Week-6 Challenge: EV Drivetrain
1. Power converter circuits employed in Electric and Hybrid-electric Vehicle:
Power converters are electrical and electronics circuits that are used to either connect DC to AC and vice versa. In simpler words, as we know that power converters are circuits which are used to convert one level of voltage into other in terms of its nature (DC to AC) and also in terms of its level so as to increase or decrease its level (DC to DC, Ac to AC converters). There are different types of power converters employed in electric and hybrid electric vehicle which we are going to discuss in detail below. Power electronic converters are used everywhere in normal daily routines at home, commercial workplace or in an industrial environment. Due to high power handling handling capacity with higher efficiencies, these converters have become integral part of industrial electrical drives, high electric power supplies, electric traction systems and automobile control equipment. There are different types of power electronic converters used to perform different functions such as rectification, inversion, etc., which are rated from few milli watts to a few kilowatts.
As we all know that a motor drive consists of a power converter and the controller. The power electronic converter handles the flow of bulk power from the source to the motor input terminals. The controller process the command input and feedback information to generate the switching signals for power converter semiconductor switches. The following are the power converters employed or used in electric and hybrid electric vehicles:
a) AC to DC Converter or Rectifier (EV Charger):
AC to DC converters are one of the most important in power electronics because there are lot of real applications are based on this conversions. The AC to DC conversion process is known as rectification. The rectifier converts AC supply into the DC supply at load end connection. Normally transformers are used to adjust the AC source to get step down transformer to reduce the voltage level to have better operation range for DC supply. As we all know that power generated from a powerplant is transmitted to our homes at a voltage of 230V at a frequency of 50Hz which we can use to power our electrical appliances. So, in the case of electric and hybrid electric vehicle, as the battery is a DC voltage source, when we have to charge the battery, we plug the charger of the vehicle on the supply or grid which is AC supply. So, rectifier is the main circuit used in the charger circuit in which the 230 V AC supply is converted into 12V or 48V DC supply. An AC to DC converter will prevent the current changing direction, so that it only flows unidirectionally. In the circuit, there is a step down transformer, a diode, a capacitor and a low dropout voltage regulator.
The step down transformer takes the high level 230V from the maine 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 AC waveform, there will be positive and negative half cycles. The diode or diodes will eliminate or remove the negative peaks so that only positive half cycles would be present as output. This process is called 'Rectification'.
Rectifiers can be classified as:
i) Half-wave and Full-wave Rectifier.
ii) Controlled and uncontrolled Rectifier.
iii) Center-tapped and bridge Rectifier.
Also, based on the input supply (AC), rectifier can be of single phase and three phase also.
Uncontrolled converter converts fixed DC voltage, it means the value of output for a given value of input AC voltage. Fully controlled converter convert adjustable DC voltage when the adjustable range can adjust in both positive and negative side. The only difference between the controlled and uncontrolled rectifier or converter is that for uncontrolled converters, we use diodes for conduction operation whereas for controlled converters, we use power electronic devices such as thyristor (SCR) for rectification purpose. As we all know that thyristors conduct when the gate pulse is applied hence, by controlling the gate pulse, which can be also called as firing angle, we can control the output voltage.
Single Phase Half Wave Rectifier:
In half wave rectification of a single phase supply, either the positive or negative half of AC wave is passed while the other half is blocked. Following is the circuit arrangement of single phase half wave rectifier. In here, diode or thyristors are used. Where the application needs fixed DC voltage, diode is used as switching element. When the application need variable voltage, thyristors are used. Working is when AC supply is given to the circuit, in half wave rectifier, during the positive half cycle, the diode will be in forward conduction mode and hence the diode conducts. So, we get the output as a positive half cycle. During the negative half cycle, the diode becomes revers bias mode and hence the diode will not conduct. So, we get a waveform as output such that only the positive half cycle is at the output whereas the negative half cycle will not be present. In the case of thyristor controlled half wave rectifier, the difference is that the thyristor will be in forward conduction mode after we apply gate pulse. Otherwise, the device will be in forward blocking mode. Here we are applying the gate pulse in terms of conduction angles which are known as firing angles. So, based on different firing angles, the output voltage would get varied. The figure below also shows the output waveforms of controlled single phase half wave rectifier.
