tommydortez
New member
an alternator on its own is single phase, if you have a three phase Gen set then usually it'll have three alternators which work in sync to produce the load between them
What is three phase?
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an alternator on its own is single phase, if you have a three phase Gen set then usually it'll have three alternators which work in sync to produce the load between them
MM5AHO
Well-known member
Looking in my archives, I found this useful explanation...
1. Most electricity is manufactured at Power Stations, where it is fed into wires that are then wound round large drums. These drums can often be seen at the roadside especially where electricity is about to be delivered to remote towns and villages.
2. Some electricity, however, does not need to go along wires. That kind is used in lighting for example, and in portable radios. This sort of electricity is not generated, but lies around loose.
3. Electricity makes a low humming noise. This noise may be pitched at different levels for use in doorbells, telephones and electric organs, etc.
4. Electricity must be earthed. That is to say it has to be connected to the ground before it can function, except in the case of ships and aeroplanes, which make their own arrangements.
5. Electricity is made up of two ingredients, positive and negative. One ingredient travels along a wire covered in brown or red plastic and the other along a wire covered in blue or black plastic. When these two wires meet together in what is known as a socket, the different ingredients mix together to form electricity.
6. Electricity may be stored in batteries. Big batteries do not necessarily hold more electricity than small batteries. In big batteries, electricity is just shovelled in whereas in small batteries it is carefully packed in flat.
7. With the invention of coloured electricity came an easing of traffic problems. In the past, policemen had to be used at road junctions. It has also made the cinema much more realistic.
8. The Light Switch: The lever in the middle controls a small vice or clamp that grips the wires very hard and thus prevents the electricity from passing that point when the switch is in the 'OFF' position.
9. The Fuse: Armourers will be familiar with this device, which is controlled by the pin on a grenade. With Electricity, the fuse usually stops things blowing up. As with all such devices, the size of the pin is important. The use of a 6" nail forced in with a hammer or pliers will normally ensure the fuse does not go off unexpectedly.
10. Changing the Bulb: Normally a Regiment of Gunners should suffice but in cases of difficulty when rotating the building, assistance may be sought from the Royal Engineers, who have special lifting and moving equipment mounted on tracked vehicles.
and also this one...
ELECTRICITY
As you probably know, electricity is the thing that happens when two clouds rub together. Lightning is produced, and in no time at all lightning conductors are sent by the Electricity Board to direct it to near-by pylons, enormous electrical lamposts found mainly in the countryside.
Nowadays we think nothing of relaxing in an electric chair while electric ovens use 'microwaves' (tiny, invisible amounts of hot water) to cook our meals. We use electrocution to help us talk properly, while in the bedroom electric blankets fold themselves. But things weren't always this easy.
It was of course Sir Isaac Walton who invented the electric cable, while waiting for the kettle to boil. He decided to suspend an apple from a wire strung bewteen two opposite poles in a magnetic field near his home. Cable or 'telegraph' poles like these are now an everyday sight in Britain.
The invention of electricity, so named after the 'electricity meters' kept underneath the stairs, meant that
previously 'wireless' radios could now be plugged in, giving them pictures. Almost overnight, television had been born. Electricity charges of 240 volts (about 10 pounds per week) are commonplace today, but electricity had been free up until the time of the Norman Conquest. Norman's brother, William the Conqueror, caused an electric storm when he announced that people would have to pay for their electricity. This earned him the nickname 'Electricity Bill', a term which is still in use today.
Most electricity is manufactured in big power stations where it is fed into wires that are wound round large drums put into cans where some special transformation takes place so the new electricity can be pumped through long hollow tubes to another can in your house. Some electricity, however, does not need to go along wires: that used for lightning, for example, and in potable radios. This kind of electricity is not generated in large power stations and pumped round the country, but is lying about loose in the air loose.
Electricity makes a humming noise. This noise may be pitched at different levels for use in doorbells, telephones, computers and electric organs. Electricity has to be grounded. That is to say, it has to be connected to the ground before it can function, except in the case of airplanes and missiles which have separate arrangements. Electricity is made of two ingredients, negative and positive. Each ingredient travels along a wire covered with different coloured plastics. When two wires meet at a socket and go into some appliance or other, the ingredients mix to form the working fluid or essence of electricity. Electricity can be stored in batteries and capacitors. In big batteries the electricity is just shoveled in, while in small batteries it is more carefully packed flat.
