Electrical Motor Diagnostics for Generators
Part 1 – The Basics
ALL-TEST Pro,  LLC 

Introduction

Electrical Motor Diagnostics (EMD) is a term for test methods and instruments designed for rotating and coil-wound machinery electrical and mechanical analysis.  These instruments are used for all motor system related analysis from the generator and prime mover, through the transmission and distribution system, to the electric motor and driven load.  These technologies, for the purpose of this paper, will include Motor Circuit Analysis (MCA), a de-energized test method, and Electrical Signature Analysis (ESA), a more advanced method of Motor Current Signature Analysis (MCSA).

In this paper, we will discuss the concepts behind the testing and analysis of both salient and turbine generators to detect some common generator faults.  These faults include bearings, winding shorts in the rotor and stator, insulation to ground faults in the rotor and stator, exciter faults, misalignment and rotating field eccentricity.

Motor Circuit Analysis

MCA is a low voltage method for testing electric machinery cables, connections, windings and rotor for developing faults.  The technique involves individual readings of DC Resistance (R), Impedance (Z), Inductance (L), Phase Angle (Fi), Current/Frequency  Response (I/F) and insulation to ground (MegOhm) testing.  Resistance is used for detecting loose connections and broken conductors, insulation to ground is used for detecting ground faults, Z and L are matched to evaluate the insulation condition for winding contamination, and, Fi and I/F are used to detect winding shorts.  One of the key aspects of MCA is the ability to detect early winding defects that can be trended over time and a time to failure can be estimated.

As a vast majority of the rotating machinery, that MCA is used to evaluate, requires balanced phases, pass fail criteria for individual readings can be developed for both assembled and disassembled machines (Reference Tables 1 and 2).  These values indicate a guideline and values outside of these guides normally identify component failures that have occurred, or are developing.  These developing issues can be compared against Attachment 1 of this paper.

In addition to the power of detecting a motor system defect, the values are trend able without the requirement of temperature adjustments for a majority of faults.  This allows for the ability to evaluate condition and provide estimates for time to failure by monitoring changes to the phase to phase unbalances over time.

Table 1: Pass/Fail Considerations for Assembled Machines

Test Result	Tolerance	Detail
Resistance (R)	<5%[1]	Used for detecting loose connections, broken wires, direct shorts and diff wire sizes
Impedance (Z) and Inductance (L)	Similar Patterns[2]	Changes to impedance that cause its phase to phase pattern to appear different from inductance are normally the result in the change to the material condition of the insulation system.  Used for detecting winding contamination, burned windings (overheated), very large phase unbalances or very poor rotor bar condition.
Phase Angle (Fi)	+/- 1 digit from average	Indicates a winding short: 74, 75, 76 OK; 74, 74, 76 suspect; 73, 73, 76 failed
I/F	+/- 2 digits from average	Indicates a winding short: -44, -45, -46 OK;  -44, -46, -46 suspect; -42, -45, -45 failed
Insulation Resistance (MegOhm)	>5 MegOhm[3] >100 MegOhm[4]	Indicates poor insulation to ground (i.e.: ground fault)

When a motor does not have a rotor in place, such as in a motor repair shop with a stator only, the tolerances change:

 

Table 2: Pass/Fail Criteria for Disassembled Machines

Test Result	Tolerance
Resistance (R)	<5%
Impedance (Z)	<3%
Inductance (Z)	<5%
Phase Angle (Fi)	+/- 0
I/F	+/- 0
Insulation Resistance (MegOhm)	> 5 MegOhms3 / > 100 MegOhms4

 

For trending and analysis purposes, MCA is a comparative tool using percent unbalance and difference between tests methods.  In the percent unbalance method, the difference between like coils (i.e.: between phases in a three phase motor) is trended over time.  This method is best for resistance, impedance and inductance.  While resistance values are impacted by temperature, for instance, the relative difference between phases is not.  By using the percent unbalance method, the user or software do not have to rely upon performing temperature correction calculations.  Impedance and inductance are not significantly impacted by temperature.  Therefore, the unbalance method is the most convenient way of detecting faults over time.  The difference between tests method is used for phase angle and I/F in which the lowest value for each is subtracted from the highest value for each.

