Traction Motor Overload

Ok, this may be a really silly question, but:

Assumption: Diesel-electric locomotives (i.e Class 60) use the same principle of transmission as diesel-electric trains (i.e Class 220) - a diesel engine drives a generator which produces current to turn traction motors which turn the driven wheels?

If so: Wikipedia tells me that modern improvements such as VVVF drives have reduced the chances of overload condition. BUT...

What actually is the reason that a locomotive needs an ammeter to show the driver the 'danger point' of load (if indeed it still does?) and can suffer traction motor overload conditions given a heavy enough train or prolonged low speed high load, whereas a DEMU doesn't apparently need one and doesn't seem to suffer overload, no matter how full of passengers the train may be?

Is it because a DEMU always has its weight within certain parameters that are allowed for? Or something else, such as a governor which locos don't have?

P.S Having never seen the cab of an 'old' DEMU such as a Class 205/207 I may be wrong and they did have ammeters fitted? But it's really the comparison between the modern DEMU and loco.

It takes more power to start something moving, than it does to keep it moving.
A modern diesel electric passenger passenger train such as a 222 isn't nearly as heavy as a 2000ton freight train, and so less power is needed to start it off. With a heavy freight, power has to be applied gradually, not only to minimise the risk of wheelslip and "snatching" which can result in snapped couplings and divided trains, but also to keep the amp's within tolerance. as the train speeds up, more inertia is overcome, and more power can be applied.
Diesel electric loco's up to Class 60 all have ammeters to tell the driver if he is getting close to the maximum so he can ease off a bit. HST's have them too, although on these they are more used to run down the power gently before shutting off and braking. We try not to shut off power until the ammeter has dropped below 500 amps to minimise jolting for our passengers. This technique is also used on freight trains, once again to give as smooth a ride as possible.

Kneedown said:

It takes more power to start something moving, than it does to keep it moving.
A modern diesel electric passenger passenger train such as a 222 isn't nearly as heavy as a 2000ton freight train, and so less power is needed to start it off. With a heavy freight, power has to be applied gradually, not only to minimise the risk of wheelslip and "snatching" which can result in snapped couplings and divided trains, but also to keep the amp's within tolerance. as the train speeds up, more inertia is overcome, and more power can be applied.
Diesel electric loco's up to Class 60 all have ammeters to tell the driver if he is getting close to the maximum so he can ease off a bit. HST's have them too, although on these they are more used to run down the power gently before shutting off and braking. We try not to shut off power until the ammeter has dropped below 500 amps to minimise jolting for our passengers. This technique is also used on freight trains, once again to give as smooth a ride as possible.

Click to expand...

67's have ammeters and they peak at about 5000, the amps cannot be constantly over 3500, I rarely use the ammeter on the 67 as it has a TMS called an EM2000 and this shows you exactly what the engine and gererator are doing.

In 50 yrs of trainspotting I have never been able to fully understand
weak field , what it does and why it is good and series and paralel for starting and running
can anyone explain in simple terms
thanks

colchesterken said:

In 50 yrs of trainspotting I have never been able to fully understand
weak field , what it does and why it is good and series and paralel for starting and running
can anyone explain in simple terms
thanks

Click to expand...

The main generator, or alternator via a rectifier, supplies dc current to the traction motors. As the current increases and the motor turns faster it starts to produce current itself which resists the current being supplied from the main generator. This is known as "Back EMF" (Electro Motive Force) To reduce the resistance from this Back EMF it is "tapped off" (Not a technical term) at various stages and redirected to the traction motor, thus adding to the traction current. This process is known as the "Weak Field" or "Field Diversion". Different loco's have different stages, for instance a 56 has 1 field diversion stage, a 47 2 or 3 (I don't remember exactly) but an HST has none as the power supplied from the main alternator is sufficient to overcome the back EMF.
Series and series parallel are just the way the traction motors are wired. IIRR 47's had both variants but the majority were series-parallel. I'm not aware of any particular advantage in either, other than in the event of an earth fault you could isolate an individual traction motor on a parallel, but had to isolate pairs on series-parallel loco's.

Thanks Kneedown and TDK for the very informative answers to my question - I think I now understand why - basically a loco has to potentially overcome a much bigger inert mass (likely to cause overload condition) than a DEMU ever would, hence the need for driver indication ammeters.

One of those things that I've always wondered about.

TDK does the DVT duplicate the screen and ammeter?

No ammeters on a DVT. Sending this volume of data over TDM was deemed a step too far. I don't recall current being available via AAR, either way no changes made to the DVTs.

Weak Field is the electrical equivalent of Overdrive.