Electricity does not leak out of an empty socket. It just sits there waiting for something to enter the socket and mix the ingredients of positive and negative in just the right way. You can find out about this by putting your finger accidentally into an empty socket when the switch is turned on. The demonstration will be most illuminating. The average Fochaberian is content to take all this for granted. He or she will press a switch and the light comes on - and that is all they know about the complex technological miracle they have just unleashed. This has never been enough for me. I have to know how things work; and if I cannot find our from some technical handbook, then I combine such information as I already have with some simple logic. And if that doesn't work, I just make it all up.
Thus it is easy to deduce that the light switch controls a very small clamp or vice that grips the wire very hard, so that the electricity cannot get through. When the switch is turned to the "on" position, the vice is relaxed and the electricity ingredients are permitted to flow to the light bulb itself where a bit of wire, called the filament, is left exposed inside the bulb. Here for the first time, we can actually see the electricity in the form of a spark on the thin filament. We can't see this on ordinary wire because of the plastic covering. This little spark is then magnified many hundreds of times by the curved bulb which is made of magnifying glass. Very special glass is used for light bulbs so that it can diffuse the light from the spark while it is magnifying it.
There are two main types of electricity. The first, which we use every day to light our rooms, comes in bulbs, a special kind of onion grown in the soil, (hence its name 'earth' electricity). 'Live' electricity , which comes from animals, is far more dangerous, as King Canute discovered when a spider burnt his cakes giving him an 'electric shock'. But it was Dr.David Livingstone, with his unusual ability to talk to animals, who first harnessed this form of electricity. His 'Davy' lamp, containing a bright yellow canary, was used to light coal mines, and these 'miner' birds are today a popular household pet.
As recently as 1966, Sir Stanley Mattews was awarded the World Cup for his discovery that the electric atmosphere found inside football stadiums could be used to power enormous 'floodlights' during periods of heavy rainfall. More recently 'damns', (so named by an architect after he'd forgotten to leave a gap for the water while building a bridge), have been used to prevent flooding. In Britain today, there are millions of 'electric fans'; people who prefer electricity to other forms of energy. For further information send a SAE to your nearest Electrical Dealer or write to the Electricity Consumer's Council, a voluntary organisation set up to help people who have consumed large amounts of electric currants etc.
The electricity that is used in transistor radios consists of two kinds; the kind that was packed into small batteries, and the free electricity in the air . The first kind is easiest to explain. When the switch is turned on, the ingredients stored in the battery flow to the transistors where they work something like the filament in the light, only they make it possible to hear the electricity that's free in the air. There isn't enough power in the battery to let us see the electricity in the transistors. There is enough to let us smell electricity in the transistors if the negative and positive electricity ingredients somehow flow backwards through the transistors. This also causes a loss of hearing in the experimenter because the sound of electricity in the air ceases as soon as the smell starts. Never smell electricity this way if you want to hear something from the electricity in the air.
The electricity in the air is universal, and very powerful. I know this is true because I tried listening to it with more that one transistor radio and all the radios I could find sounded the same. Electricity in the air isn't diminished if you listen to it. It's sort of like reading, the print isn't worn off the paper because you read it.
The really fancy electricity in the air is able to make sounds like music or voices in radios or pictures in television sets. And here's where the explanation gets very complicated. All the electricity our there in the air has all those sound and pictures in it all the time. We need the radio or TV set to hear them or see them just like the light bulb lets us see electricity.
One more important thing about electricity is that instead of flowing just straight on a wire so that a light will work, there is twisted electricity. When you connect a motor to the wires, the twisted electricity goes in the motor and unwinds, just like a rubber band in a toy airplane, only the electricity was twisted at the generating station, and it will run a motor forever while the rubber band eventually stops. I'll have to take more time to think out how the straight electricity and the twisted electricity can move on the same wire. Maybe one goes on the wire and the other goes on it, but I'll have to do an experiment to find out. I know that some electricity is along the wire because I tried taking the plastic off and there was electricity when I touched the bare wire. I don't know how to get inside the wire yet, but when I do I'll tell you about that too.
