Week-6 Challenge: EV Drivetrain

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

Nowadays power electronics become an essential part of automobiles. The power converters used based on the application for Electric vehicles and Hybrid Electric vehicle

The most common types of power converters are

Inverter(DC-AC)

Converter(DC-DC)

Rectifier(AC-DC)

Regulator(AC-AC)

Inverter(DC-AC):

The inverter converts DC to AC. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current.

Controlling the inverter can control the speed of the vehicle

The multilevel inverter has the highest efficiency.

Converter(DC-DC):

The function of a DC-DC converter is to change one level of DC voltage to another level of DC voltage. The change might be step-up(Boost) or step down(Buck) or it can be both (Buck-boost).

This converter is used to power up monitors, AC, lamb, headlight, etc.

Unidirectional converter used for powering sensor, lamb, utilities, safety equipment.

Bi-directional converter used for regenerative braking, charging, power backup.

Rectifier(AC-DC):

The function of the Rectifier is to change AC to DC. Used for slow charging (7KW) with 1 phase, fast charging (11KW) with 3 phase, regenerative action.

Regulator(AC-AC)

The function of the Regulator converts an AC waveform to another AC waveform, where the output voltage and frequency can be set arbitrarily.

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2. An Electric Vehicle's powertrain with 72V battery pack is 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 the duty cycle is 70%?

Data:

V=72 V

Ia=400 A

Ra=0.5Ω

KΦ=0.7

Chopper Switching frequency= 400 Hz

W=(V/(KΦ))-(RaT/(KΦ)^2)

W=(72*0.7/0.7)-((0.5*T)/(0.7*0.7))

W=72-1.0204T

(1)====>T=69.2-0.98W

Tv=24.7+(0.0051)W^2

Substituting (1)

0.0051W^2+0.98W-44.5=0

W=37.92 rad

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3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about the author's perspective.

Wally Rippel, author writing this blog, has experience in the General Motors, jet propulsion laboratory, Tesla. In this blog, he is saying that there is no best motor for all EV/HEV applications. He starts with the construction and working of DC brushless motor, induction motor also said the various positive and negative points on both the motors.

DC Brushless motor:

These motors operate on DC.

Merits:

1. Electrical to mechanical energy conversion is optimal
2. Can operate at unity power factor
3. Rotor cooling is easy & and peak point efficiency is higher
4. Less rotor heat is generated
5. Torque performance is high compared to induction motor
6. Higher starting and running torque compared to induction machine
7. Control is comparatively easy.

Demerits:

1. Torque remains constant
2. Without an inverter the brushless DC motor are useless motor.
3. Rotor includes permanent magnets
4. Permanent magnets are expensive
5. No starting torque when connected to the fixed frequency
6. Not easy to change magnetic field with permanent magnet
7. Efficiency is less
8. As machine size grows magnetic losses increases,load efficiency drops.

Induction motor:

These motors operate on AC supply.

Merits:

1. No magnets.
2. Has the ability to start under load when directly connected to the fixed frequency
3. Desired torque can be achieved
4. The motor has the ability to start under load, no inverter is needed
5. Efficiency is maximum over dc brushless motor when operated with smart inverter
6. The magnetic field can be adjustable
7. As machine size grows, losses are not necessary to grow.
8. Cost Advantage over PM machine

Demerits:

1. Stabilization of torque, speed, temperature is difficult.
2. Torque performance is low compared to DC motor.
3. Limited starting and limited running torque when compared to DC motor
4. The control laws are more difficult and complex to understand
5. The best power factor is about 85%

Conclusion:

After analyzing the merits and demerits of both the motor, Wally Rippel's conclusion is that DC brushless dominates in the hybrid and coming plug-in hybrid, and induction drives will likely maintain dominance for the high-performance pure electric. The selection of motors is mostly based on the application.

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