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Meeshu (20 posts)
Feb-10-02, 04:16 PM (EST) "AC Traction Motors, and NumWheels (*)" I tried searching the forums for answers to the above but didn't find anything.I am seriously considering designing locomotives for use with MSTS. But I require further information.So.1) Do modern AC traction motors use 'squirrel cage' or 'wound' rotors? And what is the mechanism or procedure for dynamic braking within these motors?2) Within locomotives '.eng' files, what is the purpose of the NumWheels (*) parameter.Thanks. Alert Edit | Reply | Reply With Quote | Top
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Subject Author Message Date ID RE: AC Traction Motors, and NumWhee... HAC Feb-10-02 1 RE: AC Traction Motors, and NumWhee... mickoo Feb-10-02 4 RE: AC Traction Motors, and NumWhee... mickoo Feb-10-02 2 RE: AC Traction Motors, and NumWhee... plainsman Feb-10-02 3 RE: AC Traction Motors, and NumWhee... Meeshu Feb-10-02 5 RE: AC Traction Motors, and NumWhee... plainsman Feb-10-02 6 RE: AC Traction Motors, and NumWhee... Meeshu Feb-11-02 7 RE: AC Traction Motors, and NumWhee... plainsman Feb-11-02 8 RE: AC Traction Motors, and NumWhee... chucksc Feb-11-02 9 RE: AC Traction Motors, and NumWhee... plainsman Feb-11-02 10 RE: AC Traction Motors, and NumWhee... chucksc Feb-11-02 11

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HAC (508 posts)
Feb-10-02, 04:48 PM (EST) 1. "RE: AC Traction Motors, and NumWheels (*)" On AC motors of the type used on locomotives, the magnetic field on the outside is in fact not stationary. While the mechanical parts are, the electric field and therefore the magnetic field are actually rotating around the rotor at a rate proportional to the frequency of the electricity. This induces voltage in the closed loops of wire in the rotor without any electrical connections being necessary.
DC traction motors have commutators and carbon brushes that are subjected to very high current loads . A DC motor having high current loads while not in motion or while moving slowly, would receive major damage or burn-out unless the high current load is kept very short. At low speeds, the high current damage would occur within minutes. In AC traction is the motors are not equipped with the commutators or brushes, eliminating the problems of the low speed-high throttle operations .AC motors allow a locomotive to have more pulling power, eliminate stall burns in the traction motors, and have very low maintenance requirements. DC traction motors require high current levels or amps. Heavy conductors and traction motor leads must be used to carry this high current and this produce a great deal of heat, further limiting the DC traction motor. AC current eliminates the ammeter in the cab, replaced by a Tractive Effort Display. The AC motor will not have large amounts of wheel slip. The motor speed is controlled by the frequency of the input voltage. Since the frequency is controlled by onboard computer systems, motors cannot overspeed (slip) as they would on a DC locomotive. The AC traction system, coupled with a sophisticated computer controlled wheel slip control system, allow the AC locomotives to achieve much higher adhesion levels than DC locomotives.
I think you could simulate AC traction in MSTS by keeping wheelslip low.cheers
Harold
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mickoo (126 posts)
Feb-10-02, 05:09 PM (EST) 4. "RE: AC Traction Motors, and NumWheels (*)" Harold,Your right, but some AC motors do indeed have coils on the rotor power is fed via slip rings and is very low as its AC, typical in the 10's of volts, the rotor current will give even greater power to the whole motor, granted there are brushes but unlike the DC commutator the slip rings are not sectioned, its the insulation between the commutator segments thats batters the brushes and produces excessive wear and dirt. Slip ring brushes can be brought outside the motor and enclosed in a seperate enclosure thus removing any dirt from the actual windings and negating flashover so common on DC motors.AC motor control is like you say, controlled via the field windings rotational speed, on a DC motor control is via current applied, if a DC motors slips then back emf is reduced from the rotor, thus allowing more current to flow in the stator, this increases the motor speed which lowers rotor back emf and so on and so on until.. bang the motor self destructs. A runway 350 HP Dc motor has the power of a 50Lb bomb, it'll destroy the motor, axel and probably damage the bogie as well, not fun at high speed as can happen on high powered light loaded multiple unit commuter stock where passenger loads are not spread evenly through out the train.Best regardsMichael Remove | Alert Edit | Reply | Reply With Quote | Top
