What advantages does ac electrical power have over dc electrical power
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What advantages does ac electrical power have over dc electrical power
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
"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
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.
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.
Last edited by Bengo; 28th Apr 2013 at 12:24.
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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.
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.
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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!,
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!,
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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 
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Capetonian:
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.
From a consumer perspective, I recall from my schoolboy physics that AC suffers from lower transmission losses over distance...
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.
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.
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.
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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.
Stuck_in_an_ATR:
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.
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?
Chu Chu:
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.
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.
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.
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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
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
Last edited by onetrack; 29th Apr 2013 at 01:07. Reason: sp ...
AC/DC versus voltage
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.
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.
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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.
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.
Last edited by Denti; 29th Apr 2013 at 05:06.
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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.
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archae86:
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.
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...
barit1:
Here is some info on DC transmission links.
List of HVDC projects - Wikipedia, the free encyclopedia
Looks like the UK-France systems have been around for a while. Now there's a second link from the UK to The Netherlands.
Here is some info on DC transmission links.
List of HVDC projects - Wikipedia, the free encyclopedia
Looks like the UK-France systems have been around for a while. Now there's a second link from the UK to The Netherlands.