What advantages does ac electrical power have over dc electrical power

[akafrank07]

Reading through my notes one of the advantages is as follows;

"Ac generators do not suffer from commutation problems associated with dc machines and consequently are more reliable, especially at high altitudes"

Could someone explain commutation? and why Ac generators are more reliable than dc machines at high altitude?

Thanks

[Bengo]

Normally, in a DC generator the stator (outside) produces the magnetic field and the output is collected from lots of coils in the rotor as it rotates.

That means that brushes are needed to collect the current and that the current in each rotor coil reverses direction twice each rotation because the magnetic field is fixed and at any point the rotor coil is one way on to the field and 180 deg later its the other way round. The brushes run on a series of copper segments arranged in a circle (the commutator) with the opposite ends of each coil connected to segments opposite each other so that as the coil current reverses so do the electrical connections and DC out is the result. That reversal is called commutation The brushes thus handle the full output current of the machine and sparks occur at each commutator segment leading to wear. Brush sparking is also a problem at high altitude, but I can't remember why.


In an AC machine the magnetic field is in the rotating part and the output current is collected from the stationary coils. The current into the rotating bit to make the magnetic field is quite small compared to the output and there is no need to reverse its direction. This makes life for the brushes much easier.

The AC generating machine can also be more mechanically robust so it can go round faster than the DC machine.

[Capetonian]

From a consumer perspective, I recall from my schoolboy physics that AC suffers from lower transmission losses over distance, and is safer as if your body shorts a DC source, you can go into spasm and hold on to it, whereas AC will repel you.

Also there was something to do with asynchronous devices used for timing not working properly on DC, as they take their timing from the number of cycles per second/Hertz, typically 50 or 60.

As I say, schoolboy physics, I'm sure someone with greater knowledge will correct the above if wrong.

[cockney steve]

Just to expand on BENGO's excellent reply.

The usual configuration for a DC (dynamo) armature commutator, is a cylinder divided into strips...each has to be insulated from the others ( imagine a straight-sided beer barrel where the staves have a strip of insulation between them)

The brushes are spring-loaded carbon -copper blocks with bonded-in leads(AKA 'tails') Because of the size of the windings and the required output, dynamic balance, cooling and structural integrity against centrifugal (centripetal?) force are major issues. brush-size and pressure against the rotating drum of segments is also an issue..insulated holders are needed to ensure accurate alignment between brushes and segments as the windings pass the pole-shoes, too much pressure causes drag and premature-wear in the Com. segments...too little and there's excessive sparking (with attendant R.F. interference) burning and erosion of Com. and brushes.
The Alternator rotor only has a single coil wound on itin the same rotational axis. balance and dynamic stresses are minimal,,the two ends of the coil can terminate in slip-rings which, as each one has a brush pressing on a smooth, continuous track,carrying only "field" current, are lighter and far longer-wearing than the brush-system on dynamos.
The fixed coils in the stator, being external, are much easier to construct and cool, no balancing required ,minimal mechanical anchoring. the outputs are normally diode-rectified...these solid-state devices are maintenance-free and have very high efficiency.

I struggle to find anything good to say about a dynamo when compared to an alternator. They are heavier and need a higher rotational speed to start generating, compared with an Alternator.they're mechanically more complex have more to wear and are less reliable....and, yes, AC transmission-losses are lower and the induction of currents in ajacent wires is less of a problem.
all sorts of odd things can happen when you get bundles of uncsreened conductors together!,

[akafrank07]

Thanks for the detailed explanations Bengo and Cockney Steve much appreciated, still some of it is going over my head though your have given me more than enough info i need, cheers :ok:

[FE Hoppy]

no brushes.

[barit1]

Capetonian:From a consumer perspective, I recall from my schoolboy physics that AC suffers from lower transmission losses over distance...

True for long-distance transmission, in which the power lines are high-voltage, low-current conductors. The enroute power losses are defined by P = I(squared) x R, so the lower current is a distinct advantage. At/near the customer's site, a transformer (or series of them) reduces the voltage to a practical (and safer) level, simultaneously making more current available.

