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This New York Times article says that on November 14, 2007 Consolidated Edison Company was going to end supplying direct current to the remaining several buildings that used those to run elevators.

I don't get why there would be DC-powered elevators.

Yes, at the end of 19th or the beginning of 20th century DC equipment was rather popular, but it's unlikely that an elevator would last a century and all the elevators I could find information for now run on AC.

Also DC is typically preferred for easier RPM control (like in trains, tramcars, etc) but this is not a problem for elevators - they typically use two-speed three-phase AC motors and speeds are changed by switching the number of poles.

Why would there still be elevators running on DC?

sharptooth
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    Sounds like a typical problem many companies face... That being, it is easier to get budget to maintain something then it is to upgrade something. – Kellenjb Feb 11 '13 at 14:04
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    It is unlikely that an elevator built *today* would last a century. – Kaz Feb 11 '13 at 14:07
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    @Kellenjb: Maybe, but maintaining a century old DC motor doesn't sound that easy. – sharptooth Feb 11 '13 at 14:07
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    Many types of AC motors that are taken for granted today probably did not exist when some of the DC-powered elevators were installed. The way many building codes are written, a building owner may often continue to use a system which was built contrary to modern practices provided no major changes are made to it, but may not make any major change without bringing it up to modern standards (which would in many cases require replacing pretty much everything). Switching from AC to DC would likely be regarded as a major enough change to trigger such a requirement. – supercat Feb 11 '13 at 17:54
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    I was associated with an institution founded 1839 and with an elevator dating from probably 1927. We were the last DC customer in the city. I believe the utility company helped with the conversion expense when they ended DC supply. – user207421 Feb 11 '13 at 23:18
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    @EJP: Yeap, the article says ConEd provided a rectifier to each disconnected customer. – sharptooth Feb 12 '13 at 06:16
  • Todays elevator don't switch poles on the machines, instead they are controlled by a frequency converter like a VLT. http://www.danfoss.com/BusinessAreas/DrivesSolutions/ – jippie Feb 12 '13 at 19:44
  • @jippie: Those are tomorrows elevators for most of users, a lot of people still use yesterdays pole switching stuff. – sharptooth Feb 13 '13 at 06:01

4 Answers4

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Century old motors were well built! And probably conservatively designed because electricity was new; they didn't know which corners you could safely cut.

In those days, everything mechanical was designed for easy maintenance; nuts, bolts, taper pins; simple tools to take the whole lot apart, adjust to take up wear, reassemble and use for another 10000 miles. Run out of parts? Turn another one to fit!

I had a 1910-era lathe still capable of turning within about 0.002" (traded it for a 1928 model!) and my 1840s watch is keeping very good time.

In an era of relatively cheap labour and expensive materials, this made sense. Who knows, we may end up back there some day!

Meantime it's worth studying how things from another era are made; partly to keep the skills alive and partly because good engineering is good engineering, from any era.

Just to clarify because this seems to have hit a nerve : I'm not simply equating long life with good engineering. What makes these motors good engineering is the skill with which they met their design goals using materials and techniques available at the time.

And long life was almost certainly one of them; reliability (not measured as MTTF but the ratio between MTTF and MTTR) i.e. easy repair, and efficiency. Swapping motors for a fix is not the issue; replacing brushes, re-lining bearings or (major job!) rewinding the motor was what happened - and what the motors were designed for. It's NOW we kinda-sorta-fix things by replacing motors.

We haven't improved THAT much on 92% efficiency in a motor in the last hundred years, but we do it with a lot less copper and iron. We can equally well admire a modern brushless motor with sealed bearings and no maintenance for ten years; they can both teach us something.

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    +1 for "good engineering is good engineering, from any era." – Shamtam Feb 11 '13 at 16:21
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    I would think the biggest change wasn't the price relationship between labor versus raw materials, but rather the differential between the labor required to produce 10,000 individually-specified parts versus 10,000 identical parts; if companies were interested in such a thing, they could use continued improvements in technology to swing that pendulum back somewhat. If a company has precise specs for its parts in machine-readable format, the cost to custom-machine replacements may go down as CAM facilities improve. – supercat Feb 11 '13 at 16:25
  • @Shamtam: If the goal of engineering is to find the cheapest solution that meets requirements, the fact that something works a hundred years after it's built is not necessarily indicative of good engineering. If there's a 50% chance that within 50 years a building would no longer meet requirements, and would be torn down and replaced with one that would, an elevator that would last 100 years would likely not be worth twice as much as one that would need replacing after 50, so if the 50-year elevator would be cheaper, it would be a good choice. – supercat Feb 11 '13 at 16:29
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    heh - I didn't specify _human_ labour! (how's that for a save?) Certainly CAM is one way to change the balance. –  Feb 11 '13 at 16:30
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    @Shamtam: I think the reason many century-old things are still around is that the only way to build something that would be 99% certain to last for 20 years was to build it in such a way that it would have a substantial likelihood of lasting 100. While it's neat to have relics that were built like tanks and are still going strong, money which is spent building something to last far beyond the point where it will get scrapped as redundant is money that can't be spent on other things. – supercat Feb 11 '13 at 16:38
  • @BrianDrummond: Another point I forgot to mention is that there's a change in people's expectations of reliability. It used to be that random appliance motors would have value, because while they were all different, they were in some sense interchangeable: one could replace a motor with an arbitrary-chosen one of suitable size and end up with something that would kinda-sorta work for awhile. And when it failed one could install another. No guarantee that any fix would last especially long, but finding motors wasn't a problem. But fewer people today like kinda-sorta fixes. – supercat Feb 11 '13 at 16:44
  • @Supercat : one of my favourite books is Cassells "New Technical Educator" (1897 edition). In the section on building carriages, they advised on how to achieve a 50 year lifetime (as opposed to cheap American imports lasting "as many months"!). On electric motors, they documented 92% efficiency as best practice. That involved a lot of copper, therefore low stress and long life, but it was justified by the price of energy at the time. –  Feb 11 '13 at 16:46
  • @supercat At the same time the marginal cost of extending the lifetime of something vs replacing it *now* often results in things being kept well past their original design life. ex thousands of depression/post ww2 era road bridges still in use despite an original 50 year design life. Feeding into that, locally in the past few years I've seen several cases where when actually replacing a bridge PennDOT chose the proposed design with the longest life expectancy over the least expensive one that hit the target in their original RFP. – Dan Is Fiddling By Firelight Feb 11 '13 at 17:00
  • @DanNeely: If the design of a bridge makes it possible to inspect it, and measure the extent to which corrosion was less severe than was conservatively predicted when it was built fifty years before, there's no reason not to take advantage of one's good fortune. Also, replacing something may have costs beyond materials and labor, especially if the thing being replaced must be taken out of service before its replacement can be installed. – supercat Feb 11 '13 at 17:42
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    Many of the early Gothic cathedrals fell down, especially in England where they were on the end of both the labour chain and the pilgrim chain that generated the money. The cathedrals that remain are very likely over-engineered in consequence. – user207421 Feb 11 '13 at 23:19
  • So basically your answer is that those people still use century old DC motors isn't it? – sharptooth Feb 12 '13 at 07:05
  • oops, I didn't explicitly say that, did I? Yes I think it's probable that, given regular maintenance, some of the original motors are still running today. –  Feb 12 '13 at 09:59
  • That's just unbelievably cool. Where I live elevators are just replaced after about 30 years no matter what - everyone likes new shiny stuff. – sharptooth Feb 12 '13 at 14:29
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    "Meantime it's worth studying how things from another era are made; partly to keep the skills alive and partly because good engineering is good engineering, from any era." Kudos to that! – Anshul Feb 13 '13 at 04:10
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I think the reason is DC motors when series connected (the armature and field coils) has a high torque at low speed (including 0) which fits well for elevators, trains and traction purposes.

Joan
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San francisco also has a existing DC network in place which supplies old elevators.

I think the answer is far more mundane ...

In the early days of power transmission there was the Edison vs. Westinghouse DC vs. AC fight with Edison being a proponent of DC (Edison promoted the electric chair in part to show how dangerous AC was). This was known as the "war of the currents".

It's likely that this is just a hold over from the days when some cities started out with DC distribution grids.

The big hint is that it's Con Ed that is supplying the power ...

from an article in IEEE spectrum magazine Nov 2012, by Peter Farley

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There's more in there about winding motors on elevators. Dynamos produce direct current.

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dc motor provide high starting torque with less transients also it enables to vary the speed at different ranges and easy control with armature voltage control and flux control. for ac induction motor, if u change the number of poles it increases the synchronous speed of the motor to control the speed it requires external auto transformer which increases the cost.

Paulraj
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