.
1. Most electricity is manufactured at Power Stations, where it is fed into wires that are then wound round large drums. These drums can often be seen at the roadside especially where electricity is about to be delivered to remote towns and villages.
2. Some electricity, however, does not need to go along wires. That kind is used in lighting for example, and in portable radios. This sort of electricity is not generated, but lies around loose.
3. Electricity makes a low humming noise. This noise may be pitched at different levels for use in doorbells, telephones and electric organs, etc.
4. Electricity must be earthed. That is to say it has to be connected to the ground before it can function, except in the case of ships and aeroplanes, which make their own arrangements.
5. Electricity is made up of two ingredients, positive and negative. One ingredient travels along a wire covered in brown or red plastic and the other along a wire covered in blue or black plastic. When these two wires meet together in what is known as a socket, the different ingredients mix together to form electricity.
6. Electricity may be stored in batteries. Big batteries do not necessarily hold more electricity than small batteries. In big batteries, electricity is just shovelled in whereas in small batteries it is carefully packed in flat.
7. With the invention of coloured electricity came an easing of traffic problems. In the past, policemen had to be used at road junctions. It has also made the cinema much more realistic.
8. The Light Switch: The lever in the middle controls a small vice or clamp that grips the wires very hard and thus prevents the electricity from passing that point when the switch is in the 'OFF' position.
9. The Fuse: Armourers will be familiar with this device, which is controlled by the pin on a grenade. With Electricity, the fuse usually stops things blowing up. As with all such devices, the size of the pin is important. The use of a 6" nail forced in with a hammer or pliers will normally ensure the fuse does not go off unexpectedly.
10. Changing the Bulb: Normally a Regiment of Gunners should suffice but in cases of difficulty when rotating the building, assistance may be sought from the Royal Engineers, who have special lifting and moving equipment mounted on tracked vehicles.
and also this one...
ELECTRICITY
As you probably know, electricity is the thing that happens when two clouds rub together. Lightning is produced, and in no time at all lightning conductors are sent by the Electricity Board to direct it to near-by pylons, enormous electrical lamposts found mainly in the countryside.
Nowadays we think nothing of relaxing in an electric chair while electric ovens use 'microwaves' (tiny, invisible amounts of hot water) to cook our meals. We use electrocution to help us talk properly, while in the bedroom electric blankets fold themselves. But things weren't always this easy.
It was of course Sir Isaac Walton who invented the electric cable, while waiting for the kettle to boil. He decided to suspend an apple from a wire strung bewteen two opposite poles in a magnetic field near his home. Cable or 'telegraph' poles like these are now an everyday sight in Britain.
The invention of electricity, so named after the 'electricity meters' kept underneath the stairs, meant that
previously 'wireless' radios could now be plugged in, giving them pictures. Almost overnight, television had been born. Electricity charges of 240 volts (about 10 pounds per week) are commonplace today, but electricity had been free up until the time of the Norman Conquest. Norman's brother, William the Conqueror, caused an electric storm when he announced that people would have to pay for their electricity. This earned him the nickname 'Electricity Bill', a term which is still in use today.
Most electricity is manufactured in big power stations where it is fed into wires that are wound round large drums put into cans where some special transformation takes place so the new electricity can be pumped through long hollow tubes to another can in your house. Some electricity, however, does not need to go along wires: that used for lightning, for example, and in potable radios. This kind of electricity is not generated in large power stations and pumped round the country, but is lying about loose in the air loose.
Electricity makes a humming noise. This noise may be pitched at different levels for use in doorbells, telephones, computers and electric organs. Electricity has to be grounded. That is to say, it has to be connected to the ground before it can function, except in the case of airplanes and missiles which have separate arrangements. Electricity is made of two ingredients, negative and positive. Each ingredient travels along a wire covered with different coloured plastics. When two wires meet at a socket and go into some appliance or other, the ingredients mix to form the working fluid or essence of electricity. Electricity can be stored in batteries and capacitors. In big batteries the electricity is just shoveled in, while in small batteries it is more carefully packed flat.
