Class 76 and 77 dual pantographs

[GordonC]

Hi,

 

this might be a daft question, but when you see photos of the Woodhead electrics Class 76 and 77 in operation, they always had both pantographs up on the overhead wire.

 

Why did they have 2 pantographs and both always in use when the AC electrics don't or the DC units didn't?

 

Thanks,

Gordon

[Xerces Fobe2]

The reason for this is that DC systems are relatively low voltage systems  such  as 1500V DC so given  and the current is higher than AC Systems with their high voltages  usually 25KV  AC  - Ohms Law helps here -  http://www.allaboutcircuits.com/vol_1/chpt_2/1.html

 

Therefore you require bigger conductors for high current and the 2 pantographs provide this on a DC electric loco such as a Class 76/77.  AC locos only have one pantograph up at a time due to the current being much lower.  Overall AC systems are more efficient than DC systems as resistance is less of an issue and  less supply points are required.

 

 

XF

[royaloak]

Because of the huge amount of current being drawn because of the lower voltage available.

 

To get the same 'power' from 1500V as you do from 25000V you need 16 times the current (power being volts x current).

[Enterprisingwestern]

Also, they put power back into the system.

 

Mike.

[Dave Scott]

Hi,

 

this might be a daft question, but when you see photos of the Woodhead electrics Class 76 and 77 in operation, they always had both pantographs up on the overhead wire.

 

Why did they have 2 pantographs and both always in use when the AC electrics don't or the DC units didn't?

 

Thanks,

Gordon

There is a furthur dynamic factor related to speed. The West Coast electric locos originally had two pantographs as per class 76 and 77. It was envisaged both would be used. However dynamic testing, cant remember if here or in France, revealed that the second pantograph was only making intermittent contact at high speed. To maintain good contact the pantograph has to exert an upward pressure which causes the contact wire to rise and creates a standing wave in the contact wire. It was discovered that the wave caused the second pantograph to loose contact and lead to arcing. As suficient power can be drawn from one pantograaph, agree with comments re Ohms law, the use of the second pantograph was discontnued on AC lines and AL1 to 5 had the second pantograph removed. I cant recall the critical speed but believe it was higher than Woodhead. (this is all from memory of an article published in Modern Railways in 60s or 70s)

[Southernman46]

ISTR Dutch locos on their system (1500v dc) would have both pantographs raised during acceleration from station stops and then drop one when the current had dropped within the capability of a single one..............not sure if that still happens, it used to when they had proper locos over there - 11's, 12's, 13's, 15's................

[phil-b259]

There is a furthur dynamic factor related to speed. The West Coast electric locos originally had two pantographs as per class 76 and 77. It was envisaged both would be used. However dynamic testing, cant remember if here or in France, revealed that the second pantograph was only making intermittent contact at high speed. To maintain good contact the pantograph has to exert an upward pressure which causes the contact wire to rise and creates a standing wave in the contact wire. It was discovered that the wave caused the second pantograph to loose contact and lead to arcing. As suficient power can be drawn from one pantograaph, agree with comments re Ohms law, the use of the second pantograph was discontnued on AC lines and AL1 to 5 had the second pantograph removed. I cant recall the critical speed but believe it was higher than Woodhead. (this is all from memory of an article published in Modern Railways in 60s or 70s)

 

However it also needs to be considered that BRs version of 25KV OHLE (even back in the mid 60s) was (except for the Styal loop) engineered to lower standard than was the norm in Europe, thus making the tendency of the contact wire to flex grater.

 

Of course the larger size contact wire needed for DC,  installations also helps mitigate the situation where two pantographs are used.

[Poor Old Bruce]

There is a furthur dynamic factor related to speed. The West Coast electric locos originally had two pantographs as per class 76 and 77. It was envisaged both would be used. However dynamic testing, cant remember if here or in France, revealed that the second pantograph was only making intermittent contact at high speed. To maintain good contact the pantograph has to exert an upward pressure which causes the contact wire to rise and creates a standing wave in the contact wire. It was discovered that the wave caused the second pantograph to loose contact and lead to arcing. As suficient power can be drawn from one pantograaph, agree with comments re Ohms law, the use of the second pantograph was discontnued on AC lines and AL1 to 5 had the second pantograph removed. I cant recall the critical speed but believe it was higher than Woodhead. (this is all from memory of an article published in Modern Railways in 60s or 70s)

 

However it also needs to be considered that BRs version of 25KV OHLE (even back in the mid 60s) was (except for the Styal loop) engineered to lower standard than was the norm in Europe, thus making the tendency of the contact wire to flex grater.

 

The "dynamic testing" was probably carried out from the BR Research outpost at Rugby. I am not too familiar with French OHE except that a lot of it was built at a constant contact wire height whereas the BR stuff went up for level crossings and down for overbridges. That meant that the French could use a pnntograph with a fairly stiff secondary suspension and just the pan head primary suspension moving up and down only a relatively small amount while BR pantographs had to accommodate much greater vertical movement.

[Xerces Fobe2]

Also, they put power back into the system.

 

Mike.

 Both AC and DC can feed back power into the respective systems and the Class 76 and 77's did.  With AC and the there is the issue of phase angles /power factor  http://www.allaboutcircuits.com/vol_2/chpt_11/3.html  where because of inductive loads the voltage and the current go out of phase and the electoral load changes so does the phase angle and efficiency. The phase angle of the AC loco acting as an AC generator has to be in phase with the incoming power supply and this has to be actively managed by onboard electronic systems in the AC loco

 

 

XF

[Talltim]

There is a furthur dynamic factor related to speed. The West Coast electric locos originally had two pantographs as per class 76 and 77. It was envisaged both would be used. However dynamic testing, cant remember if here or in France, revealed that the second pantograph was only making intermittent contact at high speed. To maintain good contact the pantograph has to exert an upward pressure which causes the contact wire to rise and creates a standing wave in the contact wire. It was discovered that the wave caused the second pantograph to loose contact and lead to arcing. As suficient power can be drawn from one pantograaph, agree with comments re Ohms law, the use of the second pantograph was discontnued on AC lines and AL1 to 5 had the second pantograph removed. I cant recall the critical speed but believe it was higher than Woodhead. (this is all from memory of an article published in Modern Railways in 60s or 70s)

This issue with multiple pantographs at speed causes problems with runing units in multiple at high speed. There were extensive testing of three coupled 350s before they were passed for 110mph running

[LNERGE]

Don't forget the cab to cab telephone that ran via the overheads on 76's and 77's.. The lead locos could talk to the bankers on Worsborough incline..

[Enterprisingwestern]

 Both AC and DC can feed back power into the respective systems and the Class 76 and 77's did.  With AC and the there is the issue of phase angles /power factor  http://www.allaboutcircuits.com/vol_2/chpt_11/3.html  where because of inductive loads the voltage and the current go out of phase and the electoral load changes so does the phase angle and efficiency. The phase angle of the AC loco acting as an AC generator has to be in phase with the incoming power supply and this has to be actively managed by onboard electronic systems in the AC loco

 

 

XF

 

Todays b*llsh*t bingo winner.

 

Mike.

[Xerces Fobe2]

Todays b*llsh*t bingo winner.

 

Mike.

Mike, With respect if you care to look at my post and the links you will see that I was putting some factual context behind what is a complex issue. Having been an electronics engineer on ship systems in the past albeit many years ago i learnt quite a bit about this types of issues etc and also gained qualifications in this subject.  So if you could  kindly identify the  "b*llsh*t" for me I would me most grateful!

 

Nigel

[Grovenor]

 

The West Coast electric locos originally had two pantographs as per class 76 and 77. It was envisaged both would be used.

Having been involved, albeit very briefly, with testing some of these locos when newly delivered to Longsight, and being a keen graduate apprentice at the time, i don't believe it was ever intended for the AC locos to run with both pans up. Provision of two pans for redundancy was normal at the time, and originally it was usual to run with the rear pan. My recollection of the reasons advanced for fitting only one pan to the AL6 and then removing one from the earlier locos was that studies had found that the unused pan wore itself out just vibrating and the benefit from the redundancy did not outweigh the extra costs of the maintenance.  Its interesting that after all these years the ECML is trying out redundant pans again to combat the severe loss to the operator in the current franchise regime when a damaged pan takes the train out of service.

Keith

[phil-b259]

Todays b*llsh*t bingo winner.

 

Mike.

 

It most certainty isn't b*llsh*t as you put it. Why do you think the interconnector under the channel that allows us to buy power from the French is DC.

 

As any power engineer will tell you before linking two AC supplies together they must be EXACTLY in phase with each other or problems will result.

[Dave Scott]

It most certainty isn't b*llsh*t as you put it. Why do you think the interconnector under the channel that allows us to buy power from the French is DC.

 

As any power engineer will tell you before linking two AC supplies together they must be EXACTLY in phase with each other or problems will result.

I did think about mentioning control of Negative Phase Sequence current by selective phase sequence selection at each feeder point but thought no one would understand. I once worked with the engineer who worked out the phase sequence for WCML.

[jamie92208]

French 1500 locos use both pans on starting and drop the 2nd one at a relatively low speed, about 10mph.   AC regeneration ahs only started in recent years as the control electronics have become available.  However ISTR reading somewhere that two descending MGR's on Woodhead brought the next one up for free.  There were large resistance grids however at the feeder stations at Penistone to dissipate excess current if nt trains were drawing the regenrated power.  The OLE on the DC is I think about 2 square inches in area in total against less than 1 sq inch for AC due to the lowere currents and voltage losses.

