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Post by q8 on Jul 4, 2005 9:00:38 GMT
Hallo Gent's. I want to tap the knowledge of the brainy ones on the forum? Now I know there are two methods of reducing back EMF of an electric motor. Field shunt and tapped field. What's the difference between the two and which gives the better result?
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Phil
In memoriam
RIP 23-Oct-2018
Posts: 9,473
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Post by Phil on Jul 4, 2005 10:47:14 GMT
Hallo Gent's. I want to tap the knowledge of the brainy ones on the forum? Now I know there are two methods of reducing back EMF of an electric motor. Field shunt and tapped field. What's the difference between the two and which gives the better result? AFAIK the only LU stock with tapped field was F stock (Steam to Silver, by J.Graeme Bruce). Right here we go. Both systems reduce the current to the field windings since it is current that determines field strength. With field divert (incorrectly called shunt) extra lower value resistors are inserted in parallel with the field windings thus reducing current to the windings themselves by providing an easier path for the field current. With tapped field the voltage to the field windings is reduced by something similar to a transformer, and by Ohm's Law this also reduces current, the field resistance remaining the same. Tapped field needs more equipment but there is less waste energy (heat) because the current is actually reduced not diverted. I'm sure someone else can add to this but I hope it is a start
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Post by q8 on Jul 4, 2005 16:44:25 GMT
Oh yes the old "F" stock had tapped field. I had the honour of riding in the cab of one not long before withdrawal (on the bunk) and I can tell you that when the tap operated the train gave a little jump and you could feel the difference much more than a weak field.
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Phil
In memoriam
RIP 23-Oct-2018
Posts: 9,473
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Post by Phil on Jul 5, 2005 9:33:53 GMT
Oh yes the old "F" stock had tapped field. I had the honour of riding in the cab of one not long before withdrawal (on the bunk) and I can tell you that when the tap operated the train gave a little jump and you could feel the difference much more than a weak field. because the transition would have had to be open circuit to avoid shorting out the internals of the tap changer. i'm just a little too young to have ridden on a 'tank'
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Post by q8 on Jul 5, 2005 9:53:43 GMT
i'm just a little too young to have ridden on a 'tank' : ------------------------------------------------------------------------ Oh man you'll never know what you missed. They were even better than my beloved "Q's" They were hand notch job like the sleet loco's. and the little "Tsit" "Tsit" sound from the controller as the driver notched up could be heard nowhere else. They were bouncy old birds too and IIRC the seats were leatherette or similar and with no armrests on the longitudinal ones. You could sit down at one end of the seat and when you next stopped could be at the other! It's debatable point but I swear they were every bit as fast as an "A" stock. I was told that when they did trips to Amersham they skated up them hills as if they were not there. Coming back down was downright hair raising I believe. Oh I would have LOVED to been there.
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