A bit of a change, i wanted to make sure that people were not getting tired by me just showing photos of metal being bashed, those blogs will continue as there seems to be a lot of interest in it.  However in the last blog i invited some questions that people might have had about how things work in a Diesel Electric locomotive like the class 26, however all of the first generation diesel electrics work on pretty much the same principles so its very relevant across the fleet.

 

@37114 asked about electrical machine maintenance and field diversion and in this blog we will talk field diversion, how it works and what it does. There will be a bit of theory first, and then some pretty pictures.

 

There has been a lot of talk about field diversion in the sound forums, mainly surrounding 37s as its very very pronounced. but all first generation diesel electric locomotives have field diversion of some sort.

 

What is field diversion, field diversion is the act of (as the name suggests) diverting the field of a motor (in our case the traction motors through a different set of resistances. It is also known as field weakening, and this is because you weaken the field of the motor.

 

The aim of field diversion in simple terms....is to make your motor spin faster based on its current load.

 

If we take the model in a OO gauge model, its a fixed field motor (permanent magnets) if you give it 12 volts it will spin at a set speed and draw a set amount of current. And for a model thats fine, we dont pull much weight with it and we rarely make it climb steep gradients. If you were to put so much weight behind the locomotive in the form of rolling stock to the point it actually stalls the motor, you will find if you push it to a certain speed it will be able to take the weight of that train on its own, but it wont be able to start it, and this is a very very crude way of explaining how field diversion helps with a locomotive starting a train.

 

When a train starts you want (in your best jeremy clarkson voice) MAXIMUM POWER, in that you want grunt, torque, tractive effort, what you dont want is speed, all you will do is simply spin your wheels. 

 

GRUNT is Current

Speed is Voltage

 

We want to trade current for voltage, by weakening the field of the motor we make it a lower strength in terms of torque, but that means we are less affected by BACK-EMF which is what puts a motor into magnetic saturation, which means the motor can spin faster before the back-emf cancels out the incoming voltage provided by the main generator.

 

In an all parallel configuration traction motors have 4 connections (compared to your 2 connections on the motor of your model train) and this is because the fields (magnets) are actually coils or better yet electro magnets and if you alter the voltage being supplied to them you can change there strength.....hence field weakening/diversion.

 

as we said before you want all of the capabilities of your motors at starting, so as a result all motors and generators start out at full field, (full strength)  the motors need full strength to have the grunt to move off, the generator needs full strength to supply those amps!

 

Lets look at the first part of this process.

 

 

the process start with the drivers power handle in the cab, you are looking inside the pedestal with the power handle above, the shrouds have been removed there are two elements in the centre you can see a group of switch contacts which are operated by cam on the left, the job of these is to make sure that various interlocks are in place (like the power control relay) which then allows the reverser to be operated (the locomotive only changes direction when power is applied NOT when the master handle is moved) the switch gear has no role in speeding up the engine or the electrical output of the main generator. On the middle right we can see an air valve (very similar in construction to the drivers straight air brake valve) as the power handle is rotated and more throttle is demanded this is air valve is pushed further and further inwards, which emits air from locomotives control air reservoir into whats called the regulating air circuit.

 

 

If you ever go into the engine-room of a working sulzer you will often see a panel like that (accept 26s as it was removed for cost saving) i re-instated that from a donor class 33. Anyway....on the bottom left is the regulating air gauge, as the driver moves the power handle at either end, that will increase/decrease as the driver moves the power handle back and forth.

 

 

That air then runs down a pipe down the side of the engine.....

 

from there it goes to the front of the engine governor, and provided all is well (all of your interlocks are in place) that magnet valve is energised when the power handle is moved to "ON" and the air is allowed to enter the fuel rack piston in the governor.

 

ERGO the engine is speed is increased by air being sent from the drivers power handle to the governor (0-53ish PSI) the more air the more the piston moves which in turn moves the rack linkage and opens the fuel pumps further and puts more fuel into the engine and speeds the engine up....

 

A lot of people think the only relationship of the engine to the electrical side is that it spins the generator....but what would happen if we had no control over the output of the generator. Basically on starting the train the generator would stall the engine.....and this is where something called the load regulator comes into play.

 

in the picture above we see an arm coming out of the governor in the background.

 

 

Theres another view this arm is connected to an oil driven servo motor inside the governor.

 

 

this arm then goes into this little box of tricks which is the back of the load regulator

 

 

the front of the load regulator, with a scale to indicate its current position (more on this later)

 

The load regulator is an extremely important device, its job is to ensure that the generator cannot overwhelm the engine, and its principles of operation are extremely simple.

 

As the driver opens the power handle the engine rotates faster, as the engine rotates faster the oil pump (inside the engine) rotates faster, this means that the oil pressure increases. Its important to note that the servo motor is not pressure specific it simply reacts to an increase or decrease in pressure, this then rotates an arm in the load regulator. By using this method, the generator cannot be allowed to increase its field without a corresponding increase in engine speed...and as a result torque...

 

 

A view of the load regulator with the cover removed, the arm is rotated by the action of the servo motor on the governor.

 

 

another view of the arm, you can see the carbon brush which completes the circuit...

 

the load regulator is connected via a very big wiring loom to a bank of resistors...

 

 

The wiring loom from the load regulator to the resistor bank in the locomotive roof.

 

So as the engine speeds up the load regulator rotates and weakens the field in the main generator, which means it generates less amps, but more volts....and the reverse happens as the engine decreases in speed back to idle.

 

THIS IS NOT FIELD DIVERSION we are simply using a potentiometer to change the field of the generator automatically inline with engine speed.

 

 

Once the load regulator rotates fully (which is quite quickly) the generator is at maximum output, and we cant supply any more volts to our motors and make the train go faster (no good) its here the field diversion process starts, when its rotated a cam operates the switch gear above, these are called (on a 26) pilot motor advance and pilot motor retard.

 

 

That switch gear operates the power to this motor in the main electrical cubicle...making it advance or retard, this then as you can see operates a cam...which in turn rotates a cam on a group of switch gear.

 

 

This then (via the switch gear supplies power to the magnet valves, which emit air to a piston, which brings in the contactors for the diver circuits, you can see here there are 3 diverts on the 26 with one contactor for each motor (so 3 groups of 4) (the number of diverts varies between locomotives high speed passenger locomotives tend to have more, class 45s have 6 diverts.

 

 

This in turn brings in different power resistors into the field circuit in order to weaken the fields of the traction motors, which then allows them to spin faster. The resistors are in banks behind the cantrail grills of the locomotive for air cooling as they get rather warm.

 

When power is shut off, the engine slows down and the oil pressure drops, the load regulator runs back which in turn brings in the switch for pilot motor retard, this then removes the diverts from the circuit, until power is required, if the speed has not dropped significantly the locomotive knows to bring in the required diverts.

 

Although the mechanics vary all of the first generation diesels follow these principles, 56,58s 37/4s and 37/7 and 37/9s have solid state electronic load regulators rather than mechanical ones, but the field diversion process is the same...

 

HSTs are the odd ball....and have NO DIVERTS this is why they have such a low tractive effort.

 

Hope that answers your question @37114 Happy to take any more questions other people have.