Дипломная работа - Проектирование 10 кВт преобразователя

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VIII

List of symbols

The voltage achieved on the DC-bus after the following smoothing

Input/Output voltage in alternating mode

Collector current

Collector-Emitter voltage

Gate-Emitter voltage

Drain-Source resistance

Drain-Source voltage

Drain current

Abbreviations

AC Alternating Current

DC Direct Current

PWM Pulse Width Modulation

PM Permanent Magnet

FET Field Effect Transistor

IGBT Insulated Gate Bipolar Transistor

SiC Silicon Carbide

PLD Programmable Logic Device

µC Micro Controller

EMI Electro Magnetic Interference

PMSM Permanent Magnet Synchronous Motor

SOA Safe Operating Area

FWD Free Wheeling Diode

LC Inductive-Capacitive

RLC Resistive-Inductive-Capacitive

RFI Radio Frequency Interference

MTBF Minimum Time Before Failure

ESR Equivalent Series Resistance

HFE small signal Forward Current Gain

MASL Meters Above Sea Level

LOP Lifetime Of Product

1. Introduction

As we all get more conscious about the damage the combustion of fossil fuels inflicts on

the environment more and more alternative solutions come to light. Hybrid drive,

biological petrol and power cells are terms most people are familiar with. Electric

propulsion in automotives is being promoted throughout the world as the most

environmental friendly solution on the market. Once electricity is produced the electric

propulsion is completely clean and it is easier to produce clean electricity in a controlled

environment such as a power plant. However, the environment issue is not the only

benefit. The increased control, performance and efficiency achieved are in some

applications the most wanted benefit, especially in avionics.

1.1. Objectives

This master thesis aims to design and evaluate a 10kW three-phase converter handling

both AC/DC and DC/DC. The function of the converter is to drive a permanent

magnetized synchronized motor possibly working as a starter motor or driving an electric

actuator possibly in an aircraft. A main focus will be to pinpoint the parts responsible for

the largest power loss, how to reduce this magnitude and estimate the weight of the

product. As an extension the characteristics for a few imaginary converters with different

power ratings will be estimated. This to in the end form up approximate equations giving

the power loss and weight based of a few input parameters.

1.2. Background

Today it is not uncommon to use highly efficient petrol driven generators which supplies

electronic equipment and motors with power and in the end reach a higher efficiency

compared driving the equipment directly with combustion engines. The middle step

between the engine and the power source is the converter, a device not seldom

responsible for a large loss due to inefficiency, demanding cooling and thereby

introducing further weight.

A new concept in aviation is the “More Electric Aircraft” which aims to replace all

hydraulics with an electric correspondence. This is to reach higher efficiency which will

reduce weight but also increase control and improved behavior.

With numerous motors demanding an individual converter its losses and weight becomes

prominent and has to be optimized. The long term goal is to remove the compressor and

improving the generator driven by the combustion engine to supply the electric power

needed.

1.3. Challenges

The environments and conditions in an aircraft such as JAS 39 GRIPEN are very

different from sea-level. This complicates the objective as electronic devices behave

differently or possibly not at all when exposed to high temperature, cosmic rays and

mechanical stress.

Cooling gets more difficult when air density decreases as it does on higher altitudes. A

higher altitude also increases the failure rate due to cosmic radiation further extending the

need for effective cooling. Since weight is an emphasized problem and cooling is

responsible for most of it a well considered tradeoff is needed.

How can we maintain a low weight while still having a high reliability? How much will

new technology reduce power dissipation? Is it possible to use the aircrafts built-in

cooling system and will it improve overall efficiency?

1.4. Research method

• Literature study

• Examine which switching topology and which components to select for the

optimum solution.

• Estimate the amounts of power dissipated, simulate if possible to confirm

• Estimate cooling needs and its size/weight

• Design and simulate filters to follow standard regulations

• Evaluate the operating area of the components and which counter measures to

take to guarantee reliability and functionality.

