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Essay: Single Phase to 3-Phase Power Conversion: Challenges and Solutions

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Abstract

Single-phase to three-phase conversion using power electronics converters is use in various aims like for traction and microgrid etc. Regarding this configuration considered over the years, it can be observed two main tendencies:

1) configurations with reduced number of semiconductor switches; and

2) configurations with  increased number of semiconductor switches.

The goal of PE in the aforementioned applications is to control the flow of energy from an electrical source to an electrical load, to take a specific need with high efficiency, high reliability, small size, light weight, high availability and low cost.

The single phase to three phase ac to ac direct conversion configurations and those which purpose to reduce the dc-link capacitor.

The conversion of variable frequency three phase supply from variable or fixed frequency single phase supply is possible without using DC link capacitor. Hence the size of this converter is small and also the weight of the converter is light.

CHAPTER 1: Introduction

1.1  Problem Summary and Introduction

The implementation of converters without DC energy storage contributes to lower costs, smaller size, greater reliability and longer working life of converters and as well as to whole systems. Converters without a DC-link are frequently seen as a future concept for specific applications, but despite intensive research over several decades they have until now achieved only low industrial penetration.

The absence of a DC-link energy storage electrolytic capacitor, increases power converter lifetime, subsequently making them more reliable.

 1.2  Aim and objectives of the project

The main purpose of the direct conversion of variable frequency three phase supply from Fixed or variable frequency single phase supply is to remove the DC filter capacitor from the power electronics circuit to reduce the weight of the circuit and also reduce the size of the circuit and it becomes more reliable due to not using electrolytic capacitor therefore converter have more life.

The another purpose is to employ the three phase drive using single phase supply and torque and speed control the three phase motor.

The three phase drive using single phase supply is extensively used in traction system to drive the three phase traction motor and also drive the auxiliary three phase motor to drive the auxiliary load such as compressor , blowers , pump etc. more than a dozen.

The single phase to three phase converter is extensively used in domestic appliance like submersible pump motor and commercial appliance.

 1.3  Why not DC link/DC Capacitor ?

The DC filter capacitor is so much Bulky.

The output of Rectifier and DC filter capacitor is not a pure DC.

The size of DC filter capacitor is Large.

 1.4  Brief literature review

In the power distribution systems, the single-phase grid has been considered as an alternative for rural or remote areas, due to its lower cost feature, especially when compared with the three-phase solution. In huge countries like Brazil, the single phase grid is common due to the large area to be covered. However in other side, loads connected in a three phase arrangement present some advantages when compared to single-phase loads. This is true when motors are considered, due to the inconstant torque, constant power, reduced size, etc. There is a required for single-phase to three phase conversion systems in this scenario.

In terms of application, the first thing that is come to mind when it is considered a single-phase to three-phase conversion, it is a three-phase motor drive system. However, the authors in claim that, in rural application, feeding a three-phase induction motor is not any more the main concern for the single phase to three-phase conversion. Due to the evolution of the farm technology, some of the local loads (such as electronic power converters, computers, communication equipments, etc.) require high power quality, that is symmetrical, sinusoidal,  and balanced three-phase voltage.

In the past, single-phase to three-phase conversion systems were made possible by using rectifier and inverter pair.

Block diagram of prior technology

Fig. 1.1 Block diagram of prior technology

Block diagram of converting single phase to three phase power supply consist of input, rectifier, filter, inverter, load, driving stage, micro-controlled and power supply. As we seen from the block diagram the first stage is input, input is given in two circuits, first one is given to rectifier and further towards the other and the second input is given to controller stage. Since the input is first works in rectifier which converts the ac supply into pulsating dc but after rectification also having some ac contain. So to remove that filters are used consisting of inductors and capacitors which helps to eliminate the ac contain and gives nearly pure dc.

In this method we have required DC filter capacitor which is high rating therefore it’s size and weight is more hence it’s cost is also more. In this method we are using aluminium electrolytic capacitor therefore it’s ageing of aluminium electrolytic capacitors is expressed by the increase in their internal resistance RESR. The electrolytic capacitor equivalent impedance Z (s) is modeled by an equivalent circuit according to a which consists of a series connection of the true capacitance C, an equivalent series resistance RESR representing all AC losses, and an equivalent series inductance LESL which depends on the winding design of the capacitor foils and how the foils are connected to the capacitor terminals.

