Comparative Performance Analysis of Thyristor and IGBT Based Induction Motor Soft Starters


Bachelor Thesis, 2012

50 Pages, Grade: 10.0


Excerpt

Table Contents

ACKNOWEDGEMENT

ABSRACT

LIST OF FIGURES

LIST OF TABLES

Chapter 1
INTRODUCTION
1.1 Project background
1.2 Literature review
Objectives
Methodology

Chapter 2
THEORETICAL BACKGROUND
2.1 Induction motor
2.1.1 Operating Principle
2.2 Starting of Induction motor
2.2.1 Direct-on-Line Starter
2.2.2 Star Delta Starter
2.2.3 Auto Transformer Starting Method
2.2.4 Rotor Resistance Starter
2.2.5 Solid State starting
2.3 Soft starters
2.3 Power electronics devices
2.3.1 Thyristor
2.3.1 Insulated Gate Bipolar Transistor (IGBT)
2.4 AC Voltage Controllers (AC Regulator)
2.4.1 Phase Control of Thyristor
2.5 Soft Starter with R-L Load
2.6 Performance parameter
2.7 Three Phase Soft Starter
2.8 Firing Circuits
2.8.1 Using ramp signal
2.8.2 Using cosine control:
2.9 Matlab/Simulink Basics
2.9.1 Matlab
2.9.2 Simulink
2.9.3 Lists of Blocks Used
2.9.4 Thyristor Characteristics
2.9.5 IGBT Characterisics

Chapter 3
SIMULATION STUDY
3.1 Thyristor with R-L load
3.2 Simulink model of IGBT with RL load
3.3 Comparative Performance
3.4 Simulink Model of Three Phase Induction Motor with Thyristor

CONCLUSION AND RECOMMENDATION

FUTURE IMPROVEMENT

REFERENCES

LIST OF ABBREVIATIONS

Abbildung in dieser Leseprobe nicht enthalten

LIST OF FIGURES

Fig 2.1: Typical Split Phase AC Induction Motor

Fig 2.2: Equivalent circuit of three phase induction motor

Fig 2.3: Block Diagram of Soft starter

Fig 2.4: Structure on the physical and electronic level and symbol

Fig 2.5: I-V characteristics of Thyristor

Fig 2.6: Static Characteristics of IGBT

Fig 2.7: ac voltage controller

Fig 2.8: Single phase full wave ac voltage controller with RL load

Fig 2.9: Gating Signal Requirements

Fig 2.10: Waveforms of supply voltage, Load Current and Voltage and Voltage across Abbildung in dieser Leseprobe nicht enthalten

Fig 2.11: Block Diagram of three-phase soft starter of induction motor

Fig 2.12: Block Diagram for firing circuit

Fig 2.13: Basic idea of ramp scheme.

Fig 2.14: Block diagram of cosine scheme of pulse generator

Fig 2.15: Load Voltage and current waveform for RL load with thyristor

Fig 2.16: Input and Output voltage characteristics with IGBT

Fig 3.1: Simulink model of thyristor with RL Load

Fig 3.2: Cosine Scheme Firing Circuit for Thyristor and IGBT

Fig 3.4: Firing pulses of Thyristor and IGBT for α=450 .

Fig 3.5: Output voltage and current of thyristor for α=450.

Fig 3.6: Input voltage, output voltage and current of R-L load with thyristor α=450.

Fig 3.7: Simulink model of IGBT with RL load

Fig 3.8: Output voltage and current of IGBT for α=450.

Fig 3.9: Input voltage, output voltage and current of R-L load with IGBT α=450.

Fig 3.10: Comparative of THD of Thyristor and IGBT for various firing angle

Fig 3.11: Comparative bar diagram of PF of Thyristor and IGBT for various firing angle

Fig 3.12: Simulink Model of 3-Phase soft starter fed to IM

Fig 3.13 Phase current and voltage for phase A at α = 700.

LIST OF TABLES

Table 2.1: Tables of block used in simulation

Table 3.1: Observation Data of THD and PF for IGBT and Thyristor

ACKNOWEDGEMENT

We would like to articulate our deep gratitude to our project Supervisor Er. Durga Prasad Oli. His valuable advice, suggestions, instructive guidance and cooperative supervision have been one of the very motivating factors for sharpening and shaping our project. The supervision and support that he gave truly helped us in the progression and smoothness of our project. The co-operation is much indeed appreciated.