Onboard a vehicle, with much sorter distance between source and load, this AC vs DC issue (low vs high voltage) is hardly worth the argument. Other factors (energy storage via battery, e.g.) are much more influential.

[Chu Chu]

Aside from ease of generation, a major advantage of AC is that it permits the use of transformers to change the voltage. A transformer won't work on DC.

This is the reason why AC can normally be transmitted with lower losses. AC can be run through a transformer and converted to a very high voltage for transmission, then converted back to a usable voltage near the load. Transmission at higher voltage means transmission at lower current; lower current means less loss to resistance.

At a given voltage, however, AC has greater transmission loss than DC. I think this has something to do with capacitance, and perhaps some other factors. Some long distance transmission lines are now DC, to minimize losses. But this requires AC to be stepped up to the transmission voltage and converted to DC, then converted back to AC and stepped down at the end of the run. This creates a need for expensive equipment and causes its own losses, so it only makes sense for long cross-country or underwater transmission lines.

None of this has anything to do with aircraft. But I have read that aircraft AC systems are 400 HZ rather than 50 or 60 because the higher frequency makes transformers more efficient. More efficient transformers can be built smaller and lighter without causing excessive losses.

[barit1]

But I have read that aircraft AC systems are 400 HZ rather than 50 or 60 because the higher frequency makes transformers more efficient. More efficient transformers can be built smaller and lighter without causing excessive losses.

Exactly right. To be more precise, the higher the AC frequency, the less iron is needed in the transformer core. Some aircraft systems have been 800 hz for this reason.

[Stuck_in_an_ATR]

With all the advantages of the AC power, why some aircraft (mostly turboprops, like eg. ATR or the Q400) use 28V DC as their primary power source?:confused:

[Tinstaafl]

Because it can be used directly from the battery without having to convert anything, I should think.

[EEngr]

Stuck_in_an_ATR (http://www.pprune.org/members/127367-stuck_in_an_atr):

With all the advantages of the AC power, why some aircraft (mostly turboprops, like eg. ATR or the Q400) use 28V DC as their primary power source?Some systems are going to require 28 Vdc so that they may also be powered by a battery (usually at 24 Vdc) as a backup. For smaller aircraft, the power requirements are low enough to be adequately supplied by a lower voltage DC system. The weight penalty of generating 115 Vac and transforming in down to 28 Vdc is not warranted.
:8

[EEngr]

Chu Chu (http://www.pprune.org/members/292170-chu-chu):

None of this has anything to do with aircraft. But I have read that aircraft AC systems are 400 HZ rather than 50 or 60 because the higher frequency makes transformers more efficient. More efficient transformers can be built smaller and lighter without causing excessive losses. This is true of all 'magnetic' devices such as motors. Also in AC to DC conversion. Ripple filtering capacitors can be much smaller at higher frequencies.

Well, sort of true. The part about lighter weight anyway. Voltage regulation is worse at higher frequencies. So are magnetic core losses. But the weight factor wins out on aircraft. Voltage regulation is a factor for longer circuit lengths. So you will see its use on long distance transmission lines, where the cost of regulation and losses offsets the higher cost of AC/DC and DC/AC conversion. These conversion costs are coming down rapidly with the advent of higher power, more reliable and cheaper solid state conversion*. You will begin to see more utility DC transmission and distribution as transformers are replaced by solid state conversion.

*Switching power supplies are an example. Old style transformer wall warts have been replaced my much lighter, smaller and more efficient switching power supplies.

[onetrack]

The "War of Currents" makes for interesting reading. Despite the story being based around domestic and industrial power supply, rather than aviation power supplies, there are many points to be considered in the supply of each type of power.

Edison refused point blank to even consider AC power, believing DC was the way to go. However, the greater efficiency of AC power won out in domestic/industrial use - and wins out in most other applications, too.

The use of modern electronics in switching and controlling AC power has advanced AC powers advantage over DC in many applications outside domestic/industrial use.

