Week-6 Challenge: EV Drivetrain
Aim:
1. To study the types of power converter circuits that are employed in an electric and hybrid electric vehicle?
2. To calculate the EV steady-state speed if the duty cycle is 70% from the vehicle speed-torque characteristics equation
3. To develop a mathematical model of a DC Motor for the below equation using Simulink.
ω = V/Kϕϕ -Ra/Kϕϕ^2 .T
4. To refer to the given blog: Induction Versus DC brushless motors by Wally Rippel, Tesla and explain in brief the author’s perspective.
Solution 1:
Power Converters:
In Electric and Hybrid Electric Vehicles (EV/HEV), the power electronic converters 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, advanced 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 that are widely used in Electric Vehicles are:
1. AC -DC Converter (On Board Charger):
AC to DC converters is most commonly used in Plug-in Hybrid Electric Vehicles (PHEV) & Plug-in Electric Vehicles (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.
2. DC-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 the DC/DC converter depends on the characteristics of the vehicle such as top speed, acceleration time from O to 100
Km/h. weight, maximum torque, and power profile (peak power, continuous power).
3. DC-AC Converter:
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 an inverter to be used in
electric motor (AC current).
Solution 2:
Rated Armature voltage (V) = 72V
Voltage after 70% Duty cycle = 0.7-72
= 50.4V
From Speed-Torque relation we know that,
`omega`= `frac{V}{Ra}- frac{Ra}{Kphi}. T`
`T = frac{K phi}{Ra}.V-frac{Kphi^2}{Ra}.omega......(1)`
Substitute Motor & Controller parameters in Equation (1)
`T= frac{0.7}{0.5}.(50.4)-frac{0.7²}{0.5}.omega`
`T = 70.56 - 0.98.omega .......(2)`
Equate the above equations, we get
`0.0051omega^2 +0.98omega - 45.89 = 0`
By using the Quadratic equation formula, we get the value of speed.
`omega = -b +- (frac{sqrt (b^2 - 4ac)}{2a})`
`omega = -0.98 +- (frac{sqrt (0.98^2 - 4xx0.0051xx45.8)}{2xx0.0051})`
`omega = -231.07 frac{radians}{sec} and 38.92 frac{radians}{sec}`
The negative value of speed shows that the Torque is transferred to the motor externally and the positive value states that the motor-generated torque reaches the wheels.
Solution 3:
The given equation is ω = V/Kϕϕ -Ra/Kϕϕ^2 .T
where,
V = Input Voltage = 100V
K Motor constant.
`phi`= Magnetic flux
Ra = Armature resistance = 0.5 ohms
T = Torque
Simulink Model:
Output: Graph of Speed (rpm) vs Torque (Nm)
The speed is inversely proportional to Torque and it shows a downward trend.
Solution 4:
Blog: Induction Versus DC Brushless motors by Wally Rippel, Tesla
The following points can be concluded from the above topic:
In the beginning, the author describes the dominance of Internal Combustion Engines (ICE) in the world of the automobile industry and also discusses 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 in the coming years.
• Then he explains the working of both a 3-phase induction motor and a brushless DC motor.
• Now the author has done a comparison between these two technologies and stated some advantages and disadvantages of each.
• Induction motor has 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 has 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 a 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 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.
References: Power Converters