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Diesel Locomotive FAQ

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Diesel-Electric Rail Engine FAQ

Also See Diesels, Electric & Turbine Prototype FAQ and RDC FAQ

Diesel FAQ by Brent Holt

How does a diesel loco really work?

Timothy Braun explains

The diesel engine in the locomotive is hooked to a generator (on old lower hp units) or an alternator (on most things except low-end switchers since the late-'60s). The electrical power is routed from the DC generator on the older units to traction motors, usually 1 per axle, which are wired in series, parallel, or series-parallel depending on the throttle notch. On the newer units, the AC power provided by the alternator is routed to a rectifier which turns it into DC power which is then routed to the traction motors in the same manner as the other units.

The newest "AC" locomotives such as General Electric's AC4400CW or EMD's SD70MAC also produce power with the diesel which turns an alternator producing AC current. These locomotives also rectify this power to DC, but then "chop" it back to AC using inverters. This AC power then goes to AC traction motors which are more expensive but much more durable than DC traction motors. The reason for the change to DC and back to AC is to make sure that the AC current is a constant 60 Hz, even though the engine is changing speeds. wrote:
> I saw the Trains Unlimited show on the History channel, and they told
> about how AC Diesel-Electric engines can pull more, compared to a DC one
> of the same engine size and power. Why is this?
Per Silas Warner at
Basically because it's a simpler motor.  A DC motor has to have a
commutator and brushes to transfer power from the stationary cage to
the rotating armature.  An AC motor does not: the induction from the
rotating magnetic field induces currents in the armature.  Commutators
are the main weak point for overheated motors: therefore an AC motor
can run hotter, therefore at greater traction at low speed.

To compensate for the simpler motors, AC traction needs a complicated
solid-state inverter box to produce current at just the right frequency
to run the motors.  But this doesn't have to be in the crowded truck,
it can be in the well-cooled electrical compartment of the carbody.
Many commuter and passenger locomotives like EMD's F40PH or GE's P32-8 use some of the power from the alternator as Head End Power for the coaches on the train. These locomotives run the prime mover at a constant speed in order to keep this power a constant 60Hz. That's why an EMD F40 sitting at a station stop is still screaming its guts out. NJ Transit is now "stretching" their F40PH units to make room for a smaller secondary generator to run HEP. This will allow the main prime mover to change speeds and save fuel. MBTA, Coaster, and many other operaters also use seperate HEP generator sets.

Most locomotives can use their traction motors for another purpose: dynamic braking. When a motor is turned by the momentum of the train, it will produce power which is then dissipated by heating grids on the top of the locomotive similar to those in an electric toaster. By using the motor as a generator like this, it takes a lot of the train's momentum away, slowing it down.

Rich Webster clarifies:

HEP does not use the main generater for power. The main generater is for traction motor use only. There is an A/C circuit in it too, but that circuit is to run the cooling fans. The HEP generater is driven by the same diesel engine as the main generater, but it's only purpose is to supply 440 volts 3 phase A/C.

John Wilson explains:

The generator runs at roughly constant frequency. (I don't know for sure that it's 60 Hz, but that may be the number. It is on AMTRAK, where the hotel power has to accommodate standard electrical appliances.) The traction motors are synchronous motors, so their rpm is directly proportional to the power frequency. The electronics provides variable frequency power for the traction motors to accommodate varying train speeds with roughly constant engine and generator rpm, hence generator frequency.

What creates "adhesion" characteristics?

From James Robinson:
Adhesion is a measure of how effectively a locomotive can translate its weight into pulling power before the wheels spin out of control. Many things can influence this including the weather conditions, the profile of the wheel tread and the rail head, contamination on the rails, the mechanical design of the trucks, and the electrical control system. The electrical system is where the greatest improvements have been accomplished.

In steam engines and early locomotives there wasn't any type of automatic wheel slip control system. It was up to the engineer to cut the power on the locomotive whenever the wheels started to slip. On these early diesel engines, most railroads felt that about 12 or 14 percent adhesion was the best you could expect for day to day operations. Steam engines could deliver more since the side rods tied the axles together, and one wheelset couldn't slip by itself with this arrangement.

The next development was the installation of a light on the control stand that indicated that wheels were slipping. This was accomplished by measuring the voltage or current imbalances between motors and lighting a light on the control stand whenever the imbalances exceeded a certain amount. (Simply stated, the voltage of a motor varies with its rotational speed, so if one motor is spinning faster than another, there will be a difference in voltage between them.) This didn't improve the dispatchable adhesion in itself, but permitted multiple unit control since the slip signal was wired between locomotives, and the engineer therefore didn't have to be sitting on the slipping locomotive to know there was a problem.