mickoo (126 posts)
Feb-10-02, 04:55 PM (EST) 2. "RE: AC Traction Motors, and NumWheels (*)" Ac Motors,I belive Squrrel cage is confined to single phase motors, most moder AC motors as 3 phase, supply is converted from single phase via chopper control to 3 phase, some older systems and most modern diesels converted to a DC bus via a rectifier then chopped that for via an inverter for AC drive usually single phase, nowadays with modern electronics its possible to split 1 into 3 quite easily.Dynamic braking, depends if were talking electrics or diesels if the latter then the DC bus is prefered ie no transformer as the prime mover is diesel thus modern diesels will usually dump power from the traction motors back into the DC bus, hwich when over excited will trip to resistor banks mounted on the roof, thus brake power is converted to heat and wasted. For electric locos with a transformer the motor becomes a 3 phase generator, origianlly power was dumped by resistors but these days can be converted back and phase aligned with the overhead power and fed back to the overhead. This can be tripped by several locos regeneratively braking and if the substation cannot handle the excess load will drop out, thus default is usually on board resistors where power is converted to heat and dumped.There are many other methods and the above is a very brief potted resumae of electric braking, as for your other question ?, havnt a clue.Best regardsMichael Remove | Alert Edit | Reply | Reply With Quote | Top
plainsman (629 posts)
Feb-10-02, 05:04 PM (EST) 3. "RE: AC Traction Motors, and NumWheels (*)" Download the SD series physics update V4.1. The SD90Mac is an example of the modern AC .eng file. The dynamic braking is also representative of EMD AC dynamic braking. sdphy41.zip
Thanks,
Bob Remove | Alert Edit | Reply | Reply With Quote | Top
Meeshu (20 posts)
Feb-10-02, 07:52 PM (EST) 5. "RE: AC Traction Motors, and NumWheels (*)" People, thanks for your replies.Actually, I am aware of the basic operating principles of AC Induction Motors. But, what is not clear to me, is the construction of the rotor used in traction motors.From replies I have received so far, it suggests that the rotor uses heavy copper wire (ie 'Wound Rotor'). If so, then my understanding of AC dynamic braking is as follows.In dynamic braking mode, the traction motor rotor windings are connected (via slip rings and via inverters/converters) to the main traction alternator. The traction alternator supplies voltage/current to traction motors rotors to provide 'excitation' (electromagnetic field).
Traction motor stator windings are connected to resistors and resistor blowers to dissipate the power generated by the traction motors.The 'excitation' voltage supplied to traction motor rotors is varied in magnitude (voltage) to vary the intensity of braking effort (ie higher voltage = greater excitation = higher induced volages (in stator) = greater power generated and dissipated = greater braking effort).The 'excitation' voltage frequency is adjusted (via inverters) to allow operation of dynamic braking over a wide range of speeds (from almost standstill to about maximum locomotive speed).Are my assumptions correct?NumWheels(*)?
I frequently see this on most custom locomotives as being NumWheels(1), regardless of the number of wheels on the locomotive!? However, I suspect that the total number of wheels on locomotives should be entered into NumWheels. So a C-C unit should have NumWheels(12), and a B-B unit should have NumWheels(8).Again is my assumption correct?Thanks. Remove | Alert Edit | Reply | Reply With Quote | Top
plainsman (629 posts)
Feb-10-02, 11:16 PM (EST) 6. "RE: AC Traction Motors, and NumWheels (*)" LAST EDITED ON Feb-10-02 AT 11:19 PM (EST)I strongly disagree on the use of numwheels. The parameter is described as a divisor which affects low speed traction. It actually appears to perform a function more related to wheelslip detection quality. Thus for a first generation diesel such as a GP7 or F3A, a numwheels equal to the wheels on the truck seems to work best. On a Dash 8 six axle 1.5 seems to work best and on Dash 9 or SD70 or newer, numwheels 1 is best. Take a look at V4.1 to see what I mean. Your approach will yield way too much wheelslip at reasonable adhesion values.
SD7=6
SD9=5
SD35=4.5
SD40=4
SD40-2=3
SD50=2 or 2.5
SD60=2
SD70=1 Remove | Alert Edit | Reply | Reply With Quote | Top
Meeshu (20 posts)
Feb-11-02, 01:01 AM (EST) 7. "RE: AC Traction Motors, and NumWheels (*)" LAST EDITED ON Feb-11-02 AT 01:05 AM (EST)Bob,Thanks for your comments.I have your 'sdphy41.zip'. I downloaded this sometime ago.