Our special thanks go to the Electrical Department and all its staffs for providing us good environment and lots of cooperation to conduct this project. We would also like to thank our Head of Department Er. Pradip Prasad Sah and DHOD, Er. Durga Prasad Oli for his co-operative efforts on providing us with the well-equipped lab and sophisticated equipments. Without their help and support, our project would have faced many difficulties.

Last but not least we would like thank to all our colleagues, friends, seniors for generously devoting their time and wise ideas to help us with project.

ABSRACT

In this project the performance of split-phase motor and three-phase induction motor drives for soft starting is evaluated. The pre systematically investigates and compares the characteristics of a variable voltage fed induction motor drive for two different types of soft starters; one based on IGBT and another based on Thyristor. The novelty of the work lies in the development of simple and flexible models for simulation purpose.

This project investigates the influence of the parameters of the machine and of the soft starter on the dynamics of the induction machine start. The situations may reproduce actual situations occurred in practice, for example the variation of initial voltage Vi, modification of the start time and load value. In this project we have investigated the relation between Total Harmonic Distortion and Power Factor of the IGBT and Thyristor based soft starter. Using an already predefined fire angle characteristic the influence of the initial voltage was also evaluated.

Discussion of these results and conclusions as to the near-optimum types of profiles are delineated based on voltage and current profile fed to induction motor, starting times, and distortion in current with change in firing angle.

Chapter 1 INTRODUCTION

1.1 Project background

With the development of electrical energy sector, the use of electricity is increasing in various sectors including industries and domestic applications. Electrical drives based on induction motors are the most widely used electromechanical systems in modern industry. Due to their reliability, ruggedness, simple mechanical structure, easy maintenance and relatively low cost, induction motors are attractive for use in a new generation of electrical transportation systems, such as cars, buses and trains. From the variety of electric energy consumers in industry one of the largest is without any doubt the induction machine operating as motor. Besides the classical destination of the induction machine as motor this machine is more and more used in the latest period as generator in the conversion chain of wind or micro-hydro-energy into electricity.

An induction motor requires draws high current during starting and can cause damage to windings when started with direct power supply. To reduce starting current, most reliable and economic method of starting in industrial application is used.

Direct online induction machine starts have many disadvantages. Torque pulsations are often large and modify from positive to negative values. These torque transients in a motor shaft are transmitted to the load, resulting in mechanical wear in the motor bearings and load couplings. Therefore, properly controlling the starting currents and torques of induction machines is of great importance in many instances. Additionally, the resulting starting currents are high, especially during the first few cycles of a starting transient. These high currents are endured by the motor and power system, causing the heating of the machines windings.

To reduce starting current, most reliable and economic method of starting in industrial application is the use of soft starter which reduces voltage at the starting time and hence reduce starting current.

Different power electronics devices such as SCR, IGBT, MOSFET are used with different technology in soft starting and here we compare the performance of the Thyristor and IGBT based soft starters.

1.2 Literature review

The basic ideas of this project were gained from the Book named “Power Electronics”, by Hasan M. Rashid. As explained by Ahmed Riyaz, Atif Iqbal, Shaikh Moinoddin, SK. Moin Ahmed, Haitham Abu-Rub in the International Journal of Engineering, Science and Techonology,Vol.1,No.1,2009.

The Basic idea of induction motor soft starter was gained from the Book named “A Textbook of Electrical Technology”, by B.L. Thereja and A.K. Thereja.

The different parameter of the system and performance can be analyzed on the basis of a dissertation submitted by Chia-Chou Yeh on “Fault Tolerant operation of induction motor drive”.

Objectives

- Comparative performance analysis of Thyristor and IGBT based induction motor soft starter based on Total Harmonic Distortion (THD) and power factor(PF)

Methodology

- Theoretical study on induction motor, power electronics devices, voltage controller and MATLAB software.
- Designing the firing circuit on MATLAB simulink.
- Implementing of firing circuit for thyristor and IGBT.
- Checking the waveforms of the output voltage and current with different firing angle with R-L load.
- Analysis of Total Harmonic Distortion (THD) and Power factor for Thyristor and IGBT based soft starter for R-L load.
- Implementation of firing circuit for three-phase and analyzing the performance of power electronics device which is good concluded from the above.

Chapter 2 THEORETICAL BACKGROUND

2.1 Induction motor

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip rings as in other types of motor. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Single-phase versions are used in small appliances. Their speed is determined by the frequency of the supply current, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common.

