An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current. Usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Alternators are used in modern automobiles to charge the battery and to power a car’s electric system when its engine is running. Automotive alternators use a set of rectifiers (diode bridge) to convert the AC output of the alternator to the DC used in vehicle’s electrical system .

Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water environment. Marine alternators are designed to be explosion proof so that brush sparking will not ignite explosive gas mixtures in an engine room environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits, the power is split between the engine starting battery and the domestic or house battery (or batteries) by use of a split-charge diode (battery isolator) or a mechanical switch (battery switch)

-adapted from Wikipedia

Anderson Connectors or Anderson Plugs as they are more commonly known, are designed for connecting large cables used in high current applications.

Each plug is a mirror of the other and are simple to assemble, There is no Male & Female. One simply connects into another of the same amperage. The plugs push together to form a very solid and reliable connection.

A common application in marine would be in the wiring that distributes AC power.

From the Anderson Power Products website:

Anderson Power Products is an industry leader in the manufacture of high current, quick-disconnect power connectors and provides a variety of interconnect solutions for the material handling, telecommunications, medical, power electronics and other industries. Our connectors are available from 10 to 700 amp maximum ratings for use through 600 Volts continuous, AC or DC operation. We are well known for our ability to develop creative solutions for our customers’ power interconnect requirements. We are flexible and will make modifications to standard products or develop complete custom solutions to satisfy particular customers’ needs. APP provides a complete engineered interconnect solution for all types of power distribution needs.

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Batteries are a common power source for many household, industrial and transportation applications.

There are two types of batteries: primary batteries (disposable batteries), which are designed to be used once and discarded, and secondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times.

Rechargeable batteries are what are used in automotive and marine applications. They can be recharged by applying electric current. Devices to supply the appropriate current are engine alternators or chargers.

The most common form of rechargeable battery is the lead-acid battery. This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging.

You might have been wondering how electrons can continuously flow in a uniform direction through wires without the benefit of these hypothetical electron Sources and Destinations. In order for the Source-and-Destination scheme to work, both would have to have an infinite capacity for electrons in order to sustain a continuous flow! Using the marble-and-tube analogy, the marble source and marble destination buckets would have to be infinitely large to contain enough marble capacity for a "flow" of marbles to be sustained.

The answer to this paradox is found in the concept of a circuit: a never-ending looped pathway for electrons. If we take a wire, or many wires joined end-to-end, and loop it around so that it forms a continuous pathway, we have the means to support a uniform flow of electrons without having to resort to infinite Sources and Destinations:

Each electron advancing clockwise in this circuit pushes on the one in front of it, which pushes on the one in front of it, and so on, and so on, just like a hula-hoop filled with marbles. Now, we have the capability of supporting a continuous flow of electrons indefinitely without the need for infinite electron supplies and dumps. All we need to maintain this flow is a continuous means of motivation for those electrons, which we'll address in the next section of this chapter.

It must be realized that continuity is just as important in a circuit as it is in a straight piece of wire. Just as in the example with the straight piece of wire between the electron Source and Destination, any break in this circuit will prevent electrons from flowing through it:

An important principle to realize here is that it doesn't matter where the break occurs. Any discontinuity in the circuit will prevent electron flow throughout the entire circuit. Unless there is a continuous, unbroken loop of conductive material for electrons to flow through, a sustained flow simply cannot be maintained.

• REVIEW:
• A circuit is an unbroken loop of conductive material that allows electrons to flow through continuously without beginning or end.
• If a circuit is "broken," that means it's conductive elements no longer form a complete path, and continuous electron flow cannot occur in it.
• The location of a break in a circuit is irrelevant to its inability to sustain continuous electron flow. Any break, anywhere in a circuit prevents electron flow throughout the circuit.

Published under the terms and conditions of the Design Science License

Disclaimer

The boat (Hammer P) is a 1966 -35 ft. OWENS flagship. She is double plank mahogany on oak frames, Teak decks and fiberglass cabin shell and fly bridge.

Equipment when we got her included mismatched and somewhat tired For V8 engines, Borg-Warner Velvet Drive transmissions all fresh water-cooled. Also outfitted with a 6.5 KW Kohler Gen- Set.

By mid 2006, I had decided that re-powering was essential. Having toyed with the electric idea for a year or so, I finally decided- "Just Do it" and began the search for a supplier for all the components. After considerable browsing catalogs and online information, this is what I chose for our application: (This is basic electric stuff)

2-Advanced DC Motors-L91-4003-13hp-72Volt

2-Curtis-PMC Motor Controllers- 72Volt-400amp

2-Merritt Inline Joystick control

2 -Albright Main Contactors

2 -Albright Reversing Contactors

3 -400 amp Ferraz/S'mut Safety Fuses

1-Link 10 E Meter

1-Onboard Charger 48-108 volt

2-Deltec Amp meter shunts

2-Westberg Ammeters

180 Lin. feet 2/0 Welding cable

90 Cable Lugs

24 -L16H Trojan Batteries

That is pretty much the electrics of the system, plus misc. hook up wire, etc.

The Mechanics portion needs some thought: Motor to shaft reduction, belt tensioner and battery placement. As you will see by the photos, I had to lengthen the propeller shaft to accommodate the larger pulley for a reduction ratio. I chose a 4:1 reduction because the motors need to turn fast enough to run cool and I need 1000rpm at the shaft to match our previous cruising speed. The rest of the mounting hardware is like a big erector set. All bolted parts were pre-drilled except for bolting to the original engine bunks. These were bolted in place after shaft augment. Motor mounts are adjustable for belt tension and tracking.

There are two flange bearings each side, and opposed for thrust bearings. This all makes a fairly compact package and we can now decide on battery placement. In our case, we are designing a 72 Volt system. Battery placement is somewhat a balancing act. Original components, (fuel) was stored behind the drive train and of course, motors amid ship. Fuel weight (160 gallons) was about 1300 lbs. Motors and transmissions about 1500 lbs. New battery pack weight: about 3000 lbs. To best balance the boat, I put 8 batteries forward and four behind each motor. With motors, batteries and electrical equipment we have an approximate weight gain of 1000 lbs.

Now, looking at performance. First we need to consider the boat Shape and Hull design. As the professionals see it, this is the second poorest hull design for electric power. Only a barge with square ends is worse. The ideal craft would have a sharp entry, a long waterline length and the transom out of the water. Boat design is always subject to compromise and the ideal form is not always practical. In our case, we will work with what is at hand and improvise, compromise and succeed.

Operation of the boat has not really changed. Still have twin props, still a hull speed just short of 6 knots (6.9 mph). What has changed is the planning and navigation. Without regenerative power we have to plan with power consumption. With batteries one should not run them totally dead. Always plan for about a 20% 'no touch' area to protect the battery life. In our case, we have about 670 usable amps that are available. Our "E' meter tells us exactly what power we have used and how long it will last based upon the rate of discharge over the last 10-12 minutes. It's nice to have that information available at the touch of a button.

For our longer distance cruises, we have adapted a temporary generator that run our battery charger at about 10 amps at best. Hi tech. chargers and modified sine wave generators are not very compatible for high output.Even the 10 amps will help some to increase longer distances. Over a 6-hour cruise, those 10 amps will return 60amps. Ifwe are traveling at 60 amps this has gained an extra hour of travel time. Of course now that we have made the trip, if it was one way, we need to allow adequate time to recharge. If we have used 550 amps and the charger output is 20 amps, potentially it could take 30-36 hours to recharge. Discharge rate and your speed are directly related and the need for speed shortens the trip. Here are some discharge rates that will help:

40-amp draw will run about 12.5 hrs.

60-amp draw will run about 8.5 hrs.

80-amp draw will run about 6.25 hrs.

Needless to say, the faster you attempt to travel, the shorter your travel time will be. This is where hull speed, weight and power storageneed to be considered.

Modern cruising vessels have high electrical demands, where refrigeration, radar, laptop computers and even plasma TVs are the norm. Keeping batteries charged is a challenge, and fitting a second alternator, says Scott Fratcher, is an easy solution

Alternator II

More charge needs either a bigger alternator, or better yet, a second alternator which adds significantly more potential to the boat than only increasing the amperage of the original alternator.

The math is easy. Imagine a typical diesel with a 55 amp alternator. If the alternator is removed and replaced with a 100 amp model, we gain 45 amps. If you add a second 130 amp alternator to the original 55 amp system, you instantly send a battery-boosting 185 amps into the electrical system.

Higher charging also increases the vessel’s safety margin. Single alternators may fail due to overwork – the constant heavy load to recharge the boat’s electrical demand takes its toll. This could lead to discharged batteries, with all electrical systems shut down. I’ve encountered many boaties who’ve spent an uncomfortable night in the shipping lanes alternating the last of the battery power between radar and navigation lights…

Many consider the decision to fit a second alternator a no-brainer. The only question is: how do you install one easily? This article discusses a technique to mount a second alternator on almost any inboard engine. And it’s worth noting that the process can be used to belt drive more than just an alternator – a hydraulic pump, dewatering bilge pump or anything else you might need to turn are also possibilities.

SIX STEPS TO ALTERNATOR II

1. Design and planning
2. Mounting a second front pulley
3. Making a base plate
4. Building a bracket from the base plate
5. Installing a belt tensioning device
6. Bolting in the alternator

DESIGN AND PLANNING
Start the design process by taking an alternator in hand and holding it next to the engine with the pulleys aligned.

There should be 12 possible positions: starboard low, starboard high, port low, port high, and above or below the drive pulley.

Face the alternator aft and you have six more possible locations – for a total of 12.

Hold the alternator in all 12 positions. Pick the best two or three positions and compare the possibilities. Choose a mounting position with the alternator as close to the engine as possible. Look for access, wire runs, mounting bolt holes in the engine and cooling air.

MOUNTING A SECOND PULLEY
We need to spin the second alternator from a second drive pulley at the front of the engine. Your engine may already have one, but usually you’ll have to add one, bolting it to the front of your existing engine pulley.

First, choose a pulley size. For a typical 30 to 75hp engine with maximum rpm of around 3600, a good drive pulley diameter is about 175mm.

I’ve experimented with larger pulleys (up to 230mm) but it’s not effective as most engines begin to hunt at low rpm. Conversely, if the pulley diameter is less than 150mm I often have to run the engine up to 1200rpm to get a good charge.

If in doubt, duplicating the original pulley size is usually a good bet.

There are two easy methods to fit a spare pulley to the front of an engine:

1. Have a machine shop make the new pulley; or,
2. Modify an existing pulley.

METHOD 1
A machine shop makes the new pulley, the simplest but most expensive option.

The machine shop will need the bolt pattern and centering ring measurements from the existing drive pulley on your engine. If you can take the measurements from the manual, the machine shop should have an easy job. If you have to take the measurements yourself, use digital calipers. Be sure to scrape away any old paint so your measurement is metal to metal.

Note: If the alternator is to produce more than 80amps, you should use a dual belt drive. You’re pushing the limit with a single belt – it will often slip, leaving gobs of black, sticky dust in your engineroom.

METHOD 2
Buy an “off-the-shelf” pulley at your local hardware store and have a machine shop make a new centering ring that fits your engine. You can even use an old car’s sheetmetal “stamped” pulley. The machine shop will combine the pulley onto a centering ring and you’re ready to install.

This option has the advantage of knowing the pulley face angles are going to be correct and smooth. It does not take much angle error, a nick or lathe marks left in the pulley face to make the belt begin to “dust”. Belt dusting is the major problem in building a dual alternator system. Twist, misalignment, rough surfaces, and drawing too much load all add to the amount of the belt dust. Commercial pulleys help solve this issue.

MAKING THE BASE PLATE
The base plate is a steel plate that gets bolted to the engine, and it becomes the base which holds the soon-to-be fabricated alternator bracket. The bracket is typically welded to the base plate.

Look for flat areas on the side of the engine block near where you want to mount your alternator. You want the plate to cover a minimum of three bolt holes – five or six is better.

