Week-6 Challenge: EV Drivetrain

Solution to Question one:

A power converter is an electrical circuit that changes the electric energy from one form into the desired form as required for the load. A converter will consist of semiconductor switches and forms a topology which helps in providing the obtained output voltage from a given input as required by the load

The power electronic converters will increases or decreases the voltage (AC or DC) of a power source depending on application. A converter that increases voltage is called a step-up converter and a converter that decreases voltage is called a step-down converter. In EVs/HEVs step-up and step-down converters are combined into one unit. An application of a step-up converter is converting EV/HEV battery voltage (typically 180-300 volts) to about 650 volts to power the traction motor. An advantage of using a converter to increase voltage from the battery is a smaller and less expensive battery may be used while still utilizing an efficient high voltage motor. An application of a step down inverter would be decreasing the high voltage direct current (DC 180-300 volts) from the HEV/EV battery to low voltage (12-14 volts) DC that can be used to charge the 12 volt auxiliary battery and operate light load devices such as lighting, radio, and windows.

• The Power Electronic Converter can be classified into various types depending on the way they change the input power either AC or DC let us discuss about some converters here:
• Diode Rectifier.
• AC to DC Converter (Controlled Rectifier)
• DC to DC Converter (DC Chopper)
• AC to AC Converter (AC voltage regulator)
• DC to AC Converter (Inverter)
• Static Switches.

In EVs, the Fuel Cell (FC) stack, battery bank, and Super- capacitors (SCs) bank are usually used as clean energy sources. . In order to design highly efficient converters for the EV's power system, advanced DC/DC , AC/DC and DC/AC converters are required to adapt the output voltage and current levels with high power quality.

DC to DC Converter (DC Chopper) :-

Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs) and Fuel Cell Electric Vehicles (FCEVs) have been typically proposed to replace conventional vehicles in the near future. ... DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels.

DC-to-DC converters are used to reduce High voltage DC input to low voltage DC output for some specific appliances. They are also used to isolate some highly sensitive components in a circuit from other components of the circuit to avoid any kind of damage.

Electric vehicles (EV) use two different power systems; a high-voltage battery (200 to 450 VDC) for traction and a low-voltage (12 V) one for supplying all the electric appliances in the vehicle. Traditionally the low-voltage battery was charged from the alternator, but in today's vehicles it gets its power from the high-voltage battery pack. However, in specific electric car architectures, this low voltage battery should be ready to help recharge the high-voltage battery pack in order to provide energy for cranking the car. This means that the on-board DC-DC converter must be bi-directional and very efficient as well as highly reliable in order to run the complex control algorithms needed to ensure an energy-efficient solution.

AC to DC Converter (Controlled Rectifier) :-

It's called the "onboard charger" though it really is a converter. It converts power from AC to DC and then feeds it into the car's battery. This is the most common charging method for electric vehicles today and most chargers use AC power. These onboard chargers are used to charge the batteries from an available power socket in Plug in Hybrid Elecric Vehicles (PHEV) and also same circuit is used in HEV as well

DC to AC Conveter (Inveter) :-

The DC to AC conveter is required to convert the dc power to a sutaible ac power for electrical machine the power ouput from the battery pack (DC current) is converted using inverter to be used in electric motor (AC current)

Converts direct current (DC) from the battery to alternating current (AC) to be used by other devices such as the traction motor and coolant pump.

Solution to question two:

The following input parameters are given:-

Battery Pack voltage(V) = 72 V

Rated armature current = 400 A

Armature Resistance(Ra) = 0.5

Motor Constant (K) = 0.7

Chopper Switching frequency = 400 Hz

Duty Cycle = 70%

.....eqn(1)

Vehicle Output Voltage = Battery Voltage(V)*Duty Cycle

= 72*0.70

= 50.4 V

We know that the torque of DC motor can be calculated by the following equation:

T =(V*k`"T=" "V×K∅" /"Ra" "-(" ("(" 〖"K∅)" 〗^"2" )/"Ra" ")×ω"`)

T = (50.4*0.7)/(0.5) – ((0.7)²/0.5)*w

T = 70.56-0.98* w……….eqn(2)

Compating equations (1) & (2) we will get a quadratic equation solving which we will get speed as

w = 38.85 rad/s

Solution to question 3:

Induction versus DC brushless motor

According to authors perspective, Firstly he gave the description of DC brushless motor about its parts and specifications and what is called and next description about Induction motor about is component and working of Induction motor. Then he compared the DC brushless and induction which he metioned that 3- phase induction motor can produce output torque and also have an ability to start under the load without any inverter,as of DC brushless motor produces no starting torque when directly connected to fixed frequency utility power and also inverter is needed to start under load.Then he discussed about the limitations of the 3-Phase induction motor 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. Finally, when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.Inverter can be added to 3-Phase induction so that we can supply power from DC source or Battery and also we can get variable speeds and can adjust the frequencies.

Then he discussed as the only main difference between DC brushless and Induction motor is of control code and also the rotor of DC brushless gets less heated at peak torque as that of Induction motor rotor.

Then he discussed about the loses when the motor runs which is 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. This advantage becomes increasingly important as performance is increased. 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. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

He concluded with 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.