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#141289 07/15/04 09:25 PM
Joined: May 2002
Posts: 381
Hutch Offline OP
Life is always interesting and today whilst discussing electric railways I came across the concept of saline solution resistance controllers of 3-phase motors.

It started off with a description being given of two electric wires above an electric railway which happened to be the still-born Porthmadog, Bergellert and South Snowden Railway – a power and railway company that attempted to connect the then North Wales Narrow Gauge Railway with Porthmadog in 1904. It will hopefully soon form part of the rebuilt Welsh Highland Railway – the original (W.H.R.) company was a successor to the P.B.&S.S.R.

From the description of the overhead, I recognized a corner grounded ac delta system (Gantz System I think after the Hungarian promoters) and this proved correct. It was however the description of the motors which left me curious. They were described as slip-ring induction motors with control resistors comprising copper rods dipped in a saline solution. As the rods were dropped further into the liquid, the resistance became less (as the motor accelerated) until zero resistance was facilitated by a dead short. Mention was made about the solution being part of the central ‘star’ of the motor windings which confused me further (the supply is delta).

It all seems a rather odd setup to have in an electric locomotive. Can anyone please elaborate?

[This message has been edited by Hutch (edited 07-15-2004).]

#141290 07/16/04 01:22 AM
Joined: Sep 2002
Posts: 1,498
Likes: 1
C-H Offline
Regulating the speed of an electric locomotive is very difficult. Tyristors and transistors revolutioned the area and the problems are gone. But in the early days there were no tyristors, let alone transistors. The the slip ring AC motors were one solution.

If I understood your description correctly (big if!), all three rods will go into the same saline bath. There is no return conductor, as the currents from the three phases cancel out in the middle. This is electrically the same thing as putting a resistor in each of the windings. Logically, is doesn't matter where in the winding you put the resistor.

If you had put the saline resistors on the other side of the windings, you would have needed a separate bath for each phase.

#141291 07/16/04 01:26 PM
Joined: May 2002
Posts: 381
Hutch Offline OP
Thanks C-H. With your help I have drawn the diagram and now ‘see the light’. Quite nifty really, and an elegant solution. One has to hand it to these old timers.

Nevertheless, the concept of controlling one of these locomotives with potentially boiling pots of salty water [Linked Image] makes me understand why steam power was so popular in 1904.

#141292 07/16/04 02:46 PM
Joined: Aug 2001
Posts: 7,520
It's certainly an interesting way to control motor speed.... [Linked Image]

I thought that most of the early electric controllers just used huge resistance banks with a stud controller (big circle of studs connecting into the resistors at different points as the controller is turned).

#141293 07/16/04 03:53 PM
Joined: Jul 2004
Posts: 200
Ah well hutch, you can just chuck your potnoodles into the controller and have all your meals cooked on the go! [Linked Image]

It seems a rather ...clumsy way of controlling motors. I too thought it was all rotary graduated resistance... You live and learn I guess! [Linked Image]

If hindsight were foresight, we'd all be millionaires!
#141294 07/16/04 04:32 PM
Joined: May 2002
Posts: 381
Hutch Offline OP
Ah well hutch, you can just chuck your potnoodles into the controller and have all your meals cooked on the go!
That's all well and done to chuck them in - it's getting them out that's the problem. By my calculation, the bubbling brine is at 1/root(3) of traction voltage! [Linked Image]

[This message has been edited by Hutch (edited 07-16-2004).]

#141295 07/16/04 04:48 PM
Joined: May 2004
Posts: 186
The old trolley bus's and trams used to be controlled via a drum resistance controller it consisted of about a 100 contacts arranged around the edge of the drum and the resistance strips were also arranged around the periphary of the drum. The contact arm was driven by a pilot motor. This made for a very smooth acceleration and braking system. Much of the control gear was made by Alln West & Co Ltd Brighton makers of the old dashpot oil starters anybody remember them??

#141296 07/16/04 05:53 PM
Joined: Jul 2002
Posts: 8,432
Likes: 3
The motor that you described above, sounds not unlike a Wound Rotor Induction motor.
The Armature is wound in Star and is fed through the slip-rings at the back of the Armature shaft.
The motor also has a normal Stator winding, as in a normal Cage induction motor.
It is the Rotor windings that have the Resistances in series with them and as the resistance is lowered the motor speeds up.
There is no actual supply to the Rotor windings, all that is varied is the currents caused by the Magnetic field in the Stator windings. [Linked Image]

#141297 07/16/04 07:48 PM
Joined: May 2002
Posts: 381
Hutch Offline OP
The discussion on the railway site was generating other discussions regarding these strange resistors ...

• On the subject of salt pot variable resistances I knew of one theatre lighting installation using salt pot dimmers. The electrolyte in these would be boiling merrily by the end of the first act and the following interval was used for topping the pots up with water.

• I seem to remember from a visit to [a utility] that the electrolyte there was caustic soda, as it took more punishment before throwing itself all over the operatives present. Theatres used salt because they were less careful and boiling caustic soda descending from the heights onto the cast was marginally less desirable from a compensation angle.

Certainly a noval electrical device! [Linked Image]

#141298 07/17/04 09:44 AM
Joined: May 2002
Posts: 381
Hutch Offline OP
Thanks Mike. A web search on ‘Wound Rotor Induction Motor” yielded the the following site which is coincidently from your part of the world! (Do they call these motors something different outside of New Zealand? [Linked Image] ).

The resultant information extinguished my falsely illuminated light (see above) but I think that I am finally starting to get the picture. A floating (w.r.t. earth) resistor potential and a change in the speed/torque characteristics produced by a changing rotor resistance – it seems. I have to admit though that the formulae going down the page explain why I am not an electrical engineer. [Linked Image]

Still, it’s amazing where - a passing online comment on a board dealing with railway restoration, numerous web searches plus of coarse helpful insight from you guys - can lead one.

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