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.1: Typical Split Phase AC Induction Motor

Abbildung in dieser Leseprobe nicht enthaltenFig 2.2: Equivalent circuit of three phase induction motor

2.1.1 Operating Principle

In both induction and synchronous motors, the stator is powered with alternating current (polyphase current in large machines) and designed to create a rotating magnetic field which rotates in time with the AC oscillations. In a synchronous motor, the rotor turns at the same rate as the stator field. By contrast, in an induction motor the rotor rotates at a slower speed than the stator field. Therefore the magnetic field through the rotor is changing (rotating). The rotor has windings in the form of closed loops of wire. The rotating magnetic flux induces currents in the windings of the rotor, similar to a transformer. These currents in turn create magnetic fields in the rotor, that interact with (push against) the stator field. Due to Lenz's law, the direction of the magnetic field created will be such as to oppose the change in current through the windings. The cause of induced current in the rotor is the rotating stator magnetic field, so to oppose this rotor will start to rotate in the direction of the rotating stator magnetic field in an attempt to make the relative speed between rotor and rotating stator magnetic field zero. For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (Abbildung in dieser Leseprobe nicht enthalten), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field as seen by the rotor (slip speed) and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as an induction generator, or it can be unrolled to form the linear induction motor which can directly generate linear motion.

2.2 Starting of Induction motor

A single phase induction motor is not self-starting; thus, it is necessary to provide a starting circuit and associated start windings to give the initial rotation in a single phase induction motor. The normal running windings within such a motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.

For small single-phase shaded-pole motor of a few watts, starting is done by a shaded pole, with a turn of copper wire around part of the pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational character to start the motor. These motors are typically used in applications such as desk fans and record players, as the starting torque is very low and low efficiency is not objectionable.

A polyphase induction motor is self-starting and produces torque even at standstill. The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Unlike a wound-rotor motor, the rotor circuit is inaccessible and it is not feasible to introduce extra resistance for starting or speed control.

2.2.1 Direct-on-Line Starter

The DOL starter switches the supply directly on to the contacts of the motor. As the starting current of an induction motor can be 6-8 times the running current the DOL starter is typically only used for motors with a rating of less than 5kW.

2.2.2 Star Delta Starter

This is the most common form of starter used for three phase induction motors. It achieves an effective reduction of starting current by initially connecting the stator windings in star configuration which effectively places any two phases in series across the supply. Starting in star not only has the effect of reducing the motor’s start current but also the starting torque. Once up to a particular running speed a double throw switch changes the winding arrangements from star to delta whereupon full running torque is achieved. Such an arrangement means that the ends of all stator windings must be brought to terminations outside the casing of the motor.

2.2.3 Auto Transformer Starting Method

This method of starting reduces the start current by reducing the voltage at start up. It can give lower start up currents than star-delta arrangements but with an associated loss of torque. It is not as commonly utilized as other starting methods but does have the advantage that only three connection conductors are required between starter and motor.

2.2.4 Rotor Resistance Starter

If it is necessary to start a three phase induction motor on load then a wound rotor machine will normally be selected. Such a machine allows an external resistance to be connected to the rotor of the machine through slip rings and brushes. At start-up the rotor resistance is set at maximum but is reduced as speed inceases until eventually it is reduced to zero and the machine runs as if it is a cage rotor machine.

2.2.5 Solid State starting

A motor solid state starter is a device used with AC electric motors to temporarily reduce the load and torque in the power train of the motor during startup. It consists of the power electronics devices to reduce the voltage at start-up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamics stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.

2.3 Soft starters

Soft starter using silicon-controlled rectifiers (SCRs) are now used extensively in the industry. This starting method essentially allows the control of the voltages applied to an induction motor and hence, control of its torque and the acceleration of a machine during its starting transient.

Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode.

The soft starters of switch all three phases are controlled can use the starting-up or shutting-down by means of voltage, current or torque control. At voltage control, is achieved a soft start-up, but it’s not generated any current or torque reaction. Appearance of soft starters produced a qualitative raise in starting, stopping or braking matter of induction motors with squirrel cage. These equipments are useless at starting-up of induction motors with phase wound rotor.

Reduced voltage starting is required when a full voltage starting creates objectionable line disturbances on the distribution system or where reduction of mechanical stress to gear boxes or belt drive systems is required.

It must be noted that when the starting torque will decrease proportionally to the percent square of voltage applied (i.e. 50% voltage produces 25% torque =0.50 squared). This phenomenon also occurs in the opposite manner when the voltage is increased.