• Form engineering rules to model scalability, power density and efficiency

1.5. Delimitations

This master thesis will not cover the physical construction or commissioning of the

converter. The programming required in the controller will not be carried out. Some very

modern applications will be neglected because it is too time consuming to estimate the

gain over conventional solutions. Modeling of reliability will not be carried out very

detailed

1.6. Structure of report

This report will be divided into seven chapters and below is a short description of each

chapter.

1. Introduction –

2. Theory – A general introduction to power inverters is given, how they operate,

difficulties etc. Also a light discussion and explanation of the concepts and behavior

of the different components and parts building up the converter is carried out as well

as some general calculations in aspect of cooling and weight of an enclosure.

3. Losses – A more specified estimation of the losses of the different components is

carried out accompanied by simulations to hopefully confirm these estimations and

illustrate their effectiveness. Loss in filter will be estimated and suggestions for

improvement will be carried out with respect to setbacks. Different snubber circuits

will be evaluated with their pros and cons.

4. Results – Results with aspect to losses and its respective cooling are presented for a

few suitable solutions along with their weight, reliability and cost.

A way of scaling is presented with their respective parameters.

5. Conclusions – Presents the conclusions of the work as well as gives answers to the

challenges mentioned concerning the converter. General thoughts on outlooks of

these applications are presented with regards to ongoing research in this field.

Also an evaluation of how the work has proceeded is included, which

problems have risen and how they were handled.

6. Future work – Discusses improvements and work to be carried out to further

optimize the converter. Which parts have been neglected etc?

7. Appendix – Various plots, spreadsheets and graphs from simulations as well as

schematics from simulations.

2. Theory

2.1. Converter principles

• Overview

Figure 1 : Inverter overview

1. Rectifier

In the first step of the inverter the rectifier rectifies the tri-phase figure 2:

Figure 2 : Rectifier stage

2. Smoothing stage

The oscillating rectified DC-bus voltage will cause problems further on and has do be

dealt with, this is done with a LC-filter (inductor-capacitor) which is low pass filtering

the voltage but also providing an extra current buffer. As explained in 2.3.3. the

oscillations can be as much as 9% of the DC-bus voltage and has to be decreased

otherwise the noise would spread further and cause loss and misbehavior. A change in

frequency or controlling the average DC-bus voltage with thyristors would alter the need

for a filter. A higher frequency would make the ripple more frequent requiring a smaller

capacitor but if the voltage is controlled a larger ripple would occur requiring larger

capacitors, more about this in 2.4.1. After the DC-bus filters the voltage will look like it

does in figure 3, with less ripple:

Figure 3 : Smoothing stage

If the DC-input would be used it would connected after the bypassed rectifier but still use

the DC-filter to relieve the source in the same manner as the V

input filter covered in

chapter 2.4.1.

3. Switching stage

Now as the transistor bridge has a much more stable voltage to process it can start

chopping the voltage and forming the pulse width modulated sinus. The way of chopping

and switching is decided by a controller which gives signals to specified drives for the

switching transistors forming the pulse width modulated sinus wave. The transistor

bridge output wave can look as described in figure 4 where the voltage is measured

between one phase and another:

Figure 4 : Switching stage

4. Filtering and output stage

This rough sinus wave formed by the transistor bridge would inflict noise and EMI both

in wires and in the load due to its step nature and has to be low pass filtered at least just

before the load but rather directly after the transistor bridge. Having long wires with large

voltage spikes would create high magnitude radio frequency disturbance which is very

unwanted, especially in an aircraft. This filtration can be done using a three phase LC-

filter but also in some cases the inductive coils of the motor can be used as a low pass

filter if the converter is close enough to the load.

In the picture 5 the pulse train has been filtered with a LC-filter. As can be seen it

resembles a sinus wave but with higher frequent noise. To remove this relative small

noise additional high power coils and capacitors has to be included further increasing

weight and size of the unit, again a tradeoff has to be made.

