Traction Motor Confusion

[Kris94]

I'm so freaking confused on how AC and DC traction motors work it's giving me headaches. So basically DC motors are less powerful than AC motors and less reliable than them and don't last as long. Also I was reading a thread from four months ago about some shutting down axles or what have you. Gave up after reading a couple of posts. Didn't understand it one bit. Too much information.

Anyways what 's the adhesion levels for the ES40DC and ES44DC? I heard someone say on the Dash8s it was 27% to the modern 39% on the AC powered units. Can anyone help me out please?

 

[kws4000]

Real quick basics:

DC motors are harder to maintain, less powerful, BUT don't require as much technology. Today, for DC, an AC generator produces electricity that is converted to DC for the motors. Control is directly by engine RPM.

AC motors are easier to maintain, more expensive AND require much newer technology. Today, for AC, an AC generator produces electricity that is converted to DC that is then run through a bank of inverters with variable settings, monitored by microprocessors. Inverters change the frequency that the electricity pulses through a motor. Your house is at 60Hz, the loco inverters can take it down to 1Hz and up to 20Hz. The ultra-modern locos can cut out a malfunctioning inverter/traction motor while keeping the others moving, therefore preserving some pullpower instead of the old-days method of cutting the entire loco and making a 200ton anchor out of it. GE provides 1 inverter per motor, while EMD provides 1 inverter per truck.


Back in the old days, DC generators were connected straight to the motors. The reason for the addition of the AC generator was because of the limitaion of 2500 input HP on a DC generator; AC generators have no such limitation.
Why was there no AC traction way back when? There was, but not really. Virtually all electrification schemes except 3rd rail are AC in the wires, which is then converted to whatever is neccessy for the loco. VGN electrics took it at industrial frequency, rectified it, then passed it through a rotary converter on their siderod electrics.

FWIW, the starting adhesion was like 45% on SD90MAC-H. But that's at unmeasurably slow speeds.
FYI, the steam engine is the only known machine to develop maximum torque at ZERO speed, where diesel engines and electric motors need to get up to speed. Amazing, huh?

 

[norfolksouthern37]

GE provides 1 inverter per motor, while EMD provides 1 inverter per truck.

This will no longer be the case soon. New EMD's will have the ability to cut out one motor at a time.

 

[kws4000]

This will no longer be the case soon. New EMD's will have the ability to cut out one motor at a time.

When? Any external differences?

 

[bahari]

I have been experiencing the derated power reported by locomotive driver frequently on EMD locomotive model no. JT22LC-2M, 2250 hp. As the locomotive arrived in my maintenance Depot, findings show the pigtail uproot/came out from carbon brush eventhough the said carbon brush newly fitted with the original EMD carbon brush.The said locomotive's have been used in my department (Malayan Railway) since 1995. The problem occured on few traction motors and locomotives on same model. Just to make sure, what is the risk of mixing of the original carbon brush EMD with different brand of carbon brush(grade almost the same with the original) on the same traction motor?Anyone can advice me to solve the said problem?Bahari IsmailMalaysia (Malayan Railway)

 

[jjanmarine3]

There can be quite a few reasons that cause pigtails to come loose.
The main problem that I know of is as follows : The traction motor brushes are fitted into the brushes by a tamping process , which is basically a process where they take fine carbon dust and 'tamp' it around the pigtails and glue it.
When traction motor armatures have flat spots on the commutator surface ( high bars, high mica etc ) the brushes basically 'jump' up and down , especially at high speed , and the pigtails tamping comes loose due to the excessive vibration.
Factors that can cause flat spots and uneven wear on commutators are stalling , which is caused by loading the traction motors and the armatures stand still for long periods before they start turning and the copper overheats beyond limits and does not return to its original form . ( Signals on gradients and bad driver behaviour)
Old age ( normal wear and tear and components worn beyond limits ).
Faulty brush spring tensions and worn brush boxes also cause 'chattering' which can lead to brush failures.
The brush box gap between the commutator surface and brush box should not be too large.
Good maintenance can prevent all this , but it is also possible that the company who manufactures the brushes you use is at fault ( for example the tamping process is not adequite ) - so you have to communicate with them if you suspect this is the case. You can check the wear limits by jacking up each wheel in the pits, fitting a dial test indicator and turning the motor by hand or using a welding machine or its own power, to make sure the commutators specs are in limits.Excessive sideplay on the bearings also causes brushes to run onto the wear ridge between brush boxes and break.
The area where the locomotives are working might also be a clue, as roadbed pounding and bad track conditions can also cause traction motor problems.