Washing machine 240, 50Hz - 240, 60 Hz?

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overvoltage
Even simpler than that. Across each lamp is a pair of spring loaded contacts that touch each other and short circuit the lamp when they are in contact. Between the contacts is placed a prepared paper disc, about the size of a quarter. The paper has been partially nitrated so it resembles guncotton in its flammability. When the lamp is good, the voltage across the contacts is low and the paper disc prevents the flow of current. When the lamp fails, the entire supply voltage appears across the disc which is unable to withstand the high voltage. A spark results, which ignites the disc, which very quickly disappears on a flash of flame. The disc having vanished, the contacts touch, short circuiting the lamp. Due to the constant-current supply, there is no effect on the remaining lamps in the circuit. When the lamp is replaced a new disc is installed.
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wrote: |
| writes: |> |> >At one time street lighting circuits were series. This required special |> >transformers with mechanically floating secondary windings (rather exotic |> >design but practical) to keep the current constant as well as | overvoltage |> >bypasses as in series Xmas tree lights which allow continued operation of |> >the string as a whole if a lamp fails. |> |> Buffalo NY has these, or at least had them in the early '80s. What is a |> transformer with a "mechanically floating secondary windings", and why is |> such useful for that type of circuit? How does bypassing a blown bulb |> work, did they parallel each bulb with a device similar to a Zener diode |> that conducts if the voltage exceeds a certain level, keeping the voltage |> drop across it constant? | | Even simpler than that. Across each lamp is a pair of spring loaded contacts | that touch each other and short circuit the lamp when they are in contact. | Between the contacts is placed a prepared paper disc, about the size of a | quarter. The paper has been partially nitrated so it resembles guncotton in | its flammability. When the lamp is good, the voltage across the contacts is | low and the paper disc prevents the flow of current. When the lamp fails, | the entire supply voltage appears across the disc which is unable to | withstand the high voltage. A spark results, which ignites the disc, which | very quickly disappears on a flash of flame. The disc having vanished, the | contacts touch, short circuiting the lamp. Due to the constant-current | supply, there is no effect on the remaining lamps in the circuit. | When the lamp is replaced a new disc is installed.
That would work for the constant current series lights. But what about the Christmas light series strings?
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     snipped-for-privacy@ipal.net writes:

They work the same way. There's a piece of wire wrapped around the lead-in wires which has a layer of thin insulation, which breaks down when mains voltage appears across the lamp.
The circuit is not constant current, so the loss of a lamp increases the power of the remaining lamps, and reduces their life. This can create a run-away effect as more die and the remaining ones burn at higher power. There's normally a fuse lamp in the set, which is a lamp which does not short out when it dies. This is to prevent a few lamps running at excessive power and igniting the Christmas tree, or the whole set shorting out. In the UK, we do get typically a couple of house fires a year caused by Christmas trees, and this maybe when people have replaced the fuse lamp with a shorting lamp.
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| snipped-for-privacy@ipal.net writes: |> That would work for the constant current series lights. But what about the |> Christmas light series strings? | | They work the same way. There's a piece of wire wrapped around | the lead-in wires which has a layer of thin insulation, which | breaks down when mains voltage appears across the lamp. | | The circuit is not constant current, so the loss of a lamp | increases the power of the remaining lamps, and reduces | their life. This can create a run-away effect as more die | and the remaining ones burn at higher power. There's normally | a fuse lamp in the set, which is a lamp which does not short | out when it dies. This is to prevent a few lamps running at | excessive power and igniting the Christmas tree, or the whole | set shorting out. In the UK, we do get typically a couple of | house fires a year caused by Christmas trees, and this maybe | when people have replaced the fuse lamp with a shorting lamp.
It's that increasing current I was wondering about.
But you guys have actual fuses in your plugs. Why not set that fuse to a bit above the proper current? Of course someone will end up replacing the fuse with one of the wrong value.
This made me think of a scenerio I described to someone once. The scenario is wiring up 60 light bulbs of our 120 volt variety in series and powering it from a 7200 volt MV source. Then wait for one to burn out, or force one to somehow, or just have one already burned out with a small filament gap. Then stand back and watch it all explode.
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     snipped-for-privacy@ipal.net writes:

