Week-6 Challenge: EV Drivetrain

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

power electronics is a technology which deals with converting and controlling flow of electrical energy in order to supply required output voltage to the load. We generally use this power converters to guide the DC/AC motors in Hybrid electrical vehicles in order to achieve variable speeds.

not only in HEV'S these convertors are used inside of so many electrical components to supply required voltages.

basically, these power electronics are used to change the form and supply of the current.

there are four types of power convertors are there they are.

AC to DC = rectifier is used to convert Alternating current in to Direct current.

DC to AC = Invertor is used to convert Direct current in to Alternating Current.

DC to DC = chopper is a power convertor used to convert the high voltage DC current to low voltage Direct current and vice-versa.

AC to AC = Regulators are used to change the frequency of Alternating current in order to achieve required speeds.

Rectifier:

Rectifier is used to convert Alternating Current in to Direct Current in order to run the DC appliances.

based on providing DC -current Rectifiers are divided in to two types

Types of Rectifiers

• Half-Wave rectifier
• Full wave rectifier

Half-Wave Rectifier:

Half wave rectifier is a type of rectifier which allows only half cycle of the AC voltage while blocking the other half cycle of the AC voltage by using a Single PN junction diode.

Simplest form of rectifier has only one diode which is forward biased while positive half of the cycle and reverse biased during negative half of the cycle. That means it doesn’t allow current to pass during negative half of the cycle. The wave form is in the form of

Full- wave Rectifier:

Full-wave rectifier is a type of rectifier which allows Alternate current to pass through the pn-junction diode even when it switches its polarity.

It allows Alternate current to flow through the diode even diode is in forward and reverse biased condition. The output waveform should be like the below one.

In the full wave rectifier, we use two circuits. First circuit has a primary coil and second circuit has a secondary coil. Emf is induced in the secondary coil when it is inside the magnetic field of primary coil.

The main use of rectifier is to convert AC- direct current without any pulses. But the output waveform has pulses to remove those pulses we have to use filter.

Invertor:

Invertor is a power convertor which is used to convert Direct current in to Alternate Current.

From the above shown image inverter is used in between the boost convertor and electric motor. In this above image electric motor used is induction motor which runs by using the Alternate Current, the input which received by the inverter from the Boost converter(Step-up-converter) is Direct current, Direct current is converted in to the Alternate current which is used by the Electric motor.

Chopper:

Chopper is the power convertor which is used to convert high voltage Direct current into low voltage Direct Current and Vice-Versa.

Chopper is divided in to two types based on the conversion of voltage, they are

• Step-up chopper
• Step-down chopper.

Step-up chopper/Boost converter:

The above shown image is the Step-Up chopper which converts low voltage dc in to high voltage Direct current.

When the switch is on the current flows to the load by passing through inductor and Capacitor.  We all know that inductor and capacitor get charged up when the current pass through the inductor and capacitor. This charged current is used when the switch puts off.

Boost converter/step up chopper converts low voltage in to high voltage by simply supplying varying and maintain the time periods. These time periods are maintained according to the control signals received from the comparator.

The on-off switching condition is maintained by the semiconductor device, because the manual switching condition is difficult to maintain.

Step-down chopper/Buck convertor:

The above shown is the step-Down chopper/Buck converter which is used to convert high voltage current. When the switch is on the current flows to the load by passing through inductor and Capacitor.  We all know that inductor and capacitor get charged up when the current pass through the inductor and capacitor. This charged current is used when the switch puts off.

Buck converter/step-down-chopper converts high voltage in to low voltage by simply supplying varying and maintain the time periods. These time periods are maintained according to the control signals received from the comparator.

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

     ……………………………………………Eq - 1

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

We know the function of chopper it works by switching on/off semiconductor according to the Control signals received from the comparator.

From the above given conditions Duty cycle is 70%

From the Formula:

Vavg   =         Vin*(Duty cycle)

Vav     =          average voltage/Output voltage

Vin      =          input voltage/Supply voltage

Vavg   =          72 * 0.70

            =          50.4 V

Voltage supplied to the motor is about 50.4V

From the Formula:

T = (V-WK/R) *K

T = Torque of the DC Motor

V = Input Voltage

R = Resistance

K = Motor Constant

W = Speed constant (Rad/Sec)

T = ((50.4*0.7)-W*(0.7*0.7))/0.5

T = (35.28 -W*0.49)/0.5

T = 70.56 – 0.98 * W…………………………………………………………………. Eq -2

The two equations 1 & 2 should equate to get W value

24.7 + (0.0051) * W² = 70.56 – 0.98 * W

0.0051W² -45.86 + 0.98W = 0

By solving above equations, we can get the speed of the vehicle at the specific steady state condition.

W = 38.8 Rad/sec

W = -230.95 Rad/sec.

We have to consider the positive value of speed of the motor W = 38.8 rad/sec

Converting rad/sec in to Rpm

(38.8 * 60)/2 * 3.14    =370.50 Rpm.

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

      ωω=   V/Kϕ -Ra/Kϕ^2.T

The above shown is the mathematical model of a Dc motor of the given equation .in this model the ramp block is used to represent the increasing torque with respect to speed and time.

The constant block is used for the constant parameters like Voltage, Motor constant, resistant, etc.

            The above graph satisfies the characteristics of a Dc- Motor, the torque is inversely proportional to the Speed of the motor. The torque is gradually decreases with increasing speeds.

4-Induction vs Dc-Motor by Wally Rippel, Tesla.

In the blog Wally-Ripple tesla explained about the Dc-Motors and Induction motors and their usage in EV-Domine. He stated the what changes should consider during selection of motors in Electric vehicle/Hybrid Electric Vehicle, and how inverters used to change the Frequency of AC-supply. he concluded that brushless Dc-Motors are used in Hybrid and coming Plug in HEV’S, and induction motors ae used in high performance pure electric vehicles but this two work side by side in coming generations.