Electricity does not leak out of an empty socket. It just sits there waiting for something to enter the socket and mix the ingredients of positive and negative in just the right way. You can find out about this by putting your finger accidentally into an empty socket when the switch is turned on. The demonstration will be most illuminating. The average Fochaberian is content to take all this for granted. He or she will press a switch and the light comes on - and that is all they know about the complex technological miracle they have just unleashed. This has never been enough for me. I have to know how things work; and if I cannot find our from some technical handbook, then I combine such information as I already have with some simple logic. And if that doesn't work, I just make it all up.
Thus it is easy to deduce that the light switch controls a very small clamp or vice that grips the wire very hard, so that the electricity cannot get through. When the switch is turned to the "on" position, the vice is relaxed and the electricity ingredients are permitted to flow to the light bulb itself where a bit of wire, called the filament, is left exposed inside the bulb. Here for the first time, we can actually see the electricity in the form of a spark on the thin filament. We can't see this on ordinary wire because of the plastic covering. This little spark is then magnified many hundreds of times by the curved bulb which is made of magnifying glass. Very special glass is used for light bulbs so that it can diffuse the light from the spark while it is magnifying it.
There are two main types of electricity. The first, which we use every day to light our rooms, comes in bulbs, a special kind of onion grown in the soil, (hence its name 'earth' electricity). 'Live' electricity , which comes from animals, is far more dangerous, as King Canute discovered when a spider burnt his cakes giving him an 'electric shock'. But it was Dr.David Livingstone, with his unusual ability to talk to animals, who first harnessed this form of electricity. His 'Davy' lamp, containing a bright yellow canary, was used to light coal mines, and these 'miner' birds are today a popular household pet.
As recently as 1966, Sir Stanley Mattews was awarded the World Cup for his discovery that the electric atmosphere found inside football stadiums could be used to power enormous 'floodlights' during periods of heavy rainfall. More recently 'damns', (so named by an architect after he'd forgotten to leave a gap for the water while building a bridge), have been used to prevent flooding. In Britain today, there are millions of 'electric fans'; people who prefer electricity to other forms of energy. For further information send a SAE to your nearest Electrical Dealer or write to the Electricity Consumer's Council, a voluntary organisation set up to help people who have consumed large amounts of electric currants etc.
The electricity that is used in transistor radios consists of two kinds; the kind that was packed into small batteries, and the free electricity in the air . The first kind is easiest to explain. When the switch is turned on, the ingredients stored in the battery flow to the transistors where they work something like the filament in the light, only they make it possible to hear the electricity that's free in the air. There isn't enough power in the battery to let us see the electricity in the transistors. There is enough to let us smell electricity in the transistors if the negative and positive electricity ingredients somehow flow backwards through the transistors. This also causes a loss of hearing in the experimenter because the sound of electricity in the air ceases as soon as the smell starts. Never smell electricity this way if you want to hear something from the electricity in the air.
The electricity in the air is universal, and very powerful. I know this is true because I tried listening to it with more that one transistor radio and all the radios I could find sounded the same. Electricity in the air isn't diminished if you listen to it. It's sort of like reading, the print isn't worn off the paper because you read it.
The really fancy electricity in the air is able to make sounds like music or voices in radios or pictures in television sets. And here's where the explanation gets very complicated. All the electricity our there in the air has all those sound and pictures in it all the time. We need the radio or TV set to hear them or see them just like the light bulb lets us see electricity.
One more important thing about electricity is that instead of flowing just straight on a wire so that a light will work, there is twisted electricity. When you connect a motor to the wires, the twisted electricity goes in the motor and unwinds, just like a rubber band in a toy airplane, only the electricity was twisted at the generating station, and it will run a motor forever while the rubber band eventually stops. I'll have to take more time to think out how the straight electricity and the twisted electricity can move on the same wire. Maybe one goes on the wire and the other goes on it, but I'll have to do an experiment to find out. I know that some electricity is along the wire because I tried taking the plastic off and there was electricity when I touched the bare wire. I don't know how to get inside the wire yet, but when I do I'll tell you about that too.
.
