4 ms·

"By the mid-20th century, large rectifiers installed at two downtown substations supplied the current, pushing 250-V DC onto a rectangular loop of 3.6-centimete

"By the mid-20th century, large rectifiers installed at two downtown substations supplied the current, pushing 250-V DC onto a rectangular loop of 3.6-centimeter-diameter cable." OK, can anyone explain why each of the drum-wind elevator motors does not have its own small rectifier and be run from local AC mains (possibly cross-phase for 1.4x voltage)? As this interesting article goes on to say the local power company is splitting the DC distribution into smaller isolated networks anyway...

asynchronous13 13y ago
Remember that these were installed ~50 years before the transistor was invented. At the time, the hardware was very expensive. It was more economical to have a single centralized rectifier than multiple distributed rectifiers. Since they were installed, it comes down to maintenance costs vs gutting and installing a brand new system. Maintaining an older system was apparently the choice these building owners made.
- cnvogel 13y ago
  At that time, rectifiers often consisted of a AC motor driving a DC generator. Which probably could have been moved to customers, but was avoided for maintenance and reliability reasons (more easy to service one big generator/motor pair than a few hunderes scattered aound the city, making noise in residential homes, ...) Nowadays it would be trivial, a complete motor control for variable-speed 3-phase motors, that could technically of course also drive DC, costs a few hundred dollars/euros. But probably not possible for certification reasons, as soon as you mess around with an old system you have to upgrade it to correspond to contemporary safety standards.
- keithpeter 13y ago
  Yes, I accept your point for when the system was originally provided. But now it strikes me that the older systems could be run from a power cabinet with a step up transformer and a rectifier running off AC mains given a supply with sufficient current capacity.
  - Aloha 13y ago
    They could have then, but mercury vapor rectifiers are/were expensive, mogen sets are noisy and need service - its far easier to put a big one in, than lots of little ones.
- bobdvb 13y ago
  Many DC elevator systems also included Mercury Arc Valve rectifiers: http://en.wikipedia.org/wiki/Mercury-arc_valve http://en.wikipedia.org/wiki/Mercury-arc_valve They are still in use in some places and a few can be found in abandoned stations on the London Underground: http://www.eetimes.com/author.asp?section_id=36&doc_id=1319738 http://www.eetimes.com/author.asp?section_id=36&doc_id=13197...
lttlrck 13y ago
It's an interesting point. Maybe it was easier to reuse the existing cable topology. Edit: or perhaps the rectifiers are an off-the-shelf variety with a capacity far beyond the requirement of a single motor so sharing cut the costs.
mrbill 13y ago
A lot of places are switching to this; see NYC for an example. Instead of getting DC from the power company they get AC and convert it to DC on-site. ConEd shut down it's DC grid in '09 I think?
jccalhoun 13y ago
I'm sure that the power company could say to them "we're not doing DC any more" and force the businesses to fix the problem by installing their own rectifiers or upgrading the elevators but obviously the power company still thinks it is more cost efficient for them to continue to provide DC to these businesses than to cut them off. I'm sure that when the time comes that the power company doesn't think it is cost effective any more then it will turn off the DC.

asynchronous13 13y ago

Remember that these were installed ~50 years before the transistor was invented. At the time, the hardware was very expensive. It was more economical to have a single centralized rectifier than multiple distributed rectifiers. Since they were installed, it comes down to maintenance costs vs gutting and installing a brand new system. Maintaining an older system was apparently the choice these building owners made.

cnvogel 13y ago
At that time, rectifiers often consisted of a AC motor driving a DC generator. Which probably could have been moved to customers, but was avoided for maintenance and reliability reasons (more easy to service one big generator/motor pair than a few hunderes scattered aound the city, making noise in residential homes, ...) Nowadays it would be trivial, a complete motor control for variable-speed 3-phase motors, that could technically of course also drive DC, costs a few hundred dollars/euros. But probably not possible for certification reasons, as soon as you mess around with an old system you have to upgrade it to correspond to contemporary safety standards.
keithpeter 13y ago
Yes, I accept your point for when the system was originally provided. But now it strikes me that the older systems could be run from a power cabinet with a step up transformer and a rectifier running off AC mains given a supply with sufficient current capacity.
- Aloha 13y ago
  They could have then, but mercury vapor rectifiers are/were expensive, mogen sets are noisy and need service - its far easier to put a big one in, than lots of little ones.
bobdvb 13y ago
Many DC elevator systems also included Mercury Arc Valve rectifiers: http://en.wikipedia.org/wiki/Mercury-arc_valve http://en.wikipedia.org/wiki/Mercury-arc_valve They are still in use in some places and a few can be found in abandoned stations on the London Underground: http://www.eetimes.com/author.asp?section_id=36&doc_id=1319738 http://www.eetimes.com/author.asp?section_id=36&doc_id=13197...

lttlrck 13y ago

It's an interesting point. Maybe it was easier to reuse the existing cable topology. Edit: or perhaps the rectifiers are an off-the-shelf variety with a capacity far beyond the requirement of a single motor so sharing cut the costs.

mrbill 13y ago

A lot of places are switching to this; see NYC for an example. Instead of getting DC from the power company they get AC and convert it to DC on-site. ConEd shut down it's DC grid in '09 I think?

jccalhoun 13y ago

I'm sure that the power company could say to them "we're not doing DC any more" and force the businesses to fix the problem by installing their own rectifiers or upgrading the elevators but obviously the power company still thinks it is more cost efficient for them to continue to provide DC to these businesses than to cut them off. I'm sure that when the time comes that the power company doesn't think it is cost effective any more then it will turn off the DC.