As I understand it:

A. Electric vehicles were/are typically powered with DC current (streetcars, LRVs, interurbans, commuter trains, freight motors etc. - and even trolley busses).

B. DC current cannot be transmitted very far, in contrast to AC current.

So my elementary questions are:

1. Why DC powering instead of AC?

2. About how far can DC be adequately transmitted in overhead wires, third rails, conduits, etc.?

3. How are these conductors kept properly energized over much longer distances? Is it with parallel AC lines feeding into the conductors at intervals through rectifiers or rotary converters?

> As I understand it:

> A. Electric vehicles were/are typically
> powered with DC current (streetcars, LRVs,
> interurbans, commuter trains, freight motors
> etc. - and even trolley busses).

> B. DC current cannot be transmitted very
> far, in contrast to AC current.

> So my elementary questions are:

> 1. Why DC powering instead of AC?

Because series wound DC motors have the best charateristics for a traction motor; variable speed, high starting tourque, etc. It is only now that AC motor control technology has surpassed the old standard.

> 2. About how far can DC be adequately
> transmitted in overhead wires, third rails,
> conduits, etc.?

Heavy duty interurban operations like the Insull properties in the Chicago area tended to have electrical sub-stations that supplied the overhead wire from parallel AC transmission lines every three to six miles. Using a higher DC voltage on the overhead could increase this somewhat; 1500VDC was common for operations that didn't have to share wire with streetcars.

> 3. How are these conductors kept properly
> energized over much longer distances? Is it
> with parallel AC lines feeding into the
> conductors at intervals through rectifiers
> or rotary converters?

Yes.

Dennis Storzek


dstorzek@elnet.com

Some answers to your questions:
The series wound DC motor best served the requirements of variable speed and high torque demanded by railway service. Motor-generator sets, booser stations, and rectifier sub stations provided the means to transmit DC power over distances. Starting early in the 20th Century 12,500 volts at 25 hz AC was adopted for overhead of class 1 railroads. It was found that series wound motors could be designed to also run on low frequency AC. The big advantage to AC is that it's voltage could be easily stepped up and down through the use of simple transformers. In Europe today, 25,000 volt AC at 16.66hz is common for railways over there. Today, power is transmitted from the Columbia River in Washington to Los Angeles via high voltage DC. Rectifiers convert the AC generated by the plants to million+ volt DC potentials and placed on the transmission lines. At LA solid state thyristor converters change it back to AC for local distribution. Today's semi conductors now make it more efficent to transmit power using DC rather than AC. They have also made the AC induction motor more adaptable for railway service. Their ability to provide variable frequency AC from DC provides variable speed control for AC induction motors.
Roger Mitchell
Master Mechanic
Fort Collins Municipal Railway

n0mcr@netzero.net

On a similar note, is there any difference in construction of the trolley wire/catenary for a 600v DC system vs. a 1500v DCsystem? If you had an existing 600v trolley wire/catenary and you wanted to energize it at 1500v, would any insulators, brackets, etc. have to be changed?

Thanks,
Tom

rrhistorian@hotmail.com

> As I understand it:

> A. Electric vehicles were/are typically
> powered with DC current (streetcars, LRVs,
> interurbans, commuter trains, freight motors
> etc. - and even trolley busses).

> B. DC current cannot be transmitted very
> far, in contrast to AC current.

> So my elementary questions are:

> 1. Why DC powering instead of AC?

> 2. About how far can DC be adequately
> transmitted in overhead wires, third rails,
> conduits, etc.?

> 3. How are these conductors kept properly
> energized over much longer distances? Is it
> with parallel AC lines feeding into the
> conductors at intervals through rectifiers
> or rotary converters?

DC power was chosen for trolley lines, etc. because the DC series wound traction motor was (and still is) ideal for the application, and AC technology did not exist at the time of development of electric street railways. Moreover early straight AC traction motors were bulky, unreliable and very inefficient.

AC power is used for long distance transmission as it can be easily transformed from one voltage level to another. (DC power cannot.) For example, if you double the transmission voltage, you can transmit four times the amount of power or transmit the same amount of power four times the original distance without increasing the line losses,etc.