Cut a piece of steel to cover the bolt holes. Using 6mm plate is the minimum – 8mm is better. Test fit the piece of steel over the area of the engine block. If it all fits and covers the bolt holes, you’re ready to start marking and drilling the holes.

Marking where the holes are to be drilled can be challenging, especially if the plate is in a difficult to reach location. Here’s an easy trick – it’s what I call the “sneak and tap” approach, and involves using a sharpened bolt screwed into each of the engine bolt holes (one at a time) to serve as a “reverse punch”.

Leave just enough bolt thread (the sharpened point) showing so you can use a pair of pliers to remove the bolt. Lay the base plate in the exact final location. Now for the tricky part – strike the steel plate with a single sharp blow from a hammer directly over the punch. Once marked you are ready to drill the hole.

Remove the bolt/punch and shift it to a new hole. Fasten the base plate (with a bolt through the newly-drilled hole) in position. Another sharp tap, remove the plate and drill the new hole. Repeat this process until all holes are drilled. This method is fast and easy and leaves no sloppy holes. If you make the base plate perfect, the rest of the job becomes much easier.

BUILDING A BRACKET
The bracket is the metal frame that holds the alternator, allowing it to pivot for tensioning the belt. I like to use a 50mm piece of 6mm flat bar. Cut two ears and weld them on the flat bar at a 90º angle (see photo).You should now have a base plate and bracket all made up. They may need to be connected with a strut (depending on the position of the alternator) to get the two pulleys in alignment. It’s usually the simplest to weld the plate, strut and bracket together.

ALIGNMENT
To find the correct alignment for the alternator, lay a wooden dowel rod in the drive pulley of the engine. Let the rod find its natural centre. You can now simply lift the rod up and down to show where an exact straight line to the slave pulley will fall. Reverse the procedure until the slave pulley is pointed directly at the drive pulley.

Remove all parts and tack the strut into position.

Replace and see if it all fits. If it does it’s time to weld it up and paint. Use this dowel rod method anytime you want an easy check of belt alignment.

MAKING A BELT-TENSIONER
Turnbuckle tension arms are an easy solution for making an alternator adjusting arm, and you only need simple tools – a hacksaw, welder and a drill. If you were making a conventional car-type tensioning arm, you’d have to cut an arc in the flat bar with an oxy-acetylene torch. Many boaties fit this turnbuckle swing arm to their existing alternator just to solve adjustment problems.

Begin with a half-inch or larger open barrel turnbuckle, and remove the studs from the barrel.

Cut the centre out of the barrel, leaving leavening 120 to 150mm of turnbuckle barrel.

Weld the barrel back together. Put the studs back in place and cut them to the length you worked out for the project at hand. Weld a flat plate to the end of each stud, drill a hole in the plates and mount to the engine.

STOP ENGINE BEFORE TURNING “OFF”
Ever notice what battery switches have printed on their faces? We all know not to disconnect the batteries while the engine is running, but what would happen if we did?

An alternator diode would fail – and that’s not good.

The reason is buried in Ohm’s Law which lays out the relationship between volts, amps, and watts. An alternator’s total power output is measured in watts. A typical high-output alternator might be charging 100amps at 14V – 100 x 14 = 1400 watts. If we had a 24-volt system, the alternator would be producing 50amps at 28 volts to make 1400 watts.

So, when we have an alternator producing 1400 watts and someone turns off the battery switch, the 1400 watts is already in the pipeline, so to speak. But the load (or current or amp draw) just dropped to zero because of disconnection to the battery.

Ohm’s Law tells us 1400 watts divided by zero (our new amp load) equals infinity. In other words, the voltage inside the alternator will climb toward infinity till it finds an escape route (the path of least resistance). That’s the shortest path to ground and typically, that’s the thin film inside a diode. Pop! The diode shorts.

Quickly switching the batteries back on might save the situation, but usually the damage is done. The boat owner may see the output of the alternator suddenly drop by a third. A typical complaint from boaties is: “My 100amp alternator is now producing 66 amps on the meter.”

This is because the alternator stator is really three-phase, and has three separate windings combining to produce 100amps. Since only the diode was ruined, each phase of the stator is still producing 33amps. If all three phases of the alternator are still producing 33amps each, why is the boat’s electrical meter only showing 66amps?

The “lost” 33amps are still being produced, but they’re not being rectified because that is the diode’s job. And un-rectified means alternating current (AC) is entering your DC system.

This is bad. At the same time that we are charging at 14 volts DC, we are also sending a battery-destroying AC “charge” into the boat’s electrical system. And because the boat’s electrical charge meter does not read AC, the owner has no clue something has gone wrong.

Those 33 AC amps are destroying the boat’s battery bank, electrical boards, and maybe even the hull zincs.

DIODE TEST
To check whether your alternator has a shorted diode you can clamp the positive alternator output lead with an AC/DC “clamp amp” meter. Switch the meter to DC amp and read the charge rate. Switch the meter to AC and we should see three or four amps.

A reading half of the DC charge rate indicates a bad diode. For example, if we were to see 66amps DC and 33amps AC, this would tell the technician it’s time to pull the alternator and change the diode pack.

Although not as accurate, we might also take a high-quality digital volt meter and measure voltage at the back of the alternator. We should see around 14 volts, but switch to AC and we should see around seven volts. Reading 14 volts AC could also indicate a faulty diode.

Adapted from Scott Fratcher’s How To Make Money With Boats, available at www.yachtwork.com

AUTO TENSIONING ARM
An automatic tensioning arm is another easy solution for a DIY installed alternator.

You can buy a “rasta” or LoveJoy arm for automatic belt tensioning from a good bearing supply house. Check out http://www.lovejoy-inc.com/

The device costs about $100. Simply mount the roller inside the unloaded belt between the drive pulley and alternator. In the photo above, the pulleys turn clockwise. Adjust the tension and ignore it for the life of the belt.