Single Phase converters can mainly classify as Bridge Converter and Center-Tapped Converter. Below figure is the circuit diagrams for bridge converter and center-tapped converter.
Single Phase Full Wave Bridge Rectifier:
As we said earlier, the half wave controlled or uncontrolled rectifier is only used to transfer the positive half cycle only. In full wave bridge rectifier, the entire positive and negative half cycle can transfer. A diode bridge is an arrangement of four (or more) diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. For each half cycle, a pair of diodes is in the forward bias mode.
Single Phase Center-Tapped Full Wave Rectifier:
In the case of center-tap full wave rectifier, only two diodes are used and are connected to the opposite ends of a center-tapped secondary transformer as shown in figure below. The center-tap is usually considered as the ground point ot the zero voltage reference point. Working of this arrangement can be as follows:
An AC input is applied to the primary coils of the transformer. This input makes the secondary ends become positive and negative alternatively. For the positive half of the AC signal, the secondary point diode D1 will be forward biased , center-tap would have 0V and diode D2 will be reverse biased condition. So, D1 conducts. Same will be the case of negative half cycle as in this case, diode D2 will be in forward biased condition and it conducts.
Three Phase Rectifier:
A three phase rectifier is a device which rectifies the input AC voltage with the use of three phase transformer and three diodes which are connected to each of the three phases of transformser secondary winding. The working is same as that of the single phase rectifier. Diode D1, D2, D3 take the positive rectangular block of current which is constant. The current are transformer secondary current and the net DC component of the same act to curve a DC magnetization of the transformer core. This is decremental because the transformer input current can then be heavily distorted due to excessive distorted and peachy measuring current. Therefore this arrangement of conversion is not suitable and not use in practice. However, if we are using a half bridge converter the direct term three phase supply (without transformer) the resulting DC current on the AC side, which is only small value for the upside distribution transformer there may not be the serious issue. Most of three phase AC to DC converters are much popular for industrial applications.
b) DC to DC Converter:
DC to DC converters in an electric vehicle may be classified into unidirectional and bidirectional converters. Unidirectional DC to DC converters cater to various onboard loads such as sensors, controls, safety equipments, etc,. They are also used in DC motor drives and electric traction. Bidirectional DC to DC converters find applications in places where battery charging, regenerative braking and backup power are required. The power flow in a bidirectional converter is usually from a low voltage end such as a battery or a supercapacitor to a high voltage side and is refered to as boost operation. During regenerative braking, the power flows back to the low voltage bus to recharge the battery (buck mode). As a backup power system, the bidirectional DC to DC converter facilitates the safe operation of the vehicle when ICEs or electric drives failto drive the motor. DC to DC converters are also known as choppers. These type of converters are operated using high frequency switching operation.
The Chopper or DC to DC converter topologies are classified as:
i) Buck Converter:
In figure(a) below, is the circuit of Buck converter. The buck converter is also known as step down converter and produces a lower output voltage than the DC input voltage. The basic operation of the buck converter has the current in an inductor controlled by two switches. In the idealised converter, all the components are considered to be perfect. Specifically, the switch and the diode have zero voltage drop when on and zero current flow when off, and the inductor has zero series resistance. Further, it is assumed taht the input and output voltages do not change over the course of a cycle. when the switch is turned on, the current flows through the load and charges the load as the load is considered as capacitive in nature. Since the voltage across load cannot rise instantly and since inductor limits the charging current, the voltage accross the load during the switching cycle is not the full voltage of the power source. When the switch is turned off, since the current in the inductor cannot change suddenly, the inductor creates a voltage across it. This voltage is allowed to charge the load and power the load through the diode when the switch is off, maintaining current output throughout the switching cycle.