Now we are using single phase three phase converter without DC link so we have compare to less component and less value of capacitor is required so we increasing its performance, smaller size, greater reliability and longer working life of converters.

Block diagram of single phase to three phase converter without DC link

Fig. 1.2 Block diagram of single to phase three phase converter without DC link

In this system we are supply single phase supply and directly convert it into three phase and give to load.

A phase converter is a device that converts electric power provided as single phase to three phase. These converters are used to produce three phase electrical power from a single phase source, thus allowing the operation of three phase equipments at a site that only has single phase electric service.

   There are drawbacks of conventional power converter using electrolytic capacitors. A large capacitance electrolytic capacitor Which must Withstand a high level of DC voltage has a relatively limited operating lifetime. The effective operating life time of a power converter as whole will in general be determined by this smoothing electrolytic capacitor. Breakdown of such an electrolytic capacitor can cause serious damage to other components of the power converter, since leakage of corrosive electrolyte may occur, or the capacitor may even explode.

Because of electrolytic characteristics, other components such as balancing resistors and high frequency capacitor are required. These additional components increase not only cost but also board space to mount them.

The smoothing electrolytic capacitors occupy a relatively large amount of space Within the power converter, thereby reducing the freedom available for mechanical design of the converter, and causing the overall size of the power converter to be large.

The power converter manufacturers and research community recognize above problem and realize improvement of AC drive product can be made if electrolytic capacitors are to be replaced.

US. Pat. Nos. 5,729,450, 5,623,399, 6,115,270 and 6,449,181B1 propose to use capacitor in place of electrolytic ones to provide ripple current capability to the inverter section. US. Pat. No. 5,623,399, US. Pat. No. 6,115,270 and US. Pat. No. 6,449,181B1 are invented for new electric vehicle applications Where a battery is readily available to serve as a huge energy storage component in front of the inverter.

The other patent US. Pat. No. 5,729, 450 is in effect teaching for inverting energy from a fuel cell DC source to AC power. The fuel cell has low impedance and again serves as the huge energy storage and imposes smoothing effort on the DC link.

Conventional AC power converter has working condition Where transient energy can come back from the motor to the DC link. In this case the DC link is easily over charged, resulting in DC link over-voltage. This over voltage may damage the converter components such as the switching transistors. Thus their solution is not readily applicable to a conventional AC power converter With a diode front end rectifier.

1.5  Project Management

Project planning and scheduling

We are going to complete this project these different steps as following :

1. Project development approaches

2. Project plan

1.5.1 Project development approaches

First, we will design the power electronics circuitry which convert the variable frequency three phase supply from fixed or variable frequency single phase supply.

Then, we will simulate this power electronics circuitry by giving appropriate sequence of gate triggered pulses to turn on or turn off the power electronics device or power electronics switches.

After the simulating the power electronics circuitry called as single phase to three phase converter, we will analyze the results or output waveforms for make sure with the single phase to three phase converter is work properly.

Now, the challenges are to design the control circuit or firing circuit to drive the power electronics device or power electronics switches to produce the appropriate output waveform at the output terminal.

We will design the gate driving circuit by using ZCD (zero crossing detector) and square wave generator or function generator such a manner that they will be drive the single phase to three phase converter in appropriate manner such that they will produce the appropriate output waveform.

Finally, we will design the AC filter such that the % THD of three phase output waveform is below the permissible limit and In other words, they will reduce the harmonics at the output waveform.

1.5.2  Project plan

Now , we will prepare the plan for the complete the project work on the single phase to three phase converter without DC link.

First , we will decide that first , we will design that the power electronics circuitry such that they convert the variable frequency three phase supply from fixed or variable frequency single phase supply.

Then , we will simulate that circuit to make sure that they will perform accurately and produced a appropriate three phase output waveform.

Now , the second step is design of control circuit or firing circuit to drive the power electronics device or power electronics switches to produce the appropriate output waveform.

In the third and final step , we will design the AC filter to produce the appropriate three phase output waveform.

In the filter design , we will design the AC filter such that the % THD of three phase output waveform is below the permissible limit and In other words , they will reduce the harmonics at the output waveform.