However, on checking a number of locomotive '.eng' files, they quote NumWheels(12) (including SD7, SD9, SD35, SD40, SD50, SD60, SD9043, SD90 plus a few others)!?I re-downloaded this file from the file library just a few minutes ago and the NumWheels value is still 12!So there appears to be a discrepancy between your quoted figures and those figures contained in the file sdphy41.zip!Any ideas?I have found that the parameters within '.eng' files appear to have names that describe a function that is usually (but not always) readily understandable.
So, the parameter NumWheels(*) I thought would mean the number of wheels on the locomotive. If, however, it has something to do with adhesion/wheel slip, then why didn't this parameter be called, say, AdhesionFactor(), SlipFactor() or something similar?OK. I am not saying you are wrong Bob, but I need more convincing that NumWheels() is NOT equal to the total number of wheels on locomotives.Still waiting for comments about my AC Dynamic Braking.
Anyone? Agree? Disagree? And why?Thanks. Remove | Alert Edit | Reply | Reply With Quote | Top
plainsman (629 posts)
Feb-11-02, 02:57 AM (EST) 8. "RE: AC Traction Motors, and NumWheels (*)" On NumWheels, we are talking about 2 different things here. The parameter in the braking section appears to be a scaler for braking action that works with the total number of wheels. The other term I was refering to is in the power and force section, and behaves somewhat differently. If you use real adhesion values, setting numwheels to total wheels in that section will not be realistic. I did use the correct numbers in that section, just as posted here in the message above, in V4.1.
On AC dynamics, for EMD locomotives the range of effectiveness is a little broader, extending down to 0.5 mph (that was what we used in the SD60Mac and SD70Mac). The GE braking is a little stronger at peak effectiveness however. I am not sure either is very significant in real world performance, not a lot of revenue time is spent at 1.6 mph, and both are so strong that they will be overshadowed by other limitations.
Bob
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chucksc (952 posts)
Feb-11-02, 10:20 AM (EST) 9. "RE: AC Traction Motors, and NumWheels (*)" Bob FYI ,
I use the following starting points for numwheels...The first occurrance I use the actual number of POWERED wheels.
the 2nd occurrance I use the number of powered axles....on locos who don't have wheelslip traction control I delete the line entirely from the ENG file (yes I know that "may" create problems on the AI locos) but if you leave the entry in as () it will behave the same as ON (1)... Another example of great coding by the Kuju Code Lizards (er wizards)... only test for one condition of three possible () (0) (1) and then ASSUME that a any of the statements equals TRUE (1)in other words merely test for the statement not the variable value....
On the locos who do have "traction control" I add the line it and set it to (1)Then I test the $hit out of the loco and check from there...
------Chuck Schneider---------

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plainsman (629 posts)
Feb-11-02, 10:46 AM (EST) 10. "RE: AC Traction Motors, and NumWheels (*)" Hi Chuck,
I agree on the traction control line.
As to the second numwheels, after a lot of testing and evaluation, I still stick to the values described above. Using the number of powered axles will not yield wildly distorted performance on many locomotives, but it results in the opposite of the intent on A1A trucks. As the value of numwheels increases, traction is reduced (traction is inversely related to this parameter). For the same traction motors and weight locomotive, the tractive effort will be greater on a Bo-Bo design than a A1A-A1A design as will the grip of the driving wheels. It will also give the wrong effect on very modern equipment, like the Dash 9 and SD70, and particularly on the AC locomotives where this should be 1.
The braking parameter does not care if the wheels are driven if they have brakes. A better statement (though most often the same result) would be the number of wheels that brake.
Thanks,
Bob
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chucksc (952 posts)
Feb-11-02, 12:18 PM (EST) 11. "RE: AC Traction Motors, and NumWheels (*)" Aha so maybe I have it backwards...
Traction would be the number of axles (assuming one traction motor per axle?)
Brakes would be the number of wheels with brakes installed...Trouble with that would be that the A1A and the C truck would have similar values for traction... That is not even close in real life as you know... Traction is divided over the numer of powered axles and adheasion is a function of weight on drivers (and other things) so maybe we need to fool around with the adheasion values on the A1A truck locos compared to the C truck locos???????------Chuck Schneider---------

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