Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Ans:
Objective: To list down the power converter circuits used in EV and HEV.
A. Power electronic circuits used in EV:
Below figure depicts the various positions where-in we use the power electronic circuits in an EV:
Apart from the converters, the figure also represents the fundamental components of an EV, such as: Battery pack, Charger, Electric Motor etc.
B. Power Electronic Circuits in EV and HEV:
Mainly there are two types of Power Electronics Circuits we use in EV or HEV. They are:
1. DC-DC converter (Chopper)
2. DC-AC converter (Inverter)
C. Choppers:
Below are some of the configurations of various types of converters; such as Buck, Boost, and Buck- Boost converter.
1. Buck Converter:
Features:
1. It is a step-down converter.
2. It`s output voltage is less than the input voltage.
2. Boost Converter:
Features:
1. It is a step-up converter.
2. It`s output voltage is greater than the input voltage.
2. Buck-Boost Converter:
Features:
1. It is either step-down or step-up converter.
2. It`s output voltage is either lesser or greater than the input voltage.
In the above all, the duty cycle is the most important parameter that decides the ON-time or OFF-time of any cycle. It helps us to maintain constant power.
D. Inverters:
The below figure depicts the application area of an inverter in the working of an EV.
Features:
1. It converts DC power to AC power as required.
2. It helps to change the speed of a vehicle by adjusting the frequency.
3. It helps to vary the motor torque also by adjusting the signal amplitude.
4. When excess energy is produced through regenerative braking, it feds that to the battery for future usage.
E. AC-DC Converter (Rectifier):
Apart from the converters mentioned above, rectifiers also have some applications.
Features:
1. It can be used for plug-in type of EV or HEV at the charging stations.
2. It is more suitable for commercial vehicles when the charging infrastructure will be available.
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2. An Electric Vehicle's Drivetrain 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 is2'. 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 V-s
Chopper Switching frequency= 400 Hz
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2
What is EV steady state speed if duty cycle is 70%?
Ans:
Objective: To calculate the EV steady-state speed for given parameters.
Given:
Rated Armature voltage (V) = 72V,
Rated armature current (Ia) = 400A,
Armature Resistance (Ra) = 0.5Ω,
Motor Constant (KΦ) = 0.7V-s,
Chopper Switching frequency= 400 Hz
Duty cycle = 70%
We have the standard torque formula as below:
So, in order to get the true V value, we need to calculate that with the duty cycle as given.
Therefore,
V = 72*70/100
V = 50.4V
So by putting all these given and calculated values in the fundamental torque equation; we get
by solving the above equation we get;
T=70.56-0.98w
Also, similar equation is given in the question that represents the vehicle characteristics as below:
TTR=24.7+0.0051w2
So, now equating both the right-hand sides; we get as:
70.56-0.98w = 24.7+0.0051w2
Further solving we get two vales for w (One positive and One negative)
So, we must take 'w' as positive which as it is more practical to have a positive speed.
Therefore; the required speed value (w) = 38.85 rad/sec.
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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 author’s perspective:
Ans:
Objective: To brief about the blog written by an author from Tesla.
Main Points:
1. There is no such thing called the best engine. According to the personal requirements, we need to adapt the engine type.
2. In the course of time, Lead-acid batteries are replaced by Lithium-ion batteries.
3. In the course of time, DC brush motors are replaced by DC Brushless or Induction Motors.
4. In the course of time, Contactor Controllers are replaced by Modulating Inverters.
5. There is a bigger concern regarding the stable use of these elements mentioned above over the next few decades.
6. The author discusses the two technologies such as: 1. DC Brushless Drives and 2. Induction Motor with it's feature of operation.
7. He says brushless motors are useless motors without the inverter.
8. He also mentioned that the 3-phase induction motor has great utility but with several limitations.
9. The major limitation being, the Induction Motor can not operate from DC, and AC is a must.
10. Further, he compares the future trends which may occur in DC Brushless as well as in Induction motor.
Conclusion:
He concludes that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.
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