War of Currents - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/War_of_Currents)

[archae86]

While easy to remember, the basic dictum that DC is bad for long distance is false. The primary issue was that at low levels of the distribution system AC could use transformers to match the voltage to requirements neatly--low, cheap to insulate, and ( almost) safe in the home, higher and more efficient for the street level, and so on.

It was NEVER true that DC was inefficient for transmission at the same voltage--what was true was that it was vastly more expensive to do voltage conversion for DC. Modern power electronics (and, no, this is not Moore's law in action) have lowered conversion costs (both from one DC voltage to another, and AC to DC and reverse), and DC is becoming more common in serious long-distance applications, with one pioneering use in the Pacific Intertie being over four decades old now. One incentive is to avoid problems of system synchronization, but an emerging one is lower (not higher) transmission losses (by the way, they are not all I-squared R losses--coronal discharge gets rapidly more significant as you scale up in serious long-distance power transmission voltages).

As to safety--at the same voltage household DC is much safer to humans than AC. As to voltages you are actually likely to encounter, the 120V AC in US home wiring can kill pretty easily but usually does not, whereas the 48 DC on which most of the US telephone system used to run was really hard to kill yourself with. When I was a callow youth working at Bell Labs as a summer co-op student about 1970, a grizzled technician taught me the lore that while it was possible to kill yourself with 48DC, it was really difficult, and vanishingly few people ever had. I'm afraid I don't know the relative safety at power transmission type voltages. I'd hazard a guess you are dead either way.

[grounded27]

The major advantage on large aircraft is weight savings in wire, this is why the TR's are in the E-bay.

[Denti]

Isn't there quite a large 270VDC system on the 787? Read somewhere that airbus was going down that route as well for some systems in upcoming airplanes.

Mid to long range power transfer via DC lines seems to be more and more en vogue nowadays, there is currently quite some discussion about it over here to transport all that wind energy from the north of the country to the south, windy north and industrial south.

Also big computer farms are often run on 12VDC as that saves all those TRs in each computer/server, the need DC anyway. Seems to save a lot of energy.

[Turbavykas]

Don't forget that AC motor 3 phase doesn't have any turning electrical parts. DC motor will need high current to be run through brushes. All modern electric cars use AC motors though power source is batteries.

[barit1]

archae86:It was NEVER true that DC was inefficient for transmission at the same voltage--what was true was that it was vastly more expensive to do voltage conversion for DC. Modern power electronics (and, no, this is not Moore's law in action) have lowered conversion costs (both from one DC voltage to another, and AC to DC and reverse), and DC is becoming more common in serious long-distance applications...

Very true, something I did not address in my earlier post. The first I heard of DC transmission was a bi-directional link between the UK and the continent, 30-40 years ago. Did this ever come to pass? The incentive claimed was peak load stagger, when one region had excess capacity, the other had excess demand.

[EEngr]

barit1 (http://www.pprune.org/members/107138-barit1):

Here is some info on DC transmission links.

List of HVDC projects - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/List_of_HVDC_projects)

Looks like the UK-France systems have been around for a while. Now there's a second link from the UK to The Netherlands.

[Uplinker]

Some good answers.

Just to reinforce what has mostly been said: AC generators are more efficient, more controllable, and capable of higher output currents without the disadvantages of the high current switching one gets with a DC generator, and the consequent sparking which could be hazardous. AC is easy to convert to different voltages with simple transformers. High power devices such as heated windscreens, fans, electric hydraulic pumps and prop blade heaters can use 'raw' AC directly.

Most avionics and electronic devices however, use DC voltages between 5 and 28 volts in their circuits. If supplied with AC, they therefore need to convert this to a lower DC voltage first, which would need a transformer and smoothing power supply circuitry in each device. Much easier to supply a suitable DC voltage to such devices. Large power convertors called Transformer Rectifier Units, TRUs, are used to produce 28VDC from the 115VAC.

