Why is LIRR third rail 750 volts?

Why is lirr third rail power is at 750 instead of 600 volts as in the nyc subway and metro north?

The third rail power was increased to 750 Volts around 1970, after the M-1's arrived on the scene. The railroad's distribution system was awful and some spots had voltage as low as 250 Volts on the rail.

This was fine with the older equipment. The ceiling fans just turned slow and the lights dimmed. Low voltage played havoc on the M-1's though. The ceiling lights would not light, the motor-alternator sets tat provide for lighting and air conditioning would slow and trip circuit breakers.

A quick fix was an increase in supply voltage.

I'm sure there is more to the story; perhaps the M-1's were actually designed for the 750 in an effort to draw less amperage. Maybe someone else has some more historical insight.

I think it's the addition of airconditioning that came with the M-1s could've required the higher voltage.

Also, I suppose the M-1's have higher speeds than the MP-54s and 72's so the extra 150 was needed for that, too. Just guessing since I'm not an expert at electric power.

BMT wrote:I think it's the addition of airconditioning that came with the M-1s could've required the higher voltage.

Also, I suppose the M-1's have higher speeds than the MP-54s and 72's so the extra 150 was needed for that, too. Just guessing since I'm not an expert at electric power.

I heard that the MP-54's could reach speeds up to 80mph, but were restricted to 65mph by PRR for safety reasons...

The voltage of MNCR third rail is anywhere between 620 to 750 volt as well.

I think the NYC Subway only uses 600v because the lines used to be 600v DC and they slowly switched over to 600v AC.

There is no AC third rail your confusing AC propulsion with DC propulsion.
Yes a lot of transit cars are AC propulsion but powered by DC

LI Tool: I recall reading that the highest power draw is from a train starting up from a station or other stop-which is why the LIRR strategically located a substation at or near every station. I remember Clem some time ago mentioned that low voltage-I wondered myself about the problems that would cause especially with the M1/3s and now the even higher power drawing M7s. Around that same early 70s time period as mentioned the LIRR increased their power supply capability for the then new M1 cars to run as they were designed-with comfort and speed.
MACTRAXX

The LIRR third rail used to be 660 vdc, as was the New York Central RR underrunning third rail. The extra 10 percent was supposed to give the trains a little extra oomph, and help counteract voltage drop with distance from the substations.
The boost to 750vdc is further in that direction. The higher voltage, coupled with the changed third rail (with aluminum side strips - aluminum is a much better conductor than steel) helps to maintain the voltage away from the substations.

Kat, was the 660VDC chosen for a practical purpose in the early 20th century? there has to be a reason.

yes average traction motor could only handle max 350 volt at that time and each truck had two motors in series.

That begs the question of how the higher voltage DC roads (South Shore, CN, MILW) handled 1500v or 3000v. Did they wire 4 motors in series for 1500v? And how was 3000v handled? CN, Lackawanna, and MILW all had 3000vDC systems.

Most of the 3000 Volt DC electrifications used 3000 Volt motors, as I believe part of the Italian Railroads do today. The DC system was selected for economies in construction.

If you care to learn more of this, high voltage motors and about vintage types of railroad electrification here's a 1916 article from Railway and Locomotive Engineering magazine:

https://books.google.com/books?id=qUtBA ... QmCh0xWQth" onclick="window.open(this.href);return false;