25 Hz traction power system (Rail transit)

[tortuga]

Does anyone here have any experience with electric rail?
I am wondering what peoples thoughts are on a 25 Hz traction power system such as this one:
https://en.wikipedia.org/wiki/SEPTA's_25_Hz_traction_power_system

Are there any benefits to installing a 25 HZ system for say 20-100 miles of electric rail?
The 12kV locomotives cost the same but they come in either 25 or 60 HZ

Thanks in advance.

 

[Andy Delle]

I have studied this as a hobby and have a few books on rail propulsion systems.

The reason I read for 25hz was the rotary AC to DC converters of the early 1900s were most efficient at 25hz. They did build 60hz units later in the century, around 1940s. Of course today the rectification is solid state and has been available since the early 1960s so there really isn't any advantage for 25hz anymore.

Study the early NYC subway system for a great primer on 25hz electrical systems. I even remember in the early 1980s seeing the bare bulb incandescent emergency lights in the stations, which were always on, flicker at 25hz. Those emergency lamps were fed from the 25hz system while the modern fluorescent lights were on normal city power.

Another neat thing is the old portable tunnel work lights. You often still see them hanging on the walls when going into tunnels. They are five Edison lamp sockets on a strip. Get it? 120 x 5 = 600v. Just clip one lead to the grounded rail and the other lead to the third rail. I wouldn't be surprised if these are now illegal per OSHA.

Here's a great book: https://www.amazon.com/exec/obidos/ASIN/1568983557/newyorkcitysubwa/ref=nosim/002-4894604-0101641

 

[Ingenieur]

I can see some advantages
a 4 pole motor will run 750 rpm vs 1800 for 60 hz

also for a given power, say 1000 hp
60 hz torque ~ 2900 lb-ft
25 hz ~ 7000 lb-ft

the whole geartrain runs slower

inductive impact would be less but not sure if that is a big deal

 

[Besoeker]

I can see some advantages
a 4 pole motor will run 750 rpm vs 1800 for 60 hz

also for a given power, say 1000 hp
60 hz torque ~ 2900 lb-ft
25 hz ~ 7000 lb-ft

the whole geartrain runs slower/QUOTE]
But will have to be designed for the muck higher torque.
Larger, heavier, more expensive. Not positives.

I don't know the answer to the OP's question. We supplied a number of inverters for the VSOE, the Orient Express, for their on board aircon units. Our inverters had to work on the different rail network voltages and frequencies as it travelled across Europe. I think the max was 3,000Vac and there was also 1,500V dc.

I think the why was historic.

 

[Ingenieur]

I can see some advantages
a 4 pole motor will run 750 rpm vs 1800 for 60 hz

also for a given power, say 1000 hp
60 hz torque ~ 2900 lb-ft
25 hz ~ 7000 lb-ft

the whole geartrain runs slower/QUOTE]
But will have to be designed for the muck higher torque.
Larger, heavier, more expensive. Not positives.

I don't know the answer to the OP's question. We supplied a number of inverters for the VSOE, the Orient Express, for their on board aircon units. Our inverters had to work on the different rail network voltages and frequencies as it travelled across Europe. I think the max was 3,000Vac and there was also 1,500V dc.

I think the why was historic.

the traction torque will be the same, train weight is the same
the gearbox will be smaller and lighter
input power is the same

the primary reason was speed control
this is single ph power therefore a universal motor

 

[Besoeker]

the traction torque will be the same, train weight is the same
the gearbox will be smaller and lighter
input power is the same

Yes, except that the input torque to the gearbox will be much higher and the gearbox has to be mechanically larger (thus heavier) to deal with that.

P = Tω

 

[Ingenieur]

Yes, except that the input torque to the gearbox will be much higher and the gearbox has to be mechanically larger (thus heavier) to deal with that.

P = Tω

wrong
the gearbox size is determined by output torque and reduction/multiplication
it will be smaller since the ratio will be lower and output torque is the same
input/output power is the same

they could essentially use dc series wound control methods

 

[Besoeker]

wrong
the gearbox size is determined by output torque and reduction/multiplication
it will be smaller since the ratio will be lower and output torque is the same
input/output power is the same

But input torque is much higher. That sizes the input shaft an pinion dimensions.
I'm not going to argue with you if you are going to blatantly disregard facts yet again.
Go in peace.

 

[Ingenieur]

But input torque is much higher. That sizes the input shaft an pinion dimensions.
I'm not going to argue with you if you are going to blatantly disregard facts yet again.
Go in peace.

the driven gear and housing will be much smaller
incorrect
the system stresses/forces are determined by the load
which is the same in both cases
it is not I constantly ignoring basic engineering principles that are obvious with a basic mathematical comprehension

 

[tortuga]

Is it that the 25HZ Engines do all the motor control with a transformer tap changer control gear, where in the 60HZ system they use a mechanical transmission of sorts like the Diesel models?

Thanks for your replies!

I have studied this as a hobby and have a few books on rail propulsion systems.

The reason I read for 25hz was the rotary AC to DC converters of the early 1900s were most efficient at 25hz. They did build 60hz units later in the century, around 1940s. Of course today the rectification is solid state and has been available since the early 1960s so there really isn't any advantage for 25hz anymore.