Figure 5 : Filtering of pulse train in output stage

2.2. Principles for Control

2.2.1. Switching frequency

Calculating and choosing a proper switching frequency of a PWM inverter is not always

trivial, there as various advantages and disadvantages of choosing a too high or a too low

switching frequency.

Low m

( 21≤=

out

m ) [12]. (2.2.1.1)

Where f

is the switching frequency at which the transistors operate and f

out

is the

intended maximum frequency at which the load operates, f

out

is thereby a low-pass

filtered version of f

. The major setback of a too low m

and too low switching

frequency is that the voltage-ripple on the sine-wave otherwise becomes prominent which

causes inefficient ripple within the load, as seen in figure 5. A synchronous PWM is

strongly advised since synchronizing the PWM means that the present output voltage is

fed back to the controller via a voltage-divider and then errors are compensated for,

greatly reducing the voltage-ripple and thus improving the efficiency. Synchronizing the

circuit requires m

to be an odd integer as well as more competent circuitry, including an

A/D-converter and perhaps hardware-support for arithmetic operations, however most

modern circuitry includes this. These voltage ripples or harmonics mainly cause loss in

effect but also can become a burden to people and mechanical devices since equipment

and especially motors can start to vibrate significantly but also generate other non

desirable side-effects. These vibrations can cause reduced lifetime in equipment due to

stress but also emit harmful and annoying sound in often heard frequencies. However if a

highly competent filter on the output is implemented many of the setbacks of a low m

can be avoided, the current ripple in the load can be reduced heavily as well as reducing

the amount of emitted electromagnetic waves. The setback here is the additional weight

and possibly significant additional loss

High m

( 21≥=

out

m ) [12]. (2.2.1.2)

When the switching frequency increases the need for a synchronized PWM decreases

since the amplitudes of the harmonics are very small and high frequent and thus

somewhat insignificant. Although when controlling an AC-motor with variable frequency

the currents at very low output frequencies can become prominent and undesired, despite

the very low amplitudes. As a result, synchronous PWM is almost always advised.

However, even if the overall negative effect of harmonics decreases with high switching

frequency the switching loss in the switching device increases in proportion to the

increased frequency which always is unwanted.

2.3. Power components

In this part mostly the semi conductive components responsible for the main part of the

loss are considered, those are the main supply rectifier and the transistor-bridge.

Mainly two types of transistors have been considered, the IGBT and the MOSFET and

their advantages and disadvantages are explained further on. A third option exists, the

bipolar junction transistor (BJT) but as this one is current controlled it would require a

driver capable of providing a high current while switching very fast. A combination

which is very difficult to realize.

2.3.1. Switching transistors

2.3.1.1. IGBT

The Insulated Gated Bipolar Transistor is a device that combines the low forward

conduction loss, especially at high voltages, of a Bipolar Junction Transistor (BJT) and

the short switching times of the Metal Oxide Semiconductor Field Effect Transistor

(MOSFET). MOSFET on its own has very high conduction loss at high voltages while

BJT turns off and on much slower [12]. The two figures 6 and 7 illustrate a simplified

model of an IGBT and the symbol layout:

Figure 6 : Simplified model

Figure 7 : Symbol layout

The upper figure consist of a Darlington coupled pnp-doped BJT and an n-channel

MOSFET with a resistor that corresponds to the drain drift region in the IGBT.

It is also common to see an npn-doped transistor between the base if the BJT and the

emitter, this is to reduce the turn of tail illustrated in figure 8:

Figure 8: Turn-off behavior

As visible in figure 8, apart from this turn off tail the switching characteristics of an

IGBT resembles the ones of a MOSFET expect for the MOSFET usually being a lot

faster.

To design and dimension the drive stage knowledge of the internal stray capacitances are

required to reach a sufficient turn on and turn off time without a too large over-shoot and

to maintain within the safe operating area (SOA). Although the stray capacitances vary

with the voltage supplied over them making it a bit harder but there are usually provided

some graphs displaying this in the datasheet. To understand the switching characteristics

a more detailed explanation has to be done, however the IGBT-symbol can be simplified

for switching evaluation as described in figure 9: