3 Phase electricity

Hi,
Could someone please explain why you have 3 phase lathes etc at home? I would have thought converting 230-240V single phase to 3 phase, then driving
a motor with the 3 phase would be less efficient that just driving a motor with single phase...Is there something I'm not getting?
Michael
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On Mon, 10 Jul 2006 00:02:09 GMT, "Michael"

Mainly because we can get three phase and the yanks can't.
I think that's reason enough ? . -- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /
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Michael wrote:

Two compelling reasons off the top.
Many of the machines being made available to the home shop world have motors that are integral or nearly so with the design of the machine. If those machines were meant to be run in an industrial environment, then like as not, they came with a 3 phase motor. Cost of converting to a single phase motor may be very high, or simply not possible.
Even when it IS possible to simply swap the motors out, the cost of a new motor may well exceed the cost of an inverter or Variable Frequency Drive. 3 phase on a VFD allows complete variable speed. An inverter can be cobbled together from a 3 phase motor and a pile of capacitors, along with some wiring (Google search "rotary phase converter build yourself") allowing one to run 3 phase gear at home where 3 phase power may not be easilly or cheaply available.
Less compelling,but a factor, 3 phase motors run a bit smoother than single phase. Reportedly, this allows better surface finishes on lathes and surface grinders.
3 phase motors have less to go wrong with them, and don't generally need more than a set of bearings to overhaul.
Were I to have to actually go and pay full retail for a new motor for a piece of gear these days, I expect that I would spring for a 3 phase motor and a VFD rather than a straight single phase motor.
As of yet, I have never had to resort to actually outright buying at retail a new motor for any of the tools that have come my way, but have accumulated an assortment that resides under one of my workbenches by keeping an eye out for opportunities at garage sales and recycling drop points.
Cheers Trevor Jones
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On Mon, 10 Jul 2006 00:02:09 GMT, "Michael"

As per other posters, cost of a replacement motor, assuming a single-phase bolt in replacement is available, and once you have the converter you can run other things as well.... :-))
Other than very small lathes like the Myfords etc., most lathes came as standard with three-phase equipment. Similarly with mills etc etc. The main reason home converters became affordable was the demand from home machinists.
It can be a real pain to convert to single-phase where there are contactors with no-volt coils etc etc that cannot be easily changed around.
We've got a 3-phase supply at home so not a problem for us.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: http://www.oldengine.org/members/diesel
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3 phase electricity is freely available, it's just that the companies do not connect all three phases to each house.
The connections are made with one house on one phase, the next house on the next phase and the next house on the last phase, this evens out the load.
Alan

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Alan Holmes wrote:

That's the case in a rural village with the old style overhead cables, but not for underground cabling in which a street is on a phase, the next street on the next phase etc. That way they only need a single phase cable down a street.
Of course situations vary but this is typical of a housing estate, unless you happen to be on the main street that feeds a lot of side streets in which case you could be lucky enough to have all three phases outside your door.
Greg
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companies do not

house on the

the load.

cables,
Not quite so Greg.
My suburban road (12 mile to Charing Cross) is wired one house on one phase, the next house on the next phase and the next house on the last phase as Alan describes - and it's certainly not a main road. I think the lighting pops in there somewhere as well. So we have all three phases in the road, but that doesn't stop EDF wanting 5K to connect it, and that's with me providing the ducting from the street.
AWEM
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On Wed, 12 Jul 2006 17:32:33 +0100, "Andrew Mawson"

We have a phase all to ourselves, direct from the transformer. Network Rail have 3-phase off the same transformer for signalling equipment. It ought to be dead easy for us to get 3-phase, but I've never asked just because I *know* it would be expensive!
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
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On Wed, 12 Jul 2006 17:32:33 +0100, "Andrew Mawson"

Our local boards in both Luton and Rushden have 3-phase cabling down streets, especially when there is street lighting as well. Haven't seen single-phase street cables since the 1950's when the demand was lower per house, probably DC supplies originally...
Luton had a DC Power Station all its own, and the remains of the station and some of the equipment foundations remain in Power Court. DC was generated 3-wire 220/440V and went out like that until replaced by AC sets in the 1930's I believe, and then onto the grid feed from Sundon Park.
Peter -- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk http://www.prepair.co.uk
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My grandfather was an engineer at Luton during the change to AC. Apparently it was quite easy to convince domestic customers that the change was a good idea, all you had to do was explain that they could power the wireless from the mains and they were happy!
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wrote:

station
was
AC sets

Park.
could
Ah but they could no longer charge the filament accumulator from the difference between earth and neutral. Convenient unmetered low voltage DC source derived from the voltage drop along the neutral !
AWEM
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wrote:

On that point, how did they meter a DC supply? Martin
--
martin<dot here>whybrow<at here>ntlworld<dot here>com



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On Wed, 12 Jul 2006 18:40:19 GMT, "Martin Whybrow"

They didn't, you paid a subscription to the company for the service, metering didn't really come in until widespread AC supplies and the induction watt meter were available.
I think the Luton company had a limited scope of supply, mainly the town centre (s it was) plus some street lighting, so it may well have been more commercial than retail as we know it these days.
Don't forget, electricity was a luxury for most in those days.
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: http://www.oldengine.org/members/diesel
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wrote:

the
DC
replaced by

Sundon
the
the
voltage
They used an interesting DC meter with a pool of mercury in the bottom - made in large quantities by Ferranti - I have a 1/2 size sales sample in the form of a calendar that came my way while employed by said company for nearly 3 decades ! I think they worked on the magnetic repulsion principle of current flowing from the axle to the rim of a copper disc but never had the opportunity to examine a working one.
AWEM
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Big Snip

A full history of Electric Meters is at http://www.watthourmeters.com/history.html I had read about the Electrode method in some early books I have.
--
Dave Croft
Warrington
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wrote:

AC.
that
they
from
employed
Google kindly provides this description:
... that of S. Z. Ferranti, who introduced it in 1883. It consists of an electromagnet within the iron core of which is a flat disk-like cavity containing mercury, the sides of the cavity being stamped with grooves. The thin disk of mercury is therefore traversed perpendicularly by lines of magnetic force when the magnet is excited. The current to be measured is passed through the coils of the electromagnet, then enters the mercury disk at the centre, flows through it radially' in all directions, and emerges at the periphery. mass of mercury is thus set in motion owing to the tendency of a conductor conveying an electric current to move transversely across lines of magnetic force; it becomes in fact the armature of a simple form of, dynamo, and rotates with a speed which increases with the strength of the current. The roughness of the surface of the cavity serves to retard it. The rotation of the mercury is detected and measured by means of a small vane of platinum wire immersed in it, the shaft of this vane being connected by an endless screw with a counting mechanism. The core of tke electromagnet is worked at a point far below magnetic saturation hence the field is nearly proportional to the square of the current, and the resistance offered to the rotating mercury by the friction against the sides of the cavity is nearly proportional to the square of the speed. It follows that the number of the revolutions the mercury makes in a given time is proportional to the quantity of electricity which is passed through the meter. In order to overcome the friction of the counting train, Ferranti ingeniously gave to the core of the electromagnet a certain amount of permanent magnetism.
from http://encyclopedia.jrank.org/MEC_MIC/METER_ELECTRIC.html
AWEM
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On Wed, 12 Jul 2006 20:25:11 +0100, "Andrew Mawson"
<snipped>

I wonder if Luton was in fact all private subscribers? That description certainly predates the period we are talking about, but my information is that Luton didn't have metering in those days of DC. I'll do some more digging...
Peter -- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: http://www.oldengine.org/members/diesel
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On Wed, 12 Jul 2006 20:44:57 +0100, Peter A Forbes

My maternal grandfather worked for many years for the Lancashire Dynamo & Crypto in Trafford Park. One of the thinks he did was designing booster stations for DC mains power. Sheds full of batteries & switchgear, I think. Somewhere I should have one of his old blueprints for such a station. Somewhere....
Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
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Ferranti of course was an advocat of AC, and built the Deptford Power Station in 1890 using AC and coaxial cable for distribution.
There is the famous safety demonstration of his cable where he hacked though a live cable (with a pick axe I think) to show that the system fused but no-one got hurt. JCBs are doing it to this day <G>
AWEM
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I seem to recall reading that during a similar debate in the US the supporters of AC pointed out that the electric chair was powered by DC, which obviously made it more dangerous than AC -otherwise they would have used AC to power it. Actually the AC suppliers had tendered for the contract to supply the power for the electric chair, but lost the contract -showing a grip of "spin" well in advance of the time!!
Regards Kevin
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