28V is the same as an aircraft battery voltage, so if the TRU derived 28 volts fails, you still have 28V from the batteries to keep the essential systems alive to get you back on the ground. Also, some devices such as radios and navigation lights will work from batteries when on the ground with no engines running.

This is why most aircraft have both AC and DC distribution systems.

[Lancman]

When aircraft started flying regularly at higher levels there were problems with heavy d.c. loads continuing to run after they were switched off; this was traced to arcs developing across the contacts of relays mounted in unpressurised areas. Using a.c. in which the voltage fell through zero many times a second alleviated this.

[CLDriver]

Great thread going here, too bad I got onto to it a bit late.

Could one of you all explain how you can have two voltages from one generator, such as the 115/200 we find in many transport category aircraft?

I've never gotten a really good explanation that I can understand how it works, and how you can have both 115 and 200 VAC being run down the same bus with different implement drawing either 115 or 200 depending on their needs.

Thanks in advance!

[Chu Chu]

I'm pretty sure that refers to 3-phase power, where there's 200 volts between any two of the three legs and 115 volts between any leg and the neutral. The numbers don't add up (i.e. it's not 115/230 volts) because the legs are 120 degrees out of phase from each other, but each is in effect 180 degrees out of phase with the neutral.

That's the best I can explain it; I'm sure someone out there can do better.

[TURIN]

Line voltage is root 3 of phase voltage.
IE. 115v x (approx) 1.72=200v.
Depends if the generator is star or delta wound. One gives equal line and phase voltage the other gives equal line and phase current.

I think?

[Uplinker]

A 3 phase alternator has three coils arranged in a circle which produce voltages that are 120 degrees from each other.

The output of each coil is a sine wave - varying from zero volts to (in this case) 115 volts, then back to zero, then minus 115 volts then zero again.

Plot a graph with voltage on the vertical axis and degrees of rotation along the horizontal axis - I don't know how to draw that on here - the first coil's output begins at zero degrees and reaches 115V at 90 degrees then zero again at 180 degrees and so on. The second coil's output starts at 120 degrees along the horizontal axis and the third coil's at 240 degrees along.

Anyway, if you plot all this, you will see that the max voltage on any single coil is 115 V and the voltage between two coil's outputs can reach 200V. This is because when one coil is at 115V, another coil will be at -85V, so the difference is 200V

I hope you followed that! A diagram would be much easier to understand.

You cannot send different voltages down the same wire - even if you tried the result would always only be one voltage, but electrical devices will draw different currents from the same wire according to their needs.

[Lightning Mate]

Someone wanted a diagram?

I used to use this for teaching:

http://i636.photobucket.com/albums/uu82/Lightning_29/starconnectionConverted_zpse0c7c7ed.jpg

[Lightning Mate]

....and here's a delta connection:

http://i636.photobucket.com/albums/uu82/Lightning_29/deltaconnectionConverted_zps3033b530.jpg

[Speed of Sound]

Plot a graph with voltage on the vertical axis and degrees of rotation along the horizontal axis

http://www.3phasepower.org/images/3PhasePowerWaveF.png

[Uplinker]

Ah thanks guys - my limited IT ability prevented me doing that!

Hopefully this all now makes sense to CLDriver?

So the output of the alternator will be carried on 4 wires*; One for each coil (known as a 'phase'), and one connected to the neutral point which serves all three phases.

A piece of equipment needing low power 115V will connect between one phase and the neutral point. A piece of equipment needing high power will use all three 115V phases, each returning to neutral - feeding either three separate heating elements in a windshield or prop de-ice boot, or three separate coils in a hydraulic pump motor for example.

A piece of equipment needing 200V would connect between two of the phases and not neutral.

(*A delta system as shown in Lightning's second diagram above is configured differently and uses only 3 wires, but the principle is similar).

[Lightning Mate]

The delta system really isn't used because of the flexibility of the star system.

[flyboyike]

Holy left winglet, I'm way too dumb for most of this.

[CLDriver]

Wow, this a completely awesome and very understandable way of explaining it. Thanks everyone!

