EV Drivetrain

TARGET>>>

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

2. An Electric Vehicle's Drivetrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration 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= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 V-s

Chopper Switching frequency= 400 Hz

The vehicle characteristics are given by the equation

TTR=24.7+0.0051w2

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

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

SOLUTION>>>

1. There are mainly four type of power converter is there. That are-

I>AC to AC II>DC to DC

III>AC to DC IV>DC to AC

Based on the motor type ,whether it is DC brushless or Induction type, power converters are used to supply the system components. DC/DC converter, usually performs the conversion from the HV energy storage to the LV legacy vehicle system. Fig- EV
In order to enable recharging from a regular electrical outlet the on-board charger (AC/DC) or from generator in case of HEV AC/DC converters are used. Fig- Plug in HEV

There are only 3 types of power converters circuits generally employed in EV or HEV. Those are-

i) BUCK Converter or Step down Chopper Circuit-

This type of converter is used to decrease voltage level from AC/DC to AC/DC.

ii) BOOST Converter or Step up Chopper Circuit-

This type of converter is used to increase voltage level from AC/DC to AC/DC.

iii) BUCK-BOOST Converter Circuit-

This type of converter is used either to increase or decrease voltage level from AC/DC to AC/DC.

2.An Electric Vehicle's Drivetrain with 72V battery pack in shown in the diagram below.The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is'd2'.

>>> Given--

Motor and Controller Parameters:

Rated Armature voltage= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 V-s

Chopper Switching frequency= 400 Hz

The vehicle characteristics are given by the equation:

Tm=24.7+0.0051w^2.........(ii)

It is given that duty cycle is 70%. So the Average voltage is Vo=[72*(70/100)]= 50.40 V

We know the DC motor torque-speed equation is given by:

Tm =[(Vo*KΦ /Ra)-{w*(KΦ)^2 /Ra}] ........ (i)

Putting all the values in eqn (i) we get:

Tm=[(50.40*0.7/0.5) - {(w*0.7^2)/0.5}]

Tm =70.56 - 0.98w ......(iii)

Now we have 2 eqns. Equating eqn (ii) & (iii) we get:

Tm=24.7+0.0051w^2 = 70.56 - 0.98w

0.0051w^2 + 0.98W -45.86=0

Using Shredhar Acharya's formula we get:

w=[-0.98±√{(0.98^2)-4*0.0051*(-45.86)}]/2*0.0051

w=[-0.98±1.377]/0.0102

solving this equation we get 2 different value of omega(w), that is:

w=38.92 rad/s & w=-231.08 rad/s(Neglected as it is negative)

Omega with positive sign is for the forward direction so we will take only w=38.92 rad/s for steady state speed.

3. Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

>>There is no best engine type, rather there are different types of engines to suit in its requirements type, such as performance and price. In today's world mainly two types of motor are used vastly either it is DC brushless type or Induction type, based on their application.
In brushless DC motor two or more permanent magnets are used that generates a DC magnetic field. This magnetic field enters the stator core and interacts with winding current and results a torque interaction between the rotor and stator. As the rotor rotates it is needed to keep continuosly varying the magnitude and polarity of the stator current such that the torque remain constant and electrical to mechanical energy at its optimal efficiency. Inverter is used to control the stator current.without the inverter the brushless motor is useless.
The 3-phase Induction motor was first invented by Nikola Tesla in the year 1889, of which forerunner is used now a days. A DC brushless motor and Induction motor are virtually identical the main difference is unlike Brushless motor, the Induction motor does not have magnets in its rotor.Induction rotor is just stacked steel laminations with buried peripheral conductors. Current flows through stator winding to produce a rotating magnetic field. The frequency of this magnetic field is equal to the difference betwwen the applied electrical frequency and rotational frequency of the rotor. A voltage is induced.In response to this voltage, current is produced within the rotor conductor that proportionate to the voltage. These current interacts with the original magnetic field to produce force for rotor torque.
3-phase induction motors have great utility, they also have some severe limitations. They cannot operate from DC; AC current is needed. Shaft speed is proportionate to line frequency.when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines. But adding an invertor it is possible to power an induction motor from battery or other DC sources. For variable speed, it is achieved by adjusting the inverter frequency and to increase the torque performance a feedback loop is added such that inverter produces the exact frequency that the motor.
In 1990s all of the electric vehicles except one were powered by DC brushless drives. Now a days all the hybrids a DC brushless powered, only in General Motors EV-1, Induction drive is notable.
Both DC brushless and induction drives use motors having similar stators.
The only differences are the rotors and the inverter controls and with digital controllers, the only control differences are with control code. Rotor of DC motor produces less heat, cooling is easier and peak point efficiency is generally heigher as compared to Induction drives.In an ideal brushless drive when maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. So, I^2R losses in minimizes and thereby optimizes efficiency. To reduce eddy and hysteresis losses magnetic field(B) should reduce. Unfortunately there is no easy way of changing field B with parmanent magnets.
But in induction machines have no magnets are there and B fields adjusted as B is proportionate to V/f.Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine.
In DC brushless, as machine size grows, the magnetic losses increase proportionately but for induction, as machine size grows, losses do not necessarily grow.Thus, induction drives may be a better approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.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.
Conclusion is 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.