 

Table 3: Reading Change Table for AC Rotating Equipment

Reading	Change From Baseline	Severity
R, Z, L	< 3%	Green
R, Z, L	>3% and <5%	Yellow
R, Z, L	> 5%	Red
Fi and I/F	<1pt	Green
Fi and I/F	>1pt and <3pt	Yellow
Fi and I/F	>3pt	Rd

 

Electrical Signature Analysis

Motor Current Signature Analysis (MCSA) refers to the evaluation of current waveforms only, including the demodulation of the current waveform and FFT analysis.  Electrical Signature Analysis (ESA) is the term used for the evaluation of the voltage and current waveforms.  This provides an increased advantage to diagnostics as power-related, motor-related and load-related signals can be quickly compared.  A key consideration when using ESA is that voltage signatures relate to the upstream of the circuit being tested (towards power generation) and current signatures relate to the downstream of the circuit being tested (towards the motor and load).

ESA uses the machine being tested as a transducer, allowing the user to evaluate the electrical and mechanical condition from the control or switchgear.  For accurate analysis, ESA systems rely upon FFT analysis, much the same as vibration analysis.

Table 4: Rotor Analysis

Condition	dB Value	Rotor Condition	Action
1	>60	Excellent	None
2	54-60	Good	None
3	48-54	Moderate	Trend
4	42-48	Rotor Fracture or High Resistance Joint	Increase Test Intervals and Trend
5	36-42	Two or more bars cracked or broken	Confirm with motor circuit analysis
6	30-36	Multiple cracked or broken bars and end ring problems	Overhaul
7	<30	Multiple broken rotor bars and other severe rotor problems	Overhaul or Replace

 

Table 5: Signature Multipliers

Type of Fault	Pattern (CF = Center Frequency)
Stator Mechanical (i.e.: loose coils, stator core movement, etc.)	CF = RS x Stator Slots Line Frequency Sidebands
Stator Shorts (shorted windings)	CF = RS x Stator Slots Line Frequency sidebands with Running Speed sidebands
Rotor Indicator	CF = RS x Rotor Bars Line Frequency sidebands
Static Eccentricity	CF = RS x Rotor Bars Line Frequency and twice Line Frequency sidebands.
Dynamic Eccentricity	CF = RS x Rotor Bars Line Frequency and twice Line Frequency with Running Speed sidebands
Mechanical Unbalance (and Misalignment)	CF = RS x Rotor Bars Line Frequency Sidebands, Space of four times Line Frequency then two Line Frequency peaks

 

Where RS = Running Speed

The pass/fail values of the signatures identified in Table 5 are presently based upon the experience of the user.  In the case of motor (downstream) analysis, these values relate to current and for generator (upstream) analysis, these values relate to voltage.

The Fast Fourier Transform (FFT) of both current and voltage signatures are normally calculated in dB instead of linear scale. Analysis of the differences in peaks is determined by comparing the dB value measured down from either the peak current or peak voltage value.

The Combined Use of MCA and ESA

MCA requires that the equipment is de-energized while ESA requires that the equipment is energized.  These differences offer the user specific analysis strengths for each technology that support each other. 

In the case of MCA, it has specific strengths in the areas of:

Control and other connections

Cable insulation system health to ground and between phases

Stator winding health to ground and between phases and conductors

Air gap issues between the stator and rotating assembly

Rotor winding health: wound, induction or synchronous

This includes the ability to provide early failure detection of insulation degradation.

ESA has specific strengths in the areas of:

Power quality

Severe insulation breakdown

Loose or open coils or stator

Loose or open rotor or rotor coils

Loose connections

Air gap problems, including static and dynamic eccentricity

Bearings and mechanical condition, including alignment

Attached mechanical systems

When used in combination, the technologies provide some overlapping capabilities, but specifically they provide a complete overview of the system being evaluated, with a high degree of accuracy.

Basic Generators

There are two basic types of generator systems.  These include turbo-synchronous machines and salient-pole synchronous machines.  There are a large variety and variation of each type, so we will cover the basic assembly of both in this paper.

The turbo-synchronous machine is most commonly used in high-speed generators (two and four pole) used for high voltage power generation.  The general assembly resembles a three phase induction motor with the following specifics:

The stator (armature) resembles a three phase motor winding.  The DC fields of the turbine rotor cut through the conductors and generate power which is supplied to the distribution system from this component.

The turbine rotor (fields) resembles the squirrel-cage rotor of an induction machine.  This component carries the DC power from the exciter and is driven by a prime mover such as a jet engine or steam turbine.  It tends to be long and narrow for horizontal machines.

The exciter can be separate from the machine, in which brushes supply power to the rotor, or brushless, in which a small DC generator is mounted directly to the turbine rotor shaft.  The exciter provides DC power to the turbine rotor.

The salient-pole machine is one of the more common smaller, low voltage, low-speed (1800 RPM or less, 4-pole) generation systems.  The distinction is that the rotor contains a series of individual wound-coils which also contain an amortissieur winding, in most cases.

The stator (armature) resembles a three phase motor winding.  The DC fields of the salient-pole rotor cut through the conductors and generate power which is supplied to the distribution system from this component.

The salient rotor (fields) incorporate an even number of pole pieces that radiate out from the rotor shaft.  These poles carry the DC current that generates the rotating DC fields.

The exciter can be separate from the machine, in which brushes supply power to the rotor, or brushless, in which a small DC generator is mounted directly to the rotor shaft.  The exciter provides DC power to the rotor.

Both machines can be evaluated in the same way that you would evaluate electric motors of similar design.  In the case of ESA, you would evaluate the signatures using voltage spectra versus current spectra.

Conclusion

Energized testing of generators requires the ability to view the voltage signature of the generator and this requires equipment that can perform Electrical Signature Analysis, not just Motor Current Signature Analysis.  The purpose of this first paper has been to provide a review of MCA and ESA, as well as a discussion of the construction of turbine and salient generators.  The general analysis of generators is performed in a similar manner of any other AC machine.

How to Buy an Assembled PC

You can buy a personal computer directly from a manufacturer or from an electronics or department store. These computers, sometimes called branded PCs, bear the insignia of a manufacturer and are often equipped with a generic hardware configuration and a suite of software applications suitable for many first-time computer users. If you're an experienced user who has very specific needs that a branded PC can't adequately meet for you, or you anticipate upgrading your personal computer after buying it, you should consider buying a custom-built PC. Also known as an assembled PC, a custom-built computer requires you to give more thought to want you want in a PC and to do more research into individual computer components than if you choose to buy a branded PC, but the potential savings and satisfaction can be worth it. Here are the things you need to consider if you're interested in buying an assembled PC.