We have 115/200 vac 400hz on the Challenger and everything on the AC buses run on 115, except, the windshield heats which are 200.

A piece of equipment needing low power 115V will connect between one phase and the neutral point.

A piece of equipment needing 200V would connect between two of the phases and not neutral.

So if I understand correctly, even though the only thing on the Main Buses that operate on 200 is the Windshield Heat, everything else being 115, it is how the equipment running on the Main Bus connects to the phases depending on what it needs.


I scanned a pic of the windshield heat electrical diagram, but it wouldn't post, I hope I described it well enough.

This was always a fuzzy subject and over the years I try and take those areas and learn more about them. Thanks again all who have contributed!

[Uplinker]

I am flying blind here in the absence of a circuit diagram for your particular situation, but as I say if a piece of equipment needs 200V, it will probably be connected across two of the three phases, instead of one phase and neutral.



For those who might not understand voltage and current:

Think of a hosepipe connected to the outside tap, with a trigger operated sprayer on the end. When you turn on the outside tap, the water pressurizes the hose, but no water flows because the sprayer end has not been switched on. The water pressure inside the hose is like the voltage in a cable. There is energy there, but nothing happens until you open the sprayer end (or draw current). When you pull the trigger on the sprayer end, water flows out. The flow of water is analogous to the current flowing through a wire. You could open the sprayer a tiny bit and a dribble of water would flow, or you could open it right up and maximum water would flow.

This is like different equipment drawing different current - a power hungry item such as a heater or a motor would draw a lot of current, but a light bulb would draw a tiny amount of current. With electrics, power is voltage times current = watts. So a piece of equipment using 10 amps from a 115V supply is using 1,115 watts, or 1.115KW of power. (To be precise; with an AC voltage it would be 1.115KVA, but don't worry about this distinction)

You could then imagine the hose having 'T' joints along it's length and different sized holes at each 'T' junction for the water to pour out. The pressure(voltage) in the hose is the same all along and at each junction, but a different amount of water flow(current) is drawn at each 'T' junction depending on the power requirement of the item.

[jimjim1]

the first coil's output begins at zero degrees and reaches 115V at 90 degrees

Thanks for the excellent posts everybody.

There is one small point that I have not noticed mentioned so far.

I now cannot recall the details (but there for sure will be someone along in a minute who does) but the peak voltage is not 115.

The voltages quoted are as I recall so called Root Mean Square (RMS) values. I think that they are used because they make V=IR work for sinusoidal AC in exactly the same manner that it does for DC.

In the UK our single phase voltage (household) is 240 volts (ish - it may have been reduced a bit) and the three phase voltage (available for business use on request - never used domestically) is 440V. Both RMS. The peaks are much higher. (On recall, from decades ago, it might be square root of 2 times higher [1.41])

[Chu Chu]

That's a good point. It isn't really necessary to explaining three-phase power, which is complicated enough already. But it probably is relevant to the earlier safety discussion. All other things (e.g. load and current) being the same, an AC system will involve voltage that spikes higher than a DC system, probably adding to the arc and shock hazard.

To add one more thing about three power: It isn't really about getting two voltages from a single system. You can do that with a "single" phase system that uses two hot conductors 180 degrees out of phase and a neutral at ground potential. The 120/240 residential power common in the United States works that way.

Where three-phase really earns its keep is in powering induction motors. Three coils spaced around the motor frame create a rotating magnetic field that starts the motor without the need for capacitors or other efficiency-reducing tricks. There may be advantages on the generation side as well, but I'm not as sure about that.

[EEngr]

The voltages quoted are as I recall so called Root Mean Square (RMS) values. I think that they are used because they make V=IR work for sinusoidal AC in exactly the same manner that it does for DC.That's basically the essence of it. The peak voltage of a sinusoidal source with RMS voltage of V is sqrt(2) * V. So for a 115 Vac RMS vooltage, the peak is 162.6 volts.

