Diesel v Electric bogies

[AM9]

I believe the maximum rating is 1,868hp for the four motors together if I recall rightly.

That shows the inequality of the way that diesel-electric trains are specified compared with pure electric. Diesel trains are usually rated by their engine's maximum power output, which includes al the ancillaries and heating/cooling. The power of electric Loco consists/EMUs is normally described as the power rating of the motors, - the presumption is that ancillaries are powered by the prime source through the transformer directly or through MG/SC equipment, but that doesn't detract from the performance of the traction system.

 

[Railperf]

That shows the inequality of the way that diesel-electric trains are specified compared with pure electric. Diesel trains are usually rated by their engine's maximum power output, which includes al the ancillaries and heating/cooling. The power of electric Loco consists/EMUs is normally described as the power rating of the motors, - the presumption is that ancillaries are powered by the prime source through the transformer directly or through MG/SC equipment, but that doesn't detract from the performance of the traction system.

Yes, i don't agree with the gross diesel engine figure. It is power at rail which is more important. A 4500hp HST effectively only delivers 3540hp to the rail. That is a huge difference! It would be more useful if the rail power figures were more commonly used.

 

[Irascible]

Tractive effort isn't usually an issue at high speeds when strong acceleration wouldn't normally be needed.
Standard DC series wound traction motors need field weakening at high rtotational speeds to reduce the back EMF and allow more rotor current to be drawn without increasing the overall applied voltage.

Or no field diverts and longer gearing - looking at you, HST - but then your starting TE is awful - looking at you, HST. Historically ( over here anyway ) field diverts have been somewhat unreliable, so the HST was a pretty careful balance to get away without using them. With better electrics & different gearing the performance might have been *really* eye opening instead of fairly eye opening ( at the time ).

Starting tractive effort is often a big issue particularity for freight as it can determine the maximum load a loco can haul. If insufficient there is a risk a train can stall on an upgrade. Passenger trains normally have a good surplus of tractive effort for good acceleration so rarely have problems (although with the HST power cars I think they have cut this a bit fine as they can have difficulties if one power car fails)

What I do find interesting is that in America some railways use Slugs. This is where they have taken the diesel engine out of an old locomotive (and replaced it with ballast) and arranged another coupled locomotive to power the locomotives motors via power cables. What this does is double the starting tractive effort so twice the load can be hauled, although at any speed the tractive effort will be no more than a single locomotive. Ideal for shunting or working slow speed industrial lines.

IIRC slugs are used to take up excess power from a locomotive at start/low speed when it's own motors can't actually use all it can provide. Which is sorta what you said, only it wasn't clear the powerplant can't actually run at full power at low speed in it's usual installation.

 

[hexagon789]

That shows the inequality of the way that diesel-electric trains are specified compared with pure electric. Diesel trains are usually rated by their engine's maximum power output, which includes al the ancillaries and heating/cooling. The power of electric Loco consists/EMUs is normally described as the power rating of the motors, - the presumption is that ancillaries are powered by the prime source through the transformer directly or through MG/SC equipment, but that doesn't detract from the performance of the traction system.

Indeed, a 3,300hp diesel won't pull the same as a 3,300hp electric.

Or no field diverts and longer gearing - looking at you, HST - but then your starting TE is awful - looking at you, HST. Historically ( over here anyway ) field diverts have been somewhat unreliable, so the HST was a pretty careful balance to get away without using them. With better electrics & different gearing the performance might have been *really* eye opening instead of fairly eye opening ( at the time ).

They were designed to incorporate one stage of field diversion, in testing it was found the motor voltage at high speed was still sufficient to give the desired performance so it wasn't incorporated in the production build, one stage was incorporated into the prototype motors but I believe it was never used, I don't know if the protodictionary PCs actually had the field diversion incorporated but isolated but I suspect not.

The divert control itself was however retained, being used to switch the high-speed/low-speed magnet valve in the two-stage brake system.

 

[Richard Scott]

Quite a few French locomotives had switchable gear ratios as I recall, the diesel equivalent of the CC6500s the CC72000s had an option between 140/160 (depending on batch) and 85km/h. The 85 setting was designed for freight but there was at least one passenger service where the stiff gradients of part of the route meant the 85 setting was used even though linespeed was higher than 85.

I believe the PV setting on 72000s was used between Roanne and Lyon. Sure many locos had these switchable bogies, 25500s spring to mind so assume 8500s and 17000s also. Likewise do 7200, 15000 and 22200s have them? Also the 67000, 67300 and 67400s possibly have this feature?

 

[hexagon789]

I believe the PV setting on 72000s was used between Roanne and Lyon.