The second advantage of using a “rasta” or LoveJoy tensioning arm is the alternator does not have to rotate away from the engine to adjust the belt. This may mean a lot if you have a tight space to work in. You simply mount the alternator as close to the engine as you can manage and let the tensioning arm take up the belt slack.

AUTHOR PROFILE
Scott Fratcher has designed and installed more than 100 dual alternator systems. More photos can be found at www.yachtwork.com

Mr. Fratcher’s highly regarded books include

• How to Make Money with Boats
• Metal Boat Repair and Maintenance
• How to Buy Your Cruising Boat Now
• Earthrace – First Time Around

Mr. Fratcher also has an excellent website that is highly recommended to all easyacdc.com readers. Please be sure to visit Team Yachtwork and thank him for his thoughtful generosity.

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by Allied Wire & Cable, Inc.

The marine environment is a hostile one for electrical wire. Wire used on board a marine vessel will potentially be exposed to numerous obstacles, such as salt water, sunlight, heat and other outside hindrances. All electrical wires are not constructed to endure the problems associated with marine conditions and therefore will not be suitable wiring on boats or ships. In these situations, marine wire or boat cable may be necessary.

Marine wire, boat cable, and marine primary wire are terms you may have heard used in reference to electrical wiring for boats or marine vessels. Wiring specified as "marine" or "boat" is different in several ways from other types of electrical wire, such as power wire used in the home, or automotive wire, etc. A main difference is that the conditions surrounding marine installations require marine wire and boat cable to perform better than other wires designed chiefly for land use.

A marine wire is specifically designed and engineered for the electrical wiring of boats and is intended for all possible uses abroad a ship. Marine wire may be distributed to the pleasure boat and commercial marine industries and is often used by boat builders. The term "boat cable" may often be used interchangeably with marine wire or marine cable. Boat cable usually refers to general electrical wiring used on a boat. Marine wire that may fall into the sweeping category of "boat cable" often starts as a single conductor cable. Extra wires are added from there into one cable, consequently creating multi conductor boat cable.

Because of the demanding marine environment, approved marine wire usually possesses a copper conductor. In addition, the jacket of the cable will most likely have been tested for flammability safety. The jacket and the insulation should be rated water resistant.

The most frequently requested single conductor boat cable styles are marine primary wire and marine battery cable. The cables are extremely similar. The main factor that differentiates the two is the AWG size of the cable. According to General Isles Marine, single conductor boat cable in sizes 16 AWG up to 8 AWG are widely known as primary wire sizes. The larger single conductor marine cables ranging from size 6 AWG up to 4/0 AWG are known as battery cable sizes.

Often times, marine wire and boat cable provided by a manufacturer or distributor will meet the requirements of UL, SAE, Coast Guard, ABYC, and NMMA. The American Boat and Yacht Council (ABYC), the United States Coast Guard (USCG), the National Marine Manufacturers Association (NMMA) and the Society of Automotive Engineers (SAE) have developed safety standards and guidelines for marine electrical installations specifically serving manufacturers, technicians, and even boat owners.

Corrosion is a primary cause of electrical failures on a boat. In order to avoid the common problem, marine wire and boat cable are built to resist quick decay. In both wet and dry conditions, marine wire needs to behave consistently in order to perform properly. Marine wire, boat cable and marine primary wire may possess PVC insulation for added defense against the elements. After all, they need all of the help they can get.

The remaining links will examine the various types of marine wire and boat cable on the market today.

Common Types of Marine Wire

Marine Primary Wire (Tinned Copper)

Marine Primary Wire may also be listed as Tinned Primary Wire. The copper conductor will usually possess a tin coating which causes the strand to be called "tinned copper." Tinned copper marine primary wire is built to reduce corrosion and prevent electrical failure.

Marine Primary Wire (Tinned Copper) can be used in 105"C marine applications, in the internal wiring of electrical equipment and for general circuit wiring. It is employed for electrical connections in the marine and automotive environments where a tinned conductor is preferred. The marine primary wire may additionally be utilized for motorcycles and other applications requiring a high temperature primary wire. Tinned copper marine wire performs well in all marine environments, even in saltwater.

You may see marine primary wire listed as UL 1426 marine grade wire. Most brands of tinned primary wire will meet the requirements of the US Coast Guard and ABYC, as well as others.

Marine Primary Wire (Bare Copper)

Marine Primary Wire (Bare Copper) can be used in 105"C marine applications, in internal wiring of electrical equipment and for general circuit wiring. The marine primary wire shares many of the same applications and properties as tinned primary wire. However, the conductor is bare copper instead of tinned copper.

SAE Primary Wire

SAE Primary Wire is General Purpose Thermoplastic (GPT) insulated primary wire that corresponds to SAE specifications, generally specifications J1128 and J378. SAE Primary Wire may be used for general purpose marine and automotive applications. It usually has a temperature range of -20"C to 105"C and voltage rating of 50 volts.

Flat Boat Cable

Flat Boat Cable is a multi-conductor marine cable that can be used for marine or brake cable. The boat cable usually meets UL Standard 1426 and UL Style BC-5W2. Flat boat cable also may meet DOT Coast Guard specs. The boat cable has a PVC insulated multi-conductor.

Round Boat Cable

Round Boat Cable is much like flat boat cable. However, round boat cable makes for easy installation where tight, jagged spaces are present. Many installers of boat cable favor round cables because they are easier to arrange. Additionally, round boat cable may be used for harsh environments.

Marine Battery Cable

Marine Battery Cable generally has a temperature range of -20"C to 105"C and a voltage rating of 50 volts. The battery cable also resists oil, fuel and acid. Marine battery cable is designed to survive the harsh marine environments. The cable normally has a high strand count cable with tin plated copper stranding. Marine battery cable may be used in battery installations.

So far we know that AC voltage alternates in polarity and AC current alternates in direction. We also know that AC can alternate in a variety of different ways, and by tracing the alternation over time we can plot it as a "waveform." We can measure the rate of alternation by measuring the time it takes for a wave to evolve before it repeats itself (the "period"), and express this as cycles per unit time, or "frequency." In music, frequency is the same as pitch, which is the essential property distinguishing one note from another.