The plug fuse is for protection of the appliance flex only, not the appliance. If an appliance requires fusing in order to remain safe, it must include appropriate provision within itself. It is not permitted to rely on the plug fuse. Plug fuses go down to 2A in value, and that's still almost 500W which is plenty enough to ignite a christmas tree.
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| snipped-for-privacy@ipal.net writes: |> It's that increasing current I was wondering about. |> |> But you guys have actual fuses in your plugs. Why not set that fuse to |> a bit above the proper current? Of course someone will end up replacing |> the fuse with one of the wrong value. | | The plug fuse is for protection of the appliance flex only, | not the appliance. If an appliance requires fusing in order | to remain safe, it must include appropriate provision within | itself. It is not permitted to rely on the plug fuse. | Plug fuses go down to 2A in value, and that's still almost | 500W which is plenty enough to ignite a christmas tree.
The string of Christmas tree lights isn't really an appliance. The ones we have over here are basically the same wires coming out of the plug and going all through the string. So there isn't any real boundary between the flex cord and the rest of the string, unless you want to define that as being the first bulb.
I remember some past light strings my family had. An older set of strings my father got from his father was parallel wired with C7 bulbs. The plug on these were strangely very flat and wide. You could easily plug several outlets spaced very close together. They were non-polarized. But the one interesting thing about these plugs is they were fused. You could twist the prongs with a bit of force and release them and the prong and fuse would come out. The prong actually connected directly to the fuse. There was a small spring behind the fuse. But once the prong was twisted in, it could not move either inward or outward. I do remember once my father managed to have one come out while in the socket. Both prongs were fused and neither were polarized (you could rotate the plug 180 degrees). I'm sure the non-polarized aspect would be prohibited today, and the fuse on the neutral might be as well (e.g. it's unsafe to have an apparently dead, but hot, wiring).
We also had a series string with blinker lights. They blinked by means of a thermal element that shorted the filament when it heated up. There was what was called a "ballast bulb" at the head of the string. It was a larger bulb with a different socket. It glowed a bit dimmer, but was always going up and down in brightness as more or fewer blinkers shorted out. There was no dead bulb bypass so if one bulb burned out the whole string went dead. There were typically about 6 burn outs per season.
Today we have some non-blinker series strings with some kind of burnout bypass. The bulbs have some very fine wire wrapped around the posts to hold the filament. I suppose this is the bypass. These bulbs indicate a 2.5 to 3.5 volt operating range.
OK, so let's assume 3 volts per bulb. You would have 80 bulbs for your 240 volts, and we would have 40 bulbs for out 120 volts. I think they are about 1 watt each. I'd have to go dig them out early to see that right now (should be doing that in about a month, anyway), as well as count exactly how many are on a string. That would mean a current of about 1/3 amp. So why not put a 1/2 amp fuse in the plug? It would be better than say a 5 amp fuse just because the cord can handle 5 amps. Then if too many bulbs burn out, and the current rises too high, the fuse will blow. Or do they just not make 1/2 amp fuses in the size to fit plugs? They should, at least for the light strings.
One thing I have found with these light strings is that about 30% of the burnouts do not result in the bypass. Then I have to find the dead bulb. The strings and lights are made in some small Asian Pacific Rim country.
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snipped-for-privacy@ipal.net writes:

In Finland I have owned several cristmas lights that have not had any fuse in the plug, but another type of protection: one of the bulbs on the christmas light is special "fuse bulb" that does not have that piece of wire wrapped around the lead-in wires. That bulb is clearly marked and instructions say that this bulb should be only replaced with the exactly same type bulb.

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Don is of course correct, as usual, but there is an even older piece of the story. The 6.6 amp series circuit originated prior to incandescent lighting in the days of arc-lamps for street lighting. Arc lamps, with their negative dynamic resistance, can not be run in parallel. The series connection was necessary to force the lamps to run at equal currrents. When incandescent lamps became practical, a lamp with a 6.6 amp rating was designed to permit retro-fitting the arc systems without re-wiring them.
The cost/benefit ratio of series street lighting escapes us today, but it was absolutely correct at the time of its inception.
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wrote:
| |>- |>At one time street lighting circuits were series. This required special |>transformers with mechanically floating secondary windings (rather exotic |>design but practical) to keep the current constant as well as overvoltage |>bypasses as in series Xmas tree lights which allow continued operation of |>the string as a whole if a lamp fails. |>Compared to parallel systems, there is no contest in terms of cost and |>safety. Note that present practice is to simply run the lights within a |>local area from a local supply so voltage drops are not a great |>concern-local photocells provide the control for the local lights. | | Don is of course correct, as usual, but there is an even older piece | of the story. The 6.6 amp series circuit originated prior to | incandescent lighting in the days of arc-lamps for street lighting. | Arc lamps, with their negative dynamic resistance, can not be run in | parallel. The series connection was necessary to force the lamps to | run at equal currrents. When incandescent lamps became practical, a | lamp with a 6.6 amp rating was designed to permit retro-fitting the | arc systems without re-wiring them. | | The cost/benefit ratio of series street lighting escapes us today, but | it was absolutely correct at the time of its inception.
What about using a series of HID lights at whatever current they need, controlled by a single current regulator (that can deal with a range of voltage)?
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wrote:

Thanks for the "as usual" which is not warranted. Thanks for the history and thanks again for the details of the bypass devices which I either never knew or had forgotten (where did I put my wine glass?? I know that it was full and really want it.).
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"Beachcomber" wrote

They still do supply DC in NYC. but I think they are trying to phase it out. As a first step: http://www.coned.com/documents/elec/158n-158p.pdf
my guess is within 10 years they will try to have all DC systems supplied as AC to the customer and then customer rectified.
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wrote:

---------------- You would have to have the pole pigs as they are the most efficient (and cheapest) way to change voltage levels. In addition you would need inverter/rectifiers and filters. Couple this with the switching problems and the present AC distribution looks damn good- the reason that AC superceded DC still holds for most situations.
As for 240DC in homes- how much more are you willing to pay for fire or accident insurance?
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-------------- Canada was/is more influenced by the US as they are (electrically) closely tied from after the original multiple frequency options shook out- sheer economics and practicality. As to TV's the line frequency really shouldn't matter provided that the refresh rate (V? can't remember) is generated internally and not from the line. Generation from the line would lead to problems with synchronisation with the signal. 50-60 Hz is OK for magnetic deflection.
I don't know the historical reason for the split system in Japan - probably the initial reason was that in some regions UK salesmen did better than US salesmen in an era where rampant salesmanship and limited regional coordination existed. In the US and Canada, there was some of the same going on where each region had its own rules (probably the main manufacturers, GE and Westinghouse had a great deal to do with the standardisation of frequency- along the lines that Henry Ford used for the choice of car colours. "you want 72 Hz"- we got 60Hz (actually 120 alternations/sec or "alts" as I have seen on a 1912 motor nameplate)-take it or leave it". This leads to a response to what you have said about DC transmission. There are 3 basic situations where DC transmission is the best choice: a) High voltage long distance point to point interconnections - cheaper lines but terminals more expensive so $ balance tips in favour of DC for long distances. (e.g Manitoba's Nelson river system) b)Potential or actual frequency differences (even of a fraction of a Hertz)between systems where an asynchronous tie is of benefit in terms of system stability Some DC ties are back to back rectifier inverters because of this (Japan-50-60Hz, Eel River New Brunswick Canada -first solid state converters in use 60-60 Hz ) Both of these are of concern in the Western DC link from Oregon south to California and 4 corners region of US ( where rapid control of power transfer aids in stability of the parallel AC system) and in ties from Northern Quebec to New York State. c) long underground or underwater lines - a rough rule of thumb is that 30 miles of cable is equivalent to 300 miles of overhead line in terms of capacitance and associated problems.
The main disadvantage of DC 1)since a station has not only transformers but also rectifier inverters and associated filters the DC stations are expensive-so that economics indicate DC for long lines or special cases as above.
2)In addition, there is a problem with circuit breakers and switching. This is far more costly and often beyond present technology for DC. With AC, circuit breakers do not normally "break" the current but simply take advantage of natural current zeros - preventing re-ignition of the arc. On a small scale look at simple switches. Open a knife switch 1/4 inch on AC at 120V, 10A and there is a small spark. Do it on DC under the same situation and there will be a sustained arc flaring out a good half inch or more from the contacts. (proven good for lighting cigarettes and if done carefully, less dangerous than smoking the cigarette- I am a reformed smoker). Why is this important? Simply put, a grid system depends on circuit breakers and transformers. Hence DC for "grid" use is, technically and economically, not a viable option except as indicated above under a)b)c).
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Just to emphasize that this was true for old TV sets, but all modern TV's have DC power supplies and stable oscillators. Those of you who remember the old vertical and horizontal sync adjustment knobs on your tv set know that this wasn't always true. Modern TV's have electronics that locks to the incoming video signal and not to the AC power line. Are you old enough to remember when sometimes they just didn't want to lock and the picture would roll?
The early TV's had "tank" oscillators that needed to synchronize with the incoming AC power line, else they would drift. Until the 3.58 Mhz color "burst" oscillator crystals became widely available, this was even more critical for the 1952 NTSC color TV system used in North America.
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| b)Potential or actual frequency differences (even of a fraction of a | Hertz)between systems where an asynchronous tie is of benefit in terms of | system stability Some DC ties are back to back rectifier inverters because | of this (Japan-50-60Hz, Eel River New Brunswick Canada -first solid state | converters in use 60-60 Hz )
Why does a frequency conversion need to be AC-DC-AC? Why not just switch pulses directly out of each phase in an appropriate balance so that you are drawing power from each steadily? Afterall, the power-sum of all three phases is flat across complete cycles.
| Both of these are of concern in the Western DC link from Oregon south to | California and 4 corners region of US ( where rapid control of power | transfer aids in stability of the parallel AC system) and in ties from | Northern Quebec to New York State. | c) long underground or underwater lines - a rough rule of thumb is that 30 | miles of cable is equivalent to 300 miles of overhead line in terms of | capacitance and associated problems.
Looking at the transmission lines as an RF problem, going between overhead and underground or underwater cable, sure looks like a change in characteristic line impedance.
| 2)In addition, there is a problem with circuit breakers and switching. This | is far more costly and often beyond present technology for DC. With AC, | circuit breakers do not normally "break" the current but simply take | advantage of natural current zeros - preventing re-ignition of the arc. On a | small scale look at simple switches. Open a knife switch 1/4 inch on AC at | 120V, 10A and there is a small spark. Do it on DC under the same situation | and there will be a sustained arc flaring out a good half inch or more from | the contacts. (proven good for lighting cigarettes and if done carefully, | less dangerous than smoking the cigarette- I am a reformed smoker). | Why is this important? Simply put, a grid system depends on circuit breakers | and transformers. Hence DC for "grid" use is, technically and economically, | not a viable option except as indicated above under a)b)c).
This sounds like the real issue for DC.
Sounds like I need to get 10 big marine batteries, a bank of light bulbs, and a knife switch.
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wrote:

