Traditional Diesel Electric - generator output when idling?

[Morpethcurve]

I'm aware that there's an aux take-off or aux gen for train supply etc., and that the loco's batteries are being charged, but I wonder about whether the main generator output at idle is more than required for these consumers. Taking a stationary light engine, what differences are there between a 1960s loco and a modern type?

Many Thanks

 

[hwl]

I'm aware that there's an aux take-off or aux gen for train supply etc., and that the loco's batteries are being charged, but I wonder about whether the main generator output at idle is more than required for these consumers. Taking a stationary light engine, what differences are there between a 1960s loco and a modern type?

Many Thanks

Older locomotives (inc 59 and 66) will tend to have a main traction alternator and 2 lower voltage alternators or generators tailored to the lower voltage electrical systems and the another for ETS if needed. Modern locos e.g 68 70 88 93 99 have single alternator with power electronics to supply the different electrical system voltages (Taking up far less volume and mass).

The governor or engine computer (depending on vintage) will adjust the fuel supply as required if there are high idle loads.

 

[Morpethcurve]

Thanks for that. I imagined the alternator/gen output even at idle would be too great for the electrical system of a light engine to absorb.

 

[fourtytwo]

Thanks for that. I imagined the alternator/gen output even at idle would be too great for the electrical system of a light engine to absorb.

I will have a stab at answering for you, the engine speed (rpm) is set by the engine governor and it adjust fuel supply to maintain it. If there is little or no load on the engine the fuel supply will only be sufficient to overcome engine frictional losses and generator windage.
A generators output voltage is proportional to rpm, ignoring windage the torque required to drive it is dependent on load current, if all the circuit breakers are open there is no current irrespective of voltage so no additional torque required from the engine.
No additional torque means the speed will not drop so the governor will not inject more fuel.
So the engine itself and it's immediate auxiliaries can absorb the idle power, no external load is needed.
Think of your car engine idling, it does not overspeed even though it has no external load because it is not getting much fuel.

 

[Morpethcurve]

I'm educated!

Thanks both, for your patient and comprehensive replies.

 

[RichardAsh1981]

Taking a stationary light engine, what differences are there between a 1960s loco and a modern type?

The big difference technically is that the 1960s loco has a DC generator with a commutator, where as a modern locomotive has an AC generator without.

It is important to understand that it is quite easy to vary the output of a generator (AC or DC) from zero up to either where the engine stalls or the generator overheats. All that is necessary is to vary the "field" or "excitation" current, which is used to set up the magnetic field for the main generator. This is a much smaller current and so easier to control. The limitation is that there is only one control, and everyone has to share whatever output is produced, and put up with a voltage that may not be right for anyone. Modern locomotives have electronic converters which solve this problem (or do if you spend the money and get the correct specs). Those with AC traction motors have electronics for them as well, so total control.

This was a big reason for multiple generators on older locos - a separate ETH generator could have the field adjusted for more or less constant voltage over the full rev range, maximum field when the engine is idling. The main generator is brought in on lower field to give a smooth start until the engine comes up to full power.