Failure to load

[BroadwayLion]

A poster over on subchat wanted to know what a "Failure to Load" was when operating a locomotive. I assumed that the prime mover was turnning the generator, but the voltage was not being generated. Looks to me like re-winding the generator is in the locomotives future.

Maybe the real issue can be explained to me.

Thanks

 

ROAR

[Randy Stahl]

There's a million reasons for an engine not loading. A bad generator is not the most likely.

 

Randy

[tree68]

I'm guessing this is on a newer locomotive.  On our antiques, we find out the locomotive isn't loading when it doesn't go when we advance the throttle (and nothing shows on the ammeter...).

In most cases, it's because the reverser is still centered or the field switch is off...

[BroadwayLion]

Ah, Yess, the Field Switch. Ergo, the Field is electric. And Where does the electric for the field come from? A Battery, I suppose, and if the battery is down then you do not go. I suppose that is another reason to leave locomotives idling.

ROAR

[Overmod]

BroadwayLion

Ah, Yess, the Field Switch. Ergo, the Field is electric. And Where does the electric for the field come from? A Battery, I suppose, and if the battery is down then you do not go.

A main generator, or alternator, is just an iron-reinforced coil of wire spinning inside other coils of wire until you create a magnetic field in the right way and the right places inside it.  Then the loops of wire cutting the 'lines' of the magnetic field have electric current induced in them.

The device that excites the required magnetic field is a small generator, appropriately enough called the 'exciter'.  This in turn can use permanent magnets to induce its field, or use a battery current, or (in one case I was looking at, in a 1948 British text on diesel-electric design) have its own little generator excite it.

"Slow to load" is a little bit more complicated than that, but you would have to read up a bit on modified Ward-Leonard and then Lemp control to get some idea of what's involved, and then have some of the railroaders explain how the computer manages some of that stuff for emissions, component longevity, company fuel-saving 'initiatives', and the like.  Basically you have to accelerate the diesel engine to get it to produce shaft horsepower before you can start exciting the generator/alternator to produce traction current.  Diesel engines do not 'like' accelerating into a load; they like to accelerate to speed and then be loaded once turning.  In addition, if they are turbocharged they need to develop adequate exhaust volume and heat energy to make the turbo boost correctly, before they are loaded.  Emissions are worse when the engine accelerates than when it stabilizes in steady state.

Meanwhile, down at the motor end, you don't want to bang the motors with a surge of current, which among other things may make them slip a set of wheels.  But you don't want to use 'resistance' to limit the current, as that wastes energy as heat.  Better to adjust the excitation relative to the engine speed and developed power (which is a function of the fuel-rack setting on a diesel) and also relative to time.

Now, GE in particular was known in the '60s for engineering-geek systems on locomotives that could introduce weird problems.  One of these was the idea that fast loading on road locomotives was a pointless idea -- save the engineer from himself by making the computer load the engine at the most 'efficient' rate net of all "stakeholder" concerns -- the pollution guys, the fuel-cost weenies, the folks who had to fix the diesel engines, etc.  This is like a dramatically expanded version of the cutout on the Krauss-Maffei diesel-hydraulics that prevented notching up until all the individual transmissions had engaged ... clunk! clunk! clunk! ... after reversing, making any kind of switching a study in agony.  Some GE engines, Dash-7 series as I recall, were reputed to take over 30 seconds to load fully; even 'normal' GEs were noted for accelerating with different characteristics from MUed EMDs. 

Baldwins were good for a different sort of fun: those things idled at about 1/5 of peak rpm, so they were producing a measurable amount of torque without any need to advance or touch the throttle.  So you could load them very fast and not encounter the usual sorts of delay associated with having to run up the diesel speed to balance the rate of excitation, and in fact you can get some Baldwins up to nearly 30mph with the engine still at idle just by tinkering with the field and motor-interconnection transition.

[M636C]

Overmod mentioned...

a 1948 British text on diesel-electric design

Many years ago I talked my way into spending my University long vacation building locomotives at English Electric's plant in Rocklea, Queensland (Australia).

English Electric had a different approach to engine control. The throttle had no notches and was called the "power lever". This was placed wherever the driver thought appropriate, and a big rotary faceplate device in the electrical cabinet (called a load regulator) looked at the load and the diesel engine and turned the diesel up until it could meet the load the traction motors wanted. This worked surprisingly well but nobody ever tried to get one of these to run in MU with an EMD or GE.

