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Matej Pacha, on 25 July 2013 - 09:13 PM, said:
Yes, the theory of electric machines speaks about quadratic relationship between the current and the force/torque but in a very ideal case. If you see the electro-mechanic characteristics of series-wounded DC machine, you'll find the characteristics is nearly linear due to magnetic saturation of the iron core and the self resistance. I've noticed that General Electric locomotives have no field weakening so the force is always given by the same current. European locomotives use this to spread the range of speed so the same force can be produced by at least two different currents. When speaking about AC machines, the current-force dependency is nearly the same because the control technique leads to the force being linear to the current and field.
See my post in the physics section on the emulation of teh current TE curve od a series motor, The data for the emulation came from to sets of graphs, the first being I to TE for an EMD D77 motor, the second another set graphs for a large series wound industrial motor. The two sets of graphs almost mirrored themselves for the current V torque relationship.
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This is interesting. Is this implemented in the DC/DC and AC/DC locos too or just AC/AC? Normally the nominal force/current is somewhere on the half way to the maximal force and the traction motor can be current-overloaded (the diesel engine cannot be power-overloaded).
AC/DC machines, info comes from the maintence manuals for the GE Dash 7 and the EMD GP40. Also the F7 drivers handbook states immediately after starting one can put the throttle up to at least notch 4 without any problems, this implies even the early machines possibly had some kind of current limiting. An interesting note here is it appears nearly all EMD drivers handbooks say this, a number of drivers a spoke to did not realise you could do this until they checked there drivers notes.
The limiting has a slightly different charteristic between the Dash 7 and the GP40, this is probably due to the Dash 7 appears to use electronics for regulation where as the EMD items of this vintage use magnetic amplifiers (a specialised form of transformer controlled by a an auxilary winding passing DC current). The Dash 7's have a specified max I for each notch, once the control gets below the constant power section. the control system will not let the current increase any further. In the EMD the current does increase beyond this point, the altenator though acts is if there is a virtual 0.4 ohm resistor in series with it, Note this is my interpretation based on the loco's altenator output current and voltage graph. I can supply these diagrams if required.
Both the EMD and the GE manuals state one should be able to go straight to Notch 8 on startup without overloading the motors (at least in the short term). EMD machines can almost certainly be set up so the driver can overload the traction motors.
Lindsay