Graham_Wright
Well-known member
Both those on my boat are three phase and on the 120A Prestolite one the phases are actually brought out to screw terminals.
prv
Well-known member
+1
Every circuit diagram I've seen shows a 3-phase alternator with a set of diodes to convert it to DC. These are the infamous diodes that are at risk if you turn off the master switch while the engine is running.
If some crazy person wanted to produce three-phase by attaching three separate alternators to a motor, how would he ensure the three phases stayed exactly 1/3 of a wave apart?
Pete
VicS
Well-known member
with a common shaft!
William_H
Well-known member
A 3 phase alternator surtely always has a rotating field and 3 sets of pick up coils each spaced 120 degreees apart. This is the case for both large power generators and alternators for cars etc. Perhaps 3 single phase gens have been used but I can't imagine why.
The concept of 3 phase power transmission is not really about electric motors that is just a happy coincidence. 3 phase power transmission comes into it's real value when you want to ship mega watts of electricity 100 miles or more. Firstly as implied they use high voltage giving less current so less resistive loss or less copper needed. Now you can't just keep increasing voltage to reduce current becuase you start losing power from arc over at insulators. Typically 660000 volts is max practical. Cost wise.
Consider that if you wanted to shift 12v at some current, a distance. Ok you have 2 wires. If you added another 12v battery and another load you would need 2 more wires. Except if you used a common wire such that +ve went down from one battery and -ve from the other battery you could share a wire. Indeed if the currents of both systems were the same you would have net 0 current so you could not even have that wire. (Yes I know this is just a 24v system.)
Well in 3 phase we have 3 different power systems each sharing a common wire. Such that if the currents are same there is no net current in the wire so you don't need that wire ( neutral). We have halved the amount of copper we need.
Now the next bit is really tricky. The current through each wire consists of its own load plus some of the current of the other 2 phases but to varying degrees out of phase so partially cancelling so that net current in each leg is of the order of 60% less than its produced current.
So using these wire saving tricks electricity supply people bring high voltage from the country generators reduce it by transformers in steps like 12kv then further transformers on poles down to 240v AC. That is the voltage each phase to neutral. Provided you can equalise the loads you have no problems.
My house has 3 phase fed in. This means that 1/3 the house is on each phase. I can load up 3 different outlets to 15 amps without popping the 15a c/b provided I get 3 sockets on each different phase. it was originally fitted for an electric instantaneous hot water heater. 3750 watts times 3.
A marina will have 3 phase available to allow for more total power with the load roughly divided into 3 parts. good lcuk olewill
The concept of 3 phase power transmission is not really about electric motors that is just a happy coincidence. 3 phase power transmission comes into it's real value when you want to ship mega watts of electricity 100 miles or more. Firstly as implied they use high voltage giving less current so less resistive loss or less copper needed. Now you can't just keep increasing voltage to reduce current becuase you start losing power from arc over at insulators. Typically 660000 volts is max practical. Cost wise.
Consider that if you wanted to shift 12v at some current, a distance. Ok you have 2 wires. If you added another 12v battery and another load you would need 2 more wires. Except if you used a common wire such that +ve went down from one battery and -ve from the other battery you could share a wire. Indeed if the currents of both systems were the same you would have net 0 current so you could not even have that wire. (Yes I know this is just a 24v system.)
Well in 3 phase we have 3 different power systems each sharing a common wire. Such that if the currents are same there is no net current in the wire so you don't need that wire ( neutral). We have halved the amount of copper we need.
Now the next bit is really tricky. The current through each wire consists of its own load plus some of the current of the other 2 phases but to varying degrees out of phase so partially cancelling so that net current in each leg is of the order of 60% less than its produced current.
So using these wire saving tricks electricity supply people bring high voltage from the country generators reduce it by transformers in steps like 12kv then further transformers on poles down to 240v AC. That is the voltage each phase to neutral. Provided you can equalise the loads you have no problems.
My house has 3 phase fed in. This means that 1/3 the house is on each phase. I can load up 3 different outlets to 15 amps without popping the 15a c/b provided I get 3 sockets on each different phase. it was originally fitted for an electric instantaneous hot water heater. 3750 watts times 3.
