EV Drivetrain

EV Drive train

Objective - To study what are the different types of power convertor circuits used in a Electric and Hybrid electric vehicle.

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 is2'. 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 – Types of power converters in HEV and EV

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.

AC to DC (Rectifier) – Used to charge the EV battery.

An AC to DC converter is also called a rectifier, which converts AC supply from main lines to DC supply for the load. The block diagram of an AC to DC converter (The Full Wave Bridge Rectifier)is shown in figure below.

This type of single phase rectifier uses four individual rectifying diodes(SCR) connected in a closed loop “bridge” configuration to produce the desired output.

The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.

The smoothing capacitor converts the full-wave rippled output of the rectifier into a more smooth DC output voltage.

They provide a smooth DC output which the other types of Rectifiers

There are also AC to DC rectifiers for Single phase and Three phase

DC to AC (Invertor) – used for a AC Induction Motor from a Battery.

These converters are connected between DC source of fixed input, and variable AC load. Most commonly, these DC to AC converters are called as inverters. An inverter is a static device that converts fixed DC supply voltage to variable AC voltage.

Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency.

This conversion from DC to AC along with variable supply is produced by varying the triggering angle to the thyristors. Most of the thyristors used in inverters are employed with forced commutation technique.

DC to DC (Chopper) – All car auxiliaries need a DC supply but with a lower voltage

Many DC operated applications need different levels of DC voltage from a fixed DC source. A DC chopper is a static device that converts a fixed input DC voltage to variable DC output or a fixed DC output of different magnitude (which can be lower or higher) than input value.

Basically there are two types of DC to DC convertors

Step Up type (Boost)- In this chopper, the output voltage is always greater than input voltage. The configuration of a boost converter is shown in figure below.

Here also a switch is used, which is connected in parallel with the load. This switch is a SCR or an SCR. A (Bipolar junction Transistor) BJT or a MOSFET can also be used depending on the application, a diode is placed in series with the load that allows the load current to flow when the SCR is turned OFF. When the SCR is turned ON, the diode is reverse-biased and hence it isolates the load circuit from the source. So the inductor charges to the maximum input voltage source.

When the SCR is turned OFF, the load gets the voltage from input as well as from inductor. So the voltage appearing across the converter output will be more than the input. Here the output voltage is equal to (1/ 1 – d) times the input voltage, where d is the duty ratio (TON / T). By varying this duty ratio, the output voltage will be varied till the load gets desired voltage.

Step Down Type (Buck)- A step-down chopper produces an average output voltage lower than the input DC voltage. The circuit for this converter is shown in figure below.

Here the switching component is a thyristor that switches the input voltage to the load when it is triggered at particular instants.

A diode acts as a free wheeling diode that allows the load current to flow through it when thyristor is turned OFF. If this diode is absent, a high induced EMF in inductance may cause damage to the switching device. The average output voltage of the converter is varied by controlling turn ON/OFF periods of thyristor. When thyristor is turned ON, the output voltage is same as the input voltage and if it is turned OFF, the output voltage is zero. The output voltage is equal to (TON / T) Vin. So, by controlling the duty ratio K = (TON / T), the output voltage will be increased.

Buck-Boost Chopper - This chopper can be used both in step-down and step-up modes by continuously adjusting its duty cycle. The configuration of buck-boost converter is shown in figure below that consists of only one switching device, i.e., one thyristor.

Along with an inductor and diode, additional capacitor is connected in parallel with this circuit.

When the thyristor is turned ON, the supply current flows to the inductor through the thyristor and induces the voltage in inductor.

When the thyristor is OFF, the current in the inductor tends to decrease with the induced emf reversing polarity. The output voltage of this converter remains constant as capacitor is connected across the load.

By varying the value of duty ratio to a certain value, the output voltage is lower than the input voltage, typically in the range 0 ≥ k > 0.5, thus a buck converter.

And the output is higher than the input voltage if the duty ratio is in the range of 0.5 > K ≥ 1, thus acts as a boost converter.

2) – To find the steady state speed at 70% duty cycle.

Let us consider a Electric vehicle which has some mass M. Now the torque speed characteristics equation is given to us which is

Traction torque is TTR=24.7+0.0051w2 -------------(1)

The above eq is derived at a steady state condition for vehicle when only Rolling resistance force and Aerodynamic drag force are acting on the vehicle

Rolling Resistance force

`F_(rr) = mu_(rr)*m*g`

Aerodynamic Drag force

`F_(ad) = 0.5*rho*A*C_d*v^2`

`F_t = F_(rr)+F_(ad)`

And Torque is equal to

`T = F_(t)*r*G`

Thus solving the we get the equation TTR=24.7+0.0051w2

motor details given as

I = 400 (armature current in amp)

Ra = 0.5 (armature resistance in oham)

V = 72 (armature voltage in volts)

K_phi = 0.7 (motor constant in v-s)

f = 400 chopper switching frequency

d = 0.7 duty cycle is 70%

Vt = d*V terminal voltage

Torque equation for motor is

T = (Vt(K_phi)/Ra)- omega((K_phi)^2/(Ra))

Substituting the values we get

T = 70.56 - omega(0.98) ------------- (2)

To find omega we equate both equations

TTR = T

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

Solving the equation for omega we get

Omega = 38.78 rad/s and Torque is 32.37 Nm.

In matlab script file

clear all;
close all ;
clc

omega = linspace(0,100,50);   % speed (rad/s)

%the given Data is
I = 400;  %(armature current in amp)
Ra = 0.5;    %(armature resistance in oham)
V = 72;      %(armature voltage in volts)
K_phi = 0.7;  %(motor constant in v-s)
f = 400;       %chopper switching frequency
d = 0.7;      %duty cycle is 70%
Vt = d*V;    %terminal voltage

%Torque equation for motor is
%T = (Vt(K_phi)/Ra)- omega((K_phi)^2/(Ra));
%substuting the values we get
%T = 70.56 - omega(0.98);

%now vehicle speed torque characteristics equation is given by
%Ttr =24.7 + 0.0051*(omega)^2;


%Now calculating the values of Both torques for various value of omega
for i = 0:length(omega)
    T = 70.56 - omega*(0.98);
    Ttr =24.7 + 0.0051*omega.^2;
end

plot (omega,T)
hold on
plot (omega,Ttr)
%axis([0 100 0 100]);
xlabel ('speed (rad/s)')
ylabel ('Torque (N-m)')
legend('motor speed torque characteristics', 'Vehicle speed torque characteristics')

3) - Induction Versus DC brushless motors by Wally Rippel, Tesla

Authors Perspective – Author discusses the evolution of batteries, changing the size and from lead acid to lithium ion type batteries, and that there are different options in engine to fulfil the need of vehicle according to the requirements of vehicle rather then one best engine type to fit them all.

Further he discusses about Electric vehicle and types of motors used in it. The two motor types predominantly used are Brushless DC motor and Induction Motor. He discusses the advantages and the disadvantages for both the types of motors. Construction, efficiency and working of motors.

DC brush less motors-the rotor includes two or more permanent magnets that generate a DC magnetic the rotor includes two or more permanent magnets that generate a DC magnetic field. In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.

Induction motors - the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors. Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

DC Brushless or Induction motor

Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this 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 B with permanent magnets. And thus reduces efficiency.

induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.

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.

Permanent magnets are expensive – something like $50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines.

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.

Conclusion - 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.

Studied Different types of Power converter circuits used in Electric and Hybrid electric vehicle.

Steady state speed for motor at 70% duty cycle is 38.78 rad/s.