Week-6 Challenge: EV Drivetrain

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

Ans:-

A power converter is an electrical circuit that changes the electric energy from one form into the desired optimized for the specific load. A converter may do one or more functions and give an output that differs from the input.

A device that increases or decreases the voltage (AC or DC) of a power source depending on the application. A converter that increases voltage is called a step-up converter and a converter that decreases voltage is called a step-down converter. In EVs/HEVs step-up and step-down converters are combined into one unit. An application of a step-up converter is converting EV/HEV battery voltage (typically 180-300 volts) to about 650 volts to power the traction motor. An advantage of using a converter to increase the voltage from the battery is a smaller and less expensive battery may be used while still utilizing an efficient high voltage motor. An application of a step-down inverter would be decreasing the high voltage direct current (DC 180-300 volts) from the HEV/EV battery to low voltage (12- 14 volts) DC that can be used to charge the 12 volt auxiliary battery and operate light load devices such as lighting, radio, and windows.

• The Power Electronic Converter can be classified into four types:
• AC to DC Converter (Controlled Rectifier)
• DC to DC Converter (DC Chopper)
• AC to AC Converter (AC voltage regulator)
• DC to AC Converter (Inverter)

In EVs, the Fuel Cell (FC) stack, battery bank, and Super capacitors (SCs) bank are usually used as clean energy sources.

In order to design highly efficient converters for the EV's power system, advanced DC/DC, AC/DC, and DC/AC converters are required to adopt the output voltage and current levels with high power quality.

DC to DC Converter (DC Chopper):-

Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs), and Fuel Cell Electric Vehicles (FCEVs) have been typically proposed to replace conventional vehicles in the near future. DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels.

DC-to-DC converters are used to reduce High voltage DC input to low voltage DC output for some specific appliances. They are also used to isolate some highly sensitive components in a circuit from other components of the circuit to avoid any kind of damage.

Electric vehicles (EV) use two different power systems; a high-voltage battery (200 to 450 VDC) for traction and a low-voltage (12 V) one for supplying all the electric appliances in the vehicle. Traditionally the low-voltage battery was charged from the alternator, but in today's vehicles, it gets its power from the high-voltage battery pack. However, in specific electric car architectures, this low voltage battery should be ready to help recharge the high-voltage battery pack in order to provide energy for cranking the car. This means that the onboard DC-DC converter must be bi-directional and very efficient as well as highly reliable in order to run the complex control algorithms needed to ensure an energy-efficient solution.

AC to DC Converter (Controlled Rectifier):-

It's called the "onboard charger" though it really is a converter. It converts power from AC to DC and then feeds it into the car's battery. This is the most common charging method for electric vehicles today and most chargers use AC power.

DC to AC Conveter (Inveter):-

The DC to AC converter is required to convert the dc power to suitable ac power for the electrical machine the power output from the battery pack (DC current) is converted using the inverter to be used in the electric motor (AC current)

Converts direct current (DC) from the battery to alternating current (AC) to be used by other devices such as the traction motor and coolant pump.

The power converter is made of high-power, fast-acting semiconductor devices, such as BJT, MOSFET, IGBT, SCR, GTO, and other thyristor families. these devices are used as ON-OFF electronics switch to convert the fixed supply voltage into the required variable voltage and with variable frequency supply.

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Q:- 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%?

Ans:-

Vehicle Output Voltage (Vv) is given by: -Vv = Battery Voltage (V) * Duty Cycle

Vv = 72 * 0.70

Thus, Vehicle voltage = 50.4V

Duty cycle is the ratio of the Ton and total time (Ton+Toff)

Duty cycle = Ton / (Ton+Toff)

Now, the Equation for Vehicle torque is given by: -

Where;

Tv= Vehicle Torque (N-m)

Vv = Vehicle voltage (Volts)

ω= steady-state speed of vehicle (rps)

K ∅ = Motor constant

Substituting all these values in the equation, we get:

Tv = ((50.4 - O.7 * ω)*O.7)/0.5

Tv = 70.56 − 0.98 ⋅ ω ------- (i)

Given vechicle torque equation

Tv = 24.7 + 0.0051 ⋅ ω^2 ----- (ii)

equating these two equations,

70.56 − 0.98 ⋅ ω = 24.7 + 0.0051 ⋅ ω2

We get a quadratic equation:

0.0051 ⋅ ω2 + 0.98 ⋅ ω − 46.94 = 0

Since quadratic equation, we get two values of ;

ω= 38.85 rad/s and ω = -230.95 rad/s

We take the positive value;

Steady state speed of vehicle = = 38.85 rad/s

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

ω= V/Kϕ -Ra/Kϕ^2 .T

Ans:-

ω= V/Kϕ -Ra/Kϕ^2 .T by using this equation I create a mathematical model of a dc motor the screenshot of the model is attached below and the Simulink model is also attached below.

in this model, you have to give the inputs like the voltage, constant value, armature resistance, and torque values

based on the above result you will get the speed.

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Q:- 4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Ans:-

• In the blog of induction versus DC brushless motor, the author has mentioned about advantages and disadvantages of both induction and DC brushless motor. In the blog, the basic construction of both the induction and DC brushless motor are explained.
• In case of DC brushless motor, permanent magnets are used which produces a rotating magnetic field. This magnetic field when induces a current in the conductor. This current results in force generation in the rotor and the result in rotation of the rotor.
• whereas in case of an induction motor, the current is supplied to the stator which results in the generation of emf, when the emf comes in contact with the rotor which is generally a squirrel cage type, this results in current induced in the rotor. unlike the DC motor, the rotor consists of conductors plates and is placed in a cage-like structure. This also consists of bars around them which current gets induced and the rotor starts rotating.
• one of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and at peak point, efficiency is generally higher for this drive. It is mentioned that the DC brushless Can drive at unit power factor. whereas the best power factor for the induction drive is about 85% (0.85).
• In case of dc brushless motor, the strength of the magnetic field is adjustable. when at low speed the maximum torque is required magnetic field should be maximum, i.e, inverter and motor current maintained at their lowest optimize efficiency. whereas there is no such adjustments in the induction motor.
• As machine size grows, magnetic losses increases proportionally and part load efficiency drops. In case of induction motor, as machine size grows, losses do not necessarily increases.
• The construction of dc brushless motor is costly compared with an induction motor, due to the use of permanent magnets.
• however thoughts the speed variations of the induction motor is depends upon the frequency input, it is difficult to control.

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