Power Electronic Circuits and Controller for EV
TARGET-
- Why power electronics circuits are efficient? In practice, which types of losses occur in power electronics circuits?
- Using a suitable MATLAB model, show how duty cycle control signal can be generated for a simple power converter circuit?
- Make a comparison of square wave inverter output voltage with sine modulated one.
- Explain the difference between VFD and Vector control drive. Can we use VFD method for induction motor in EV application?
SOLUTION-
1.Efficient Power Electronics Circuits>
Power electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact and robust manner for convenient utilisation. In the broader sense it implies that part of electronics used in electric power. This technology is used in the systems for control and regulation of electric power supplies and in the systems for the regulation of electric drives. Power electronics is based fundamentally on the switching of the power semiconductor devices. With the development of power semiconductor technology, the power handling capabilities and the switching speed of the power devices have improved vastly. The development of microprocessors/microcomputer technology has a significant impact on the control and synthesizing the control strategy for the power semiconductor devices.Power electronics combine power, electronics and control. Power deals with the static and rotating power equipment for the generation, transmission and distribution of electric power. Electronics deals with the solid state devices and circuits for signal processing to meet the desired control objectives. Control deals with the steady-state and dynamic characteristics of closed loop systems. The figure below shows the interrelationship of Power electronics with power, electronics and control.
Fig-Relationship of Power electronics with Power, Electronics and Control The power component, serving for the transfer of energy from the source to the load consists of power electronic switches, electric chokes, transformers, capacitors, fuses and sometimes resistors. A combination of these elements combine to make different converter circuits adjusted to the mode of the primary supply and the character of the load. Energy losses within a converter should be as small as possible. Consequently, the semiconductor elements of the converter are mainly operating in the pulse (switching) mode. They could be either controllable (transistors, thyristor) or non- controllable (diodes). The control or information block controls the regulating (mostly switching) elements of the converter. The control or regulation is accomplished on the basis of the information the control block has collected from the power part of the apparatus. Mostly the information concerns the output voltage, load current or current/voltage of a critical element of the converter (e.g. transistor). The control block can functionally be a very complex electronic assembly consisting of either analog or digital elementary assemblies.
There are four possible types of power electronics converters namely
[1]AC to DC converters (controlled rectifiers).
[2] AC to AC converters (AC voltage controllers).
[3] DC to AC converters (Inverters).
[4] DC to DC converters (DC choppers).
Fig- Generalized Power converter system
[1]AC to DC converters>
The process which converts AC to DC is known as Rectification; hence these converters are also called as Rectifiers. Usually the AC input to the circuit is a sinusoidal voltage source that operates at 120V, 60Hz or a 230V, 50 Hz, which are used for power distribution applications. The AC voltage is rectified into a unidirectional DC voltage, which can be used directly to supply power to a DC resistive load or control a DC motor.
Fig-Full wave Rectifier
Rectification can be half wave or full wave. In the first case (half wave rectification), only one half of the input waveform can be employed to reach the desired output. Therefore only this half AC wave (positive or negative) is converted. Hence it is not useful for power transfer. The full wave rectification can convert the whole of the input waveform to achieve the constant output signal. Hence it becomes more efficient and useful.
[2] AC to AC converters>
These converters are used to transform an AC input signal to another AC output signal with arbitrary amplitude. Moreover depending on the converter complexity, the frequency can be changed as well. Converters which have the same fundamental input and output frequencies are called “AC controllers”.
Fig- Voltage controller with Waveform Fig-Cycloconverter Circuit with Waveform Fig-Matrix Converter
The conversion is from a fixed voltage fixed frequency (FVFF) to a variable voltage fixed frequency (VVFF). Applications include light dimmers and control of single phase AC motors that are typically used in home appliances. When both voltage and frequency are changed, the circuits are called “cycloconverters”, which convert a fixed voltage fixed frequency (FVFF) to a variable voltage variable frequency (VVVF) and when fully controlled switches are used, this class of circuit is called “Matrix converter”.
[3] DC to AC converters>
These types of converters are also referred to as Inverters, which can obtain a certain amplitude and frequency of the AC voltage and/or current without using normally an intermediate DC stage. Inverters transform DC voltage fromsources such as batteries, solar cells, or fuel cells or output of a rectifier into AC, for powering motor drives, providing stand-alone AC output, or interconnecting to the AC grid.
