Week-6 Challenge: EV Drivetrain

1. Which types of power converter circuits are employed in electric and hybrid electric vehicle.

The main components of an Electric Vehicle are DC-AC inverter, DC-DC converter, battery, and electric motor.

There are two major power electronic units in the general configuration of an EV and HEV;

• DC-DC converter
• DC-AC inverter

The converter topologies are classified as:

• Buck Converter: The buck converter is step down converter and produces a lower average output voltage than the dc input voltage.

• Boost converter: The output voltage is always greater than the input voltage.

• Buck-Boost converter: The output voltage can be either higher or lower than the input voltage.

2. GIVEN;

Motor and Controller Parameters:

Rated Armature voltage= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

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

Q. What is EV steady state speed if duty cycle is 70%?

DC Motor torque equation;

`T=(k_m*psi*E_S)/R_a - (k_m*psi)^2/R_a *omega`

since the duty cycle is 70% ;

Supply voltage Es = 72*0.7

Es = 50.4V

Now;

equating the load torque and motor torque equation to find the steady state speed;

24.7 + (0.0051`omega`^2) = 70.56 - 0.98`omega`

`0.0051omega^2` + `0.98omega` - 45.86 = 0

`therefore omega = 38.91`

Thus, the EV steady state speed at 70% duty cycle is 38.91rpm.

3. A Brief understanding is to what Mr. WALLY RIPPEL says about the future of electric and hybrid electric vehicles and the importance of DC brushless and Induction motor drives.

We know that, in the recent times lead acid batteries , DC brush motors and contactor controllers have been replaced with Lithium , DC brushless or induction and modulating inverters respectively. However, the author says induction and DC brushless drives are likely to exist for many years to come.

Lets take a closer look on what are DC brushless drives and Induction motor drives;

In brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field In turn, this magnetic field enters the stator core and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. The device that provides this current control is called an inverter. Without it, brushless motors are useless.

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. The essential difference between the two machines is with the rotor.When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset; the motor has the ability to start under load. No inverter is needed. 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. They really need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor. We must also know that, induction motors cannot operate form DC;AC , and when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.

But there is one possibility, if feed back loops are added such that the inverter produces the same frequency that the motor requires, so that the induction motors can now compete with the DC and DC brushless drives in vehicle appliction.

So now, the most important question; Which drive machine is better?

The answer is actually Undefined as they both have their own Pros and Cons and favor their use in their respective areas. Induction motor drives are prefered for High performance vehicles , whereas; DC brushless drives are used for a more economic drive. But there is one important factor that we must take into concideration, Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless. This means added development costs, but likely little or no recurring costs.

Finally the author concludes by saying 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, and when the time comes when, hybrids become more electrically intensive and their performance levels increase,we would be likely to see induction and DC brushless live and work side by side during the coming golden era of hybrid and electric vehicles.