Edit Steps

1. Consider whether you want a desktop or laptop computer. If you like to work from a number of locations in your house or outside of it, you'll want a laptop computer. If you prefer to work from a dedicated office inside your home or at a remote location, you'll want a desktop computer. Presently, assembled PCs are more likely to be desktop computers than laptop computers, but the increasing popularity of laptops and wireless Internet connections is causing more assembled laptop PCs to be built than previously.
   If you divide your time between working in 1 location and working remotely, you may want to consider one of these options: If you spend more of your time at home than working remotely, you may want to have both a desktop computer and a laptop computer, with the desktop being an assembled PC and the laptop a branded PC. If you spend more time working remotely than at home, you may want to have a laptop with a docking station in your home office to plug the laptop into so you can use it with a desktop-style keyboard and monitor.
2. Determine what you're going to use the computer for. You can use either a branded PC or an assembled PC for most all-around needs. If you have specific needs for which you're buying a computer, you may want to buy an assembled PC, as you can have the computer built to your specifications.
   • For playing video games, you'll want the fastest processor and the most random access memory (RAM) you can get, a fast hard drive, and the best graphics card you can afford. You may want to have multiple graphics cards, which will require additional expansion slots and a bigger power supply.
   • For digital editing of photographs and videos, you'll want a high-capacity hard drive, a fast processor, and a good graphics card. If you plan to connect the computer to your TV set to view your digital images, you'll want to have the computer equipped with the same type of connectors (S-video, DVI, or HDMI) that your TV has. For video editing, you'll also need either a USB or Firewire port to connect a video camera to your computer and a quality sound card.
   • For a home recording studio, you'll want a high-quality sound card, a fast processor, and a high-capacity hard drive to handle the sound files you create. You'll also want to have a MIDI connector if you plan to connect the computer to a MIDI keyboard.
   • For running a home business, the ideal configuration depends on the kind of business you run. For a graphic design business, you'll want a quality graphics card and a large monitor. For a business that requires you to run a number of software applications at once, you'll want a fast processor and hard drive. You may also want a large monitor if you keep a number of screens open at once or spend most of your time in front of the display.
3. Decide how much you're willing to spend and on what. Assembled PCs often run cheaper than branded PCs, in part because they don't come loaded with software you won't use. However, depending on how you configure an assembled PC, it can cost as much or more than a branded PC of equal processing power. The main costs come from the speed of the processor, the storage capacity and speed of the hard drive, how many slots the motherboard includes, and how many of them are filled with memory, sound, or graphics cards.
   • The faster the processor you buy, the faster your computer runs the software on it, but also the more cost it adds to your computer system. Faster processors now feature multiple processing cores (dual-core, quad-core, 6-core, and 8-core).
   • The larger the capacity of your hard drive, the more data (documents, images, sound files) you can store on it and the faster its retrieval speed in revolutions per minute, the sooner you can start working with your data. Hard drives are increasingly being made with a speed of 7,200 rpm, with older drives operating at 5,400 rpm. The fastest hard drives are solid-state drives, which have no moving parts, but these cost a good bit more than other hard drives.
   • The motherboard holds the processor, memory boards, and other peripheral cards. It should be compatible with the processor you buy and have enough expansion slots to upgrade the computer as you see fit. It should also have enough connection ports to your peripherals, at least 2 or 3 USB ports, and connectors for a TV, MIDI keyboard, or video camera as needed.
   • Dedicated sound and graphics cards have their own processing chips, allowing them to produce better sounds and image displays than memory chips within the computer's motherboard.
   • Memory comes relatively cheap, but it is important. The more memory you have, the more applications you can run at once, and the better your computer will perform overall. Be sure you get enough memory to run your computer's operating system; in general, the more recent the operating system, the more it can do and the more memory you'll need. You can buy an assembled PC with plenty of available memory card slots with the intent to upgrade later, but it may be better in the long run to buy the additional memory before you need it.
4. Look for reputable manufacturers of computer components. The advantage of buying an assembled PC over a branded PC is that you can choose which components you have installed in your computer when you buy it. The disadvantage is that you have to become knowledgeable about the manufacturers of components instead of looking for a familiar brand name for the computer. You can research component manufacturers on the Internet or by asking around at computer stores or by asking other computer users.
   • The 2 main processor manufacturers are AMD (maker of the Athlon chip) and Intel (makers of the i, T, and G series chips and best known for the Pentium and Celeron chips). Some processors have a reputation for running hotter than others, requiring more and bigger fans to keep the processor from overheating.
   • Reputable hard drive manufacturers include Seagate, Western Digital, and Iomega.
5. Look for a reputable computer assembler. You can build a personal computer yourself by ordering the components from distributors, but most buyers of assembled PCs prefer to have someone else assemble the computer for them. You can either deal with a computer manufacturer who assembles PCs to order and sells them over the Internet, or you can buy from a local computer assembler.
   • Buying direct from a manufacturer may offer you the comfort of a brand name for your personal computer while still having a choice in the components that are used in it. However, your choices can be limited to a narrow range of processors, hard drives, or other components that may be what the manufacturer got a good deal on at the time and may or may not meet your needs. Also, you may have to send the computer back to the manufacturer or to another servicer if the computer needs servicing while under warranty.
   • Buying from a local assembler lets you ask questions directly of the company, choose from a broader range of components, and, in most cases, have local access to service and support if you have problems with your computer. You can also upgrade your computer more easily, usually without worrying about whether a hardware or operating system upgrade will void your warranty.
   • You can vet the quality of a computer assembler by getting a list of that assembler's customers and interviewing them at random, as well as searching Internet websites and forums. You want an assembler with an established track record, decent warranties, and excellent technical support.