 

Jamie

[Titan]

Todays b*llsh*t bingo winner.

 

Mike.

Again, I worked with the class 92 project team, back in my good old BR days so I got more familiar than most with the locos intricacies, including the regenerative braking and how it works and can confirm it is not b*llsh*t but pretty much spot on.

[david.hill64]

Interesting question and the basic answer - because of the larger currents involved - is correct.

 

The problem of leading pantographs setting up standing waves in the contact wire is well known. If you ever see a video of Japanese Shinkansen trains which have multiple pantographs you will usually see a great deal of arcing, even with the heavy compound catenary that they use. The Japanese also use metallic contact strips on the pantographs rather than carbon which is the European norm.

 

BR Research did many studies into the mechanics of current collection and a spin off form this was the development of the sophisticated Brecknell-Willis pantographs that were pretty much standard for BR. These use aerofoils to help control the upward contact force which tends to rise as speed increases. The continental approach was to accept that the force would increase with speed and stiffen up the overhead accordingly. Hence when spare eurostars were used on ECML duties they had to be fitted with a BR pantograph so that they did not bring down the OHLE.

 

Network rail is now discovering that for the new electrification schemes such as GWML and Midland main line, replacement of the existing OHLE will be necessary as it will not be able to accept a standard European pantograph. I assume that this is because the TSI is based around UIC practice, but this is only a guess. It does mean that the proposed schemes are becoming significantly more expensive.

[Enterprisingwestern]

I didn't say it was bulshit, If you find out what bulshit bingo actually means then you will understand my post.

Posts 2 & 3 were a quite adequate explanation for the vast majority of us on the forum, there is no need for willy waving.

When you have, I will expect removal of the disagree icons and an apology to me.

 

Mike.

[stovepipe]

I think perhaps you need to look up what bullsh*t bingo means. It is a way debunking management speak, which was not present in the answer. God helps us if anyone using technical terms on a specialist forum is to be slapped down. I found the answers given informative and useful, as distinct from you own response, which was not either.

[Xerces Fobe2]

I didn't say it was bulshit, If you find out what bulshit bingo actually means then you will understand my post.

Posts 2 & 3 were a quite adequate explanation for the vast majority of us on the forum, there is no need for willy waving.

When you have, I will expect removal of the disagree icons and an apology to me.

 

Mike.

 

Mike, I note that continue to be insulting and ignorant , so I suggest that  if you cannot compose a civilised reply that you don't bother as you will only demean yourself further

 

Nigel

[Wheatley]

Posts 2 & 3 were a quite adequate explanation for the vast majority of us on the forum, there is no need for willy waving.

I had wondered why AC locos did not regenerate when the 1940s technology 76s could, now thanks to XF I know. Supplementary information, not willy waving.

[jamie92208]

There was a form of AC regeneration in 1907.  When the Midland electrified the lancaster Morecambe Heysham lines they used their own power station at Heysham with DC generators and then rotary converters that put out 6.25Kv at 25Hz.   The units used Siemens and Westinghouse AC motors so didn't need to carry their own rotary converter, just a step down transformer.   However the system struggled a bit if all three units were in action and heavily loaded so a large traction battery was installed at Heysham.  This fed into a rotary converter and automatically topped up the line voltage if it dropped.  I don't know how they coped withe phasing but it worked apparently.  As to pantogrpahs the Westinghouse unit used one diamond Pan but  the Siemens one used two bow collectors and often ran with both up and a high voltage line along the roof.   To stop bounce they had a subsidiary bow with a deflector/aerofoil on it at the opposite end of the frame that the collector stood on.

 

I also think that some of the early three phase lines could regenerate but I'm not certain.

 

Jamie

 

Edited for typos

[Xerces Fobe2]

There was a form of AC regeneration in 1907.  When the Midland electrified the lancaster Morecambe Heysham lines they used their own powere station at Heysham with DC generators and tehn rotayr converters that put out 6.25Kv at 25Hz.   The units used Siemens and westinhouse AC motors so didn't need to carry their own rotary converter, just a step down transformer.   However the system struggled a bit if all three units were in action and heavily loaded so a large traction battery was installed at Heysham.  This fed into a rotary converter and automatically topped up the line voltage if it dropped.  I don't know how they coped withe phasing but it worked apparently.  As to pantogrpahs the westinghouse unit used one diamond Pan but  the Siemens one used two bow collectors and often ran with both up and a high voltage line along the roof.   To stop bounce they ahd a subsidiary bow with a defelctor/aerofoil on it at the opposite end of the frame that the collector stood on.

 

I also think that some of the early three phase lines could regenerate but I'm not certain.

 

Jamie

 

I had totally forgot about the above Jamie  thanks for the reminder!

 

Nigel