Inverters can be usually classified according to their ACoutput as single phase or three phase and also as half and full bridge converters.
[4] DC to DC converters>
These converters are electronic circuits that change the DC operating voltage or current. Normally, they are designed in order to transfer power from the input to the output in one direction. These converters are also known as “Choppers” or “switching regulators”.
The circuit will change the level voltage available from a DC source such as a battery, solar cell or a fuel cell to another DC level, either to supply a DC load or to be used as an intermediate voltage for an adjacent power electronic conversion such as a DC to AC converter. DC to DC converters coupled together with AC to DC converters enable the use of high voltage DC (HVDC) transmission which has been adopted in transmission lines throughout the world.
Efficiency of Power Electronic Circut>
Power electronics deals with the conversion and control of electrical energy with the help of power semiconductor devices that operate in a switching mode, and, therefore, efficiency of power electronic apparatus may approach as high as 98–99%. Power electronics is very important in modern high-efficiency energy processing systems, such as HVDC, SVC, flexible ac transmission system (FACTS) for active and reactive power flow control, uninterruptible power supply (UPS), and industrial process control with variable-frequency drives for improving productivity and quality of products in modern automated factories. After SCR, a great many power electronic devices have emerged like GTO, IGBT, SIT, MCT, TRIAC, DIAC, IEGT, IGCT and so on. These devices are rated for several hundreds of volt and ampere unlike the signal level devices which work at few volts and mill amperes. In order to achieve the purpose of power electronics, the devices are made to work as nothing more than a switch. All the power electronic devices act as a switch and have two modes, i.e. ON and OFF. Silicon (Si), gallium- nitride (GaN), and silicon-carbide (SiC) are the commonly used materials for power electronic devices with their specific advantages. SiC is also used to manufacture power transistors, but because SiC does not have an electron-gas structure, only vertical conduction devices are practical. With a vertical conduction device in GaN or SiC, 1- to 2-kV breakdown voltage levels are easier to reach than with Si. As silicon (Si), gallium-nitride (GaN), and silicon-carbide (SiC) processes are maturing, so, too, are their suppliers’ expertise and creativity towards power electronics. GaN and SiC are wide-bandgap (WBG) materials, which mean the energy required for an electron to jump from the top of the valence band to the bottom of the conduction band within the semiconductor is typically larger than one or two electron volts (eV). GaN transistors are extremely fast. As a result, the system is far more sensitive to the layout than it is with slower Si devices. In particular, stray inductance plays a larger role in the overall system efficiency.
Losses Occur in Power Electronics Circuits>
The losses are calculated in Matlab under the assumption that the three-phase currents on the rotor- and line- side are perfectly sinusoidal, which can be assumed as the current ripple in average will not generate any additional losses. The total losses consist of conduction and switching losses in the IGBT and clamping diode modules. Conduction Loss--Conduction losses occur when the device is in full conduction. The current in the device is whatever is required by the circuit and the voltage at its terminals is the voltage drop due to the device itself. These losses are in direct relationship with the duty cycle.
The conduction losses Pcond depend on the on-state voltage drop across the device and the current through it. They can be calculated from the on-state threshold voltage Vce, the onstate slope resistance Ice, and the device current Ice according to
Switching Loss--Switching losses occur when the power switch or rectifier is transitioning between the ON state to the OFF state and vice versa. These are frequency depend- ent since there are more transitions per second at higher frequencies.
The switching loss can be calculated from the characteristic turn-on and turn-off energy (Eon, respectively Eoff ) given in the datasheetsThe switching losses are dependent on the switched current and the device temperature. The switching losses Psw can be calculated by summing up the switching events during a fundamental period according to
Dead Time Loss--The dead time loss is the loss that occurs due to the forward voltage of the body diode of the low-side switch (MOSFET) and the load current during dead time. Here, the symbol Pdead_time is used.
When the switches on both sides are in the off state, the body diode of the low-side MOSFET is directed in the forward direction with respect to the load current, and so a current flows to the load through this body diode. The resulting loss, Pdead_time, can be calculated using the following equation
Gate Charge Loss-- Gate charge loss is consider as losses relating to driving of the gate of a MOSFET power switch.
Gate charge losses are losses that, in this example, are due to the Qg (total gate charge) of external MOSFETs. They occur because, when a MOSFET is switched, the gate driver in the power supply IC charges the parasitic capacitance of the MOSFET . This is an issue for study in all applications that use MOSFETs as power switches, regardless of the switching power supply.
