, stacks of copper and selenium oxide plates, semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches.

The rectifiers can be commonly classified as follows:

Single Phase Rectifiers
- Half Wave Rectification
- Full Wave Rectification
Three Phase Rectifiers
- Three Phase, Half Wave Rectification
- Three Phase, Full Wave Rectification using Center-Tapped Transformer
- Three Phase Bridge Rectifier Uncontrolled
- Three Phase Bridge Rectifier Controlled
- Twelve-pulse Bridge

DC to AC Converter: It is commonly known as a “Power Inverter or Inverter”. A power inverter, or inverter, is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. 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. These can be single phase or three phase inverters 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 Converter: It is commonly known as a “Chopper”. A Chopper is a power electronic Static device, that converts power directly from fixed DC to variable DC. It gives smooth speed control and fast dynamic response. Some of the applications include subway cars, DC traction systems, control of large DC motors, battery operated vehicles, trolley buses, etc. They can be unidirectional or bidirectional devices based on the application it is intended for. DC choppers are classified into three basic types based on input and output voltage levels. They are:

Step-down or buck converter
Step-up chopper or boost converter
Buck or boost converter

AC to AC Converter: A solid-state AC to AC converter converts an AC Waveform to another AC waveform, where the output voltage and frequency can be set arbitrarily. These are usually called “Cycloconverter” or “Matrix Converters”. A cycloconverter constructs an output, variable-frequency, approximately sinusoid waveform by switching segments of the input waveform to the output; there is no intermediate DC link. With switching elements such as SCRs, the output frequency must be lower than the input. Very large cycloconverters (on the order of 10 MW) are manufactured for compressor and wind-tunnel drives, or for variable-speed applications such as cement kilns. In order to achieve higher power density and reliability, it makes sense to consider Matrix Converters that achieve three-phase AC-AC conversion without any intermediate energy storage element.

Usage of the converters in EV and HEV:

The most of the converters used in EV and HEV are DC-AC and DC-DC converters. These converters are used in most of them applications. But AC-DC converter is used for only one application in these vehicles. AC-AC converter is not used in the vehicles. The applications of various converters used in EV and HEV are shown below.

AC to DC converter	DC to AC converter	DC to DC converter
Charging the battery from an external supply. The output is AC but the battery is DC.	To supply power to Electric Vehicle Induction Motor	Various onboard loads such as, sensors, controls, entertainment, utility, and safety equipment.
	Plays a vital role in feeding the battery through regenerative braking.	Used in DC motor drives and electric traction.
	It accepts the high voltage from the traction battery and converts it typically in the 3-phase AC voltage suitable for the traction electric motor.	Used in regenerative braking.
	The inverter controls the amount of the energy generated by the motor and consequently the intensity of the braking.	Backup Power
		Charging the battery from a different voltage level.

TASK-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 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

T = 24.7 + (0.0051 * ω^2)

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

SOLUTION:

Given input parameters:

Rated Armature voltage (Vbatt) = 72 V

Rated armature current (Ia) = 400 A

Armature resistance (Ra) = 0.5Ω

KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

Duty cycle (d) = 70% = 0.7

Torque speed characteristic is given as T = 24.7 + (0.0051 * ω^2) … (1)

Total Voltage (V) = Vbatt * d = 72 * 0.7 v

We know steady state speed is ω. And it’s given by,

ω = (V/Kϕ) - ((T*R)/(Kϕ)^2)) … (2)

Substituting the values in (2).

ω = ((72 * 0.7)/0.7) - ((T * 0.5)/(0.7)^2)

ω = 72 - (0.5T/0.49)

72 - ω = 0.5T/0.49

0.49(72 - ω) = 0.5T

35.28 - 0.49ω = 0.5T

T = 70.56 - 0.98ω … (3)

Substitute (1) in (3)

24.7 + (0.0051 * ω^2) = 70.56 - 0.98ω

0.0051 * ω^2 + 0.98ω = 70.56 - 24.7

0.0051 * ω^2 + 0.98ω - 45.86 = 0

It is a polynomial equation with second degree. By solving the factors (4), we get a positive and a negative value as factors.

i.e., ω = 38.915 and -231.072

Since the speed should be positive, therefore ω = 38.915 rad/s

Therefore, the steady state speed of EV if duty cycle 70% is 38.915 rad/s

TASK-3: Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

ABOUT THE AUTHOUR:

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.

OBSERVATION:

In this blog, the author, Wally Rippel explains how induction and DC brushless motor used now a days. He also explains in detail of both motors like advantages, requirements, etc. He explains these motors as following.

Since we have various kinds of engines according to usage, prices and performance, in the same way electric drive vehicles also have different peripherals for different purpose and standards.
Previously lead acid batteries, DC brush motors and contactor controllers are used. Now-a-days those were replaced and we use lithium batteries, DC brushless motors and modulating inverters. He states that technology is developing in each and every component or field.
He starts with DC brushless motor. When the motor runs, it is necessary that the magnitude and polarity of the stator currents be continuously varied and the torque should be constant and electrical to mechanical conversion is optimally efficient. This can be done by the inverter and motors are useful without these.
He later explains about induction motor which was invented by Nikola Tesla before 1889. Both motors contain same type of stators which contains three sets of distributed windings. The main difference between these two is the rotor.
Induction rotor doesn't have magnets just stacked steel laminations with buried peripheral conductors which forms a shorted structure.
The speed difference can be said. Since the current produced within the rotor is proportional to the voltage. The currents interact with magnetic field and produces force.
When induction motors are used, there is no need of inverters. Induction motors are directly compatible with conventional utility power, where, inverters are needed for DC brushless motors to develop torque. The phase is maintained in step with the angular position of the rotor.
When operated from utility power, induction motors have limited starting torque and somewhat limited running peak torque capabilities.
Variable speed also becomes possible simply by adjusting the inverter frequency. This shows that induction motors are competent with DC motors in applications.
Since mostly all vehicles contain DC brushless motors now-a-days, only some vehicles like GM EV1 and tesla roadster etc uses induction motors.
The differences between these two kinds are too small. Since the differences are in rotors and inverter controls. DC motors require absolute position sensor and induction requires speed sensor.
The best power factor in induction is 85%. He also states the motor properties regarding eddy currents and hysteresis.
He states regarding permanent and ferromagnets used in EV's. Their cost and forces they gain in motors. Added development costs, but likely little or no recurring costs.
Induction motors are difficult to control since laws are more and also achieving stability over the entire torque speed range is difficult.
He finally concludes that, both motors are used for EV's and almost gives same overall results. DC motors have more dominance in hybrid and plug-in hybrid vehicles. Where induction motors have dominance in pure EV. the technology develops more in future where induction can also be used in hybrid vehicles. Both motors should work side-by-side in future.

REFERENCES:

EV Drivetrain

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