However, we encounter a measurement problem if we try to express how large or small an AC quantity is. With DC, where quantities of voltage and current are generally stable, we have little trouble expressing how much voltage or current we have in any part of a circuit. But how do you grant a single measurement of magnitude to something that is constantly changing?

One way to express the intensity, or magnitude (also called the amplitude), of an AC quantity is to measure its peak height on a waveform graph. This is known as the peak or crest value of an AC waveform: Figure below

Peak voltage of a waveform.

Another way is to measure the total height between opposite peaks. This is known as the peak-to-peak (P-P) value of an AC waveform: Figure below

Peak-to-peak voltage of a waveform.

Unfortunately, either one of these expressions of waveform amplitude can be misleading when comparing two different types of waves. For example, a square wave peaking at 10 volts is obviously a greater amount of voltage for a greater amount of time than a triangle wave peaking at 10 volts. The effects of these two AC voltages powering a load would be quite different: Figure below

A square wave produces a greater heating effect than the same peak voltage triangle wave.

One way of expressing the amplitude of different waveshapes in a more equivalent fashion is to mathematically average the values of all the points on a waveform's graph to a single, aggregate number. This amplitude measure is known simply as the average value of the waveform. If we average all the points on the waveform algebraically (that is, to consider their sign, either positive or negative), the average value for most waveforms is technically zero, because all the positive points cancel out all the negative points over a full cycle: Figure below

The average value of a sinewave is zero.

This, of course, will be true for any waveform having equal-area portions above and below the "zero" line of a plot. However, as a practical measure of a waveform's aggregate value, "average" is usually defined as the mathematical mean of all the points' absolute values over a cycle. In other words, we calculate the practical average value of the waveform by considering all points on the wave as positive quantities, as if the waveform looked like this: Figure below

Waveform seen by AC "average responding" meter.

Polarity-insensitive mechanical meter movements (meters designed to respond equally to the positive and negative half-cycles of an alternating voltage or current) register in proportion to the waveform's (practical) average value, because the inertia of the pointer against the tension of the spring naturally averages the force produced by the varying voltage/current values over time. Conversely, polarity-sensitive meter movements vibrate uselessly if exposed to AC voltage or current, their needles oscillating rapidly about the zero mark, indicating the true (algebraic) average value of zero for a symmetrical waveform. When the "average" value of a waveform is referenced in this text, it will be assumed that the "practical" definition of average is intended unless otherwise specified.

Another method of deriving an aggregate value for waveform amplitude is based on the waveform's ability to do useful work when applied to a load resistance. Unfortunately, an AC measurement based on work performed by a waveform is not the same as that waveform's "average" value, because the power dissipated by a given load (work performed per unit time) is not directly proportional to the magnitude of either the voltage or current impressed upon it. Rather, power is proportional to the square of the voltage or current applied to a resistance (P = E²/R, and P = I²R). Although the mathematics of such an amplitude measurement might not be straightforward, the utility of it is.

Consider a bandsaw and a jigsaw, two pieces of modern woodworking equipment. Both types of saws cut with a thin, toothed, motor-powered metal blade to cut wood. But while the bandsaw uses a continuous motion of the blade to cut, the jigsaw uses a back-and-forth motion. The comparison of alternating current (AC) to direct current (DC) may be likened to the comparison of these two saw types: Figure below

Bandsaw-jigsaw analogy of DC vs AC.

The problem of trying to describe the changing quantities of AC voltage or current in a single, aggregate measurement is also present in this saw analogy: how might we express the speed of a jigsaw blade? A bandsaw blade moves with a constant speed, similar to the way DC voltage pushes or DC current moves with a constant magnitude. A jigsaw blade, on the other hand, moves back and forth, its blade speed constantly changing. What is more, the back-and-forth motion of any two jigsaws may not be of the same type, depending on the mechanical design of the saws. One jigsaw might move its blade with a sine-wave motion, while another with a triangle-wave motion. To rate a jigsaw based on its peak blade speed would be quite misleading when comparing one jigsaw to another (or a jigsaw with a bandsaw!). Despite the fact that these different saws move their blades in different manners, they are equal in one respect: they all cut wood, and a quantitative comparison of this common function can serve as a common basis for which to rate blade speed.

Picture a jigsaw and bandsaw side-by-side, equipped with identical blades (same tooth pitch, angle, etc.), equally capable of cutting the same thickness of the same type of wood at the same rate. We might say that the two saws were equivalent or equal in their cutting capacity. Might this comparison be used to assign a "bandsaw equivalent" blade speed to the jigsaw's back-and-forth blade motion; to relate the wood-cutting effectiveness of one to the other? This is the general idea used to assign a "DC equivalent" measurement to any AC voltage or current: whatever magnitude of DC voltage or current would produce the same amount of heat energy dissipation through an equal resistance:Figure below

An RMS voltage produces the same heating effect as a the same DC voltage

In the two circuits above, we have the same amount of load resistance (2 Ω) dissipating the same amount of power in the form of heat (50 watts), one powered by AC and the other by DC. Because the AC voltage source pictured above is equivalent (in terms of power delivered to a load) to a 10 volt DC battery, we would call this a "10 volt" AC source. More specifically, we would denote its voltage value as being 10 volts RMS. The qualifier "RMS" stands for Root Mean Square, the algorithm used to obtain the DC equivalent value from points on a graph (essentially, the procedure consists of squaring all the positive and negative points on a waveform graph, averaging those squared values, then taking the square root of that average to obtain the final answer). Sometimes the alternative terms equivalent or DC equivalent are used instead of "RMS," but the quantity and principle are both the same.

RMS amplitude measurement is the best way to relate AC quantities to DC quantities, or other AC quantities of differing waveform shapes, when dealing with measurements of electric power. For other considerations, peak or peak-to-peak measurements may be the best to employ. For instance, when determining the proper size of wire (ampacity) to conduct electric power from a source to a load, RMS current measurement is the best to use, because the principal concern with current is overheating of the wire, which is a function of power dissipation caused by current through the resistance of the wire. However, when rating insulators for service in high-voltage AC applications, peak voltage measurements are the most appropriate, because the principal concern here is insulator "flashover" caused by brief spikes of voltage, irrespective of time.