IC Operation Loss-- This loss occurs due to consumption of Power electronic components.
Where, VIN= Input voltage & Icc=IC consumption current.
TOTAL POWER LOSS= Pavg.cond+PSW+Pdead_time+PGate+PIC
2. BOOST CONVERTER>
A boost converter (step-up converter) is a DC-to-DC power converter that steps up voltage (while stepping down current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) containing at least two semiconductors (a diode and a transistor) and at least one energy storage element: a capacitor, inductor, or the two in combination. To reduce voltage ripple, filters made of capacitors (sometimes in combination with inductors) are normally added to such a converter's output (load-side filter) and input (supply-side filter).
Operation 1 When Square wave signal is High--
The circuit action during the initial high period of the high frequency square wave applied to the MOSFET gate at start up. During this time MOSFET conducts, placing a short circuit from the right hand side of L1 to the negative input supply terminal. Therefore a current flows between the positive and negative supply terminals through L1, which stores energy in its magnetic field. There is virtually no current flowing in the remainder of the circuit as the combination of D1, C1 and the load represent a much higher impedance than the path directly through the heavily conducting MOSFET.
Operation 2 When Square wave signal is Low--
The current path during the low period of the switching square wave cycle. As the MOSFET is rapidly turned off the sudden drop in current causes L1 to produce a back emf. in the opposite polarity to the voltage across L1 during the on period, to keep current flowing. This results in two voltages, the supply voltage VIN and the back e.m.f.(VL) across L1 in series with each other.
This higher voltage (VIN +VL), now that there is no current path through the MOSFET, forward biases D1. The resulting current through D1 charges up C1 to VIN +VL minus the small forward voltage drop across D1, and also supplies the load.
SIMULINK BOOST CONVERTER MODEL> This example shows the operation of a boost converter.
Description-- A boost converter is a DC/DC power converter which steps up voltage from its input (source) to its output (load). In continuous conduction mode (current through the inductor never falls to zero), the theoretical transfer function of the boost converter is: (Vout/Vin)=1/(1-D) where D is the duty cycle.
In this example, the converter is feeding an RC load from a 24 V source and the PWM frequency is set to 20 kHz.
SIMULATION-- Running the simulation and observe waveforms on Scope, that the mean value of the load voltage (Vout) is very close to the theoretical value of: Vout = 24 / (1 - 0.5) = 48 V.
3.Square Wave and Modulated Sine Wave>
The inverter converts battery DC electricity to AC electricity. DC signal looks like a unidirectional straight line and AC signal looks like waves of ocean. DC wave cannot travel long so all our home electricity is supplied in AC manner. But the battery stores the power as DC, the inverter converts this DC current to some what similar to AC current. Depending on the inverter circuit, either inverter converts the DC current to Sine wave AC current or Square wave AC current.
Square Wave Inverter>
The square wave inverter is a type of inverter which converts DC power to AC power. The square wave inverter is simpler in design and more efficient than a sine wave inverter. However, the square wave will not be appropriate for some loads, because the large harmonic content can cause interference. Some electromechanical devices do not work well with square wave power. If the load is reactive, there will be loss of efficiency due to reactive power loss, so the efficiency advantage of the inverter may be nullified. Resistive loads also may suffer from radiated interference due to harmonic content.
A switching circuit is used in the conversion of DC voltage to an alternating (or bipolar) square wave voltage. One method is the use of the inverter bridge (also known as an H-bridge), which is illustrated in figure shown above.The switch symbols are used to represent switching transistors (IGBTs or MOSFETs) or other types of electronic switching devices. In Figure a, switches S2 and S3 are on for a specified time and S1 and S4 are off. The direct current is through the load, which creates a positive output voltage. In Figure b, opposite switches are on and off. The current is in the opposite direction through the load, and the output voltage is negative. The complete on/off cycle of the switches produces an alternating square wave.
Modified Sine Wave Inverter>
The operation of a basic H-bridge is enhanced to produce the misnamed modified sine wave, which is shown in figure below.
The resulting wave is far from resembling a sine wave despite the name and cannot be used for some types of loads. The wave is created from a square wave and adds a short dead time between positive and negatives excursions of the square wave. Although the resulting output is definitely not a sine wave, the harmonic distortion is reduced to 24% compared to a pure square wave’s 45%. This is still a significant amount of distortion. Filtering can reduce this distortion; however, filtering also significantly reduces the power to the load and thus reduces the efficiency. With PWM and low-pass filtering, a nearly pure sine wave can be formed. The PWM process produces a pulse waveform with varying pulse widths proportional to the amplitude of a signal (sine wave in this case).