1.6  Materials / Tools required :

Mosfet or gto

Op amp

Inductors

Capacitors

Timer IC555

PCB

Dead band generator

Zero crossing detector

Auto coupler

CHAPTER 2: Design Methodology and implementation strategy

We started with developing a way to directly convert ac to ac single phase to three phase converter without dc link hence converter becomes more reliable, less in weight and reduced size comparatively prior AC-DC-AC converter.

2.1 AEIOU Summary

Fig 2.1 : AEIOU Summary sheet

Activity

Drilling

Welding

Milling

Polishing

Pressing

Shopping

Farming

Teaching

Testing

Travelling  

Environment

Noisy

Dusty

High temperature

Natural cooling

Heating

Radiation

Thermal discomfort

Ventilation

Humidity

Objects

Railway

Hoist

Air conditioner

Compressor

Submersible pump motor

Grinder

Lift

Pump

Interaction

Observations

Engineers and labours on construction site

Lab assistant and student

Managers and employee

Welder and supervisor

Professor and student

Elements

Welding machine

Lift

Drilling machine

Hoist vehicles

Testing instruments

Scene

Wiring

Driving

Testing

Measuring

Office work

Users

Metal working workshop

Railway employee

Industry worker

Welder

Engineers

Driller

Farming

Irrigation system

Vehicle servicing

2.2 EMPATHY :

Users

Irrigation system

Farmers

Vehicle servicing

Metal working workshop

Large building

Industry

Stakeholders

Farmer

Supervisor

Vehicle owner

Instrument engineer

Electrical engineer

Industry worker

Activities

Drilling

Welding

Milling

Polishing

Pressing

Driving

Farming

Wiring

Testing

Teaching

Fault repairing

Researchin

Fig. 2.2 empathy sheet

2.3 Ideation Canvas :

Fig 2.3 Ideation canvas

People

Electrician

Driver

Engineer

Farmer

Manager

Professor

Lab assistant

Researcher

Student

Railway engineer

Scientist

Supervisor

Technician

Worker

Activities

Wiring

Driving

Learning

Teaching

Traction

Inventory system

Maintenance

Working on safety system

Estimating cost

Travelling

Testing

Farming

Milling

Planting

Polishing

Drilling

Situation / Context / location

Measuring

Water pumping

Industry

Testing

Irrigation

Railways

Maintenance

Rainy

Laboratories

Cleaning

Construction site

Props / Possible Solutions

Light in weight

Less THD

Reduce in size

Reducing harmonics

LC filter required

2.4 Product Development Canvas :

Purpose

Convert single phase to three phase supply

To employ single phase to three phase electric drive

Provide variable frequency three phase supply

Reduce weight of converter

Drive three phase traction motor

Product Experience

Light in weight For Higher Frequency

Portable

Better performance Maintenance free

Efficient

Product Functions

To convert single phase to three phase supply

Light in weight

Reduce in size

Drive three phase motor

Fig 2.4 : Product Development Canvas

Customer Revalidation

Cost ****

Reliability   *****

Performance *****

Portability   *****

Efficiency of power transfer  ****

People

Engineer

Manager

Electrician

Worker

Farmer

Lab assistant

Technician

Professor

Railway engineer

Product Features

Reliable

Portable

Better performance

Efficient

Light in weight

Components

Semiconductor devices

Op-Amp

Zero crossing detector

Indicator

Adder

Capacitor

Transformer

GTO / MOSFET

Inductor

Substractor

Dead band generator

Reject , Redesign and Retain

AC filter is required.

Harmonics

Switching loss

Converter is small in size and light in weight for higher frequency.

LC filter is small in size and light in weight for higher frequency.

This converter is implementing for less industrial application because of semiconductor’s behaviour. This converter is now research and development under scientist and researcher and students. Ac to ac  converter is demanded in industry with better performance because of more reliability.

For design this converter we study converters which are single phase to three phase converter (prior technology), ac to ac converter,  rectifier (ac to dc), inverter(dc to ac). For single phase to three phase converter we are using prior technology we have to use two converters rectifier and inverter and also required to large dc filter.

These canvases show the stage by stage progress that we have made for implementing  the  idea..