Peak and peak-to-peak measurements are best performed with an oscilloscope, which can capture the crests of the waveform with a high degree of accuracy due to the fast action of the cathode-ray-tube in response to changes in voltage. For RMS measurements, analog meter movements (D'Arsonval, Weston, iron vane, electrodynamometer) will work so long as they have been calibrated in RMS figures. Because the mechanical inertia and dampening effects of an electromechanical meter movement makes the deflection of the needle naturally proportional to the average value of the AC, not the true RMS value, analog meters must be specifically calibrated (or mis-calibrated, depending on how you look at it) to indicate voltage or current in RMS units. The accuracy of this calibration depends on an assumed waveshape, usually a sine wave.

Electronic meters specifically designed for RMS measurement are best for the task. Some instrument manufacturers have designed ingenious methods for determining the RMS value of any waveform. One such manufacturer produces "True-RMS" meters with a tiny resistive heating element powered by a voltage proportional to that being measured. The heating effect of that resistance element is measured thermally to give a true RMS value with no mathematical calculations whatsoever, just the laws of physics in action in fulfillment of the definition of RMS. The accuracy of this type of RMS measurement is independent of waveshape.

For "pure" waveforms, simple conversion coefficients exist for equating Peak, Peak-to-Peak, Average (practical, not algebraic), and RMS measurements to one another: Figure below

Conversion factors for common waveforms.

In addition to RMS, average, peak (crest), and peak-to-peak measures of an AC waveform, there are ratios expressing the proportionality between some of these fundamental measurements. The crest factor of an AC waveform, for instance, is the ratio of its peak (crest) value divided by its RMS value. The form factor of an AC waveform is the ratio of its RMS value divided by its average value. Square-shaped waveforms always have crest and form factors equal to 1, since the peak is the same as the RMS and average values. Sinusoidal waveforms have an RMS value of 0.707 (the reciprocal of the square root of 2) and a form factor of 1.11 (0.707/0.636). Triangle- and sawtooth-shaped waveforms have RMS values of 0.577 (the reciprocal of square root of 3) and form factors of 1.15 (0.577/0.5).

Bear in mind that the conversion constants shown here for peak, RMS, and average amplitudes of sine waves, square waves, and triangle waves hold true only for pure forms of these waveshapes. The RMS and average values of distorted waveshapes are not related by the same ratios: Figure below

Arbitrary waveforms have no simple conversions.

This is a very important concept to understand when using an analog meter movement to measure AC voltage or current. An analog movement, calibrated to indicate sine-wave RMS amplitude, will only be accurate when measuring pure sine waves. If the waveform of the voltage or current being measured is anything but a pure sine wave, the indication given by the meter will not be the true RMS value of the waveform, because the degree of needle deflection in an analog meter movement is proportional to the average value of the waveform, not the RMS. RMS meter calibration is obtained by "skewing" the span of the meter so that it displays a small multiple of the average value, which will be equal to be the RMS value for a particular waveshape and a particular waveshape only.

Since the sine-wave shape is most common in electrical measurements, it is the waveshape assumed for analog meter calibration, and the small multiple used in the calibration of the meter is 1.1107 (the form factor: 0.707/0.636: the ratio of RMS divided by average for a sinusoidal waveform). Any waveshape other than a pure sine wave will have a different ratio of RMS and average values, and thus a meter calibrated for sine-wave voltage or current will not indicate true RMS when reading a non-sinusoidal wave. Bear in mind that this limitation applies only to simple, analog AC meters not employing "True-RMS" technology.

REVIEW:

• The amplitude of an AC waveform is its height as depicted on a graph over time. An amplitude measurement can take the form of peak, peak-to-peak, average, or RMS quantity.
• Peak amplitude is the height of an AC waveform as measured from the zero mark to the highest positive or lowest negative point on a graph. Also known as the crest amplitude of a wave.
• Peak-to-peak amplitude is the total height of an AC waveform as measured from maximum positive to maximum negative peaks on a graph. Often abbreviated as "P-P".
• Average amplitude is the mathematical "mean" of all a waveform's points over the period of one cycle. Technically, the average amplitude of any waveform with equal-area portions above and below the "zero" line on a graph is zero. However, as a practical measure of amplitude, a waveform's average value is often calculated as the mathematical mean of all the points' absolute values (taking all the negative values and considering them as positive). For a sine wave, the average value so calculated is approximately 0.637 of its peak value.
• "RMS" stands for Root Mean Square, and is a way of expressing an AC quantity of voltage or current in terms functionally equivalent to DC. For example, 10 volts AC RMS is the amount of voltage that would produce the same amount of heat dissipation across a resistor of given value as a 10 volt DC power supply. Also known as the "equivalent" or "DC equivalent" value of an AC voltage or current. For a sine wave, the RMS value is approximately 0.707 of its peak value.
• The crest factor of an AC waveform is the ratio of its peak (crest) to its RMS value.
• The form factor of an AC waveform is the ratio of its RMS value to its average value.
• Analog, electromechanical meter movements respond proportionally to the average value of an AC voltage or current. When RMS indication is desired, the meter's calibration must be "skewed" accordingly. This means that the accuracy of an electromechanical meter's RMS indication is dependent on the purity of the waveform: whether it is the exact same waveshape as the waveform used in calibrating.

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Using an electrical meter safely and efficiently is perhaps the most valuable skill an electronics technician can master, both for the sake of their own personal safety and for proficiency at their trade. It can be daunting at first to use a meter, knowing that you are connecting it to live circuits which may harbor life-threatening levels of voltage and current. This concern is not unfounded, and it is always best to proceed cautiously when using meters. Carelessness more than any other factor is what causes experienced technicians to have electrical accidents.