There are three main reasons to apply reduced voltage to motors:

1. To reduce the mechanical jerks during starting and stopping
2. To limit the inrush current inherent with full voltage starting
3. To reduce the effects of pressure surges and water hammer in pumping systems.

Electronic Soft Starter is also known as Solid State Reduced Voltage Starter. The use of solid state Reduced Voltage Starting can provide a smooth stepless method of acceleration and smoothly decelerating a squirrel cage induction motor. This type of starting method, when properly applied can provide an efficient and reliable means of smoothly starting and stopping a motor and load. The use of solid- state reduced voltage starting will perform, in most cases, more efficiently than field coupling, eddy current drives and clutches. The step less ramped acceleration and deceleration capabilities of these types of starter reduces the inrush currents to the motor, eliminating transitional shocks to the load and reducing voltage flicker on the distribution system. Electronic soft starter has other advantages related to the technology aspect. Some of them are as follows:

- Precision Soft Starting
- Kick Start facility
- Soft Stopping
- Current Limited Starting (Range- 1.5 x FLC to 4.5 X FLC)
- Reduction in Sizing of Generator / Transformer
- Top of Ramp (full voltage) indication
- Facility to use with BYPASS Contactor
- Energy Saving at Partial Loads
- Improvement in Power Factor of Motor at Partial Loads
- Reduction in Reactive component KVAR at Partial Loads
- Reduces spikes impulses, surges on supply side during motor starting process-
- Reduced Wear & Tear on Electrical & Mechanical Components
- A Reduction in Maintenance Cost
- Motor Life is increased due to Lower motor temperature running
- Use with Slip Ring Motors with single stage Resistance in Rotor

The Soft starter consists of back-to back thyristor or IGBT and the firing circuit controls the output voltage which is fed to the induction motor or other load by reducing the current at start-up. The soft starter has smooth current curve than DOL starting and also torque speed characteristics. The soft starter provides a variable output voltage, but the frequency of the output voltage is fixed and in addition the harmonic content is high, especially at low voltage range.

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.3: Block Diagram of Soft starter

The Soft-starter is fed with constant ac power supply. The Firing control unit of soft starter generates trigger of pulses at specific time which is determined by variable angle and the output from soft starter is the variable voltage form which can vary with varying the firing angle of the firing control unit.

2.3 Power electronics devices

2.3.1 Thyristor

A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Robert N. Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.

This device is generally used in switching applications. In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch "off". If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly, and the partially triggered SCR may dissipate more power than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device.

Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode
2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
3. Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.4: Structure on the physical and electronic level and symbol

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.5: I-V characteristics of Thyristor

2.3.1 Insulated Gate Bipolar Transistor (IGBT)

The insulated-gate bipolar transistor or IGBT is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching.

The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium- to high-power applications such as switched-mode power supplies, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amperes with blocking voltages of 6000 V, equating to hundreds of kilowatts.

Abbildung in dieser Leseprobe nicht enthalten

Collector Emitter Voltage (V)

Fig 2.6: Static Characteristics of IGBT

2.4 AC Voltage Controllers (AC Regulator)

AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value of the alternating voltage applied to a load circuit by introducing Thyristors between the load and a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the Thyristors in the ac voltage controller circuits.

In brief, an ac voltage controller is a type of thyristor power converter which is used to convert a fixed voltage, fixed frequency ac input supply to obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power flow to the load is controlled by varying (adjusting) the trigger angle ‘a’.

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.7: ac voltage controller

2.4.1 Phase Control of Thyristor

In phase control the Thyristors are used as switches to connect the load circuit to the input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using Thyristors during a part of each input cycle.

The thyristor switch is turned on for a part of every half cycle, so that input supply voltage appears across the load and then turned off during the remaining part of input half cycle to disconnect the ac supply from the load.

By controlling the phase angle or the trigger angle ‘a’ (delay angle), the output RMS voltage across the load can be controlled.

The trigger delay angle ‘a’ is defined as the phase angle (the value of wt) at which the thyristor turns on and the load current begins to flow. Thyristor ac voltage controllers use ac line commutation or ac phase commutation. Thyristors in ac voltage controllers are line commutated (phase commutated) since the input supply is ac. When the input ac voltage reverses and becomes negative during the negative half cycle the current flowing through the conducting thyristor decreases and falls to zero. Thus the ON thyristor naturally turns off, when the device current falls to zero.