How to Choose Your First Computer

So you want to buy your first computer? This will show you how to find the right computer for you. You have to buy a computer that fits your budget, your needs. You may or may not need a powerful computer depending on your needs.

Edit Steps

1

Make a budget. Think about how much money you are willing to pay for a computer.
So you want to buy your first computer? This will show you how to find the right computer for you. You have to buy a computer that fits your budget, your needs. You may or may not need a powerful computer depending on your needs.

Edit Steps

1. Make a budget. Think about how much money you are willing to pay for a computer.
2. What purpose will it serve. If it is for basic applications like word processing, then look for a cheap model. If you want to play games, you'll need something with a fast processor, lots of RAM, a large hard drive, and good sound and graphics cards.
3. Where will it be used. If you need to take it all over the place, you'll need a laptop. If you intend to leave it in the office or at home on the desk, then buy a desktop.
4. Who is going to use it? If it's for an office and you have to share it, you might need one with more memory and Windows 7 Professional or Ubuntu. If you will be the only one using it, and mostly for playing games, then Windows 7 Home Premium or Vista Home Basic will serve you well.
5. Choose an operating system.
   • Linux (Ubuntu, Debian, Fedora, OpenSuse and Mandriva) is stable, fast and free.
   • Microsoft Windows (98, XP, Vista, 7) is the most popular platform in general and for gaming.
   • Mac OS X (Tiger, Leopard, Snow Leopard) is popular for creative endeavours and multimedia.
6. Choose the hardware. Decide whether you want a desktop machine, a laptop that you can take with you, or a netbook. A netbook is a smaller version of the laptop but more portable and with fewer built-in devices such as CD and DVD drives.
7. Decide on the amount of memory. Decide how much RAM memory you want your computer to have. Many software have a minimum RAM requirement in order to run. So if your software say it requires 2 GB of memory, and your computer has only one GB of memory, it will not run. The more RAM the better, but it will be more expensive. You should at least have 2 GB or more RAM, unless the only thing you are using the computer for is connecting to the internet (in which case you can do with only 1 GB).
8. Research. Scan the Internet, and become more knowledgeable. Compare prices, and whether the computer can be upgraded as needed. Then you are ready to order, either online, or in a computer store.

How to Choose the Rolling Laptop Case for Your Macbook

Steps

1. Find the correct size.Just as rolling laptop case computer computer systems are produced in different dimensions, laptop computer scenarios are produced to maintain computers of diverse measurements. Come across a laptop computer bag that holds a personal computer at least as massive as yours.
2. Choose the sum of safety you want. Most laptop computer sleeves and totes will defend your notebook from dirt, water and slight bumps and scratches. Gentle-sided bags and scenarios normally provide a tiny far more padding for your computer and generally have a semi-rigid compartment to securely hold your laptop computer. But if you want the most protection you can get, a challenging-shell notebook situation is greatest.
3. Decide on your design. Notebook computer bags are styled to seem and act like most other bags. There are laptop bags that operate considerably like briefcases, tote bags, messenger bags, suitcases and backpacks. If you feel cozy just grabbing your PC bag and really going, a tote or briefcase model will be best. Messenger bags and backpack-style notebook computer bags are perfect for college students and bike commuters due to the fact they are palms-free of charge and supply room for a several textbooks.
4. Pick your selections. Several much larger notebook scenarios will have wheels to make finding by way of the airport easier. Some personal computer bags will supply a whole lot of further area for other papers or publications. Also contemplate how quite a few pockets you want for equipment, producing utensils or your MP3 player. You will also want to choose if you want the security of zippers and buckles or the simplicity of open pockets.
5. Choose your shade and substance. Notebook computer situations arrive in just as many substance and colour alternatives as any other piece of luggage. Black is preferred and goes with virtually something. Leather is also preferred and reveals course. Aluminum can be striking no matter whether it is painted or not. Choose a thing that you will be delighted with.

How to Buy a Laptop

Laptops are everyday items for business, home and study use. If you haven't ventured into purchasing one for yourself yet, do you feel confident enough to work out what type will best suit your needs? After all, they come in many different sizes, colors, and types, making it a challenging choice to find the "right one". This article is aimed at walking you through the key considerations prior to purchasing a laptop, to ensure that you end up with one that will serve your needs well.

