Week-6 Challenge: EV Drivetrain

AIM

1. To identify the types of power converter circuits employed in electric and hybrid electric vehicle.
2. To calculate the steady state speed of an EV with given inputs if duty cycle is 70%.
3. Refer to the blog and explain in brief about author's perspective.

Study#1:

Q1. Which type of power converter circuits are employed in elctric and hybrid electric vehicle?

Power Electronic Convertors:-

In Electric and Hybrid Electric Vehicle (EV/HEV), the power electronic convertors are considered as the key elements that interface their power sources to the drivetrain of the EVs. In order to design highly efficient converters for the EV's power system, advance DC/DC and DC/AC converters are required to adapt the output voltage and current levels with high power quality.

The 3 types of converters which are widely used in an Electric Vehicle are:

AC-DC Converter (On Board Charger):-

AC to DC converters are most commonly used in Plug-in Hybrid Electric Vehicle (PHEV) and Plug-in Electric Vehicle (PEV). These Vehicles get their propulsion power from an external electricity source (Charging Stations) which is alternating current. The battery needs to be charged using a direct current source. For this purpose AC to DC converter is employed in a PHEV & PEV. 

DC to DC Converter:-

DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels. By introducing DC/DC converters one can choose the voltage variation of the devices and the power of each device can be controlled. The power of DC/DC converter depends on the charecteristics of the vehicle such as top speed, acceleration time frpm 0 to 100km/h, weight, maximum torque and power profile (Peak Power, Continuous Power).

DC-AC Converter (Inverter):-

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

Study#2:

 Q2. An Electric Vehicle's powertrain with 72 V battery pack is shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for braking operation duty ratio is 'd2'.

The other parameters of the Electric Vehicle is given below,

Motor and Control Parameters;

Rated Armature Voltage = 72V

Rated Armature Current = 400A

R`alpha`=0.5`Omega`, K`phi`=0.7 Volt Second

Chopper Switching frequency = 400Hz

The vehicle speed-torque characteristics are given by the below equation

What is EV Steady State speed if duty cycle is 70 %? 

Given:-

The following input parameters are given:

Battery pack voltage (V) = 72 V

Rated Armature Current = 400 A

Armature Resistance (R_α) = 0.5Ω

Motor Constant (K_Ø) = 0.7

Chopper Switching frequency = 400Hz

Duty Cycle = 70%ω

Solution:-

Vehicle Output Voltage (V_v) is given by:-

V_v = Battery Voltage (V) * Duty Cycle

V_V = 72*0.70

Thus, Vehicle Voltage = 50.4V

Now, Equation for Vehicle torque is given by:-

Where;

T_v = Vehicle Torque (N-m)

V_v = Vehicle Voltage (Volts)

ω = Steady state speed of Vehicle (rps)

K_Ø = Motor Constant

R_a = Armature Resistance

Substituting all these values in equation, we get:

T_v = 70.56 – 0.98 ω

T_v = 24.7 + 0.0051 ω²

Solving 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

Study#3:

Q3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

The following points can be concluded from the above topic:

• In the beginning, author describes the dominance of Internal Combustion Engines (ICE) in the world of automobile industry and also discuss different types of engines. He explains that all engines were similar and it was up-to the user to select according to their requirement. He also speaks about induction motor and brushless DC motors and presents his view that these two technologies will be widely used for the coming years.
• Then he explains the working of both 3-phase induction motor and a brushless DC motor.
• Now the author has done comparison between these two technologies and stated some advantages and disadvantages of each.
• Induction motor have some advantages like: it has no magnets. The motor has the ability to start under load. No inverter is needed. Some disadvantages of induction motors are: 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. When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
• Brushless DC motor have some advantages like: much less rotor heat is generated with the DC brushless drive. The peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive. Some disadvantages of Brushless DC motor are: as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.
• Finally the author couldn't choose which drive is better than the other but makes it clear that the 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.