Explanation about Power Electronics, Duty Cycle Control Signal, Inverter
AIM :
1. To explain about efficiency of Power Electronics and also types of losses occur in Power Electronic circuits.
2. To generate a duty cycle control signal for simple power converter circuit by using suitable MATLAB model.
3. To make a comparision of square wave inverter output voltage with sine modulated one.
4. To explian the difference of VFD and Vector Control Drive and for Induction motor in EV application VFD is used or not.
STUDY - 1 :
Power Electronics :
Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean compact and robust manner for convinient utilisation. Power Electronics is the application of Solid state electronics to the control and conversion of electric power. The Conversion is performed with Semiconductor Switching devices such as diodes, thyristors and power transistors such as the Power MOSFET and IGBT. The power range can be from miliwatts, for example mobile phones to megawatts in electric power transmission system.In contrast to electronics system concerned with transmission and processing of signals and data, in power electronics substantial amount of electrical energy are processed.
The Power Conversion systems can be classified according to the type of input and output power.
- AC to DC(Rectifier)
- DC to AC(Inverter)
- DC to DC(Chopper)
- AC to AC(Cycloconverter)
Efficiency of Power Electronics :
Power Electronics (PE) is the technology associated with efficient conversion, control and conditioning of electric power by static means from its available input into the desired electrical output form. Electric energy conversions carried out by PE circuits are diverse and apply to power varying from tens of watts up to hundreds of megawatts. This by particular wide range of power is reflected, among others, in the varying overall dimensions of PE arrangements. Some of the arrangements are hand size, while others require spaces especially designed for their utilization.
Our attention should be drawn to the fact that the definition of PE emphasizes high conversion efficiency. That is connected to the fact that the principle of operation of any PE circuit consists in periodical linking of a power source to the electrical energy consumer – load. In such a case, assuming ideal switches (immediate switching, null resistance in the “on” state and infinitesimally high resistance in the “off” state) as well as the lack of other dissipation elements, power losses equal zero. therefore, efficiency of power electronic apparatus may approach as high as 98–99%.
Reliability of power converters become a key industrial focus. Electronic devices and control circuits must be highly robust in order to achieve a high useful life. A special accent must be set on the total efficiency of the power electronics circuits. Firstly, because of the economic and environmental value of wasted power and secondly, because of the cost of energy dissipated that it can generate. Power electronics converters have evolved rapidly both in terms of available electronic switching devices and converter topologies. Power converters exhibiting higher efficiency, faster response and better control.
Power Losses of Power Electronics :
According to energy conversation, power losses of power electronic circuits are:
"Input Power = Output Power + Power Losses"
Higher power losses cause lower efficiency and higher temperature of the circuits. Larger cooling components are needed for preventing the components of the circuit from overheated so that the circuits are larger and heavier, and hence, the production costs are high. Power losses of power electronics generally include:
- Conduction losses
- Switching losses
Fig : Power losses in Power Electronics
Conduction Losses :
Conduction losses occur when the device is in full condition. The current in the device is whatever is required by the circuit and the voltage at its terminals is the voltage drop due to device itself. These losses are in direct relationship with the dutycycle.
Fig : MOSFET Switch
When the MOSFET switch is 'ON' Current(`I_(rms)`) flows through resistor(`R_D`), conduction losses will occur. Conduction losses can be calculated by following steps.
`P_(cond) = I_(D(on))^2*R_(DS(on))*D`
---> `P_(cond)` - Conduction losses
---> `I_D` - Current
---> `R_(DS)` - Resistance
---> `D` - Duty cycle
Switching Losses :
Switching losses occur when the device is transitioning from the blocking state to the conducting state and vice-versa. This interval is characterized by a significant voltage across its terminals and a significant current through it. The energy dissipated in each transition needs to be multiplied by the frequency to obtain the switching losses.