- In the inverter, a low-power reference 60 Hz sine wave and a higher-frequency triangular wave are used to produce the PWM waveform.
- The sine wave amplitude values are sampled by the triangular wave to produce the PWM waveform.
- The PWM signal is passed through a low-pass filter (equivalent to the mathematical process of integrating) and reproduces the sine wave with very little distortion, as shown in Figure below.
The sine wave inverter uses a low-power electronic signal generator to produce a 60 Hz reference sine wave and a 60 Hz square wave, synchronized with the sine wave.The reference sine wave goes to the PWM circuit along with a triangular wave that is used to sample the sine wave values to produce a PWM control output. This PWM control signal operates an electronic switch that converts the DC input to a PWM voltage.
The PWM voltage and the 60 Hz square wave are inputs to the H-bridge. The output of the H-bridge is an alternating PWM voltage, as shown in Figure below-
The filter converts the alternating PWM voltage to a utility-quality sine wave that goes to a transformer that steps it up for use by the load.
Square wave Inverter vs Sine Modulated Inverter>
A Modulated Sine Wave inverter actually has a waveform more likely square with an extra step . A modulated sine wave inverter will work fine with most equipment although the efficiency or power of the equipment will be reduced with some.
1.Motors, such as refrigerator motor, pumps, fans etc will use more power trom the inverter due to lower efficiency. Most motors will use about because a fair percentage ot a modified sine wave is higher frequencies - that is not 60 Hz - so the motors cannot use it.
2. Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises.
3.Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the peak of the modified sine has a flat top rather than a peak this may cause the occasional double clocks and timers may run taster or not work at all. trigger Because the modified sine wave is noisy and roug
4.ltems such as bread makers and light dimmers may not work at all - in many cases appliances that use electronic temperature controls will is on such things as variable speed drills will only have speeds-on and off.
5 most equipment will operate without any noticeable difference, and because the lower cost, makes this the most common inverter sold .
INVERTER WAVE TYPE-- (i)Square Wave Inverter>
- Safety of appliances: Less
- Noise level: High, creates humming noise in inverter as well as in appliances
- Supported appliances: Motors
- Price: Economical
(ii)Modulated Sine Wave Inverter>
- Safety of appliances: High
- Noise level: Normal
- Supported appliances: Computers, laptops, refrigerators, ovens
- Price: Little expensive
Conclution> Modulated Šine wave inverter output is very close to the AC current. It is very pure form compared to square wave output. Many devices gets heated up if you use on Square wave inverter and make humming noise too. Because of this reason square wave inverters in few European countries is banned.
Induction Motor Characteristic under Variable Frequency Drive--
In an induction motor induced in stator, E is proportional to the product of the slip frequency and the air gap flux. The terminal voltage can be considered proportional to the product of the slip frequency and flux, if stator drop is neglected. Any reduction in the supply frequency without a change in the terminal voltage causes an increase in the air gap flux which will cause magnetic saturation of motor. Also the torque capability of motor is decreased. Hence while controlling a motor with the help of VFD or Variable Frequency Drive we always keep the V/f = ϕ ratio constant. K=f/frated (i)For operation below K < 1 i.e. below rated frequency we have constant flux operation. For this we maintain constant magnetization current Im for all operating points.
(ii)For K > 1 i.e. above rated frequency we maintain terminal voltage V rated constant. In this field is weakened in the inverse ratio of per unit frequency ‘K’.
(iii)For values of K = 1 we have constant torque operation and above that we have constant power application.
Vector Control Drive> The "vector" that is the name of this drive technology is the rotating space vector that describes the flux in the motor. Since flux and current are in phase, it also describes the current in the stator.
Control of speed and torque--
An induction motor is very similar to a DC motor. It needs a magnetizing current and a torque producing current. In a DC motor, these two currents are fed to two different windings; the field winding and the armature winding. In an induction motor, there is only one set of windings: the stator winding. So the vector drive has to separate the two components some other way.