A marina will have 3 phase available to allow for more total power with the load roughly divided into 3 parts. good lcuk olewill
Iliade
Well-known member
Just to bring this back to practical boating, I understand that three-phase is available on the pontoons of the better class of marina to enable ones' superyachts' air conditioners to function without the need to raise steam onboard.
Slightly scarily, our yard uses three-phase sockets to deliver differing amperages of single phase supply, depending on which lead you plug in.
Slightly scarily, our yard uses three-phase sockets to deliver differing amperages of single phase supply, depending on which lead you plug in.
prv
Well-known member
That seems slightly odd - do they just have different ratings of breaker attached to the different "phase" pins?
I know some places use 3-phase plugs and sockets for normal 16-amp single-phase supplies purely because visitors are unlikely to carry such a plug with them. It means they have to tell the office, and pay the required fee, in order to receive an adaptor and be able to plug in.
Pete
Buck Turgidson
Well-known member
Almost all aircraft generators are 3 phase AC normally 400Hz 115V line to earth 200V Line to line. It's the lightest application available. Well except for the 400Hz bit, that needs a constant speed drive and they are heavy. Latest jets are going for frequency wild to reduce weight.
For those who are interested in AC electrics google star or delta wound motor/generator. That should bring up enough to get you going.
For those who are interested in AC electrics google star or delta wound motor/generator. That should bring up enough to get you going.
superheat6k
Well-known member
A most interesting thread, especially MM5AHOs response.
VicS thread response earlier in your thread gave an excellent précis of what three phase means. the point I would add is that virtually all AC power station generators produce a three phase wave form (including our own engine mounted alternators), by using this wave form at the consumer end the power use is simple and efficient, it is a true rotating supply.
The vast majority of significant loads met by a power station are rotating motors so these simply mimic the prime mover at the power station.
If you observe any three phase distribution cabling you will see two sets of three phase wires, with one set dangled each side of the pylon - the top wire is a simple bonding wire.
The single phase power we draw from our familiar 3 pin sockets is produced from a star connected secondary of a transformer from a normally far higher voltage supply from the power generator. By connecting three coils in a star pattern the 'star-point' becomes the 'neutral'. Power distribution to houses is grouped with several of each from each phase in turn connected to the common neutral. The idea is that each group will draw approximately the same amount of power so the supply balances itself out, which is good for supply energy efficiency.
Marina distribution works in exactly the same way.
Generally the earth conductor is also connected to this star point, but only at the power source, which is why Neutral appears at earth potential (no voltage between, but note on an imbalanced system or on the individual group sections the power conducting along a neutral can be considerable, so any neutral is electrically is potentially lethal as the live conductor.
Now the most confusing bit - the UK standard supply is a nominal 415VAC 3 phase at 50hz frequency, but the single phase drawn from that is 240volts, not 207. this is because the single phase voltage is derived from the 1 / 'square root of 3' x the phase to phase voltage.
Also be aware that the nominal AC voltage is not the actual voltage, which is continually varying, but is a specially assessed average known as the RMS or Root of the Mean of the Square value, as this continually varies between a peak level of almost 600volts positive then the same negative, 50 times each second in the UK. In the US they operate at 440v and 60 hertz frequency. The straight average of an AC voltage is zero, so the RMS works this out to an approximate DC equivalent (average) level by first squaring the voltage, which as multiplying two negatives provides an overall positive series of sine waves, the Mean averages out this squared all positive voltage, the Root coverts this back to an average level but now it is the root of a mean positive number.
Hope the above makes some sense and adds to the explanations provided so far.
VicS thread response earlier in your thread gave an excellent précis of what three phase means. the point I would add is that virtually all AC power station generators produce a three phase wave form (including our own engine mounted alternators), by using this wave form at the consumer end the power use is simple and efficient, it is a true rotating supply.
The vast majority of significant loads met by a power station are rotating motors so these simply mimic the prime mover at the power station.
If you observe any three phase distribution cabling you will see two sets of three phase wires, with one set dangled each side of the pylon - the top wire is a simple bonding wire.