Fig : MOSFET Switch and `V_(DS),I_D,P_(on)`Characteristics
When the MOSFET is 'OFF' Voltage(`V_(DD)`) is blocked by the device, No current flowing through the drain so (`I_D` = 0), In this case the energy dissipated by the MOSFET is zero. When the MOSFET is turned 'ON' the current (`I_D`) starts to rise but Voltage (`V_(Ds)`) is not start to fall until the current reached its final value, then (`V_(Ds)`) starts to fall until it reaches its final value which is close to zero, with this MOSFET is completely 'ON' and the conduction losses takes place. If we consider the instantaneous power during transition we see a triangular spike, the area between the spike represents the energy dissipated during turn 'ON'. This energy can be caluculated as
`E_(on) = I_(D(on))*V_(DD)*(t_(on)/2)`
where, "Total time `t_(on)`" is "time it takes for the current to rise `t_(ri)`" plus "time it takes for the Voltage to fall `t_(fv)`"
`t_(on) = (t_(ri)+t_(fv))`
Turn 'OFF' losses is similar to turn 'ON' losses. when the MOSFET is 'ON' (`V_(Ds)`) is very small is close to zero and there is a current flowing through drain of the MOSFET. When the device is turned 'OFF' Voltage (`V_(Ds)`) is starts to rise but the current (`I_D`) flows to the device. when the voltage(`V_(Ds)`) between the drain and source reaches its final value, the current starts to drop until it reaches it final value close to zero.The area between the spike represents the energy dissipated during turn 'OFF' and can be determined as
`E_(off) = I_(D(off))*V_(DD)*(t_(off)/2)`
where, "Total time `t_(off)`" is "time it takes for the Voltage to rise `t_(rv)`" plus "time it takes for the Current to fall `t_(fi)`"
`t_(off) = (t_(rv)+t_(fi))`
Therefore Switch losses can be calculated as
`P_(switch) = P_(on)+P_(off)`
`P_(switch)=(E_(on)*f_(sw))+(E_(off)*f_(sw))`
where,
---> `P_(switch)` - Switching Losses,
---> `E_(on) & E_(off)` - Turn-on & Turn-off Energy,
---> `V_(DD)` - Voltage,
---> `t_(on)&t_(off)` - Total on-time & off-time
Total Power Losses :
The Total Power Losses is combination of Conduction losses and Switching Losses.
Power Losses = Conduction Losses + Switching Losses
`P_(Loss)=P_(cond)+P_(switch)`
STUDY - 2
For generating a Duty cycle control signal for simple Power Converter is by using Boost Converter.
A boost converter (step-up converter) is 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).
Fig : Boost Converter Schematic
When a boost converter operates in continuous mode, the current through the inductor never falls to zero. From the above fig, that the left hand side of 'L' is at 'Vi' and the right hand side of L sees the 'Vs'. The average value of Vs is "(1-D)Vo" where 'D' is the duty cycle of the waveform driving the switch. From this we get the ideal transfer function:
Vo = Vi/(1-D)
where,
Vo - Output Voltage,
Vi - Input Voltage,
D - Duty Cycle
MATLAB Model of Boost Converter :
The above model is the example which shows the Operation of a boost converter, which duty cycle is 0.5(50% of duty cycle) is connected with PWM generator.
Pulse Width Modulation(PWM) - speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.
Fig : Duty Cycle input
Fig : Constant Frequency
In the above Boost Converter Model, Duty Cycle is 0.5 and Frequency is 20e3 Hz. Here the Duty Cycle of PWM power supply is set to 50%, then the pulse is 'ON' for the 50% of the time and it is 'OFF'' for the 50% of time. Duty Cycle refers the amount of time it is 'ON".
Case - (i) : For 50% duty cycle, Output Voltage is
Vo = Vi / (1-D)
Vo = 24 / (1-0.5)
Vo = 48 V
The above simulation results shows for 50% duty cycle and Output Voltage(Step-Up-Voltage) is 48V.
Case - (ii) For 25% Duty Cycle, Output Voltage is
Vo = Vi / (1-D)
Vo = 24 / (1-0.25)
Vo = 32 V
The above simulation results shows for 50% duty cycle and Output Voltage(Step-Up-Voltage) is 32V.
