Week-6 Challenge: EV Drivetrain
Q1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
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 to 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 charger, laptop charger etc. An AC power supply has an alternating waveform which repeatedly reverses direction. An AC to DC converter will prevent the current changing direction, so that it only flows unidirectionally. In the circuit of an AC to DC converter, 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 a 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. 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 are stored in batteries in the DC form and this can power a house which uses an AC power supply so that we can utilise 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 which 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 a 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 if suppled with too little or too much voltage, it wont 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 will turn it back in to 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 type. The non-isolated type can be subdivided to 5 types:
• Buck - to step down the voltage
• Boost - to step up the voltage
• Buck/Boost - this can do both step 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 in to 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 thyistors, 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.
Power Converters :
Q2. 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%?
Answer :
As Torque formula is given, We need to find speed in rad/sec. Using other epression available which is,
`T = (V*k)/R - (K^2*w)/R`
T = Torque
V = Armature Velocity
R = Resistance
w = Speed rad/sec
with comparing to the formula and deriving a equation is,
`24.7 + 0.0051*w^2 = (V*k)/R - (K^2*w)/R`
`24.7 + 0.0051*w^2 = (72* 0.7*0.7)/0.5 - (0.7^2/0.5)*w`
`24.7 + 0.0051 *w^2 = 70.56 - 0.98 * w`
`w^2 + 192.2 *w - 8992.2 = 0`
In order to find roots we following formula , For calculating Quadratic equation of form `ax^2 + bx + c = 0` to calculate solution are,
`x = (-b +- root2 (b^2 + 4*a*c))/(2a)`
As speed value can not be in negative,
`w = (-192.2 +- root2 (192.2^2 + 4*1*8992.2))/(2*1)`
`w = (-192.2 +- 270)/2`
`w = (-192.2 + 270 ) /2`
`w = 38.9` rad/sec
Q4. Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Answer :
The author writes this blog to entail the differences between the two most commonly used motor drives in the
automobile industry: the DC brushless motor drive and the induction motor drive. Rippel goes in to detail about the
differences in construction, working, and cost between the two drive types. However, with what he has written, he has
not arrived at the conclusion that one type is better than the other. In the last lines of the opening paragraph, Rippel
writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to
choosing an engine. It is in this same regard that he ends the blog stating that there is still'no winner' when it comes
to motor drive selection either. In spite of both motor drive types having similarities, they also have their benefits and
drawbacks.
In case of DC brushless motor, permanent magnets are used which produces a rotating magnetic field. This
magnetic field when induces a current in the conductor. This current results in force generation in the rotor and this
results in rotation of the rotor.
Whereas in case of induction motor, the current is supplied to the stator which results in the generation of emf,
when this emf comes in contact with the rotor which is generally a squirrel cage type, this results in current induced
in the rotor. Unlike the DC motor, the rotor consists of conductors plates and placed in a cage-like structure. This also
consists of bars around them in which current get induced and the rotor starts rotating.
Induction motors can be used directly with power from the mains and does not require an inverter to start the drive
and generate the starting torque. It has the ability to start under the mains load applied. However, DC brushless
drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They
require an inverter to do this.
Induction motor drives cannot run on a DC power supply and are solely dependent on an AC power supply. Also,
when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a
limited starting torque as well as peak torque. In order to rectify this, an inverter can be used to operate the induction
drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no
longer an issue - it can be varied.
DC Brushless drives use permanent magnets, and cannot change the strength of their magnetic fields. It would be
desirable if they could, because when the maximum torque is needed by the rotor, the magnetic field strength
should also be at a maximum level. This allows for the inverter and motor currents to be at a minimum, thereby
reducing the power loss by heat dissipation. Similarly, when the torque required is minimum, the magnetic field
strength should also be minimum to reduce hysteresis and eddy current losses. The permanent magnets used are
also quite expensive costing $50 per kilogram and their magnetic fields are difficult to control if ferromagnetic
material gets close enough to the field.
On the other hand, induction drives do not have permanent magnets and so they can vary their magnetic field
strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and
maximum efficiency as the magnetic field strength can be changed according to the requirement by the rotor.
These losses do not increase if the size of the induction drive also increases but as a DC brushless drive gets bigger,
its losses increase and their efficiency decreases. So induction drives have a cost advantage compared to DC
brushless drives
Although, DC Brushless drives generate less heat at the rotors. So rotors more easily and have a higher peak point
efficiency than induction drives. In addition, the DC brushless motor can operate at unity power factor i.e. I, whereas
induction drives can operate up to a power factor of approximately 85%.
In conclusion, Rippel has elaborated the different aspects to compare the induction and DC brushless motor drives
and has detailed the advantages and disadvantages of both types. As mentioned earlier, at the end of the day it
depends on the preference of the user. If a user is seeking a high performance with comparatively lesser costs, then
the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high
power factor and easier control that a user seeks, then the DC brushless drive would be the smarter choice. Both
types have their place in the automobile industry and will retain their relevance in their respective applications.
Rippel believes that due to the increasing performance of vehicles, one day both may be used side by side under the
hood of a vehicle in order to get the best of both drives. This would result in a new kind of hybrid that combines the
characteristics of an ordinary vehicle e.g. sedan with that of a sports car.
Learning Outcome:
- Types of converters used in HEV and EV vehicles.
- To find the velocity using the torque-speed expression.
- To get to know about the induction and DC motor and try to find the best options available.