Phase control Thyristors which are relatively inexpensive, converter grade Thyristors which are slower than fast switching inverter grade Thyristors are normally used.

For applications upto 400Hz, if Triacs are available to meet the voltage and current ratings of a particular application, Triacs are more commonly used.

Due to ac line commutation or natural commutation, there is no need of extra commutation circuitry or components and the circuits for ac voltage controllers are very simple.

Due to the nature of the output waveforms, the analysis, derivations of expressions for performance parameters are not simple, especially for the phase controlled ac voltage controllers with RL load. But however most of the practical loads are of the RL type and hence RL load should be considered in the analysis and design of ac voltage controller circuits.

2.5 Soft Starter with R-L Load

In this section we will discuss the operation and performance of a single phase full wave ac voltage controller with RL load. In practice most of the loads are of RL type. For example if we consider a single phase full wave ac voltage controller controlling the speed of a single phase ac induction motor, the load which is the induction motor winding is an RL type of load, where R represents the motor winding resistance and L represents the motor winding inductance.

A single phase full wave ac voltage controller circuit (bidirectional controller) with an RL load using two thyristors Abbildung in dieser Leseprobe nicht enthalten and Abbildung in dieser Leseprobe nicht enthalten (Abbildung in dieser Leseprobe nicht enthalten and Abbildung in dieser Leseprobe nicht enthaltenare two SCRs) connected in parallel is shown in the figure below. In place of two thyristors a single Triac can be used to implement a full wave ac controller, if a suitable Traic is available for the desired RMS load current and the RMS output voltage ratings.

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.8: Single phase full wave ac voltage controller with RL load

The thyristor Abbildung in dieser Leseprobe nicht enthalten is forward biased during the positive half cycle of input supply. Let us assume that Abbildung in dieser Leseprobe nicht enthalten is triggered at Abbildung in dieser Leseprobe nicht enthalten, by applying a suitable gate trigger pulse to Abbildung in dieser Leseprobe nicht enthalten during the positive half cycle of input supply. The output voltage across the load follows the input supply voltage when Abbildung in dieser Leseprobe nicht enthalten is ON. The load current Abbildung in dieser Leseprobe nicht enthaltenflows through the thyristor Abbildung in dieser Leseprobe nicht enthalten and through the load in the downward direction. This load current pulse flowing through Abbildung in dieser Leseprobe nicht enthalten can be considered as the positive current pulse. Due to the inductance in the load, the load current Abbildung in dieser Leseprobe nicht enthalten flowing through Abbildung in dieser Leseprobe nicht enthalten would not fall to zero at Abbildung in dieser Leseprobe nicht enthalten, when the input supply voltage starts to become negative.

The thyristor Abbildung in dieser Leseprobe nicht enthalten will continue to conduct the load current until all the inductive energy stored in the load inductor L is completely utilized and the load current through Abbildung in dieser Leseprobe nicht enthalten falls to zero at Abbildung in dieser Leseprobe nicht enthalten, where Abbildung in dieser Leseprobe nicht enthalten is referred to as the Extinction angle, (the value of Abbildung in dieser Leseprobe nicht enthalten) at which the load current falls to zero. The extinction angle Abbildung in dieser Leseprobe nicht enthalten is measured from the point of the beginning of the positive half cycle of input supply to the point where the load current falls to zero.

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.9: Gating Signal Requirements

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.10: Waveforms of supply voltage, Load Current and Voltage and Voltage across Abbildung in dieser Leseprobe nicht enthalten

We obtain the final expression for the inductive load current of a single phase full wave ac voltage controller with RL load as

Abbildung in dieser Leseprobe nicht enthalten ; Where Abbildung in dieser Leseprobe nicht enthalten.

The output voltage is given by

Abbildung in dieser Leseprobe nicht enthalten

2.6 Performance parameter

2.6.1 Total harmonic distortion (THD) and power factor(PF)

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz for American power). One of the major problems in electric power quality is the harmonic contents. There are several methods of indicating the quantity of harmonic contents. The most widely used measure is the total harmonic distortion (THD). Various switching techniques have been used in static converters to reduce the output harmonic content.

Measuring and monitoring quality parameters of AC power systems requires several calculations, among them the total harmonic distortion (THD) of voltage and current can be considered. This calculation is performed with samples of the monitored waveforms, at sample frequency equal to a power of two multiple of the frequency of waves. The samples are converted to digital values by analog-to-digital converters, with a finite number of bits. Numeric algorithms applied to these digital values insert some errors in the final results, due to the number of bits used in calculations.