Edit Steps

1

Consider whether a laptop has the advantages you're looking for. If you haven't had a laptop before, it can be a good idea to consider the possible benefits of owning one. When compared with a desktop, laptops have the following advantages (and disadvantages):

• You can take a laptop with you wherever you go, even abroad provided that you take a power adapter as well.
• Laptops are now more affordable and competitive in their pricing.
• Many laptops can do what we expect most desktops to do.
• Laptops save on space and are easy to move out of the way.
• There are a few negatives: Laptops are easy to steal if you forget to take care when traveling with them. Their battery life isn't extraordinarily long and can be frustrating if you want to work without electricity for a long period of time, such as on a plane or sitting at the shore near your vacation cottage. The average lifetime of laptops is also significantly lower than for desktop computers. If you plan on using your laptop exclusively, you will probably have to upgrade after two years. Due to the proprietary nature of laptop hardware, even something as silly as a broken mouse button or gummed up keyboard can't be fixed without mailing the laptop back to the manufacturer, usually at cost to you.
• Set your budget. It's important to know how much your budget is before you start looking or you might be swayed by long-term interest-free sweeteners to purchase something beyond your means. There is a large range of laptops available and setting a limit will ensure that you enjoy the laptop you can afford, without being prevented from upgrading later on because you're still paying off the old one! Decide what aspects are important to you and fit these within your budget:
  • If you're only after a laptop that can cope with sending emails and browsing the internet, then you will be fine getting a netbook or laptop that doesn't cost a lot. A refurbished laptop with a good warranty is great for this purpose.
  • If you're into playing games online, you'll need to spend more to get a faster graphics card. If online gaming is a hobby, you've probably already factored in this extra expense.
• 3
  Consider what Operating System(OS) you prefer: Linux, Mac OS or Windows. It is really up to your personal preference, but these are some important considerations:
  • Go with what you know? If you're used to one OS it'll be 'easier' to continue with a familiar interface than give something new/fresh a chance. But don't let your first OS decide every subsequent OS and computer buy.
  • Why stay with an OS that doesn't fit you? Some programs you may need/want are only available for your old OS but can be made to work through an emulator. Others can't. Your Mileage May Vary.
  • If you are inexperienced with computers and will need help, buy the type of computer that helpful family members or friends know and will help you with. Otherwise you'll have to rely on call-center 'tech support'.
  • Try out all three types in the store or friends computers. See what feels most innate and natural to your way of using a computer. Even within the same make, keyboards, track pads, etc., can feel very different under your personal touch.
  • Give Linux a try. Some laptops can be purchased with Linux pre-installed. To try before making the purchase download a free live distro (distributed demo). It allows you to run your computer using a Linux operating system out without making any changes to the computer. Most Linux operating systems are free of charge as are thousands of programs and apps. A program named WINE lets you run many Windows software on Linux systems. You can install and run these applications just like you would in Windows. Wine is still under active development. Not every program works yet. However there are several million people using WINE to run their Windows software on Linux operating systems. Linux has virtually no threats from viruses. Linux is the perfect choice for children because the operating system is free, the programs are free, and there is virtually no threats of viruses. If the the kids mess the operating system up just reinstall the operating system and start over fresh. Linux Mint looks and works the most like Windows. Ubuntu Linux is the most popular.
  • Macs cost more for relatively the same hardware/features. This is a big drawback if you're on a budget. On the plus side, they match well with iPhone, iPods, iPad, etc., they are also considered fairly easy to use for the less technically inclined, and have good support in Apple Stores. Read up the pros and cons of Mac ownership on relevant internet forums. Macs are also generally less prone to viruses, though they are not immune. Another thing about Macs is that if you still have Windows applications that you need, you can buy Windows emulation software and install it on your Mac, which will allow you to run Windows on your Mac. All recent Macs come with software like this, called BootCamp, but using it requires a restart to boot into Windows mode.
  • Windows netbooks/laptops can be quite affordable, and there are many options from a number of manufacturers to fit virtually every need or want. Windows 7 includes the ability to connect to an in-built GPS device and has good power. However a lot depends on the manufacturer, so it's important to look at what each manufacturer offers in terms of price, features and support, and then to read reviews and other sources for information on how reliable the manufacturers products are.
• 4
  Think about the size of laptop that will suit you best. There are three different size/weight ranges for a laptop: a netbook, a laptop, or desktop replacement. Although all of these fall into the broader concept of a "laptop", their end usability differs and can impact your choice. Here is a brief rundown of each:
  • Netbook (also known as a mini notebook or an ultraportable) - with a portable small screen of 7"-13"/17.79cm-33.3cm. This has a compact size, is light in weight, and is usually suitable for emailing and browsing or light internet usage as their memory is small. Since netbooks tend not to have as much RAM as laptops, their ability to run sophisticated applications is limited. However, changes are occurring all the time, so always ask the retailer's advice.
  • Laptop - with a screen of 13"-15"/33.3cm-38.1cm. This is of medium weight, is still thin and light, and is able to hold a lot of memory. The decisions to make about a laptop's capacity really come down to your own preference as to screen size, and the amount of RAM you think you'll need.
  • Desktop replacement - screen of 17"-18"/43.8cm-45cm. This is larger and heavier, has full features, and tends to be bound for the desk rather than being lugged around in your backpack. While not as portable as the other two, however, it's still very mobile when needed and the added weight when carrying it isn't really a big issue for many people. If you're not sure about this size, weigh up your desk and portable needs, noting that it is possible that the desktop replacement will eventually do away with desktop computers altogether.^[1]
• 5
  Consider your durability needs. Decide on whether you'd prefer a metal or plastic exterior. Nowadays the choice of casing is mostly an issue of personal preference, as the weight of each outer-casing is fairly similar, with well-made metallic laptops being no heavier than plastic ones. In terms of durability, metallic casing is probably best for a laptop liable to get knocked around a bit but it's still best to ask your retailer for advice.
  • If you are doing fieldwork or a lot of "rough travel" with a laptop, you might need customized extras to protect it. Ask for a stronger screen, shock mounting of internal components and "ruggedizing" of the whole laptop against water, spills, and drops.^[2].
  • If you are a professional in the field and you really need it to last then there is a class of laptops called Toughbooks, which tend to be quite expensive, but you can run them over with a truck or bake them in the oven without damaging them.
  • Most consumer model laptops in retail showrooms are not built for durability. Look for a corporate model laptop built from metal or composite materials if durability matters.
• 6
  Look for features to match your needs:
  • Check the CPU or central processing unit. Higher end, faster processing laptops will have a multi-core CPU such as Intel, AMD, and now ARM. This won't usually be found in netbooks or lower-end laptops. The difference impacts the speed of your laptop's performance.
  • The size of the Hard Disk Drive (HDD) is virtually unimportant, as long as it can hold your OS and programs, which even the smallest drives today probably can. To store your media (e.g. pictures, music) no amount is enough and since you can't put an extra HDD in a laptop, external/network storage becomes almost a must. What does matter for performance is the speed of the HDD. Of course, more speed = less battery life.
  • Alternatively, Solid State Drives (SSDs) offer much higher performance, no noise and extended battery life, but have much smaller capacities (usually 30GB to 256GB at the time of writing) and cost more. If you are looking for the best possible performance, an SSD is a must, but you'll probably need to purchase an external hard drive for things like music, photo and video libraries.
  • Consider how much RAM you actually will need in your new unit. While it may be tempting to get a whole ton of RAM when you purchase the laptop, often retailers will put massive amounts of RAM in a unit to disguise the fact that the rest of the components are sub-par (slower processor, etc). Since it is quite easy to upgrade RAM (even in a laptop), this should not be a huge consideration for a specific laptop. The amount of RAM memory can be an important spec to consider. Often the amount of memory can limit the applications that you can run. Large applications will require more memory to run. In general, the more memory you have, the faster the laptop will run. The typical memory numbers come in amounts like 1 GB, 2 GB, 3 GB, and 4 GB, where GB means gigabytes. Try to get at least 3 GB. Laptops with 4 GB are ideal but make sure that your operating system is 64 bit for anything 4GB and over, as a 32 bit operating system cannot recognize more than 4GB. Most manufacturers will not sell you a laptop with a 32 bit operating system if you choose 4 or more gigabytes of RAM, though.
  • If you are playing games, check the graphics memory. You must have a graphics card with discrete video memory for 3D games, though this is not necessary for most casual games. A discrete graphics card will consume more battery power as well.
  • How many USB ports are in place for adding your peripherals (memory sticks, iPod, Firewire, etc.).
  • Check the laptop's optical drives. If you want to be able to burn CDs and install software from discs, you will need a DVD drive. If your laptop does not have one, you can always purchase an external DVD drive to plug in when needed. Blu-Ray drives are also options now in many laptops. If you want to be able to play blu-ray movies, make sure you choose a blu-ray drive (sometimes called "bd-rom") instead of a dvd drive.
  • Look for the right screen resolution. While not impossible, it's difficult and expensive to change the screen that comes with your laptop once purchased, so get the right amount of pixels from the start. More pixels increases the cost but if the resolution is important for your activities, it is worth getting this part right.^[3] Ask how the laptop screen performs under sunlight; cheaper screens will often be "invisible" in outdoor light, making their "portability" a little less useful for you.
  • Check the wi-fi connectability. Your laptop should be wi-fi enabled.
  • Test the keyboard. It should feel right to your way of typing or tapping. Same for the track pad.
• 7
  