In the UK our single phase voltage (household) is 240 volts (ish - it may have been reduced a bit) and the three phase voltage (available for business use on request - never used domestically) is 440V.Close. The relationship between the line to neutral voltage or (phase voltage) and the 3 phase line to line voltage (line voltage) is:

V L-L = sqrt(3) * V L-N

V L-L = sqrt(3) * 240 = 416

Three phase systems are typically identified by their line to line voltage.

Confusing enough for you?
:8

[Lightning Mate]

The other big advantage with AC, of course, is that any ac voltage may be obtained via a transformer, and any DC voltage via a transformer/rectifier unit (TRU).

[Uplinker]

Yep, as already stated: check out post #21 :ok:


As for root mean square, RMS; yes, all true, but don't worry about it too much because it is not required to improve your understanding of the basic 3 phase system. Designers need to know because they need to know the highest voltages in the system, in order to correctly specify wire insulation, and they need to be able to compute the power and heating effects in the cables too.

[TURIN]

RMS value of AC is equal to the power dissipated by the equivalent value of DC.

[Chu Chu]

VK2TVK,

Thanks for the compliment. I do have to say, though, that I disagree on altitude. Air is a pretty good insulator most of the time. Ionized air is a pretty good conductor, though, and air gets ionized when electricity flows through it.

The typical problem with a switch is that when the contacts open, there's always an instant when they're still very close together. An arc will jump that gap, starting ionization, then grow as the contacts spread further apart and the ionization process continues.

Simple switches are spring loaded, so the contacts will separate faster than ionized air can fill the gap and maintain an arc. Some high-voltage switches actually keep the contacts in a vacuum to avoid ionized gas (though the contacts themselves can also create enough ionized particles to sustain an arc in some circumstances).

Other high-voltage switches use a blast of air to physically blow out the arc. These might not work well at altitude, but I don't think they're used for typical aircraft voltages anyway.

Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.

*** Should have googled before posting. Apparently electrons that bump into too many gas molecules get slowed down and aren't effective at ionizing. So up to a point -- somewhere around 150,000 feet -- less air means more ionization and more arcing.

[LASJayhawk]

Keep in mind when avionics started, we Had to have AC to make the avionics work. Synchros and resolvers need AC in most applications. These are the gyzmos that drove the compass cards pointers and such in the steam gauges. And the tube type radios were able to me made much lighter with an AC power supply that could be easily stepped up and down in voltage.


Some aircraft like the newer King Airs with the proline 21 avionics are now DC only, getting rid of the weight associated with the inverters.

[james ozzie]

To refer back to the earlier posts on poly-phase systems: a major advantage of these is that for a balanced system, there is no neutral return current. For example: 3 single phase AC circuits would need 6 conductors; the same result can be achieved with 3 conductors if connected 3 phase. In practice, the 3 phases would not balance perfectly so there would be a residual neutral current but it can be of reduced section (it is also possible to use a 3 wire system with no neutral). But wait there's more...because there is no neutral current, the total volt drop across the circuit is halved for each phase, allowing further economies in conductor size.

This great idea was invented by Tesla in the late 1880s and works so well that it has not changed in principle since. And I think these systems still will be around in another 100 or 200 years time.

[garrya100]

And we haven't even started with 3rd harmonic currents, high frequency iron core saturation, power factor and switchmode psu waveform distortion yet! :ok:

[EEngr]

Chu Chu (http://www.pprune.org/members/292170-chu-chu):

Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.The effect you refer to is described by Paschen's Law. Here is a curve describing breakdown voltage per inch vs air pressure:

Breakdown Voltage | Paschen Curve | Altitude and Pressure | Corona | High Voltage Spacing | Insulation | from High Voltage Connection (http://www.highvoltageconnection.com/articles/paschen-curve.html)

Short of [no pun intended] an complete arc, this breakdown voltage also affects something called corona discharge around charged conductors. That's the crackling noise you hear in the vicinity of EHV transmission lines. For long lines (utility) this actually represents a small amount of power loss and inefficiency. But even on relatively short lines in an aircraft, this effect can produce a considerable amount of RF interference as well as ozone, which can affect some materials adversely.
:8