In another thread David Hamley wrote, “The DC system was certainly a fascinating contraption with that underground conduit scheme.”

I’ve often wondered how that conduit system (and those in other cities too) ever functioned with reliability when it was subjected to all that could be washed into it from the streets - mud, salt, snow, ice, rain downpours, and maybe more. Seems that such stuff could easily interfere with sufficient electrical contact by the streetcar collectors/shoes/pickups, and could even lead to shorting out the current. Any thoughts?

Road salt was prohibited in Washington, DC on streets that had conduit. The story is told about snow removal for President Kennedy's 1961 inaugural. There was a huge storm within days of the event and the streetcar system was in the throes of abandonment. As a result the military folks and city staff insisted on using road salt and the conduit system was a mess afterward.

The conduit system was built with storm drains to collect run off. There were access hatches evenly spaced to allow for access to the conduit slot for cleaning etc. Overall it was fairly reliable for about 60 years.

Wesley

Conduit streetcar operation was in cities like Washington and Manhattan that had extensive cable car operation before electrification. City fathers wanted to avoid those ugly overhead wires. Most cable car lines were converted to streetcar operation soon after electric operation was perfected because it was so much cheaper. At one time thirty cities in the United States had cable cars. Cable cars survived only in those places like San Francisco and Seattle where hills were too steep for the electric trolleys.

Flat land cable cars survived in Melbourne, Australia, until 1940 because the government was too poor to convert them.

Contact with the third rail was from the side, not like top or underiding third rail shoes commonly used on electric railways.

Special work cars scraped ice out of the slot in winter weather.

Shoreline Trolley Museum in East Haven CT has an excellent display in the Sprague building of conduit operation.

Here is a link to a photo of a conduit section displayed at National Capital Trolley Museum.

http://www.flickr.com/photos/mr_t_in_dc/7347026390/

Washington did not have slot scraper cars for winter weather. Instead, the Company ran limited service at all hours to keep the slot open. In summer, the slots would expand and catch the conduit plow. A pulled plow would easily back up traffic until a trouble truck could address the problem.

One double-end cars, the motorman had to know where the plow was in order to activate the street switches.Cars were marked as No. 1 and No. 2 end to indicate the location of the plow.

A Youtube video link showing conduit operations.

http://www.youtube.com/watch?v=A5H2r3AwqjQ

Wesley

If someone can find the missing photo links, here is an article about artifacts at NCTM.
[url]
http://www.rypn.org/briefs/single.php?f ... 003158.txt[/url]

Wesley

Here is a photo of our conduit yoke artifact. The image was taken during construction of our visitors center. The yoke was moved in place and the building was completed around it.

NCTM's conduit artifact was removed from R street in the vicinity of 4th Street, NE in the Eckington neighborhood. The yokes and rails were encased in concrete as a single piece. When removed from the street, the artifact appeared to be some kind of vault, perhaps a catch basin for draining run off from the slot rail. Contractors chipped away the concrete to reveal the yokes as they appear today. The slot rails were welded closed with a section of rebar (since removed). At the time the yokes were recovered from the street, pieces of the slot rail and an insulator were still in place, but appear to have been lost after the artifact was moved to outdoor storage at the Museum. Cracks and chips in the cast iron yokes demonstrate the brittle qualities of the iron used in the casting. This brittle quality made previous efforts to acquire conduit artifacts impossible since the iron would easily break during excavation attempts.

Wesley

Did most/all conduit systems use 4th rails (so the running rails were not used for power return)?

Steve

Who else besides in Washington, D.C. and Manhattan (TARS) used conduit systems? I'm curious.

There were successful conduit systems in Budapest and London.

Wesley

Actually, does a 4th rail or double trolley create polarity problems? If you turn the car around and go the opposite direction, the power polarity is reversed. Would this be a problem for accessories and directional controls?

Generally, old fashioned trolley cars and trackless trolleys weren't sensitive to polarity.

The direction is set by the relationship of the current flow through the field coils and the armature. The reverse drum in the controller (old K style) or under the floor (MU cars, etc) sets up the connections.

If you have permanent magnets for the field (like in models) then changing the polarity will change direction.

My original engineering degree is Civil, not EE Rotating. but I don't see that as much of a problem. Anyone who has successfully wired an HO scale model railroad with a wye or a return loop in the days before DCC can work out the basic circuitry. Unless I am sorely mistaken the normal method of reversing a direct current motor is simply to reverse the polarity. All you would need for a loop on a single track is what modellers call a 'reversing section', that is a section of wire which is is electrically separate and can be energized from either end but not both at the same time. The wire is energized from the direction of approach until the car is in the segment. The car stops momentarily while the wire is de-energized from the direction of approach and then re-energized in the direction of departure. Meanwhile the operator throws a switch (if necessary) on the car reversing the polarity of the motors.

Most of the trackage in double wire systems was operationally one way so this would not have been a major problem. The only regular bidirectional operation I can think of under a single set of wires was a short around-several-blocks trolley-bus route in New Orleans where direction was alternated periodically (odd-even years as I recall).

Some systems have had sections of interlaced (gantlet) wirework which appear to have vehicles operating in different directions off of one set of wires but electrically this is not the case.

GME

In Washington, polarity of the powered slot rails could change between sections. This required special accessories like dual line switches on the PCCs and a K66 version of the K35 controller. There is some discussion of this subject in Roy King's 100 Years of Capital Traction. I will get my copy and provide additional information in a future post.

Wesley

A brush commutator motor doesn't care if the polarity changes because it changes for both armature and field coils. Such motors can also run on AC with some minor changes in the design of the magnetic structure and, for larger motors, the AC frequency is low enough.

How do you change direction then? You change the polarity of either the armature or field connections but not both.

The motors that do reverse when power polarity reverses are the permanent magnet motors because the magnet providing the field obviously remains the same when the power polarity is changed so the motor reverses.

You can also arrange for a motor that has wound field coils to behave like a PM motor by feeding either the armature or the field (but not both) via a bridge rectifier so its polarity stays the same while the incoming power is switched. Then it will reverse direction like a real PM motor.

Steve