  Find a suitable retailer for purchasing a laptop.
  Find a suitable retailer for purchasing a laptop. The main thing is that you find knowledgeable people or adequate information to satisfy your questions about the laptop you'd like to own. There are a number of options for a retailer:
  • Specialized stores that sell only computers and/or computer parts.
  • Brand stores that sell only sell that brand.
  • Generalized electronics stores that sell everything from fridges to computers rarely have staff with specialized computer knowledge
  • Department stores with specialist computer sections.
  • Online electronic or specialist retailer stores - and you can even check out the laptop in a real store and then order it online if preferred - be prepared to pay shipping charges, of course.
  • Online auction sites - be sure you know the reputation and trustworthiness of the vendor by checking their ratings.
  • Refurbished laptop dealers - make sure they have been in business for a while and give a one year warranty. Beware of short warranties. A physical presence means you can go and have a look at the laptops.
• 8
  Know what to avoid. There are some laptop products you should avoid buying:
  • Know the risks before you buy used, re-certified, or refurbished laptops. It is very important that the laptop comes with a good warranty and from a reputable dealer. Durable, corporate grade laptops can be a bargain when refurbished. The risk is that the laptop has been mistreated and is in poor condition. If the price is right, and especially if their is a one year warranty, then the risk may be negligible.
  • Don't buy discounted floor stock laptops unless they come with a good warranty from a reputable dealer.^[4] These can be several generations older than current models on full sale, and it's likely that they have been powered up all day every day, as well as being exposed to store dust, grimy fingers, and endless pressing and banging from bored kids or confused customers.
• 9
  Once you have your laptop, take good care of it. While it depends on the brand and type of laptop, a laptop that is cared for well should last a good few years before you need to invest in another laptop.

How to Make a Scrolling Marquee in HTML

1. Open your HTML document, if you haven't already.
   
   
2. 
   
   Where you want your marquee to be, place this code:
   
   

   <marquee style="background-color: #000080;" direction="right" loop="20" width="75%">
   This is a scrolling marquee
   </marquee>
   

   • The style attribute controls the background color for the marquee. Leave the tag out if you don't want one. You should use either a color name, a hexadecimal (hex) value, or a RGB (Red Green Blue) value. A full list of colors and their values can be found here.
   • The style attribute inside the <div> tag controls the color of the text. See this list for colors.
   • Change the direction attribute to any direction you want out of the following: right, left, up or down.
   • The loop attribute can be changed to suit your needs - it controls how many times the marquee should loop. If you want it to run continuously, you can leave out this attribute.
   • The text inside the <marquee> tag and the </marquee> tag is the bit you need to change to put your text in.
   
   

Edit Tips

• Marquees tend to be a little annoying on websites.
• Be careful when you change the attributes on your marquee.
• For a more complete definition of the <marquee> tag, take a look here.