But this was the early 1970s and somebody in the UK decided that big mechanical components should be replaced by solid state control. Given the state of development of these things, this was not a good idea at the time.

However, the last of a batch of roadswitchers, no. 1347 received this new controller. It must have worked at the factory (after I'd left) but the owner couldn't get it to do anything. I remember seeing 1347 being driven up and down the yard light (which it could do easily). It just couldn't haul anything. I think they fitted an old type load regulator and it disappeared into the coal traffic with all the others.

I had to monitor a test on an earlier unit, maybe 1332. It was set up, idling with the generator connected to a load box and the power lever at the upper stop. As the the plates were pulled out of the load box, the engine growled into life and roared away convincingly, with nobody at the controls.

One reason I remember the day was that the engine hadn't run at much more than idle before and there was an undetected problem with the cooling system. The radiator header tank was missing a plug, but the the hole was covered with masking tape from the time the tank was painted. I was walking around taking temperatures from glass thermometers placed in strategic locations. I had just opened the door nearest the header tank, read the temperature and had the door almost shut when the whole hood filled up with steam as the masking tape let go. Someone, maybe me, ran to the cab and pulled the power lever back to the idle stop and the plates were returned into the no-load position in the load box.

Queensland Railways had EMDs as well. One type was a (very) light lines loco, type 1720. It was a model GL18C where the L stood for "light" (they weighed 58 long tons, and had six D36 motors (suitable for 3'0" gauge, although the QR was 3'6").

These were used to replace steam in suburban service. They were 1000 HP with an 8-645E and coud do good work with light passenger trains. However to maximise performance, they had a "passenger" switch on the control system that isolated the delay EMD had built into the system to prevent overloading if the driver pushed the throttle straight to notch 8, in order to speed up the stopping passenger trains. I'm sure that switch stayed in the "passenger" position for switching unless a really heavy train was involved.

M636C

[oltmannd]

BroadwayLion

And Where does the electric for the field come from? A Battery, I suppose,

Yes.  Well, at least for locomotives with DC main generators.  On locomotives with traction alternators, EMD 40 series and newer, the excitation comes from the companion alternator - a separate set of windings within the tractioni altenator.  

 

BroadwayLion

if the battery is down then you do not go.

No.  Kind of like your car.  Once you get the engine going, you have battery charging from the auxilliary generator.

 

[mvlandsw]

[quote user="M636C"]others.

 It was set up, idling with the generator connected to a load box and the power lever at the upper stop. As the the plates were pulled out of the load box, the engine growled into life and roared away convincingly,

 

    How does a load box work?  I thought they are just big resistors with a cooling fan.

 

 Mark

[M636C]

mvlandsw

M636C

 It was set up, idling with the generator connected to a load box and the power lever at the upper stop. As the the plates were pulled out of the load box, the engine growled into life and roared away convincingly...

 

    How does a load box work?  I thought they are just big resistors with a cooling fan.

 

 Mark

 

You can use a dynamic brake unit or two to test a locomotive and many locomotives have a self test feature where the engine can be run against its own dynamic brakes.

 

The unit at Rocklea was a big tank full of salty water which conducted the electricity to terminal plates in the tank from a number of steel plates suspended in the salt solution. The upper steel plates could be pulled out slowly by winch control, and the smaller the area of plate in the solution, the higher the resistance to the current and the higher the load seen by the locomotive. The salty water boiled away, the whole thing being like a huge electric kettle and needed to be topped up from time to time. Since the salt didn't boil away, you had to check how salty the solution was from time to time.

 

When that load box was built, none of the locomotives had dynamic brakes. The little 58 ton EMDs didn't have dynamic brakes, neither did the 61.25 ton English Electric units for light branch lines. They had dual controls, but one direction was a dummy connected to the real controller with two bicycle chains under the cab floor. 61.25 tons was the MAXIMUM. One set of units stayed at the builder while lighter truck frames were fitted, then aluminium hood doors and finally hollow aluminium handrails. Then they weighed 61.25 tons.

 

More recenly, most locomotive repair points have dynamic brake units removed from scrapped units that can be used to test locomotives following repair. They take up a lot less room. The old tanks of salt solution were ten feet square at least and twice that high with the plate lifting mechanism.

 

M636C

[Randy Stahl]

Search for "water rheostat" it should provide some background.

 

Randy