Case - (iii) For 75% Duty Cycle, Output Voltage is
Vo = Vi / (1-D)
Vo = 24 / (1-0.75)
Vo = 96 V
The above simulation results shows for 50% duty cycle and Output Voltage(Step-Up-Voltage) is 96V.
Result :
From the above Operation of Boost Converter using Duty Cycle, observed that by varying Duty Cycle Output Voltage also varies. Therefore Duty Cycle Control Signal is generated for Boost Converter Model.
STUDY - 3
Inverter :
Alternating Current (AC) power supply is used for almost all the residential, commercial and industrial needs. But the biggest issue with AC is that it cannot be stored for future use. So AC is converted into DC and then DC is stored in batteries and ultra-capacitors. And now whenever AC is needed, DC is again converted into AC to run the AC based appliances. So the device which converts DC into AC is called Inverter. The inverter is used to convert DC to variable AC. This variation can be in the magnitude of voltage, number of phases, frequency or phase difference.
Inverter can be classified into three types based on output,
* Square wave Inverter
* Sine Wave Inverter
* Modified Sine Wave Inverter
Fig : Different Types of Inverter
Square Wave Inverter :
The output waveform of the voltage for this inverter is a square wave. This type of inverter is least used among all other types of inverter because all appliances are designed for sine wave supply. If we supply square wave to sine wave based appliance, it may get damaged or losses are very high. The cost of this inverter is very low but the application is very rare. It can be used in simple tools with a universal motor.
Sine Wave Inverter :
The output waveform of the voltage is a sine wave and it gives us a very similar output to the utility supply. This is the major advantage of this inverter because all the appliances we are using, are designed for the sine wave. So, this is the perfect output and gives guarantee that equipment will work properly. This type of inverters is more expensive but widely used in residential and commercial applications.
Modified Sine Wave Inverter :
The construction of this type of inverter is complex than simple square wave inverter but easier compared to the pure sine wave inverter. The output of this inverter is neither pure sine wave nor the square wave. The output of such inverter is the some of two square waves. The output waveform is not exactly sine wave but it resembles the shape of a sine wave.
Modified sine wave inverter is a classification of power inverters whose wave form and power quality is superior to basic square wave inverter. The modified sine wave inverter’s wave form stay at zero volt for some time before any change in polarity.
Comparision of Square Wave Inverter Output Voltage with Sine Modulated Wave :
Below given wave form is comparison between square wave and modified sine wave:
Fig : Square Wave
Fig : Modified Sine Wave
The wave form in green colour represents square wave and the red colour represents the modified sine wave.
The square wave changes its polarity abruptly, but the modified sine wave stays at an intermediate point (zero volts) before any change in polarity. By simply keeping the wave form at zero point for some milliseconds it will reduce noise at the output, but modified sine wave cannot be compared with pure sine wave.
Modified Sine Wave (quasi-sine) A modified sine wave inverter actually has a waveform more like a square wave, but with an extra step. A modified sine wave inverter will work fine with most equipment, although the efficiency or power of the equipment will be reduced with some.
---> Motors, such as refrigerator motor, pumps, fans etc will use more power from the inverter due to lower efficiency. Most motors will use about 20% more power. This is because a fair percentage of a modified sine wave is higher frequencies – that is, not 60 Hz – so the motors cannot use it.
---> Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises.
---> Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the peak of the line power – basically, the modified sine has a flat top rather than a peak – this may cause the occasional double trigger. Because the modified sine wave is noisier and rougher than a pure sine wave, clocks and timers may run faster or not work at all.
---> Items such as bread makers and light dimmers may not work at all – in many cases appliances that use electronic temperature controls will not control. The most common is on such things as variable speed drills will only have two speeds – on and off.
---> most equipment will operate without any noticeable difference, and because the lower cost, makes this the most common inverter sold and generally the only type found at your local retailer.
Square Wave Very few but the very cheapest inverters any more are square wave. A square wave inverter will run simple things like tools with universal motors with no problem – but not much else. These are seldom seen any more except in the very cheap or very old ones.
