The single phase AC field can be looked at as two counter-rotatin fields. The torque produced from both combined with the rotor at rest cancels The motor won't start. The starter windings produce a slightly larger field rotating in on direction, and the motor begins to run. ( Or you can manually spin the motor in one direction or the other) Due to the shape of the torque-slip curve, once it is running in on direction the torque produced by the field rotating in the opposit direction becomes very small, while the torque produced from the field rotating in the same directio increases.
Ahh, a willy waver!, I can do that with the best of em: a Masters degree, currently designing multi megawatt generator control systems that are sold world wide and work with practically every AC system in use, have designed
400Hz systems for various military and aviation applications around the world, oh and a fully qualified electrical inspector in the UK.
If you don't say what you mean you cant expect people to know what you mean!. Greg
It seems that there has been a miss interpretation of a little sloppy phrasing...
Could I ask you to forgive and forget... there appears to be a danger of starting a totally unnecessary flame war.
As I understand it the Americans ( mainly ) use a 220 3 phase distribution system ( 120 degree phase separation ) this is then supplied to domestic users as a 110-0-110 supply, by centre taping a phase to earth. Effectively the 110-0-110 supply is a 110 V two phase supply with a 180 degree phase difference. The Europeans centre tap to earth a three phase 400 V system to give a 230 V single phase domestic supply.
A 3 phase supply will allow a motor to self start, a single or two phase supply will not unless the motor is equipped with capacitors to start / run it.
I think if you both agree with the above, peace can be declared... If not I fear I may have opened a second front :-(
Agreed, I have no desire for that, I just wanted to correct a bit of mis-information in case it causes people grief.
Well sort of, the Americans do indeed use a 3 phase high voltage distribution system and drop it down by transformer to 3 phase 4 wire at low voltage for industial supplies, as does virtually every country in the world, though the low voltage varies considerably depending on the state (New York uses a whopping 277V phase-neutral) which is a bit of a problem for industry. Oh and the neutral (star point of the transformer secondary) is earthed.
The big difference comes with the domestic supply, in the UK and indeed most countries of the world each house gets one phase and the neutral from a 3 phase supply supply, the houses are spread across the three phase to roughly ballance the load. Typically one street gets the red phase, the next the yellow phase, the third the blue phase and then it repeats (yes I DO know the colours have chagned 8-))
The US and some other countries that followed them are very different, a group of houses has it's own transformer, the ones you always see exploding on top of a pole in the movies 8-), this has a single phase high voltage primary and a single phase low voltage secondary which is centre tapped to produce the 110V-0-110V (or whatever voltage the state uses) three wire supply to the houses, the centre tap being earthed. Their socket outlets are connected from one end (or 'hot' wire) to the centre tap and domestic appliances being permenantly wired across both 'hot' wires. They believe this to be safer, though I don't know if there is any evidence to back it up, it is definitely inferior form a technical point of view as they have to draw twice the current from a socket outlet for a given appliance thus increasing the conductor sizes.
As I said before some apartment blocks are different but lets not get into that again.
Yes exactly, which is no use for powering a 3 phase induction motor, they have to use a single phase motor, in this case rated 220V, across the 'hot' wires of the supply.
Yes exactly, but not only self start, 3 phases makes the motor small and efficient, single phase motors are generally larger, less efficient, less reliable and more expensive than a similarly rated three phase motor. The three phase motor is the mainstay or industry world wide and for very good reasons.
Well I really hope we can leave it there, this is a UK group after all so this is of academic interest. The UK distribution system is by far the most consistent system in the world, we all get the same type of supply within the specified tolerance, with only the type of earth supplied varying somewhat.
What people do with their news feed is up to them and beyond anyone's control, but this group is in the UK hierarchy of Usenet and therefore a UK group. I'm not saying that readers from the US are not welcome, but they should not treat it as a US group. It is not up to us to opt out of something that was done, I presume, without the agreement of all here.
I cannot see any linkage to any other groups other than uk.rec.models.engineering, and I have been back through the last 10 posts, so one must assume that
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is doing this of its own volition, we certainly aren't cross-posting from here as far as I can see.
Peter
-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:
The purpose of this group is defined in its charter. You can read a copy here:
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The first line is as follows:
"uk.rec.models.engineering is for the discussion of model engineering within the UK."
If you wish to change the charter I expect you will find the procedure for doing so on the site above.
How about I get Al Jazeera to reproduce some RCGroups forums on their website, then ask you all to speak in Arabic so their readers can understand. I'm sure you'll be accommodating.
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