Week-6 Challenge: EV Drivetrain

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

In Electric and Hybrid Electric vehicles we use three types of power converter circuits

i)AC to DC converter (Rectifier)

A power electronics circuit that converts alternating current (AC) power to direct current (DC) power is called a rectifier.

In Electric and Hybrid Electric vehicles we use the Rectifier circuit to charge the battery, where the external AC power is converted to DC power which is required to charge the battery. Required DC voltage and current is obtained with the help of this circuit.

ii)DC to AC converter (Inverter)

A power electronics circuit that converts direct current (DC) power to alternating current (AC) power is called an Inverter.

In Electric and Hybrid Electric vehicles normally BLDC motors and Induction motors are used. We use the Inverter circuit as motor drives, where the external DC power of the battery is converted to AC power to drive the motors. Inverters can be designed as both Single Phase Inverter and Three Phase Inverter as per the motor we use.

iii)DC to DC converter (Chopper)

A power electronics circuit that converts a source of direct current (DC) from one voltage level to another voltage level is called Chopper.

Mainly we use two types of Chopper circuits in Electric and Hybrid Electric vehicles

>Buck Converter (Step-down DC-DC converter)

This Converter reduces the DC voltage level to the required one. We require different DC voltages for Head Lamps, Side Lights, or any other electrical load inside a vehicle so we use Buck Converter here to avoid usage of different batteries individually.

>Boost Converter (Step-Up DC-DC converter)

This Converter increases the DC voltage level to the required one. In some Electric and Hybrid Electric vehicles, we use Boost converters in charging circuits and regeneration circuits.

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

Given,

Rated Armature voltage= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

T = 24.7+0.0051`omega^2`----- (1)

Also given Duty Cycle is 70%

At Duty Cycle 70%, Volatge = 72*0.7=50.4 V

We know the torque equation is given by

`T=(V*K phi)/(Ra)-(K phi)^2/(Ra)*omega`

Here, `T=(50.4**0.7)/(0.5)-(0.7)^2/(0.5)**omega`

`T=70.56-0.98 omega` ----- (2)

Compareing both equations (1) and (2)

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

`0.0051omega^2+0.98omega-45.86=0` -----(3)

Solveing the equation (3)

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

`omega=38.915 or −231.072`rad)/s

We can neglect the negative value and consider the `omega=38.915`rad)/s

3. Refer to the blog on the below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about the author’s perspective.

In the blog, the author Wally Rippel compares the Induction motor and Brushless DC Motor concerning the Electric Vehicle driving application.

In the first section, he tells that the change is mandatory in any field. He starts with a comparison with conventional gas/fuel-powered vehicles where we have a verity of engines available as per the user requirement. He says that there is nothing which engine is best, he also adds that this also applies for electric vehicle drives. He then tells about the transition that happened in the industry from using Lead-acid batteries, DC brushed motors, and contactor controllers to Lithium batteries, BLDC or Induction motors, and modulating inverters respectively.

Moving to the second section he describes the working of Brushless DC Motors and Induction Motors. He also compares the structure model of both the motor types. He also describes the limitations of both the motors. In the case of BLDC motor, without inverter it is useless. The inverter is essential to the current control to ensure smooth functioning. Even though it is a DC motor, the inverter is a must. The limitations of a 3-phase induction motor are pointed like, "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."

In the third section, the author tells that in 1990s all electric vehicles except some like General Motors EV-1 and Tesla Roadster were using BLDC drives. He then takes us to a more in-depth comparison between two motors. He describes the rotor specifications, the usage of sensors, the heat generated, and the losses in the two motors. The author also compares the efficiency like "The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive." Discusing stability and controllability Induction motor is less stable than BLDC motor .Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless.

The author concludes with a title "Still No Winner" , he says that BLDC motor drives will likely continue to dominate in the hybrid and plug-in hybrid vehicles and Induction motors will maintain dominance for the high-performance pure electric vehicles. He foresees a golden era of hybrid and electric vehicles by using both Induction and BLDC drives side by side.

CONCLUSION

1.types of power converter circuits employed in electric and hybrid electric vehicles are discussed.

2.Value of EV steady state speed at 70% of duty cycle is calculated from the given conditions.

3.The author’s perspective in the given blog is briefly explained.