CHAPTER: 3  IMPLEMENTATION

3.1 Firing Circuit OR Control Circuit

Waveform Of Firing Pulses (Output Three Phase Supply Frequency is 100 Hz)

3.2 Firing angle table

Firing Angle

MOSFET 1

0 to 180

MOSFET 2

180 to 360

MOSFET 3

MOSFET 4

MOSFET 5

120 to 180

MOSFET 6

300 to 360

MOSFET 7

0 to 120

MOSFET 8

180 to 300

MOSFET 9

0 to 60

MOSFET 10

180 to 240

MOSFET 11

60 to 180

MOSFET 12

240 to 360

Note : The Firing angle is only for the Frequency of input single phase supply is 50 Hz and the frequency of output three phase supply is 50 Hz.

3.3   Single phase to Three Phase converter(MOSFET)

   3.3.1  Power Circuit

    3.3.2   Firing Circuit

    3.3.3  Voltage Controlled Circuit

  3.3.4  Speed of Three Phase induction Motor (1000rpm)

    3.3.5 Waveform of Three Phase Output Current

   3.3.6  THD of Three Phase Output Current

   3.3.7  Waveform of Three Phase Output Voltage

   3.3.8   THD of Three Phase Output Voltage

CHAPTER: 4  SUMMARY

4.1 Advantages of our work

1. It converts single phase to three phase with a slight unbalance in voltage.

2. Imagine a three phase motor running from three phase input then one line gets disconnected. What happens may surprise many ; the motor continues to run and deliver the load (But reduce torque) with slight drop in speed.

3. It converts single phase to three phase with a slight unbalance in current.

4. DC filter capacitor is not required.

5. Light in weight for higher output frequency because of reduce the weight of AC filter.

6. Portable

7. Reliable

Because here  we are using small value of capacitor for reducing harmonics. Hence we use are not using electrolytic capacitor. Because of electrolytic characteristics, other components such as balancing resistors and high frequency capacitor are required. These additional components increase not only cost but also board space to mount them.

4.2  Limitations

1. Harmonics will be present in output waveform. AC filter (L-C filter) is required.

2. To employ VVVF (variable voltage variable frequency) drive or variable frequency drive is difficult. It can be solved by control circuit.

4.3  Unique features of project

The single phase to three phase converter is successfully drive the three phase motor using only single phase supply.

The single phase to three phase converter is successfully operate a three phase submersible pump motor on a single phase line. Proper installations can be achieved only if the recommendations and specifications of both the single phase to three phase converter and submersible pump motor manufacturers are followed.

The conversion of variable frequency three phase supply from fixed or variable frequency single phase supply is possible using only single power electronics circuitry called single phase to three phase converter. There is no need of DC filter capacitor for filter the rectifier output. So the size of the single phase to three phase converter is small and also the weight of the single phase to three phase converter is light.

If the one of the phase of three phase line is disconnected then the large voltage unbalance is not hazardous effect on power system because of the only single phase electric power is feed to the electric load.

4.4 future work.

The major scope of single phase to three phase converter in traction system to drive the three phase traction motor using only single phase supply. So only one conductor is used in track electrification. (the neutral conductor is as ground used as rail-track) So the conductor required to track electrification is reduce. So the cost of conductor is reduced and finally the cost of track electrification is reduced. And also the torque and speed control of three phase traction motor is achieved.

The another scope of single phase to three phase converter in implementation of three phase electric drive using only single phase supply. And also the torque and speed control of three phase motor.

The single phase to three phase converter is extensively used in domestic appliance like submersible motor. The single phase to three phase converter is successfully operate a three phase submersible pump motor on a single phase line. Proper installations can be achieved only if the recommendations and specifications of both the single phase to three phase converter and submersible pump motor manufacturers are followed.

The single phase to three phase converter is used extensively on railways electric locomotives for driving three phase motor load of auxiliary drives like compressors , blowers , pumps etc. more than a dozen.

The single phase to three phase converter is used extensively on Hoist, welder, wheel balancer, pumps, compressor, irrigation system, air conditionin, refrigeration unit, conveyor, crane, grain mill lathe machines, drilling machine, milling machine, grinder, saw, polisher, shaper, CNC machine etc.

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