Sine Wave vs Square Wave Inverter :
The sine wave inverter is vastly different from the square wave inverter simply because their core functioning itself is very different from each other. The way these two inverters convert DC into AC current is different which ultimately results in both these inverters having their own unique features. The following are some of the major areas where these inverters differ:
Supported Appliances -Square wave inverters are usually used to support motors alone.Sine wave inverters are used to support household appliances such as refrigerators ovens, computers, laptops, etc.
Overall Safety Level - Square wave inverters are less reliable and also unsafe to use for appliances.Sine inverters are highly safe to use.
Noise Interference - Square wave inverters produce a very loud noise when used.Sine wave inverters produce normal sound only.
Cost - Square wave inverters are less expensive than Sine wave inverters.Sine wave inverters are more expensive than Square wave inverters.
Conclusion :
A square wave inverter is recommended for those users who just need an inverter for powering up very basic electrical equipment such as lights and fans or who need it for running motors. If users are planning to connect sensitive electrical equipment such as television, microwave, refrigerator etc. with the inverter as well, you will have to go for the sine wave inverter because of its compatibility with the electronics as well as safety features.
Modified sine wave inverter whose functioning is kind of in-between that of a sine wave and square wave inverters. A modified sine wave inverter does not produce a pure sine wave but rather simulates a sine wave in its most important aspects.This ensures that most of the appliances can still be connected to this inverter and will not heat up easily. This kind of inverter is preferred by those who want the benefits of a sine wave inverter but without having to spend too much money. However, if your priority lies in ensuring that your appliances are absolutely safe, the most foolproof way of ensuring the same is to invest in a pure sine wave inverter.
STUDY - 4 :
Variable frequency Drive (VFD) :
A variable frequency drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage of its power supply. The VFD also has the capacity to control ramp-up and ramp-down of the motor during start or stop, respectively.
Scalar control (frequency control) is a control technique of a brushless AC motor, which is to keep the voltage/frequency ratio (V/Hz) constant throughout the full operating speed range, with control only the magnitude and frequency of the supply voltage.
Fig : Variable Frequency Drive System
Vector Control Drive :
Vector control, also called field-oriented control (FOC), is a variable-frequency drive (VFD) control method in which the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. FOC is used to control AC synchronous and induction motors.
Vector Control Technique used in 3 phase AC induction motor output by means of controlling the voltage magnitude, voltage angle and the frequency. It was used initially for applications that require constant torque over full speed range, constant speed irrespective of load variations, and full torque at zero speed.
Fig : Vector Control System
Difference Between VFD and Vector Control Drive :
The Scalar Control Method is based on varying two parameters simultaneously. The Speed can be varied by Increasing or decreasing the Supply frequency, but this results in change of impedances. The Change of impedances eventuates the increase or decrease of current. If the current is small, torque of the motor decreases. If the frequency decreases or Voltage increases, the coils can be burned or saturation can occur in the iron of coils. To avoid this problems, it is necessary to vary the voltage and frequency at the same time.
Field Oriented Control (FOC) is possible to uncouple the field components. Uncoupling establishes two independent and single controlled currents : the flux-producing current and the torque-producing current. Using these currents the flux and torque can be independently controlled. Moreover, a 90 degree electric angle is ensured between the uncoupled control currents. As a result, the model of induction motor losses its complexity and a high performance drive can be realized.
Conclusion :
Scalar control(VFD) is cheap and easy to implement, though it is not sufficient in controlling drives with dynamic behaviour. Overall Scalar is low performance, but stable. And in other hand with Field Oriented vector control the current independently, allowing it to operate move fast responses and making a much more appropiate for a dynamic drive. However Field Oriented Control is more costly and complex, but it is the higher performing mode of the two method.
VFD is not preferred for Induction motor in EV application. Vector Control drive is most preferred because it's precise control of machine's torque and is speed over a wide range including high speed control above rated speed. Vector control has more accuracy compared to VFD control. Vector control is one kind of VFD where speed and torque of motor is controlled using PWM techniques in Inverter. A Vector Control drive has high power factor, high-speed capability, and the ability to use induction motors are important advantages.