The average output voltage is given by,
Vo= (Ton/T)*Vs= DVs
ii) Boost Converter:
In figure(b), a boost converter circuit is shown. The output voltage is always greater than the input voltage. The working of boost converter can be explained here. The input voltage source is connected to an inductor. The solid state device which operates as a switch is connected across the source. The second switch used is a diode. The diode is connected to a capacitor and the load and the two are connected in parallel as shown in the figure(b). The inductor connected to input source leads to a constant input current and thus the boost converter is seen as the constant current input source. The boost converter has two modes of operation. The first mode is when the switch is on and conducting which represents a short circuit ideally offering zero resistance to the flow of current so when the switch is on all the current will flow through the switch and back to the DC input source. Here, the inductor is charged. When the switch is off, the polarity of the inductor is reversed. The energy stored in the inductor is released and is ultimately dissipated in the load resistance and this helps to maintain the flow of current in the same direction through the load and also step-up the output voltage as the inductor is now also acting as a source in conjunction with the input source.
The average output voltage is given by,
Vo=(Vs)/(1-D).
iii) Buck-Boost Converter:
In figure(c), a buck-boost converter is shown. The output voltage can be either higher or lower than the input voltage. The average output voltage is given by,
Vo=(DVs)/(1-D)
D can be varied from 0 to 1. When D=0, Vo=0;
When D=0.5, Vo=Vs
When D=1, Vo= infinity.
Hence, in the interval 0<=D<=0.5, the output voltage varies in the range 0<=Vo<=Vs and we get step down or Buck operation. Whereas, in the interval 0.5<=D<=1, output voltage varies in the range Vs<=Vo<=infinity and we get step up or Boost operation. The amount of power flow between the input and the output can be controlled by adjusting the duty cycle. Usually, this is done to control the output voltage, the input current, the output current or to maintain a constant power. While DC to DC converters can be based on a number of different designs, the underlying function remains the same. We might be talking about a step-up converter, which turns a low voltage input into a high voltage output or a step-down converter which does the opposite. In electric vehicles, which uses a DC motor, running the motor can use up to three times the voltage provided by the battery. With the help of the right converter we can bridge this gap without having to possibility to a larger, prohibitively heavy battery. Auxiliary like radio, headlamps works on DC supply thus this chopper is used to supply the auxiliary equipments through battery.
c) DC to AC Converter (Inverter):
A DC to AC converter or an inverter is a device that converts DC power from the battery to AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current. The use of inverters can increase or decrease the power or torque of the motor by adjusting the amplitude of the signal. It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery. The output waveform of ideal inverters should be sinusoidal. However, the waveform of practical inverters are non-sinusoidal and contain certain harmonics.
- AC to DC Converter (Rectifier)- Used in EV charger.
- DC to DC Converter (Chopper)- Used to supply DC power to Auxiliary like radio and car headlights.
- DC to AC Converter (Inverter)- Used to supply Power to Motor.
2. Observation of an electric vehicle's Drivetrain:
Given,
Motor and controller parameters:
Rated Armature voltage= 72V
Rated armature current= 400A
Armature Resistance Ra= 0.5ohm
k`phi`= 0.7 Volt Second
Chopper Switching frequency= 400Hz
The vehicle speed-torque characteristics are given by the equation= Tv= 24.7+(0.0051)(`omega`)^2-----(1)
We need to find thee steady state speed of EV at 70% of duty cycle.
We know, the torque equation,
T= ((Vo*k`phi`)-((k`phi`)^2*`omega`))/R-----------------------------------------------------------------(2)
where,
T= torque
V= Armature voltage
R= Armature Resistance
`omega`= Angular speed
Comparing equations (1) and (2),
24.7+(0.0051)*(`omega`)^2=((Vo*k`phi`)-((k`phi`)^2*`omega`))/R
From the question, we have to calculate the motor speed at 70% of duty cycle. So, the output voltage , here, Vo= D *Vin
here, Duty cycle D= 0.7 (70%)
therefore, Vo=(0.7)*(72) (since V=72V given)
= 50.4V
equation (2) becomes,
T=((50.4*0.7)-((0.7)^2)*`omega`)/0.5
T=70.56-0.98`omega`---------------------------------------------------(3)
Now, Comparing equations (3) and (1),
70.56-0.98`omega`= 24.7+(0.0051*(`omega`)^2)
0.0051(`omega`^2)+0.98(`omega`)-45.86=0-----------------------------------(4)
Solving the above quadratic equation, using the formula for solving the quadratic equation we get roots as,
`(-b+sqrt(b^2-(4*a*c)))/(2*a)` and `(-b-sqrt(b^2-(4*a*c)))/(2*a)`
Therefore,
`omega`=(-0.98+sqrt(0.98^2-(4*0.0051*-45.86)))/(2*0.0051)` and
(-0.98-sqrt(0.98^2-(4*0.0051*-45.86)))/(2*0.0051`
`omega`=38.915 rad/sec and `omega`=-231.072 rad/sec
Since the speed cannot be negative value, as the vehicle is moving in forward direction, therefore the steady speed of the motor is,
`omega`=38.915 rad/sec.