I couldn't remember which route it was but that sounds like it

Sure many locos had these switchable bogies, 25500s spring to mind so assume 8500s and 17000s also. Likewise do 7200, 15000 and 22200s have them? Also the 67000, 67300 and 67400s possibly have this feature?

25500s, 8500s and 17000s do (either 90 or 100 PV & 140 GV), I don't think the 67xxx classes do though I believe they were all fixed at either 140km/h or 90km/h.

 

[Richard Scott]

I couldn't remember which route it was but that sounds like it



25500s, 8500s and 17000s do (either 90 or 100 PV & 140 GV), I don't think the 67xxx classes do though I believe they were all fixed at either 140km/h or 90km/h.

Ok, thanks for info; sure 67xxx are monomotor bogies but with fixed gearing then?

 

[hexagon789]

Ok, thanks for info; sure 67xxx are monomotor bogies but with fixed gearing then?

Seems like it, they were originally mixed-traffic locos geared for 140km/h later reductions in the sphere of passenger operations saw downgrading to freight locos with 90km/h gearing:

"Avec l'arrivée de machines plus modernes et la réduction de leur aire d'action suite aux électrifications, la vitesse limite des BB 67000 a été réduite à 90 km/h"

(With the arrival of more modern machines and a reduction in their areas of activity after electrifications, the speed limit of the BB 67000s has been reduced to 90km/h.)

 

[43096]

I believe the PV setting on 72000s was used between Roanne and Lyon. Sure many locos had these switchable bogies, 25500s spring to mind so assume 8500s and 17000s also. Likewise do 7200, 15000 and 22200s have them? Also the 67000, 67300 and 67400s possibly have this feature?

7200, 15000 and 22200s have fixed gearing, being one of 100km/h, 160km/h or 200km/h.

 

[hexagon789]

7200, 15000 and 22200s have fixed gearing, being one of 100km/h, 160km/h or 200km/h.

I thought the 15000s were geared for 180? (Though I believe they've always been 160 in service.)

 

[Richard Scott]

7200, 15000 and 22200s have fixed gearing, being one of 100km/h, 160km/h or 200km/h.

Thought they possibly were as knew they swapped bogies around between 7200 and 22200 at some stage. Still monomotor bogies, though?

== Doublepost prevention - post automatically merged: 28 Mar 2021 ==

Seems like it, they were originally mixed-traffic locos geared for 140km/h later reductions in the sphere of passenger operations saw downgrading to freight locos with 90km/h gearing:

"Avec l'arrivée de machines plus modernes et la réduction de leur aire d'action suite aux électrifications, la vitesse limite des BB 67000 a été réduite à 90 km/h"

(With the arrival of more modern machines and a reduction in their areas of activity after electrifications, the speed limit of the BB 67000s has been reduced to 90km/h.)

That would explain why 67200s quoted as 90km/h max speed then, wondered why it was so low.

 

[43096]

Thought they possibly were as knew they swapped bogies around between 7200 and 22200 at some stage. Still monomotor bogies, though?

Yes, still monomotors.

 

[hexagon789]

That would explain why 67200s quoted as 90km/h max speed then, wondered why it was so low.

They performed a number of similar alterations to other classes as I recall, and in the past some members of certain classes have been swapped between gearings more than once. I believe the third-rail equipped CC6500s for the Ligne de Maurienne went from 160 to 140 and then back to 160km/h after the third rail capability was removed

 

[The Lad]

Sorry, going back to post 33.
I thought the ambition for the production HST's was to uprate the engines on the Eastern sets from 2250 up to 2500hp ie 1876Kw.
Sadly it didn't happen.

 

[ac6000cw]

Sorry, going back to post 33.
I thought the ambition for the production HST's was to uprate the engines on the Eastern sets from 2250 up to 2500hp ie 1876Kw.

It was proposed - due to the Eastern Region sets having an extra coach compared to the earlier Western sets - but BR decided that the performance would be OK without doing that.

Given the subsequent HST engine and cooling system problems I think it was a wise decision...

== Doublepost prevention - post automatically merged: 29 Mar 2021 ==

The French BB 26000 'Sybic' design was a 7'500hp design dsigned to haul 16 corail (longer and heavier than a UK MK3) coaches at 200km/h (124mph) or 2,050t freight at 80 km/h (50mph) on a 0.88% or 1 in 115 gradient. Not sure any UK loco can match that.
I wonder what advantage the monomotor bogies played a part in that? The B-B configuration being similar to a bo-bo except one large motor per bogie rather than two!

