Week-6 Challenge: EV Drivetrain
Answer 1 :
A power converter is an electrical circuit that changes the electric energy from one form into the desired form optimized for the specific load. A converter may do one or more functions and give an output that differs from the input. It is used to increase or decrease the magnitude of the input voltage, invert polarity, or produce several output voltages of either the same polarity with the input, different polarity, or mixed polarities such as in the computer power supply unit.
The DC to DC converters is used in a wide range of applications including computer power supplies, board-level power conversion and regulation, dc motor control circuits, and much more.
The converter acts as the link or the transforming stage between the power source and the power supply output. There are several kings of converters based on the source input voltage and the output voltage and these fall into four categories namely the AC to DC converter known as the rectifier, the AC to AC cyclo converter or frequency changer, the DC to DC voltage or current converter, and the DC to AC inverter.
The power converter circuits understudy, used in electric vehicles, and hybrid electric vehicles are regulator type and power electronics switching type.
Regulator - the element of control.
The regulator is a resisting device that acts as a variable resistor and decreases the voltage of the battery according to the needs of the motor. eg. Consider the required load at the motor is 20 volt and the battery peak voltage is 100 volts. The regulator varies its resistance accordingly. The applied resistance at the regulator then results in a voltage drop of 80 volts at the regulator and the resulting 20 volts is passed on to the motor. At full load, when the required voltage at the motor is 100 volts the regulator doesn't resist at all and all the battery voltage is supplied to the motor. The heat generated at the regulator due to voltage drop is then dissipated.
Extreme heat losses occur which makes the regulator an inefficient type of power converter circuit. Therefore we use a semiconductor switching type power converter circuit.
Switching Circuit:
The required voltage at the motor is achieved by using the semiconductor type switching where the switch is switched on and off according to the inputs from the control signals. The switch is kept on and off for a specific time unit. The average voltage over the specific time unit is the required voltage at the motor.
Consider the required voltage at the motor is 20 volts and the batter peak voltage is 100 volts. The Switch is then switched on for 2-time units and switched off for 8-time units. The average voltage over the total time unit is 20 volts which is the required motor voltage.
Average voltage = [100v(peak battery voltage) * 2(unit time) + 0v * 8(unit time)]/ 10(unit total time)
Also, Average voltage = Battery voltage * Duty cycle
The duty cycle is the percentage time for which the switch was kept on.
Answer 2:
* It wasn't possible to input mathematical equations in the answer block and also the math equation command isn't working. So apologies and please excuse the inconvenience.
Answer 3 :
Authors perspective
As we understand from their types based on their performance and price similarity, the EV drives also have different types of motors and drivetrains according to their utility. So it is difficult to predict the exact future of current technology. According to the author, there is no clear winner between brushless DC motor and induction motor as both have their own grounds.
It's important that we emphasize the utility of the motors rather than argue which one-ups the other. Both the motors have a lot in common except for rotors and inverter controls. After comparison, we finally conclude that the plug-in hybrid market will be dominated by brushless DC motors and induction motors will drive the high-performance pure electric so that they both have their own significance in the coming age of electric automobiles. Let's discuss the advantages and disadvantages of both the motors.
Advantages of brushless DC motor over Induction motor:
- Much less heat is generated with DC brushless drive
- Brushless DC produces no starting torque when directly connected to fixed frequency utility power. They really need to aid in an inverter.
- Rotor cooling and easier and peak point efficiency is generally higher for this drive.
-The DC brushless drive can also operate at a unity power factor, whereas the best power factor for the induction drive is about 85 percent.
- The peak point of energy efficiency of a brushless DC 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. Unfortunately, there is no easy way of changing the magnetic field with permanent magnets.
-Peak efficiency will be a little higher than with an induction motor.
-Induction motor is more difficult to control as the control laws are more complex and difficult to understand.
Advantages of induction motor over brushless DC motor.
-Induction motors are directly compatible with conventional utility power.
-They cannot operate from DC, AC is a must.
-When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
-Torque performance is low compared to DC machines.
-Induction machines have no magnets and magnetics fields are adjustable. 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 the efficiency is optimized. This advantage becomes increasingly important as performance is increased.
-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. Thus induction drives may be the favored approach where high performance is desired.
-Average efficiency may actually be better.
-Permanent magnets are expensive and permanent magnets rotors are also difficult to handle due to very large forces.
-Induction motors will likely retain a cost advantage over PM machines. Due to the field weakening capabilities of induction machines, inverter rating and cost appear to be lower.