Week-6 Challenge: EV Drivetrain

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

Power train of a general vehicle is a system, which consists of power plant, clutch or torque converter, gear box, differential gear, drive shaft and drive wheel. In case of BEV, there are six possible configurations as shown in Figure.

Figure Possible configurations of BEV power train
whereas: ______Electrical link ====== Mechanical link
EM = Electric Motor, ESS = Energy Source and Storage FG = Fixed Gear

In figure a to c there is only one converter and figure d to f two converters are used. Hence following are some coveters are used in EV:

1. AC – DC Converter: Rectifier
2. DC – DC Converter: Chopper
3. DC – AC Converter: Inverter

The electricity generated by this generator can be utilized to supply power to the main powertrain motor or charge the EV battery. The electric motor is the only element driving the wheels. The motor can be an induction motor, permanent magnet motor, or switched reluctance motor and can be fixed on the vehicle in the same way as in conventional vehicles, without the necessity for a transmission. In-wheel hub motors are also used. In the series configuration, the motor is designed to deliver the required torque to drive the vehicle in all conditions.

Auxiliary units, such as pressure pumps, air-conditioning units, and auxiliary batteries, require a DC-AC inverter or a DC-DC converter at a lower power rating. A DC-AC power inverter converts a DC input voltage to the required magnitude and frequency of AC output voltage. Generally, AC power is more efficient, reliable and widely utilized in many commercial, domestic, and industrial applications, such as the renewable energy system (RES), motor drives system (MDS), and uninterruptible power supplies (UPSs)

2. An Electric Vehicle's powertrain 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 Volt second

Chopper Switching frequency= 400 Hz

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

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

Given: -

The following input parameters are given: -

Battery pack voltage (V) = 72V

Rated armature current = 400 A

Armature Resistance (R_a) = 0.5Ω

Motor constant (K_Φ) = 0.7

Chopper Switching frequency= 400 Hz

Duty Cycle = 70%ω

Solution: -

Vehicle Output Voltage (V_v) is given by: -

V_v = Battery Voltage (V) * Duty Cycle

V_v = 72 * 0.70

Thus, Vehicle voltage = 50.4V

Now, Equation for Vehicle torque is given by: -

T=70.56−0.98⋅ω

Tv=((Vv−ω.K∅)⋅ K∅)/ Ra

Where;

Tv = Vehicle Torque (N-m)

Vv = Vehicle voltage (Volts)

ω = steady state speed of vehicle (rps)

K∅ = Motor constant

Ra = Armature Resistance

Substituting all these values in equation, we get:

Tv=(50.04−0.7⋅ω)⋅0.70 / 0.5

Tv=70.56−0.98⋅ω

Tv=24.7+0.0051 ω ²

Solving these two equations,

70.56−0.98⋅ω=24.7+0.0051⋅ ω ²

We get a quadratic equation:

0.0051⋅ ω ²+0.98⋅ω−46.94=0

Since quadratic equation, we get two values of ;

ω = 38.85 rad/s and ω = -230.95 rad/s

We take the positive value;

Steady state speed of vehicle = = 38.85 rad/s

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla (https://www.tesla.com/blog/induction-versus-dc-brushless-motors)

Explain in brief about author’s perspective.

Initially author describes world of automobile industry and how the dominance of Internal Combustion Engines (ICE) in the automobile industry and also elaborated various types of engines. But he is not criticize on any on engine, He explains that all engines were similar and it was up to the user to select according to their requirement. Author also discuss about two technologies will be widely used such as induction motor and brushless DC motors.

Author also do comparative analysis for these two technologies with its pros and cons.

As per author perspective:

Induction motor
Advantages	Disadvantage
no magnets ability to start under load	Not operate with DC supply Shaft speed is proportionate to line frequency
BLDC
Advantages	Disadvantage
less rotor heat is generated	machine size grows

Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.

Finally author suggested 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.