The most common piece of electrical test equipment is a meter called the multimeter. Multimeters are so named because they have the ability to measure a multiple of variables: voltage, current, resistance, and often many others, some of which cannot be explained here due to their complexity. In the hands of a trained technician, the multimeter is both an efficient work tool and a safety device. In the hands of someone ignorant and/or careless, however, the multimeter may become a source of danger when connected to a "live" circuit.

There are many different brands of multimeters, with multiple models made by each manufacturer sporting different sets of features. The multimeter shown here in the following illustrations is a "generic" design, not specific to any manufacturer, but general enough to teach the basic principles of use:

You will notice that the display of this meter is of the "digital" type: showing numerical values using four digits in a manner similar to a digital clock. The rotary selector switch (now set in the Off position) has five different measurement positions it can be set in: two "V" settings, two "A" settings, and one setting in the middle with a funny-looking "horseshoe" symbol on it representing "resistance." The "horseshoe" symbol is the Greek letter "Omega" (Ω), which is the common symbol for the electrical unit of ohms.

Of the two "V" settings and two "A" settings, you will notice that each pair is divided into unique markers with either a pair of horizontal lines (one solid, one dashed), or a dashed line with a squiggly curve over it. The parallel lines represent "DC" while the squiggly curve represents "AC." The "V" of course stands for "voltage" while the "A" stands for "amperage" (current). The meter uses different techniques, internally, to measure DC than it uses to measure AC, and so it requires the user to select which type of voltage (V) or current (A) is to be measured. Although we haven't discussed alternating current (AC) in any technical detail, this distinction in meter settings is an important one to bear in mind.

There are three different sockets on the multimeter face into which we can plug our test leads. Test leads are nothing more than specially-prepared wires used to connect the meter to the circuit under test. The wires are coated in a color-coded (either black or red) flexible insulation to prevent the user's hands from contacting the bare conductors, and the tips of the probes are sharp, stiff pieces of wire:

The black test lead always plugs into the black socket on the multimeter: the one marked "COM" for "common." The red test lead plugs into either the red socket marked for voltage and resistance, or the red socket marked for current, depending on which quantity you intend to measure with the multimeter.

To see how this works, let's look at a couple of examples showing the meter in use. First, we'll set up the meter to measure DC voltage from a battery:

Note that the two test leads are plugged into the appropriate sockets on the meter for voltage, and the selector switch has been set for DC "V". Now, we'll take a look at an example of using the multimeter to measure AC voltage from a household electrical power receptacle (wall socket):

The only difference in the setup of the meter is the placement of the selector switch: it is now turned to AC "V". Since we're still measuring voltage, the test leads will remain plugged in the same sockets. In both of these examples, it is imperative that you not let the probe tips come in contact with one another while they are both in contact with their respective points on the circuit. If this happens, a short-circuit will be formed, creating a spark and perhaps even a ball of flame if the voltage source is capable of supplying enough current! The following image illustrates the potential for hazard:

This is just one of the ways that a meter can become a source of hazard if used improperly.

Voltage measurement is perhaps the most common function a multimeter is used for. It is certainly the primary measurement taken for safety purposes (part of the lock-out/tag-out procedure), and it should be well understood by the operator of the meter. Being that voltage is always relative between two points, the meter must be firmly connected to two points in a circuit before it will provide a reliable measurement. That usually means both probes must be grasped by the user's hands and held against the proper contact points of a voltage source or circuit while measuring.

Because a hand-to-hand shock current path is the most dangerous, holding the meter probes on two points in a high-voltage circuit in this manner is always a potential hazard. If the protective insulation on the probes is worn or cracked, it is possible for the user's fingers to come into contact with the probe conductors during the time of test, causing a bad shock to occur. If it is possible to use only one hand to grasp the probes, that is a safer option. Sometimes it is possible to "latch" one probe tip onto the circuit test point so that it can be let go of and the other probe set in place, using only one hand. Special probe tip accessories such as spring clips can be attached to help facilitate this.

Remember that meter test leads are part of the whole equipment package, and that they should be treated with the same care and respect that the meter itself is. If you need a special accessory for your test leads, such as a spring clip or other special probe tip, consult the product catalog of the meter manufacturer or other test equipment manufacturer. Do not try to be creative and make your own test probes, as you may end up placing yourself in danger the next time you use them on a live circuit.

Also, it must be remembered that digital multimeters usually do a good job of discriminating between AC and DC measurements, as they are set for one or the other when checking for voltage or current. As we have seen earlier, both AC and DC voltages and currents can be deadly, so when using a multimeter as a safety check device you should always check for the presence of both AC and DC, even if you're not expecting to find both! Also, when checking for the presence of hazardous voltage, you should be sure to check all pairs of points in question.

For example, suppose that you opened up an electrical wiring cabinet to find three large conductors supplying AC power to a load. The circuit breaker feeding these wires (supposedly) has been shut off, locked, and tagged. You double-checked the absence of power by pressing the Start button for the load. Nothing happened, so now you move on to the third phase of your safety check: the meter test for voltage.

First, you check your meter on a known source of voltage to see that it's working properly. Any nearby power receptacle should provide a convenient source of AC voltage for a test. You do so and find that the meter indicates as it should. Next, you need to check for voltage among these three wires in the cabinet. But voltage is measured between two points, so where do you check?

The answer is to check between all combinations of those three points. As you can see, the points are labeled "A", "B", and "C" in the illustration, so you would need to take your multimeter (set in the voltmeter mode) and check between points A & B, B & C, and A & C. If you find voltage between any of those pairs, the circuit is not in a Zero Energy State. But wait! Remember that a multimeter will not register DC voltage when it's in the AC voltage mode and vice versa, so you need to check those three pairs of points in each mode for a total of six voltage checks in order to be complete!

However, even with all that checking, we still haven't covered all possibilities yet. Remember that hazardous voltage can appear between a single wire and ground (in this case, the metal frame of the cabinet would be a good ground reference point) in a power system. So, to be perfectly safe, we not only have to check between A & B, B & C, and A & C (in both AC and DC modes), but we also have to check between A & ground, B & ground, and C & ground (in both AC and DC modes)! This makes for a grand total of twelve voltage checks for this seemingly simple scenario of only three wires. Then, of course, after we've completed all these checks, we need to take our multimeter and re-test it against a known source of voltage such as a power receptacle to ensure that it's still in good working order.