----------- Great -try switching 1000A backed by a 500KV source at any part of the cycle? Try it- attempting to switch at any point in the cycle will mean that actual switching occurs at current zero which is not the same instant in each phase. In back to back, the AC-DC-AC works well. In long distance transmission the use of pulses doesn't eliminate the problems that favour DC transmission.

----------- True- but the main problem is that cables have a much higher capacitance/mile/phase and a corresponding increase in charging current and charging MVAR as well as increased Ferranti effect. ------------

---------- It is a very big issue in terms of "grid" flexibility.
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| |
| wrote: |> |> | b)Potential or actual frequency differences (even of a fraction of a |> | Hertz)between systems where an asynchronous tie is of benefit in terms |> of |> | system stability Some DC ties are back to back rectifier inverters |> because |> | of this (Japan-50-60Hz, Eel River New Brunswick Canada -first solid |> state |> | converters in use 60-60 Hz ) |> |> Why does a frequency conversion need to be AC-DC-AC? Why not just switch |> pulses directly out of each phase in an appropriate balance so that you |> are |> drawing power from each steadily? Afterall, the power-sum of all three |> phases is flat across complete cycles. | ----------- | Great -try switching 1000A backed by a 500KV source at any part of the | cycle? Try it- attempting to switch at any point in the cycle will mean that | actual switching occurs at current zero which is not the same instant in | each phase. In back to back, the AC-DC-AC works well. In long distance | transmission the use of pulses doesn't eliminate the problems that favour DC | transmission.
Sure, you can switch the AC at zero crossover easy enough. But what are they doing with the DC? Either the DC is a bunchof half-cycles at twice the incoming frequency if no filtering is used, or it is filtered and relatively flat, in which case the thyristors doing the switching won't have any zero crossovers to switch at anywhere. If they can switch that much power in DC, they can switch as much power in mid-cycle AC.
Whatever they can use to switch the DC with, I suggest switching the AC with that as described before, three phase 50 Hz to three phase 60 Hz or the reverse.
|> | Both of these are of concern in the Western DC link from Oregon south to |> | California and 4 corners region of US ( where rapid control of power |> | transfer aids in stability of the parallel AC system) and in ties from |> | Northern Quebec to New York State. |> | c) long underground or underwater lines - a rough rule of thumb is that |> 30 |> | miles of cable is equivalent to 300 miles of overhead line in terms of |> | capacitance and associated problems. |> |> Looking at the transmission lines as an RF problem, going between overhead |> and underground or underwater cable, sure looks like a change in |> characteristic |> line impedance. | ----------- | True- but the main problem is that cables have a much higher | capacitance/mile/phase and a corresponding increase in charging current and | charging MVAR as well as increased Ferranti effect. | ------------
The far end of the transmission line changes its impedance different than the characteristic empedance of the line. It still looks like an RF problem ... on a very large scale.
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wrote:

