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283 comments on DrumBeat: June 30, 2008

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GreenMan on June 30, 2008 - 10:07am Permalink | Subthread | Parent | Comments top

What's the advantage of using a different frequency?

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AlanfromBigEasy on June 30, 2008 - 11:13am Permalink | Subthread | Parent | Comments top

It allows you to use motors with fewer poles (or get greater torque from a given AC motor) AFAIK. I would like to know more.

Also some long distance transmission advantages.

Amtrak's NorthEast Corridor operates at 25 Hz south of New York City to Washington DC. With modern electric locos, Hz is no longer an issue. They convert single phase AC from wire to DC and then 3 phase AC

Hopes that helps,

Alan

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TJ on June 30, 2008 - 12:46pm Permalink | Subthread | Parent | Comments top

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

Ns = 120F / p

where

Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding

(from Wikipedia)

The lower the frequency the lower the motor speed, and the higher the torque available. Also easier gearing and better power factor in both the motor and the transformers.

Also, in general (with lots of exceptions):

DC motors > AC motors in cost
Synchronous AC motors > Induction AC motors in cost
Cheapest motors in big sizes around are squirrel cage induction motors. Three phase types are self-starting and can act as generators, for regenerative braking.

AFAIK, most traction motors used to be series wound DC, but now tend to be wound-rotor induction. Wikipedia notes the TGV is a synchronous AC, though.

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AlanfromBigEasy on June 30, 2008 - 7:05pm Permalink | Subthread | Parent | Comments top

Simple schematic diagram of a modern electric locomotive

http://www.railway-technical.com/elec-loco-bloc.shtml

FYI for those curious,

Alan

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RBM on June 30, 2008 - 7:36pm Permalink | Subthread | Parent | Comments top

Neat !

BTW,
One time you mentioned a 40 pole motor was the most you knew of in locomotive applications.

I told one of my HVAC/R instructors about that (he's a retired nuke engineer) and he expressed an interest in seeing a schematic of that.

Have you come across such a thing ?

I poked around google a bit.

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AlanfromBigEasy on June 30, 2008 - 8:19pm Permalink | Subthread | Parent | Comments top

I think it was hydroelectric. Matches optimum RPM for the turbine with the generator by increasing the # of poles.

2 pole 50 Hz = 3000 rpm

2 pole 60 Hz = 3600 rpm

40 pole 60 Hz = 180 rpm

40 pole 50 Hz = 150 rpm

www.andritz.com/hydro-media-media-center-hydro_news_2003_05_en_1_.pdf
1.3 Mb pdf

40 pole is not the limit of those in use, I think I once heard of a 62 rpm 116 pole hydroelectric generator. Not economic today (and rewinding it would be "problematic").

Alan

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283 comments on DrumBeat: June 30, 2008

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