Using a multimeter to check for resistance is a much simpler task. The test leads will be kept plugged in the same sockets as for the voltage checks, but the selector switch will need to be turned until it points to the "horseshoe" resistance symbol. Touching the probes across the device whose resistance is to be measured, the meter should properly display the resistance in ohms:

One very important thing to remember about measuring resistance is that it must only be done on de-energized components! When the meter is in "resistance" mode, it uses a small internal battery to generate a tiny current through the component to be measured. By sensing how difficult it is to move this current through the component, the resistance of that component can be determined and displayed. If there is any additional source of voltage in the meter-lead-component-lead-meter loop to either aid or oppose the resistance-measuring current produced by the meter, faulty readings will result. In a worse-case situation, the meter may even be damaged by the external voltage.

The "resistance" mode of a multimeter is very useful in determining wire continuity as well as making precise measurements of resistance. When there is a good, solid connection between the probe tips (simulated by touching them together), the meter shows almost zero Ω. If the test leads had no resistance in them, it would read exactly zero:

If the leads are not in contact with each other, or touching opposite ends of a broken wire, the meter will indicate infinite resistance (usually by displaying dashed lines or the abbreviation "O.L." which stands for "open loop"):

By far the most hazardous and complex application of the multimeter is in the measurement of current. The reason for this is quite simple: in order for the meter to measure current, the current to be measured must be forced to go through the meter. This means that the meter must be made part of the current path of the circuit rather than just be connected off to the side somewhere as is the case when measuring voltage. In order to make the meter part of the current path of the circuit, the original circuit must be "broken" and the meter connected across the two points of the open break. To set the meter up for this, the selector switch must point to either AC or DC "A" and the red test lead must be plugged in the red socket marked "A". The following illustration shows a meter all ready to measure current and a circuit to be tested:

Now, the circuit is broken in preparation for the meter to be connected:

The next step is to insert the meter in-line with the circuit by connecting the two probe tips to the broken ends of the circuit, the black probe to the negative (-) terminal of the 9-volt battery and the red probe to the loose wire end leading to the lamp:

This example shows a very safe circuit to work with. 9 volts hardly constitutes a shock hazard, and so there is little to fear in breaking this circuit open (bare handed, no less!) and connecting the meter in-line with the flow of electrons. However, with higher power circuits, this could be a hazardous endeavor indeed. Even if the circuit voltage was low, the normal current could be high enough that an injurious spark would result the moment the last meter probe connection was established.

Another potential hazard of using a multimeter in its current-measuring ("ammeter") mode is failure to properly put it back into a voltage-measuring configuration before measuring voltage with it. The reasons for this are specific to ammeter design and operation. When measuring circuit current by placing the meter directly in the path of current, it is best to have the meter offer little or no resistance against the flow of electrons. Otherwise, any additional resistance offered by the meter would impede the electron flow and alter the circuit's operation. Thus, the multimeter is designed to have practically zero ohms of resistance between the test probe tips when the red probe has been plugged into the red "A" (current-measuring) socket. In the voltage-measuring mode (red lead plugged into the red "V" socket), there are many mega-ohms of resistance between the test probe tips, because voltmeters are designed to have close to infinite resistance (so that they don't draw any appreciable current from the circuit under test).

When switching a multimeter from current- to voltage-measuring mode, it's easy to spin the selector switch from the "A" to the "V" position and forget to correspondingly switch the position of the red test lead plug from "A" to "V". The result -- if the meter is then connected across a source of substantial voltage -- will be a short-circuit through the meter!

To help prevent this, most multimeters have a warning feature by which they beep if ever there's a lead plugged in the "A" socket and the selector switch is set to "V". As convenient as features like these are, though, they are still no substitute for clear thinking and caution when using a multimeter.

All good-quality multimeters contain fuses inside that are engineered to "blow" in the event of excessive current through them, such as in the case illustrated in the last image. Like all overcurrent protection devices, these fuses are primarily designed to protect the equipment (in this case, the meter itself) from excessive damage, and only secondarily to protect the user from harm. A multimeter can be used to check its own current fuse by setting the selector switch to the resistance position and creating a connection between the two red sockets like this:

A good fuse will indicate very little resistance while a blown fuse will always show "O.L." (or whatever indication that model of multimeter uses to indicate no continuity). The actual number of ohms displayed for a good fuse is of little consequence, so long as it's an arbitrarily low figure.

So now that we've seen how to use a multimeter to measure voltage, resistance, and current, what more is there to know? Plenty! The value and capabilities of this versatile test instrument will become more evident as you gain skill and familiarity using it. There is no substitute for regular practice with complex instruments such as these, so feel free to experiment on safe, battery-powered circuits.

REVIEW:

• A meter capable of checking for voltage, current, and resistance is called a multimeter,
• As voltage is always relative between two points, a voltage-measuring meter ("voltmeter") must be connected to two points in a circuit in order to obtain a good reading. Be careful not to touch the bare probe tips together while measuring voltage, as this will create a short-circuit!
• Remember to always check for both AC and DC voltage when using a multimeter to check for the presence of hazardous voltage on a circuit. Make sure you check for voltage between all pair-combinations of conductors, including between the individual conductors and ground!
• When in the voltage-measuring ("voltmeter") mode, multimeters have very high resistance between their leads.
• Never try to read resistance or continuity with a multimeter on a circuit that is energized. At best, the resistance readings you obtain from the meter will be inaccurate, and at worst the meter may be damaged and you may be injured.
• Current measuring meters ("ammeters") are always connected in a circuit so the electrons have to flow through the meter.
• When in the current-measuring ("ammeter") mode, multimeters have practically no resistance between their leads. This is intended to allow electrons to flow through the meter with the least possible difficulty. If this were not the case, the meter would add extra resistance in the circuit, thereby affecting the current.

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