Mathematically,

Abbildung in dieser Leseprobe nicht enthalten

2.7 Three Phase Soft Starter

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.11: Block Diagram of three-phase soft starter of induction motor

Torque surges entail high mechanical stress on the machine, which results in higher service costs and increased wear. High currents and current peaks lead to high fixed costs charged by the power supply companies (peak current calculation) and to increased mains and generator loads.

A soft starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated in a gentle manner. Service life, operating behavior and work flows are positively influenced.

The three phase induction motor drive scheme is shown in fig 2.11 and the power switches are may be thyristor or IGBT. The three phase sinusoidal voltages are used as input which are 1200 electrically apart and fed to induction motor drive via power switches for controlling the starting inrush current and also for speed control. The Firing Circuit is used to control the voltage and hence current fed to induction motor.

2.8 Firing Circuits

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.12: Block Diagram for firing circuit

The figure demonstrates with the help of a single line diagram, the major blocks necessary to generate firing pulses for any scheme. The converter is organized from a.c. power. Since the firing pulses must be synchronized with the a.c. supply, a.c. power also goes to the isolation and synchronizing blocks. Isolation is essential as because the control circuit uses very low power devices such as various chips, logic gates etc. The logic circuit block uses few logic gates to implement a particular firing scheme. The strength of the pulse obtained from logic gates may not be sufficient to drive the gate of a thyristor, so amplification of the pulse along with isolation is used at final stage.

Variety of firing circuits available but mainly two most popularly used control circuits that are namely using ramp signal and using cosine signal.

2.8.1 Using ramp signal

In this scheme a ramp signal is generated in synchronism with the a.c. supply. Vs by using two comparators and an approximate ramp generator circuit using a transistor and capacitors as described in Fig.2.13

Abbildung in dieser Leseprobe nicht enthalten

Fig 2.13: Basic idea of ramp scheme.

2.8.2 Using cosine control:

In this scheme, the supply voltage Vs is first transformed into a cosine wave and then so obtained is compared with a reference d.c. voltage (Vref). Therefore square pulses will be generated at the output terminal and is fed to the comparator. This signal is synchronized with the pulse and is delayed from the supply zero crossing by an angle α. Obviously, the value of α can be varied a range of 00≤ α ≤ 1800.

The output of the comp-1 will be square wave and it goes to high state from the instant when Vr becomes greater than the cosine voltage value. However the width of the pulse will vary as Vr is varied. Our first aim will be to make the width of the pulse to be 1800. This is achieved in the following way. The output of the Comp-1 is fed to a block mono-1. Output of the mono-1 will be a pulse of small width at positive going edge of the input square wave. The output of mono-1 will thus give small pulses separated by 3600.

The voltage Vb0 is similarly processed, i.e., it is transformed into cosine wave then compared with the same variable d.c. with the help of comparator-2, output of COMP-2 will be a square wave and will be shifted by 1800 from the output square wave of COMP-1. This is because of the fact that Vb0 lags Va0 by 1800. The output of the COMP-2 is now fed to a block mono-2. Output of mono-2 will be a pulse of small width at positive going edge of the input square wave. The output of MONO-2 will thus give small pulses separated by 3600. This is important to know that the fixed width pulse waveforms at the output of mono-1 and mono-2 are shifted by 1800 as shown in Fig. 9. The outputs of mono- 1 and mono-2 can be used in conjunction with to two S-R flip flops so as to generate two square waves each having a fixed width of 1800 and mutually separated by 1800.

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Title
Comparative Performance Analysis of Thyristor and IGBT Based Induction Motor Soft Starters
College
Tribhuvan University  (Institute of Engineering)
Course
Electrical Engineering
Grade
10.0
Authors
Year
2012
Pages
50
Catalog Number
V376772
ISBN (eBook)
9783668577060
ISBN (Book)
9783668577077
File size
1685 KB
Language
English
Tags
comparative, performance, analysis, thyristor, igbt, based, induction, motor, soft, starters
Quote paper
Ajay Singh (Author)Anil Kumar Sahani (Author)Mahanji Yadav (Author), 2012, Comparative Performance Analysis of Thyristor and IGBT Based Induction Motor Soft Starters, Munich, GRIN Verlag, https://www.grin.com/document/376772

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Title: Comparative Performance Analysis of Thyristor and IGBT Based Induction Motor Soft Starters



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