Week-6 Challenge: EV Drivetrain
Aim - Ev Drivetrain.
Objective -
1. Which types of power converter circuits are employed in an electric and hybrid electric vehicle?
2. An Electric Vehicle's powertrain with a 72V battery pack is 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 are 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 the duty cycle is 70%?
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
ωω= V/Kϕϕ -Ra/Kϕϕ^2 .T
4. Explain in brief about author’s perspective.
1. Which types of power converter circuits are employed in an electric and hybrid electric vehicle?
There are four main categories of power converter circuits that are used in electric and hybrid electric vehicles. They are discussed below.
AC to DC converter -
This type of converter takes an AC power supply and converts it to a DC one. Power generated from a power plant is transmitted to our homes at a voltage of 230v at a frequency of 50Hz which we can use the power our electrical appliances. But our applianced operate on a DC supply, not an AC one and so we need a converter for this application. Examples of AC to DC converters are phone chargers, laptop chargers, etc. An AC power supply has an alternating waveform that repeatedly reverses direction. An AC to DC converter will prevent the current charging direction so that it only flows unidirectionally. In the circuit of an AC to DCconverter there is a step-down transformer, a diode, a capacitor, and a low dropout voltage (LDO) regulator.
The step-down transformer takes the high-level 230V from the mains supply and steps it down to a lower level e.g. 12 or 13V for our use, but it is still in AC form. Then, a diode or group of diodes is implemented to restrict the current to flow in just one direction. In an AC waveform, there will be positive and negative peaks at each half cycle like a sine wave. The diode(s) will remove the negative parts of the waveform so that there are positive peaks only. This is called 'rectifying' and this part of the circuit is referred to as a 'rectifier'. This can be a half-wave, full-wave, or bridge rectifier. Then, the rectified output voltage waveform is smoothened by a filter capacitor. Finally, the LDO regulator will regulate the output for our use.
In the case of EVs or HEVs, the battery pack which operates with a DC power supply is charged at home or at a charging station both of which are supplied power in AC form from the electric grid. So, the chargers for these vehicles are an example of the AC to DC power converter.
DC to AC converter-
This type of converter takes a DC supply and converts it to an AC one. The power which is supplied from a battery or capacitor can have its form changed to the AC type for use in homes or systems which have an AC load. For instance, the power generated by solar panels is stored in batteries in the DC form and this can power a house that uses an AC power supply so that we can utilize this power from the mains. This type of converter is referred to as an 'inverter' and is widely used in homes and in EV/HEVs.
The DC supply is changed to an AC waveform using an electronic switch, which is basically a set of semiconductor transistors e.g. IGBTs, MOSFETs. These switches are controlled by a controller to either be in an open or closed position to direct the flow of current in a circuit. The controller sends signals to the switches to tell it to open/close, and even decides the frequency for the same. By doing this operation, the DC current mimics an AC current and can be fed to the load which requires it.
In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer that consists of a primary and secondary winding. The current from the battery is first supplied to the primary winding where the electronic switches will repeatedly reverse the direction of the current to create an AC waveform. This current will return back the way it came from. The AC current in the primary winding will induce an AC current in the secondary winding which then supplies this current to an AC load e.g. the AC traction motor.
DC to DC converter-
This type of converter is similar to a transformer in that it can change the voltage from its current level to another level. It can take an input supply of voltage and increase it or do the exact opposite and decrease it similar to a step up or step down transformer. This kind of converter is called a 'chopper'. The purpose of using this type of converter is to provide the necessary level of power to a particular component and for the safety of the appliance and the user. If a component is supplied with too little or too much voltage, it won't operate due to lack of power or it will get damaged, so it all depends on the load requirement.
It works by taking the DC supply and passing it through a switching element which converts the waveform to a square wave which is actually AC in nature. Then this AC is passed through a filter which will turn it back into a DC supply for use in a load. In EVs, HEVs, and conventional vehicles, this type of converter is used for various purposes. E.g. In the case of stepping down the voltage, the car radio may require a stepped-down voltage of 12V to operate and would not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of a car may require 500V to operate, but the vehicle may need more voltage than the battery pack can provide.
There are 2 main types of DC to DC converters which are isolated and non-isolated types. The non-isolated type can be subdivided into 5 types:
Buck - to step down the voltage.
Boost - to step up the voltage.
Buck/Boost - this can do both steps up and step down operations but is mainly used to reverse polarity.
Cuk - similar to the Buck/Boost type.
Charge Pump - to step up/step down the voltage for low power sources.
The isolated type can be subdivided into 2 types:
Flyback converter - similar to the Buck-Boost converter except it uses a transformer instead of an inductor to store energy.
Forward converter - this type will use a transformer to send energy between the input and output in a single step.
AC to AC converter -
This type of converter takes an input AC waveform with a particular voltage and frequency and outputs a different AC waveform with another voltage and frequency. The output voltage and/or frequency can be changed or maintained depending on the application. This converter has many applications in our daily life e.g. lamp dimming, motor speed control. There are 4 main types of AC to AC converters:
Cycloconverters
This type is also known as a frequency changer as it will change the frequency of an input AC waveform and output an AC waveform with another frequency but it can also change the magnitude of the power. This type of converter does not have any DC linkages and avoids any conversion between AC and DC to prevent losses and save costs. The input wave's frequency is changed by changing the firing angle applied to a thyristor. By switching the positive and negative limb thyristors, we can get an output waveform that has a stepped-up/down frequency compared to the input.