The single phase power we draw from our familiar 3 pin sockets is produced from a star connected secondary of a transformer from a normally far higher voltage supply from the power generator. By connecting three coils in a star pattern the 'star-point' becomes the 'neutral'. Power distribution to houses is grouped with several of each from each phase in turn connected to the common neutral. The idea is that each group will draw approximately the same amount of power so the supply balances itself out, which is good for supply energy efficiency.
Marina distribution works in exactly the same way.
Generally the earth conductor is also connected to this star point, but only at the power source, which is why Neutral appears at earth potential (no voltage between, but note on an imbalanced system or on the individual group sections the power conducting along a neutral can be considerable, so any neutral is electrically is potentially lethal as the live conductor.
Now the most confusing bit - the UK standard supply is a nominal 415VAC 3 phase at 50hz frequency, but the single phase drawn from that is 240volts, not 207. this is because the single phase voltage is derived from the 1 / 'square root of 3' x the phase to phase voltage.
Also be aware that the nominal AC voltage is not the actual voltage, which is continually varying, but is a specially assessed average known as the RMS or Root of the Mean of the Square value, as this continually varies between a peak level of almost 600volts positive then the same negative, 50 times each second in the UK. In the US they operate at 440v and 60 hertz frequency. The straight average of an AC voltage is zero, so the RMS works this out to an approximate DC equivalent (average) level by first squaring the voltage, which as multiplying two negatives provides an overall positive series of sine waves, the Mean averages out this squared all positive voltage, the Root coverts this back to an average level but now it is the root of a mean positive number.
Hope the above makes some sense and adds to the explanations provided so far.
Iliade
Well-known member
I guess so...
Iliade
Well-known member
I was obliged to study a fair amount of 'power supply' electricity at university. This served only to reinforce my impression that it is all a matter of black magic and wishful thinking!
Meters running backwards! I ask you!
Meters running backwards! I ask you!
William_H
Well-known member
AC voltage is described as RMS as said Root mean squared. This can be simplified to reflect the fact that 250v RMS has the same heating power as 250v DC. Quite confusing when you think that the actual AC voltage varies from a peak of 380v in one direction then to a peak of 380 volts in the opposite direction all at 50 times per second.
However the advantages of AC being easy to transform make it almost universal for power generation and distribution. Many years back some small town systems were 240v DC but was quickly replaced as demand grew.
Consider the average demand for a home is in the order of 2 or 3 Kw sometimes more. Take a city of 1million homes and you are looking for 10 million amps at 240v. That would take a lot of copper if the power station was 100 miles away. Compare how much copper you need for 400 amps for your starter on your boat and just a few metres .
The American folks have a real disadvantage settling early on for 115vAC so the current is twice as much. But also any volt drop in wiring is twice as significant.
It shows as really heavy wiring into houses and very bulky cords on things like air conditioners. olewill
However the advantages of AC being easy to transform make it almost universal for power generation and distribution. Many years back some small town systems were 240v DC but was quickly replaced as demand grew.
Consider the average demand for a home is in the order of 2 or 3 Kw sometimes more. Take a city of 1million homes and you are looking for 10 million amps at 240v. That would take a lot of copper if the power station was 100 miles away. Compare how much copper you need for 400 amps for your starter on your boat and just a few metres .
The American folks have a real disadvantage settling early on for 115vAC so the current is twice as much. But also any volt drop in wiring is twice as significant.
It shows as really heavy wiring into houses and very bulky cords on things like air conditioners. olewill
Leighb
Well-known member
I have read this thread with great interest, and have , I hope, a better understanding.
However I still can't relate what I have read to an incident here a few years back when we had a local power failure. Our semi-detached cottage had a big voltage drop, the lights went dim, electric kettle took for ever to boil and, as we discovered later, the freezer motor was wrecked. Our neighbour still had full power, and another cottage close by which gets its supply from the same overhead lines had no power at all.
I checked our voltage with a meter and if I remember correctly it was around 80V.
Can any of the electrical gurus offer an explanation?
However I still can't relate what I have read to an incident here a few years back when we had a local power failure. Our semi-detached cottage had a big voltage drop, the lights went dim, electric kettle took for ever to boil and, as we discovered later, the freezer motor was wrecked. Our neighbour still had full power, and another cottage close by which gets its supply from the same overhead lines had no power at all.