3. To develop a mathematical model of a DC Motor using Simulink:
According to the blog "Induction versus DC Brushless motor" by Wally Rippel, Tesla, the author wants to highlight topics related to EV technology especially related to DC brushless motor and induction motor. According to him, DC is replaced by DC brushlesss motor or induction motors. Also, lead acid batteries have been replaced by lithium ion batteries. He also tells us briefly about the future of EV technology especially in the field of motor drives and main competition between different motors specifically DC brushless and Induction motor. He said that there will be the future of both the components induction and brushless DC motor depends upon their demands and uses.
DC Brushless Motor:
The technology behind the DC brushless motor is that it has more than two permanent magnet which generates a DC magnetic field. For better torque generation in rotor and stator, a current flow in winding and this happen due to magnetic field near the stator. According to him, the brushless motor needs a proper flow of current into rotor and stator if it will be not in proper way then the current will not generate and there will be decrease in torque. To maintain electrical to mechanical conversion, there should be smooth flow of current otherwise the brushless motor is of no use. A brushless motor does not create any starting torque. The advantage of DC brushless motor is that there is low heat generation in the rotor, its cooling capacity is better as compaared to induction motor drives. Also, peak power efficiency is high. Another difference is that DC brushless motors can operate at unity power factor whereas the best induction motor drives is about 85% of efficiency.
Induction Motor:
Induction motor was discovered by Nikola Tesla in 1889. In induction motor, there is the use of steel laminated device instead of permanent magnets. Currents fflowing in the stator produces a rotating magnetic field that enters into the rotor. Since the rotor is like a short circuited path, a current flows through the rotor which satisfied the condition required for the motor's working principle, "When a current-carrying conductor is placed in a magnetic field, the conductor experiences a force". In an induction motor current interacts with the magnetic field generated by stator to produce a force which is required for rotor torque. The induction motor is compatible with a power source. An inducttion motor cannot be operated in DC. It runs on AC supply only. The induction motor does not have any magnets and in this the magnetic field is adjustable. In an induction motor magnetic field is proportional to V/f(Voltage to Frequency). This technology is used to reduce the magnetic losses and conduction losses due to which efficiency will maintain at maximum peak. This all lead to increase in performance. Even though the peak efficiency of induction motor is less compared to DC brushless motor, they are very useful for high performance as it operates smoothly. The controlling in induction motor is more complex. Also the torque generated is low and the motor temperature increases when it works at high speed.
Conclusion:
Induction motors were used during 1990s general Motors EV-1 and Tesla Roadster and rest all HEV were using DC Brushless motors. The only difference is in rotoe and inverter controller of the motors. According to sensors, the DC brushless motor requires absolute position sensor whereas induction motor only needs speed sensors. Another factor is related to cost of permanent magnets, it is costly in Kg. Also, the permanent magnet rotor needs more careful maintenance and it is difficult to handle. So, cost of induction motor is less. Also, as the machine size grows, magnetic losses increases and part load efficiency drops. In case of induction motor, as machine size grows losses do not necessarily grow. The conclusion according to the author is that the induction motor drives is only best for pure high performance EV whereas DC brushless drive are beneficial for hybrid as well as Plug-In Hybrid Vehicle.