It does this by keeping in mind that magnetizing current always lags (inductive) the voltage by 90 degrees and that the torque producing current is always in phase with the voltage. It controls the magnetizing current (usually named Id) in one control loop and the torque producing current (Iq) in another control loop. The two vectors Id and Iq, which are always 90 degrees apart, are then added (vector sum) and sent to the modulator, which turns the vector information into a rotating PWM modulated three-phase system with the correct frequency and voltage.
As soon as a deviation from correct speed or torque or magnetizing current is detected by the control loops the corresponding variable will be changed by the controller to correct the variable.
If for example the speed is wrong, the output frequency will be corrected and also the voltage so that the correct magnetizing current is maintained. And correspondingly, if the stator winding heats up the magnetizing current would go down if the decrease wasn't detected and corrected by the controller. The action in this latter case is that the voltage goes up (PWM adjusted), but not the frequency (the speed was already correct.
Vector drives are among the most complex standard equipments that exist. But keeping in mind that there are always two control loops, one for magnetizing current and one for speed/torque.
Vector drives come in 2 types, Open Loop and Closed Loop, based upon the way they get their feedback information.
(i)Open Loop or Sensor-less Vector Drive-- Open Loop is actually a misnomer because it is actually a closed loop system, but the feedback loop comes from within the VFD itself instead of an external encoder. For this reason there is a trend to refer to them as "Sensor-less Vector" drives.
Fig-Sensor-less Vector Control Drive
The microprocessor creates a mathematical "model" of the motor operating parameters and keeps it in memory. As the motor operates, the microprocessor monitors the output current (mainly), compares it to the model and determines from experience what the different current effects mean in terms of the motor performance. Then the microprocessor executes the necessary error corrections just as the closed Loop Vector Drive does. The only drawback is that as the motor gets slower, the ability of the microprocessor to detect the subtle changes in magnetics becomes more difficult. At zero speed it is generally accepted that an Open loop Vector Drive is not reliable enough to use on cranes and hoists. For most other applications though it is just fine.
(ii)Closed Loop Vector Drive-- A true Closed Loop Vector Drive uses a shaft encoder on the motor to give positive shaft position indication back to the microprocessor.So when the microprocessor says move x radians, the encoder says "it only moved x-2 radians". The microprocessor then alters the PWM signature on the fly to make up for the error.
Fig-Closed loop Vector Control Drive
For torque control, the feedback allows the microprocessor to adjust the pattern so that a constant level of torque can be maintained regardless of speed, i.e. a winder application where diameters are constantly changing. If the shaft moves one way or the other too much, the torque requirement is wrong and the error is corrected. A true closed Loop Vector Drive can also make an AC motor develop continuous full torque at zero speed, something that previously only DC drives were capable of. That makes them suitable for crane and hoist applications where the motor must produce full torque before the brake is released or else the load begins dropping and it can't be stopped. Closed Loop is also so close to being a servo drive that some people use them as such. The shaft encoder can be used to provide precise travel feedback by counting pulses.
Choosing Vector Control Drive Over VFD-- A standard VFD (a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent upon the load conditions. The Scalar drive knows nothing about that, it only tells the motor what to do. If for example it provides 43Hz to the motor, and the motor spins at a speed equivalent to 40Hz, the Scalar Drive doesn't know. You can't do true torque control with a scalar drive because it has no way of knowing what the motor output torque is (beyond an educated guess). These problems associated with the scalar VFDs inability to alter its output with changes in the load gets worse as the speed reference goes down, so the "rule of thumb" in determining the need for which technology to use is that scalar drives work OK at speed ranges between 5:1 (50Hz applications) or 6:1 (60Hz applications). So if your application will need accurate control below 10Hz, scalar may not work.
A Vector Drive uses feedback of various real world information (more on that later) to further modify the PWM pattern to maintain more precise control of the desired operating parameter, be it speed or torque. Using a more powerful and faster microprocessor, it uses the feedback information to calculate the exact vector of voltage and frequency to attain the goal. In a true closed loop fashion, it goes on to constantly update that vector to maintain it. It tells the motor what to do, then checks to see if it did it, then changes its command to correct for any error.
VFD for EV Application> Practically, below 10 Hz (50 Hz motor) motor cannot start with full load when using V/F method. One solution is to add some boost voltage proportional to speed but it may result in saturating the motor (flux = v/f) but if it is wanted to run the motor down to zero speed it is not possible for VFD so we have to use Vector Control Drive or Direct Torque Control. So the precesize control of EV cannot be achieved and the overall efficiency of the vehicle will decrease as VFD is low efficient.