I checked our voltage with a meter and if I remember correctly it was around 80V.
Can any of the electrical gurus offer an explanation?
halcyon
Well-known member
All the high power distribution is 3 phase, 3 positive wires and a common neutral, the power supply to your street will be 3 phase, but your house will only use one of them and a tapping to common neutral.
So you could be on one phase, the chap next door on one of the other phases, the fault can be only on one phase, if that's your you have no power, but the chap next door may still have a supply on his phase.
Brian
GEB
Member
This may also happen if the supply neutral becomes disconnected (or gets nicked) on the distribution network. The system voltages can fluctuate wildly and, in the worst case, could subject your domestic appliances to over 400 volts.
It is an unstable situation so when you measured 80 volts that could have changed without warning to a much higher value. BTW I hope you were using a professional quality voltmeter as the cheap ones can suffer a catastrophic failure if used on the supply network when a voltage surge occurs.
If it happens again turn everything off and phone your supply company.
JumbleDuck
Well-known member
Some applications can't be protected with an RCD. The most common, domestically, is the standard sort of electric cooker with flat black coiled heating elements. The insulation inside them (silica, I think) is always very slightly damp, so they always leak very slightly to earth, irritating any inquisitive RCDs. That's why it's quite common to see fuse (you know what I mean) boxes with some circuits protected by RCD and some by MCB.
Almost all alternators are three phase. For a start, they absorb constant torque, just as three phase motors produce constant torque. That means less torsional vibration and fatigue in whatever is driving them. They are also much more economical in their use of copper, like all three phases systems. Trying to use three single phase alternators to make three phase would be a tad eccentric.
I have Ferranti artificial horizon in a box somewhere which I was going to fit to my glider. It comes with a 3-phase 400Hz inverter to produce aircraft standard supply. I wouldn't be surprised if some boat gyrocompasses didn't have synthesised 3-phase somewhere them.
Power in a resistive load is proportional to the square of the voltage applied or the current passing. When these are varying, the average powere therefore depends on the average of the square of the voltage or current. Take the square root to get the effective DC equivalent, so 230V DC applied across a resistor will dissipate the same heat in it as 230V rms AC which is, as you say, much less than the peak-to-peak AC voltage.
ianj99
Active member
The undersea cables supplying the UK from Scandinavia now use high voltage DC. Due to the distance - several hundred km, the reactive losses using AC were far greater than the purely resistive ones using DC.
prv
Well-known member
Not allowed under the 17th, unless the cable is either buried more than two inches into the structure of the house, or protected by steel trunking or similar. Since this is rarely the case or easy to retro-fit, in practice everything needs to be protected by a 30mA RCD.
Pete
rhumlady
New member
Both the houses I have lived in have had the three phase input from the street. We use one and the houses next door on each side the other two. This is fine till you get a knock on the door from the guy from the supply network asking to check the cables. The first time was n a council house that had been wired using aluminium cables from the street and the problem was a brownout on one of the phases so next door had problems. It turned out that the cable into our house had a loose connection on that phase and the cable was getting pretty hot. When asked what would happen if I hadn't been in he said that the police would have been called and entry would have been forced:-( I was lucky that the cable was just long enough to be remade or it would have meant digging up the garden. This house has fuses for two others and I have been similarly disturbed to have them replaced on a couple of occasions.
The loss of or disconnection of the neutral has certainly caused problems in my line of work. The first time blowing the PSUs on about 70 PCs after a big weekend switchboard rewire exercise. Another one was due to loose connection on a switchboard which blew 6 PSUs on a major computer system. Both were the result of electricians checking live to earth but not live to neutral. Both caused multiple loud bangs as the supplies blew and upped my overtime for the weekend I question.
The loss of or disconnection of the neutral has certainly caused problems in my line of work. The first time blowing the PSUs on about 70 PCs after a big weekend switchboard rewire exercise. Another one was due to loose connection on a switchboard which blew 6 PSUs on a major computer system. Both were the result of electricians checking live to earth but not live to neutral. Both caused multiple loud bangs as the supplies blew and upped my overtime for the weekend I question.
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