Week-6 Challenge: EV Drivetrain

Objective:

• To understand types of power converter circuits employed in EV and HEVs.
• To determine the steady state speed of the vehicle.
• To develop the mathematical model for given equation in simulink.
• To breif about wally Rippel's Induction versus DC brushless motors blog.

Description:

1. Power converter circuits:

The primary task of power electronics is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads. Modern power electronic converters are involved in a very broad spectrum of applications like switched-mode power supplies, and vehicular technology, etc.

Power electronic converters can be found wherever there is a need to modify the electrical energy form with classical electronics in which electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Some examples of uses for power electronic systems are DC/DC converters used in many mobile devices.

                     Power conveter circuits in Electric vehicle.

Types of power converter circuits:

a. AC to DC Converters or Rectifiers:

The task of turning alternating current into direct current is called rectification, and the electronic circuit that does the job is called a rectifier. The most common way to convert alternating current into direct current is to use one or more diodes, those handy electronic components that allow current to pass in one direction but not the other.

Although a rectifier converts alternating current to direct current, the resulting direct current isn’t a steady voltage. It would be more accurate to refer to it as “pulsating DC.” Although the pulsating DC current always moves in the same direction, the voltage level has a distinct ripple to it, rising and falling a bit in sync with the waveform of the AC voltage that’s fed into the rectifier.

This converter circuit can be observed in EV charger.

There are three distinct types of rectifier circuits as follows,

• Half wave rectifier:

The simplest type of rectifier is made from a single diode. This type of rectifier is called a half-wave rectifier because it passes just half of the AC input voltage to the output.

• Full-wave rectifier:

A full-wave rectifier uses two diodes, which enables it to pass both the positive and the negative side of the alternating current input. The diodes are connected to the transformer.

• Bridge rectifier:

The problem with a full-wave rectifier is that it requires a centre-tapped transformer, so it produces DC that’s just half of the total output voltage of the transformer.

b. DC to DC converter:

A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission).

DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

Used in auxiliay of Electric vehicles.

c. AC to AC converters:

AC to AC converters is used for converting the AC waveforms with one particular frequency and magnitude to AC waveform with another frequency at another magnitude. This conversion is mainly required in case of speed controlling of machines, for low frequency and variable voltage magnitude applications as well.

d. DC to AC converters or Inverters: 

DC to AC converters is mainly designed for changing a DC power supply to an AC power supply. Here, DC power supply is comparatively stable as well as positive voltage source whereas AC oscillates approximately a 0V base stage, typically in a sinusoidal or square or mode. The common inverter technology used in electronics is to convert a voltage source from a battery into an AC signal. Generally, they operate with 12 volts and commonly used in applications like automotive, lead-acid technology, photovoltaic cells, etc.

A transformer coil system & a switch is the simple circuit used for an inverter. A typical transformer can be connected toward the DC signal’s input through a switch to oscillate back quickly. Due to the current flow in bi-directional in the primary coil of the transformer, an alternating current signal is an output throughout the secondary coils.

Used in Motor drive of electric vehicle.

2. Given,

Rated Armature voltage= 72 V

Actual voltage = 0.7 * 72 = 50.4V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

w.k.t

 `omega = (V/(K*phi)-R_a/(K*phi)^2)*T`

`T_v = ((V*Kphi)/R_a-(Kphi)^2/R_a)*omega`   

`omega`=Steady state speed of vehicle.

V= Actual voltage.

`T_v`= Vehicle torque

`Kphi`= Motor constant.

`R_a`= Armature resistance.

Sub all the values in eq(1)

`T_v = ((50.4 *0.7)/0.5-(0.7^2)/0.5)*omega`

`T_v = 70.56-0.98omega`

`T_v= 24.7+ 0.0051omega^2`  

w.k.t

`omega = (-b +-sqrt(b^2-4ac))/(2a)`

`omega = (-0.98+-sqrt(0.98^2+(4*0.0051*45.86)))/(2*0.0051)`

`omega` = -231.071 rad/sec`

`omega` = 38.914 rad/sec.

Therefore the EV steady state speed of vehicle for 70% duty cycle is 38.914 rad/sec.

3. Mathematical model of a DC Motor for the below equation using Simulink.   

`ω=   V/(Kϕ) -R_a/(Kϕ)^2    .T`   

Below is the simulink model for the given equation and the following parameters were used in the model.

Rated Armature voltage= 50.4 V

Ra= 0.5Ω,

KΦ= 0.7 Volt second

Torque was increased from 0 to 10 in 10 sec.

Output:

The simulation is run for 10 seconds and the following output was obtained.

In the above snap we can see that with increase in torque the speed decreases.

4. https://www.tesla.com/blog/induction-versus-dc-brushless-motors

• In the above blog the author tries to differentiate between DC brushless drives and Induction motor and their advantages and disadvantages in modern days.
• 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.
• Adding feedback loops for the induction motor would produce the exact frequency that the motor desires and now, the induction motor is capable to compete with the DC & DC brushless motors. 
• Discussion on how battery technology has changed in EV domain from lead acid batteris to Li-ion batteries and the future in battery technology for EV and HEVs was done.

Advantages of DC brushless motors:

• Better speed versus torque characteristics.
• High dynamic response.
• High efficiency.
• Long operating life due to a lack of electrical and friction losses.
• Noiseless operation.

Disadvantages of DC brushless motors:

• The cost of a brushless DC motor is comparatively higher as compared to brushed DC motor and the electronic controller also increases the cost of overall setup, as in a traditional motor, low-cost mechanical commutation setup involving brushes is used.
• When brushless DC motor is operated at low speed, slight vibrations occur during low-speed rotation. However, vibrations reduce at high speed.

Advantages of Induction motors:

• Induction motors are simple and rugged in construction. Advantage of induction motors are that they are robust and can operate in any environmental condition
• Induction motors are cheaper in cost due to the absence of brushes, commutators, and slip rings
• They are maintenance-free motors unlike dc motors and synchronous motors due to the absence of brushes, commutators and slip rings.
• Induction motors can be operated in polluted and explosive environments as they do not have brushes which can cause sparks.

Disadvantages of Induction motors:

• The power factor of the motor is very low during the light load condition.
• The change in speed of the motor is very low during different loading conditions. So, the speed control of IM is difficult.

At the end the author cannot conclude that whether Brushless or induction drives are better but he predicted that in future the brushless DC motor may dominate in EV & HEV domain.

Conclusion:

• Types of converters used in EV and HEVs were mentioned.
• Steady state speed of vehicle was calculated.
• Mathematical model for the given equation was developed in simulink.
• Brief on Wally Rippel's Induction versus DC brushless motor was done.