------- Thyristors turn off at current zero or when the next phase takes over. This is true for inverters as well as rectifiers-the current in the individual thyristors is AC but not sinusoidal. The total from a bridge is relatively smooth DC- better than a bunch of half cycles- with notches. (Turn on is not the problem as there inductance works for you to mitigate the rate of rise of current. Interrupting current by any means is more difficult.) * Note that there are no circuit breakers involved on the DC side. The thyristors can be used to interrupt at current zeros in case of a fault on the DC side. For the AC side breakers are satisfactory. However, for grid switching and system configuration changes, this is not satisfactory as the whole DC link must be shut down at all sources (inverters as well as rectifiers require an AC source for frequency and voltage control as well as reactive needs) rather than simply switching in or out a line as done with AC. There have been HVDC circuit breaker attempts in order to get the switching flexibility of the AC grid- many introduce a resonant circuit to cause oscillations and current zeros -then an AC breaker cuts in. Generally not practical. Note that the cost of filtering at a DC station is quite high (filters on both AC and DC sides. Now consider the increased cost of such filtering for a chopped wave- if you could chop it successfully. Also consider that such filtering would be needed everywhere you have such a link. Someday there may well be a satisfactory DC breaker but not today.
There are many references available on HVDC converters and their control. -------------

--------- There is a relationship in that the telegraphers equations are valid but the operating regime is quite different. For example, a rather long 60Hz line without compensation may be about 1/8 of a wavelength. At that distance, an open circuited line will have a receiving end voltage which is about 40% higher than the sending end voltage and the charging MVAR level will be high and may cause other problems. To hold the voltage to a decent level, there has to be a reactive source at each end and often in the middle. Even at this length, there will be shunt reactors used and possibly series capacitors as well -simply to ensure a reasonable power transfer capability. Quebec Hydro's 735KV lines handle (old data) about 12 GW and require 25GVAR compensation. Hence there is a point where the AC line costs exceed the costs of going to DC.
For a 500KV underwater cable (near my home) of about 30 -40 miles, there are shunt reactors at each end and on an island in the middle. A 40 mile 500KV line is considered extremely short. In RF circuits, impedance matching is important as you are dealing with multiple wavelength lines as well as deliberate 1/4 wavelength lines In power systems these don't occur and there is no attempt to match impedances. Impedance mismatches are of importance when considering lightning switching surges (not that any attempt is made to do such matching) but not for 60Hz. There is a ballpark measure called surge impedance loading which is based on the characteristic impedance. For some shorter lines the loading can exceed this and for longer lines it must be considerably lower. No power line is operated anywhere near the maximum power point. - too inefficient and generally unstable.
* Interruption in breakers involves arcing after the contacts open. A brute force approach is to lengthen the arc until it becomes unstable. This is done with many lower voltage breakers which have arc chutes and magnetic deflection. Blowing helps by cooling the arc. AC circuit breakers - particularly at high voltages have a number of contacts in series and each may only open about an inch . No attempt is made to chop the current and the intention is to blow out arc products at current zero, replacing them with clean dielectric. Chopping can occur when small currents such as transformer magnetising currents are involved. This results in a series of very high overvoltages- not good. For DC the breaker option is brute force or finding some satisfactory way to generate current zeros. In spite of efforts this doesn't seem to have been done at a satisfactory cost.
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| Thyristors turn off at current zero or when the next phase takes over. This | is true for inverters as well as rectifiers-the current in the individual | thyristors is AC but not sinusoidal. The total from a bridge is relatively | smooth DC- better than a bunch of half cycles- with notches. (Turn on is not | the problem as there inductance works for you to mitigate the rate of rise | of current. Interrupting current by any means is more difficult.) * | Note that there are no circuit breakers involved on the DC side. The | thyristors can be used to interrupt at current zeros in case of a fault on | the DC side. For the AC side breakers are satisfactory. However, for grid | switching and system configuration changes, this is not satisfactory as the | whole DC link must be shut down at all sources (inverters as well as | rectifiers require an AC source for frequency and voltage control as well as | reactive needs) rather than simply switching in or out a line as done with | AC.
If you have DC coming in to a DC-to-AC system, how that system converting the DC to AC and creating the zeros if it doesn't cutout the DC? Is this with a resonant tank on the DC side of the thyristors?
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The AC side needs a source of AC voltage at the desired frequency. It also needs to be able to supply reactive. If you are working with a low power setup, an oscillator (as you suggest) will do but an AC grid able to accept what is being transmitted works better when dealing with hVDC systems. Rather than try to deal with this textually (as diagrams do help) I suggest that you go to http://www.hvdc.ca/pdf_misc/dcsum.pdf as a start. There are more detailed explanations (fraught with equations) of commutation in various EE texts (both for motor control and for power systems).
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