AC to AC convert with DC Link
As the name suggests, this type includes a DC link along with a rectifier and an inverter. The input AC waveform is first rectified by the rectifier to remove the negative portions of the AC wave and output only the positive portions. This creates a DC waveform which is then stored in the DC link and is then converted back to an AC waveform using the inverter.
Matrix Converter
This type converts from AC to AC directly without using a DC linkage to reduce costs and losses that would otherwise be incurred with a DC-link storage element. It is used mainly to increase the reliability and efficiency of a system. Matrix converters use bidirectional switches (IGBTs) to conduct current and block voltages of both polarities.
Hybrid Matrix Converter
This type is very similar to the matrix converter in that they combine the AC//DC/AC design with the matrix design. It uses unidirectional switches and 2 converter stages, without the DC link to reduce the size and weight of the converter.
2. An Electric Vehicle's powertrain with a 72V battery pack is 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 are 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 the duty cycle is 70%?
Given -
The following parameter are given -
Battery pack voltage - 72v
Rated armature current - 400A
Armature resistance (Ra) - 0.5
Motor constant (K) - 0.7
Chopper switching frequency - 400Hz
Duty cycle -70%
Vehicle output voltage = Battery voltage (v) * Duty voltage
= 72*0.70
= 50.4V
The vehicle voltage is 50.4v.
Now equation for torque is ,
Tu = Vu*(kΦ/Ra)- ((kΦ)^2/Ra)*ω ...........(1)
where,
Tu = Vehicle Torque (N-M)
Vu = Vehicle Voltage (volts)
ω = Steay state speed of vehicle (rps)
kΦ = Motor constant
Ra = Armature resistance
Substituting all value in equation (1) we get ,
Tu = 50.4*0.7/0.5 - ((0.7)^2 /0.5) *ω
Tu= 70.56 - 0.98ω ..........(2)
Now, the vehicle speed-torque characteristics are given as,
Tu = 24.7+0.0051ω^2 ........(3)
At steady state
Equation (2) = Equation (3)
24.7+0.0051ω2 = 70.56 - 0.98ω
0.0051ω2 + 0.98ω -45.85 = 0
To solve the above equation we have to apply the quadratic equation formula,
x = −b ± √(b2 − 4ac)/ 2a
x = -0.98 ± √(0.98^2 - 4(0.0051)(-45.85) ) / 2(0.0051)
ω = 38.92 rad/ sec.
The steady-state speed of the vehicle is 38.92rad/sec.
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω= V/KΦ -Ra/KΦ^2 .T
let,
V = 220v
KΦ = 0.7
Ra = 1
Model -
Output -
4. Explain in brief about author’s perspective.
Wally Ripple is a long-time proponent of electric vehicles. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.
In the beginning, the author describes the dominance of Internal Combustion Engines(ICE) in the world of the automobile industry. He explains that all engines were similar and it was up to the user to select according to their requirement. Next, the author highlights topics related to EV technology especially related to DC Brushless Motor and Induction Motor.
According to him, DC Brush Motors is replaced by either DC Brushless Motor or Induction Motor. Also, the Lead Acid Battery is replaced by lithium-ion batteries for EVs.He also quoted that there will be the future of both the technology (DC Brushless Motor and Induction Motor) depending upon their demands and uses.
DC Brushless Motor:
With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). 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.
The author also said that 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.
Advantage:-
The advantage of DC Brushless Motor is that there is low heat generation in the rotor and its cooling capacity is better as compared to Induction Motor. Also, peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at the unity power factor, whereas the best power factor for the induction drive is about 85%. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.
Disadvantage:-
As Machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.
Induction Motor:
The technology behind the induction motor is that it basically uses a 3-phase induction motor which was invented by Nikola Tesla sometime before 1889. Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In an Induction motor current interacts with the magnetic field generated by the stator to produce a force that is required for rotor torque.
When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success.
Advantage:-
The induction motor does not have any magnets and in this the B field is adjustable. The motor has the ability to start under load and there is no need for an inverter. In an induction, the motor B field is proportionate to V/f (Voltage to frequency). This technology is used to reduce the magnetic losses and conduction losses, due to which efficiency will remain at maximum peak. This all leads to increasing the performance, an induction motor is very useful and efficient for high performance.
Disadvantage:-
An induction motor cannot operate from DC Voltage it always requires an AC voltage supply. In an induction motor, the Shaft speed is proportionate to line frequency. Hence, when used with utility power they are constant speed machines. When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC-type machines.
Inference:-
In the end, the author couldn't choose which drive is better than the other but makes it clear that DC Brushless motor drives will likely continue to dominate in the Hybrid and upcoming Plug-in hybrid markets and induction motor drives will likely maintain dominance for the high-performance pure electric.
Conclusion:
- We have discussed and studied the different types of power converter circuits employed in electric and hybrid electric vehicles.
- EV Steady-state speed of an EV Drivetrain was determined with the given input conditions.
- The Mathematical model for DC motor is developed.
- Lastly, we have studied a Blog on the Topic:- Induction Versus DC brushless motors by'' Wally Rippel'', ''Nikola Tesla and explained, in brief, the author’s perspective.