How far you can transmit DC power for railway purposes depends on the voltage level, the load and the size of the conductors. The proper voltage level is maintained by substations which convert hight voltage three phase AC to say 600 volts DC using rotary converters, mercury arc rectifiers, or (today) silicon diode rectifiers.

aeriola@adelphia.net

> On a similar note, is there any difference
> in construction of the trolley wire/catenary
> for a 600v DC system vs. a 1500v DCsystem?
> If you had an existing 600v trolley
> wire/catenary and you wanted to energize it
> at 1500v, would any insulators, brackets,
> etc. have to be changed?

> Thanks,
> Tom

A simple answer is, yes. Insulators used in urban light rail and trolleybus systems are typically rated at 750 volts. These would have to be replaced with insulators rated for higher voltage.

Also, clearances between the live part of the trolley system and adjacent structures would have to be greater to avoid the possibility of arcing, particularly in damp conditions.


artschwartz@sprintmail.com

After reading thie entire thread very carefully, I realized just how little I know about electricity. I grew up near the MILW electrified line and remember hearing the stories of the locomotives putting "power back in the cantenary" going down mountain passes (a process called "regeneration" by the MILW).

It is always fascinating to be in company of such skilled folks and to realize how much there is to learn about this fascinating industry.

Thanks.

The 600 volts DC at the Western Railway Museum have gone about as far as they can go. There is about 5 miles of wire in the air.

A sub-station is being designed so that additional
power can be taken from Pacific Gas and Electric and converted to 600 volts DC. This is not helped by a younger generation of electrical engineers who know from nothing about railroad practice.

At is peak, the Sacramento Northern was running 1500 volts DC which helped in the transmission of the current.

Ted Miles

ted_miles@nps.gov

> On a similar note, is there any difference
> in construction of the trolley wire/catenary
> for a 600v DC system vs. a 1500v DCsystem?
> If you had an existing 600v trolley
> wire/catenary and you wanted to energize it
> at 1500v, would any insulators, brackets,
> etc. have to be changed?

Typical 1200V practice was to double-insulate.
Where you would normally have a single wood or
porcelain strain insulator in a span, you'd have
two in "series".

webmaster@bera.org

Many thanks to all of you for your expert information in answer to my questions! I've learned a lot, and I certainly agree with Scott about all the "skilled folks" that can be contacted on this board!

Dick

> The 600 volts DC at the Western Railway
> Museum have gone about as far as they can
> go. There is about 5 miles of wire in the
> air.

We just ran a continuous voltage/current recorder on a Key System unit to the end of wire, and were down to 195 volts at the end.

The low voltage is a concern for proper lubrication of air compressors (they are splash lubricated and don't get as much oil on the bearings at slow speeds).

> A sub-station is being designed so that
> additional
> power can be taken from Pacific Gas and
> Electric and converted to 600 volts DC.

We also will probably run a feeder wire down the line to reduce resistance loss through the trolley wire.

We've already welded bonds across all the rail joints to provide a good return path to the substation.

(For the original poster, the return path is a serious concern in cities for safety reasons as well as performance. A poor return path can cause some of the return current to travel on other underground utilities such as steel gas and water lines. This can lead to leaks in the pipes.)

> At is peak, the Sacramento Northern was
> running 1500 volts DC which helped in the
> transmission of the current.

Only certain parts of the SN were 1500 volts. The cars were equipped for dual voltage operation. Our restoration of SN combine 1005 in cludes the voltage changeover equipment, although we will never have to use it.

> Ted Miles


forags-no_spam@nature.berkeley.edu

Based on our experience at the Pennsylvania Trolley Museum, the feeder cable alone would produce a substantial improvement. We put in a 250mcm cable the length of our line (about 2 miles). No engineering calculations to back this up, but my gut feeling is that a 350mcm feeder would help reduce voltage drop quite a bit, and that a 500mcm feeder would enable you to defer the substation until you add that additional 3 miles of line you are planning.

artschwartz@sprintmail.com