Week-6 Challenge: EV Drivetrain
1.
Power Electronic 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 efficieht 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 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 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.
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).
DC - AC Converter (Inverter) -
The DC/AC converter is required to convert the de power to a suitable ac power for the electrical machine. The power output from battery pack
(DC current) is converted using inverter to be used in electric motor (AC current).
2.
The following input parameters are given: -
Battery pack voltage (V) = 72V
Rated armature current = 400 A
Armature Resistance (Ra) = 0.50
Motor constant (Ko) = 0.7
Chopper Switching frequency= 400 Hz
Duty Cycle = 70%w
Solution: -
Vehicle Output Voltage (Vy) is given by:
Vy = Battery Voltage (V) * Duty Cycle
Vy= 72 * 0.70
Thus, Vehicle voltage = 50.4V
Now, Equation for Vehicle torque is given by:
(V,
T.=
- W. Kg) - Ke
Ra
Where;
T.= Vehicle Torque (N-m)
V.= Vehicle voltage (Volts)
wE steady state speed of vehicle (rps)
Kg= Motor constant
R,= Armature Resistance
Substituting all these values in equation, we get:
Tw = (50.04 - 0.7 • w) - 0.7/0.5
T. = 70.56 - 0.98 - T, = 24.7 + 0.0051 - w?
Solving these two equations,
70.56 - 0.98 - W = 24.7 + 0.0051 . w2
We get a quadratic equation:
0.0051 - w7 + 0.98 - w - 46.94 = 0
Since quadratic equation, we get two values of ;
wE 38.85 rad/s and w=-230.95 rad/s
We take the positive value;
Steady state speed of vehicle = = 38.85 rad/s
3.
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 ner
disadvantages of Induction motors are: They cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequ
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 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.
There is no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance. This is also true for electric vehicle drives. Back when I had hair on my head and carried a slide rule, there were lead acid batteries, DC brush motors, and contactor controllers. Lead has been replaced by lithium and DC by either DC brushless or induction.
Let’s start with DC brushless drives. With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field . Let’s move on to induction motor drives. A forerunner of the 3-phase induction motor was invented by Nikola Tesla sometime before 1889.
Curiously, the stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a «shorted structure.» Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success. In contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power.
Add some feedback loops such that the inverter produces the exact frequency that the motor «desires,» and the induction motor is now capable of competing with DC and DC brushless for vehicle applications.
Brushless or Induction -
Back in the 1990s all of the electric vehicles except one were powered by DC brushless drives. Today, all the hybrids are powered by DC brushless drives, with no exceptions. The only differences are the rotors and the inverter controls. One of the main differences is that much less rotor heat is generated with the DC brushless drive.
Rotor cooling is easier and peak point efficiency is generally higher for this drive. 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. In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable.
When maximum torque is required, especially at low speeds, the magnetic field strength should be maximum – so that inverter and motor currents are maintained at their lowest possible values. In contrast, induction machines have no magnets and B fields are «adjustable,» since B is proportionate to V/f . This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized. Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine – magnetic and conduction losses can be traded such that efficiency is optimized.
With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. This means that induction motors will likely retain a cost advantage over PM machines. Also, due to the field weakening capabilities of induction machines, inverter ratings and costs appear to be lower, especially for high performance drives.
Since spinning induction machines produce little or no voltage when de-excited, they are easier to protect.