Week-6 Challenge: EV Drivetrain
Types of power converter circuits employed in an electric or hybrid electric vehicle are:
AC to DC converters: It takes in an AC supply and converts it into DC supply. It will prevent the current from changing directions. An AC-DC converter comprises of step down transformer, diode, capacitor and a low dropout voltage regulator.
The step down transformer takes in 230 V AC supply and steps it down to 12/24 V for normal use but it is still AC. Diodes in the circuit restrict the flow of current in one direction. The combination of diodes leads to many types of rectifiers: 1) Half wave 2) Full Wave 3) Bridge rectifier
The smoothening of waves is done by filter capacitor which is regulated by the LDO for use.
IN EVs or HEVs, the battery which operates as a DC power supply is charged with an AC source at home or at charging station. Here is where AC-DC converter is needed.
DC to AC converters: It takes in a DC supply and converts it into AC. Power which is supplied from a battery or a capacitor can have its form changed to AC for use in houses or transmission. This type of converter is also known as 'inverter'.
DC supply is changed to an AC waveform using semi-conductor transistors (IGBT,MOSFETs). These switches are controlled by a controller which sends signals to them and decides their frequency of switching from open to close.
DC to DC converters: It changes the voltage from one level to another i.e. it can increase or decrease the voltage supplied to it. Also known as chopper. It is neccessary to supply the appliances with their required voltage or else it wont operate due to lack of power or in an overload conditon.
There are 2 main types of DC to DC converters which are isolated and non-isolated type.
Non-isolated types:
1) Buck: to step down the voltage
2) Boost: to step up the voltage
3) Buck/Boost: can do both stepup & down voltage(mainly used to reverse polarity).
4) Cuk: same as Buck/Boost but uses capacitor to store energy opposite to the storage of Buck/Boost where it is inductor.
5) Charge pump: to stepup-down the voltage for low power source.
6) Flyback converter: similar to Buck/Boost converter but uses transformer to store energy.
7) Forward converter: uses transformer to send energy between input and output in a single step.
AC to AC converter:
It takes in an input of AC waveform with a particular voltage and frequency and gives out an AC waveform of different voltage and frequency. Avoids conversion between AC and DC to prevent losses. The frequency of the input wave is changed by altering the firing angle applied to the thyristor.
AC to AC converter with DC link: It includes a DC link along with a rectifier and an inverter. The input waveform is first rectified and stored in a DC link and then it is converted through an inverter.
Matrix Converter:
It converts AC to AC without any DC link to reduce cost and losses. It is used mainly to increase reliability and efficiency of the system.Uses IGBT bidirectional switches to conduct and block voltages of both polarities.
Hybrid Matrix Converter: It uses unidirectional switches with converter stsges, without the DC link to reduce the size and weight of the converter.
2) Given:
Armature Voltage = 72V
Armature current= 400 A
Solution:
Vm = Vb.d
=> Vm = 50.4V
Armature resistance: Ra = Va/Ia => Ra = 0.18 ohms
Torque-speed realtionship is given by:
T = V.KΦ/Ra - (KΦ)^2.w/Ra
where KΦ is motor constant
Ra is armature resistance,
w is angualar speed in rad/s
T is the motor torque
Substituting data we get,
T = 70.56 - 0.98w .....(1)
Given torque speed equation
T = 24.7 +0.0051(w^2) .....(2)
Combining (1) & (2), we get
0.0051w^2 + 0.98w - 45.86 = 0
Applying quadratic formula we get two values for 'w' i.e. 38.82 rad/s and -230.98 rad/s.
The steady state angular speed of the motor is 38.82 rad/s.
3)
The author talks about the two motor drives generally used in automobile industries: DC brushless motor and Induction motor. He tells us about the differences in cost and working of both the motors.
At the end he leaves it to the personal preference of the individual for choosing the motor drive selection as in case of engines "no winner".
He continues to plot differences between the two drive types:
For DC brushless drives, it contains to permanent magnet at the rotor to generate a magnetic field. It consists of a stator core.
The drive uses an inverter as it cannot run on DC supply since it requires current through it to change repeatedly to maintain constant torque.
For induction drives, it has no magnets but stacked steel laminations instead. It is the current in the stator winding that generates the rotor magnetic field that interacts with the rotor.
It does not require an inverter to start the drivee and generate initial torque. Induction motor also cannot run on DC supply.
DC Brushless drives use permanent magnets and cannot alter the strength of the magnetic field. To reduce power loss by heat dissipation and edd current losses it would have been good to control the strength of the magnetic field.
On the other hand, induction motor does not use permanent magnet and can vary their magnetic field strength. If used with a smart inverter they can operate with minimum losses and maximum efficiency.These losses do not increase with size but for a brushless DC motor size increases the losses and decreases the efficiency.
Although, DC brushless drives generate less heat at the rotor as compared to induction drives giving higher peak point efficiency. DC brushless motor can operate at unity power factor, whereas induction drives can operate up to power factor of 85%.
Conclusion: If a user is seeking high performance with comparatively lesser costs, then induction drive is advisable. However,if high peak point efficiency, high power factor and easy control are the features required by the user then DC brsuhless motor is a better choice.
Both have their own places in automobile industries and will retain their relevance in the industry. Rippel believes that due to increasing performance of the vehicle , one day they may be used side by side to get the best out of both drives. This will result in new kind of hybrid.