Study the early NYC subway system for a great primer on 25hz electrical systems. I even remember in the early 1980s seeing the bare bulb incandescent emergency lights in the stations, which were always on, flicker at 25hz. Those emergency lamps were fed from the 25hz system while the modern fluorescent lights were on normal city power.

Another neat thing is the old portable tunnel work lights. You often still see them hanging on the walls when going into tunnels. They are five Edison lamp sockets on a strip. Get it? 120 x 5 = 600v. Just clip one lead to the grounded rail and the other lead to the third rail. I wouldn't be surprised if these are now illegal per OSHA.

Here's a great book: https://www.amazon.com/exec/obidos/ASIN/1568983557/newyorkcitysubwa/ref=nosim/002-4894604-0101641

Thank you I'll order that book.
Would resistive losses over long distances not be lower since 20 HZ is closer to DC?
I have read that "Pantograph contact with the overhead wire is also more tolerant of high speeds and variations in track geometry at 25HZ"
(The Pantograph is the grass hopper thing that pops up off the train car and contacts the primary wire.)

Thanks again.

 

[winnie]

I don't know about gearbox weight, but lower frequency won't improve the torque/size of an induction motor.

A 100 Hp 25 Hz 4 pole motor will produce roughly 60/25 the torque of a 100 Hp 60 Hz 4 pole motor. However this 100 Hp 25Hz motor will be approximately the same size as a 240 Hp 60Hz motor. To a good approximation the size of an induction motor is set by its torque rating.

-Jon

 

[drcampbell]

OK, everybody slow down.
If you're buying -- not building -- locomotives, motor selection and gearbox design are irrelevant; somebody else will have done that.
If you are building locomotives, reducing the weight of the gearbox -- or any other component -- isn't an advantage. Reducing a locomotive's weight will reduce its wheel-rail grip and reduce its tractive effort.
You can't use a fixed-speed motor as a locomotive's traction motor. It must be variable speed.

Using a different frequency than your power company supplies will require conversion equipment. An added expense with no benefit.

 

[tortuga]

OK, everybody slow down.
../..
Using a different frequency than your power company supplies will require conversion equipment. An added expense with no benefit.

Ok cool, I was wanting to explore the 'no benefit' aspect of this assuming the supply was not a issue or added cost (amazingly to me).
Would lower resistive losses over long distances @ 25 HZ make any difference?
Any maintenance advantage to that better contact with the overhead wire at 25HZ?

Thanks all for your continued thoughts
Have a great weekend.

 

[zbang]

I have studied this as a hobby and have a few books on rail propulsion systems.

Yep, same here. Maybe 3' of shelf for the electric trains books .

The reason I read for 25hz was the rotary AC to DC converters of the early 1900s were most efficient at 25hz.

Before the railroads, 25Hz was the more-standard industrial frequency, IIRC the motors of the time had better commutation at lower frequencies and since electricity was for motors, not lights, any flicker wasn't important. The all of the equipment was build to the standard frequency of the time.

Now? It makes negative sense to install 25Hz. None of the gear is off-the-shelf and the railways don't need to run long transmission lines because they tap the existing 50/60Hz grid. All "new" systems are either 50 or 60Hz and usually 25KV. (Amtrak's Northeast Corridor locos can run at 25Hz/11KV and 60Hz at either 12.5KV or 25KV. And a few of the European railways run 16.66Hz.)


The book you really want about this--
https://www.amazon.com/Westinghouse...nsportation-Various/dp/B006N37MAY/ref=sr_1_44

 

[Besoeker]

I don't know about gearbox weight, but lower frequency won't improve the torque/size of an induction motor.

A 100 Hp 25 Hz 4 pole motor will produce roughly 60/25 the torque of a 100 Hp 60 Hz 4 pole motor. However this 100 Hp 25Hz motor will be approximately the same size as a 240 Hp 60Hz motor. To a good approximation the size of an induction motor is set by its torque rating.

-Jon

And the gearbox sized appropriately to handle that 60/25 times torque. Just basic physics.

 

[Tony S]

Some of you may find this interesting https://www.nycsubway.org/wiki/Rotary_Converter_Power_Technology

 

[drcampbell]

... Would lower resistive losses over long distances @ 25 HZ make any difference? ...

DC is advantageous for long-distance power transmission when the "long distance" is thousands of miles, not dozens.

The losses would need to be truly astonishing to justify installing frequency converters.

 

[Besoeker]

DC is advantageous for long-distance power transmission when the "long distance" is thousands of miles, not dozens.

The losses would need to be truly astonishing to justify installing frequency converters.

London underground is DC 630V.

 

[Ingenieur]

And the gearbox sized appropriately to handle that 60/25 times torque. Just basic physics.

the torque passed thru the box is the same for a given mass and velocity
power in = power out
basic mechanical engineering

the primary reason for the 25 Hz was speed control
the transition could take advantage of dc speed control
a universal motor is single phase, ~ ac series

 

[zbang]

London underground is DC 630V.

Virtually all urban transit systems are 600-750VDC, but they distribute AC (or more often pull from the AC grid).

IIRC, the Underground has something like 15-20 separate traction current sections per line and each like has at least 10 separate rectifier substations.