technical question about locomotives

[truckman060473]

ok so a diesel electric loco has one huge engine driving an alternator which feeds the traction motors. sounds simple enough. but is there anything else taking place here that allows the loco to have such a wide speed range? (an articulated lorry may have 18 or more gears) ok so we know the loco has no gear change since there is one pinion on the traction motor driving one large spur on the wheel axle so the ratio can't be altered. so if we look at the electrical machines - the alternator; does this have a switching system, lets just say for arguments sake, it may have two main windings which are switched in a parallel set up for lower voltage high current that applies when the train is moving off and when the train is up to medium speed the windings are switched to series for more voltage allowing the loco to gain 70mph+. i may be wrong and this does not occur (although i have heard of this principle in electrical engineering), but i have noticed when watching DVD's of cab rides that with a class 37, when it has reached about 30-40mph there follows a change in engine tone as if the load on the alternator has increased.

 

[starrymarkb]

Good question, is it the case that the new DEMUs (and the Class 70) the output from the Alternator is fed into Inverters which then vary the frequency fed to the AC traction motors?

 

[truckman060473]

newer stuff is very likely to use electronic switchgear to be able to control motor speed in this way. i have a good knowledge of the latest radio control electric cars which use a battery pack and a speed controller/ inverter which changes the D.C. to A.C. running a A.C. brushless motor. the efficiency here compared to a motor with commutator+brushes is extraordinary and these model racing cars can achieve 40 to 70mph. back to the locos and the ones originating from the 1960's - 37, 47 etc would possibly use a mechanical switch. when the driver is seen operating the throttle he just seems to go from 0 to full power with one action, but there may be automatic electrical things happening back in that engine room that change the way the alternators' windings are connected. i would love it if a maintenance guy out there could say for sure if this is what happens.

 

[Sir_Clagalot]

Same with any electric motor, increase the voltage and current and it will spin faster up to it's maximum, hence why the engine revs up when power is asked for

 

[ailsa]

ok so a diesel electric loco has one huge engine driving an alternator which feeds the traction motors. sounds simple enough. but is there anything else taking place here that allows the loco to have such a wide speed range?

Older English Electric locos used 'field weakening' to allow the traction motors to run at a wider speed range. As the speed of the motor rises it starts acting as a generator and produces 'back EMF', in other words a voltage opposing that applied to the motor. Reducing the current to the traction motor field windings reduces the back EMF produced by the motor which increases the balancing speed. Southern region Mk.1 EMUs operated on the same principle, and I assume that Brush locos did also.

Class 66s effectively have two alternators built in the same housing. At low speed they are connected in parallel for maximum current. At higher speeds the control gear re-connects them in series to 'shove' (technical term there ) more voltage into the traction motors, overcoming the back EMF. Crude but it works!

As mentioned previously, Class 70's, Networkers etc have AC motors which are driven by electronics, producing a rotating magnetic field. To increase speed the electronics switch more rapidly, causing the magnetic field to rotate more swiftly and the train to speed up. The early electronics had a fairly limited maximum switching speed, hence the 'gear changing' mee mee mee Networker sound. No actual gears are being changed, just 'virtual' ones.

 

[paul1609]

Same with any electric motor, increase the voltage and current and it will spin faster up to it's maximum, hence why the engine revs up when power is asked for

Up to a point, In an ac motor its actually the frequency of the supply that mainly determines the speed of the motor although the voltage and current do vary as well.

 

[truckman060473]

Re: the 66's with the twin alternators. it may explain the difference in noise this loco produces. when i have filmed these in 2003 pulling away from a red signal in Newport with 35+ HAA hoppers, the tone of the loco rises and rises until you can't hear it anymore as the noise of the clanking wagons takes over. the same train coming up through Pyle is making a different roaring sound, nothing like the sound when pulling away. maybe the engine just sounds different when the mechanical load on it changes, as it would dramatically, if the alternators windings were put in series meaning it now has to work much harder turning over the alternator to try and force a lot more voltage across the 6 traction motors

 

[ailsa]

I've noticed that hard working 66s do make some noise, and that noise sounds like electrical machinery. It's possible that as the loco has been working a while, bits of it have become hot, and the radiator fans are roaring whilst trying to keep the engine cool. Not having driven one though, I couldn't say for certain.