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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?

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    \$\begingroup\$ 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. \$\endgroup\$ – Kellenjb Feb 11 '13 at 14:04
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    \$\begingroup\$ It is unlikely that an elevator built today would last a century. \$\endgroup\$ – Kaz Feb 11 '13 at 14:07
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    \$\begingroup\$ @Kellenjb: Maybe, but maintaining a century old DC motor doesn't sound that easy. \$\endgroup\$ – sharptooth Feb 11 '13 at 14:07
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    \$\begingroup\$ 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. \$\endgroup\$ – supercat Feb 11 '13 at 17:54
  • \$\begingroup\$ 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. \$\endgroup\$ – user207421 Feb 11 '13 at 23:18
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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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    \$\begingroup\$ +1 for "good engineering is good engineering, from any era." \$\endgroup\$ – Shamtam Feb 11 '13 at 16:21
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    \$\begingroup\$ 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. \$\endgroup\$ – supercat Feb 11 '13 at 16:25
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    \$\begingroup\$ heh - I didn't specify human labour! (how's that for a save?) Certainly CAM is one way to change the balance. \$\endgroup\$ – Brian Drummond Feb 11 '13 at 16:30
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    \$\begingroup\$ @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. \$\endgroup\$ – supercat Feb 11 '13 at 16:38
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    \$\begingroup\$ "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! \$\endgroup\$ – Anshul Feb 13 '13 at 4:10
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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

enter image description here

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

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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.

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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.

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protected by Dave Tweed Jul 9 '15 at 11:37

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