Week-6 Challenge: EV Drivetrain

Question 1) Which type of power converrter circuits are employed in electric and hybrid electric vehicle?

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

There are 3 type of converters used in Electric Vehicles.

AC-DC converter Or Rectifier (On Voard 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 elctricity source(Charging Stations) Which is alternating Current. The battery needs to be charged using a direct current source. For this purpose, AC-DC convertor is employed in a PHEV & PEV.

DC-DC conveter or Chopper

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 fo 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 0 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 dc 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).

Question 2) An Elevtric vehicle's powertrain with 72 v Battery pack in shown in diagram below. The duty ration for acceleeration operation is 'd1' and for the braking operation the duty ratio is d2

The other parameters of the electric vehicle is given below,

Motor and Controller Parameters:

Rated armature Voltage=72V

Rated armature current=400A

Ra=0.5ohm, kpi=0.7 volt second

Chopper switching frequency=400Hz

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

Tv;=24.7+(0.0051)w^2.............eq1

solution

Duty Cycle = 70% = 0.7

Actual voltage = 0.7*72=50.4V

we have torque equation for motor,

T=Vkphi/Ra-((kphi)^2/Ra).w

T = 50.4.0.7/0.5-((0.7)^2/0.5).w

T = 70.56-0.98.w...................eq2

Solving equation 1 & 2

comparing both torque of vehicle and torque of motor,

24.7+(0.0051).w^2 = 70.56-(0.98).w

0.0051w^2 + 0.98w-45.86 = 0

by solving this by quadratic equation we get,

we geet two w values as

w = 380915 rad/sec and w = -233.072 rad/sec

so the EV steady state speed if duty cycle is 70% = 38.915 rad/sec or 371 RPM

Question 3) Inducton versus DC brushless motor

In the beginning author describes the dominance of internal combustion engines(ICE) in the world of automobile industry and also discuss differenct types of engines. He explains that all engines were similar and it was up to the user to select according to thr requirement. He also speaks about induction motor and brushless DC motors and presents his view that thses two technoloies will be sidely 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 thses two technolgies and stated a some advantages and dfisadvantages of each

Induction motor advantages

• No magnets
• The motor has the ability to start under load.
• No inverter is needed.

Induction motor Disadvantages

• Cannot operate from DC, AC is a must.
• Shaft speed is proportionate to line frequency.
• Used with utility power, they are constant speed machines.
• When operated form utility power, they have limited starting torque and somewhat running peak troque capabilites.

Brushless DC motor

Advantages

• Much leass 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.

Disadvantages

• As machine size grows, the magnetic losses increase proportionatley and part load efficiency drops.
• Permanenet magnets are expensive and idfficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them

finally the author couldnot choose which drve 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 hig-perfromance pure electrics.