Monomotor bogies have the advantage (like diesel hydraulic locos) that all the axles in a bogie are mechanically coupled together, which helps reduce wheelslip - individual axles can't slip within a bogie, only all or none of them.

Individual axle slip is the bane of classic DC-traction drives (i.e. those without Sepex or similar individual axle torque control) and that ultimately limits their maximum tractive effort performance.

Modern AC-traction drives achieve the same 'axles locked together' effect electrically (and can operate efficiently over a wider speed range), so do any manufacturers still offer loco designs using monomotor bogies?

 

[apk55]

Tyne and Wear metro cars use monomotor bogies with a longitudinally mounted motor with right angle drives from each end to the axles. Sheffield supertram I think also uses monomotor bogies in a similar arrangement.

DC series motors in series behave a bit like a car differential. If one motor slips then it absorbs all the volts and can under some circumstances over-speed to the point of busting the armature.

 

[ac6000cw]

DC series motors in series behave a bit like a car differential. If one motor slips then it absorbs all the volts and can under some circumstances over-speed to the point of busting the armature.

AFAIK - when EMD moved from DC-generators to alternators and electronic wheelslip control they also changed to using an all-parallel (DC) traction motor connection arrangement, with multiple switched alternator field windings. At low speeds the windings were connected to provide a lower voltage/higher current output and a higher voltage/lower current output at higher speeds (the switchover point is known to crews as the 'transition'). I think they stopped using motor field weakening at the same time.

Earlier, DC-generator equipped, EMD locos I think switched the motor connection groupings between series-parallel and all-parallel arrangements at the 'transition' point (and used motor field-weakening). As engine power increased the complexity of the (relay-based) electrical control system increased (more field weakening stages etc.) to the point where the last of the DC-generator loco models acquired a reputation for electrical unreliability. Moving to alternators was a real game-changer in the late 1960s.

In the UK, what were the motor connection and field weakening arrangements like on the 40's, 45's and 47's?

 

[Richard Scott]

AFAIK - when EMD moved from DC-generators to alternators and electronic wheelslip control they also changed to using an all-parallel (DC) traction motor connection arrangement, with multiple switched alternator field windings. At low speeds the windings were connected to provide a lower voltage/higher current output and a higher voltage/lower current output at higher speeds (the switchover point is known to crews as the 'transition'). I think they stopped using motor field weakening at the same time.

Earlier, DC-generator equipped, EMD locos I think switched the motor connection groupings between series-parallel and all-parallel arrangements at the 'transition' point (and used motor field-weakening). As engine power increased the complexity of the (relay-based) electrical control system increased (more field weakening stages etc.) to the point where the last of the DC-generator loco models acquired a reputation for electrical unreliability. Moving to alternators was a real game-changer in the late 1960s.

In the UK, what were the motor connection and field weakening arrangements like on the 40's, 45's and 47's?

Can tell you 45s are all parallel with five stages of field weakening. The motors themselves are series so armature and field in series with each other. Think a 47 has three stages of field weakening and some have three series pairs of motors in parallel and some are all parallel. Guessing with a 40 but if same as a 37 then the three series pairs in parallel with three stages of field weakening.

 

[ac6000cw]

Can tell you 45s are all parallel with five stages of field weakening. The motors themselves are series so armature and field in series with each other. Think a 47 has three stages of field weakening and some have three series pairs of motors in parallel and some are all parallel. Guessing with a 40 but if same as a 37 then the three series pairs in parallel with three stages of field weakening.

Thanks - an interesting variety of arrangements from the different builders/designers.

IIRC the class 58's had something of a reputation for slipping, and Roger Ford commenting that they had a rather more 'series' than 'parallel' arrangement of traction motors (but I don't know the reasons for that).

 

[Richard Scott]

Thanks - an interesting variety of arrangements from the different builders/designers.

IIRC the class 58's had something of a reputation for slipping, and Roger Ford commenting that they had a rather more 'series' than 'parallel' arrangement of traction motors (but I don't know the reasons for that).

Believe 56s and 58s were same arrangement as the 40s i.e. three series pairs of motors in parallel. Seems this arrangement is more prone to slipping than an all parallel arrangement as motor voltage is same across each motor in all parallel arrangement so slipping has an element of self control. Assume the series-parallel arrangement allows for a small generator/alternator due to lower current supply (thus should, in theory, be more efficient as higher current means higher energy loss due to heat) but if generator fitted more chance of flashover due to higher voltage. Think the other issue with 58s was a problem with centre axle on each bogie suffering lower adhesion, think partially solved by softer springing on that axle?

 

[hexagon789]

Think a 47 has three stages of field weakening and some have three series pairs of motors in parallel and some are all parallel.

I believe the split was between no-heat locos intended for freight and those with heating intended for passenger/mixed-traffic duties. The field diverts on the 47701-17 series were adjusted and optomised for 100mph running by, at the cost of poorer acceleration from rest and little ability to significantly exceed 100mph as with unmodified 47s.

 

[ac6000cw]

Out of interest/curiosity, I've just come across this PDF of a 1944 vintage EMD FT operators manual - https://www.alternatewars.com/BBOW/Railroads/EMD_FT_OM.pdf . As this was aimed at crews coming from steam traction, there are warnings about being careful when starting trains due to the high tractive efforts the diesels can generate...

The FT's were EMDs first proper 'road' freight diesel design (in 1939), and had all-manual motor grouping, field weakening and dynamic braking control.

I think our class 76 electrics also had predominately manual control of motor arrangements as well (they were a similar design vintage).

 

[Richard Scott]

I believe the split was between no-heat locos intended for freight and those with heating intended for passenger/mixed-traffic duties. The field diverts on the 47701-17 series were adjusted and optomised for 100mph running by, at the cost of poorer acceleration from rest and little ability to significantly exceed 100mph as with unmodified 47s.

They were fairly mixed. Lots of series-parallel and all parallel boilered and ETH locos. Only all parallel series was the 47/3. They found the all parallel arrangement to be more reliable hence the later ones were built this way and lasted longer, on the whole.

 

[ac6000cw]

According to the 'Decline' section of the class 47 Wikipedia page - https://en.wikipedia.org/wiki/British_Rail_Class_47#Decline :

BR drew up a 'hit-list' of locomotives for early withdrawal, mainly including those with non-standard electrical equipment, known as series parallel locomotives.

...which implies the 'all-parallel' variety were the majority (and preferred) version. Maybe the 'series-parallel' versions were more prone to motor flash-over issues in wheelslip conditions?

 

[Richard Scott]

According to the 'Decline' section of the class 47 Wikipedia page - https://en.wikipedia.org/wiki/British_Rail_Class_47#Decline :

...which implies the 'all-parallel' variety were the majority (and preferred) version. Maybe the 'series-parallel' versions were more prone to motor flash-over issues in wheelslip conditions?

The voltage across a generator in series-parallel is higher so risk of generator flashover higher. Other disadvantage is if a traction motor does fail you need to isolate the pair (so down to four motors) whereas in all parallel only one needs isolating (still have five motors).

 

[hexagon789]

They were fairly mixed. Lots of series-parallel and all parallel boilered and ETH locos. Only all parallel series was the 47/3. They found the all parallel arrangement to be more reliable hence the later ones were built this way and lasted longer, on the whole.

I should've specified 47/3s rather than freight for all-parallel

 

[AM9]

According to the 'Decline' section of the class 47 Wikipedia page - https://en.wikipedia.org/wiki/British_Rail_Class_47#Decline :

...which implies the 'all-parallel' variety were the majority (and preferred) version. Maybe the 'series-parallel' versions were more prone to motor flash-over issues in wheelslip conditions?

Assuming the standard method of transition between series and parallel, one of the significant failure modes is the failure of contactors. I've not seen anything about loco S/P designs, but the 1500V EMUs had motors, wired in series with resistors across the 1500V supply up to about 20mph, at which speed the resistances would all be switched out. Then, to avoid having to break a nearly purely inductive circuit with a high running current, the centre point was switched to ground momentarily to allow the lower motor to be changed over to its own 1500V feed, with the resistor chain back in circuit. This meant that there was no arcing across the contactors by trying to open a circuit. The problem was that the sequencing of contactors was failrly critical as the short to ground produced a current surge before the reasitors on the upper motor could be switched back in. Sometimes there would be a repeated tripping of contactors more than 5 times before the drive settled down again.

 

[Richard Scott]

Assuming the standard method of transition between series and parallel, one of the significant failure modes is the failure of contactors. I've not seen anything about loco S/P designs, but the 1500V EMUs had motors, wired in series with resistors across the 1500V supply up to about 20mph, at which speed the resistances would all be switched out. Then, to avoid having to break a nearly purely inductive circuit with a high running current, the centre point was switched to ground momentarily to allow the lower motor to be changed over to its own 1500V feed, with the resistor chain back in circuit. This meant that there was no arcing across the contactors by trying to open a circuit. The problem was that the sequencing of contactors was failrly critical as the short to ground produced a current surge before the reasitors on the upper motor could be switched back in. Sometimes there would be a repeated tripping of contactors more than 5 times before the drive settled down again.

Series-parallel locos are permanently wired as such, the only switching will be field diversion.