Some bright designer then decided to use the slip signal to automatically vary the power of the locomotive: Whenever the system detected a slip, the power of the locomotive would be automatically cut to stop the slip, then carefully reapplied. The engineer still had to modulate to throttle to keep the system from working too hard, but since the automatic system could react more quickly than the engineer could, the railroads increased the dispatch adhesion to about 16 or 18 percent. Refinements in the system eventually permitted dispatch adhesions of 18 to 20 percent.

At this point the electrical control systems graduated from slip control to adhesion control. This is because newer control systems were designed that actually allow the wheels to slip slightly, on purpose. It is called creep control. On a heavy pull the wheels will typically by turning a couple of miles an hour faster than the locomotive is moving, and thereby gain the greatest pulling power from the locomotive. You can sometimes hear the effect on a heavy pull when the wheels begin to sing. This means the control system is working hard. The theory is that the slipping wheels will burn off any contamination between the wheels and the rail and thus give a better grip on the rails. With this type of system installed on DC locomotives the adhesion ratings jumped to 25 or 28 percent dispatch adhesion.

Finally, AC traction technology permitted even finer control of wheel creep. These types of locomotives typically are rated at between 31 and 34 percent dispatch adhesion.

> Also, if AC motors do not have brushes, how does current get to the
> internal rotating part?  (I assume all electric motors have two
> electro-magnets--a stationary part on the outside and a rotating part on
> the inside).

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.

In a way, it's similar to how a transformer works: There are no physical connections between the primary and secondary windings of a transformer, but the changing electical field of the primary will induce a voltage in the secondary windings.

The following is from Doug Allen

"The AC traction motor is really no different than any other 3 phase motor, such as those used for cooling on a locomotive. Simply, it is a standard asyncronimous (squirrel cage) 3 phase wound motor.

The advantages of the AC motor is:

Other Facts: The stator is stationary in an AC Traction Motor. Only the "Rotor" rotates, as with any other standard AC motor design. AC motors do work the same way as a transformer. Current from the Stator is "induced" to the rotor, an electrical properity that occurs in AC, therefore there is no need for brushes to transfere power to the rotor.

A rotor HAS windings: The lamaneted iron is stacked the same as a DC armature. Copper bars are installed in the slots in the iron, and brazed to conductive plates on both ends of the iron. The individual segments, or bars, of the rotor's iron become electro-magnets, causing the attraction/repulsion dynamics that results in the rotation of the rotor.

All induction motors are "frequency depended" for their speed. The speed of an AC induction motor is determined by the amount of poles in the windings, and the frequency applied. AC traction motors have 4 poles, just like commerical 3 phase motors. Example: 4 poles with 60 hz = 1800 rpm, increase the frequency to 120 Hz = 3600 RPMs. A 2 pole winding, such as a swimming pool pump motors, with 60 Hz = 3600 rpms. It is simple math!

The inception of semi-conductors that can be gated OFF, as well as gated ON, such as GTO & IGBT semiconductors, has made compact, efficient 3 phase invertors fesible. These invertors produce variable frequency power to the AC Traction Motors. Current and Voltage are also controlled to enhanse torque, control heat, and power factor usage.

Gate firing of the power semiconductors IS controlled by computers. It is the most efficient, compact, and reliable means to accomplish the precision timing required to create a synthetic AC wave form from a DC supply. On all locomotives with AC traction motors, whatever their power source: be it from a AC catenary system on an electric locomotive; or the AC alternator on modern Diesel-electric locomotives; they all rectified and regulated this power to a DC buss refered to as the DC Link. The DC link's power is fed into the invertors. They in turn produce the variable frequency, 3 phase power that is fed to the AC traction motors. In this day of computers, even the power distribution to your house is controlled by computers.

Interesting History of variable frequency AC motors: EMD (General Motors) diesels have used variable frequency AC motors for their radiator cooling since the 40s (F7s, GP9s, etc). As the diesel engine increased in RPM, it turned the generator faster to produce higher DC voltage to the traction motors. A small AC winding was also installed in the rear section of the generator's stator to supply 3 phase AC power for the roof mounted cooling fan motors. As the generator's RPM increased, the frequency of the AC power also increased, increasing the RPM of the cooling blower motors. This feature enable not only longer cooling motor life, but supplied increased cooling as the diesel engine worked harder. Variable frequency/ speed AC motors is not new.

So... It's NOT the AC traction motor that is cutting edge technology, it's the new semi-conductors, capable of switching higher voltages, higher currents, producing less heat and power losses, in more compact packaging, that enabled this new AC drive technology.

Railroad's maintenance dollars are saved, and power is used more efficiently as a result. It would not be in any Railroads best interest today to order a new locomotive without an AC traction motor drive. Would you prefer your automobile be built with a carburetor instead of fuel-injection? ...or a 2-speed automatic transmission instead of a computer controlled 4-speed transmission? New technology is simply more efficent."

Also see: Traction Motors FAQ

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