Of ice and electricity

Photo of what winter weather may look like, westbound at Hawthorn Farm

Steven Vance recently forwarded me a question about why CTA‘s rails have been sparking. Unlike MAX trains which get power from an overhead wire system, their trains get power through a third rail system. However, arcing in both kinds of power systems during winter weather is typically caused by ice. Ice building up on the wire (or third rail as the case may be) acts as an electrical insulator, preventing the carbon shoe from making contact with the wire where the ice accumulates. This interruption in the flow of the current from the wire to the pantograph is visible as arcing.

Here’s a video of a MAX train pulling into & out of a platform where there was some ice on the catenary, and you can easily see the effect that it has:

To help clear ice from the overhead wire & prevent it from accumulating, a few of the Type 1s (107-112) are equipped with ice cutters which are put into use for major freezing rain/ice events. I don’t have a picture of any of them in use (though I’m willing to accept donations!), but they look like a second pantograph and function by heating/scraping ice from the overhead wire. Unlike the pantographs, ice cutters only draw current to heat the elements and not to provide power to the train, so they won’t arc the same way the pantographs do in ice. Their function is strictly to clear ice from the wire.

Pantograph (left) raised, ice cutter (right) lowered

If you’d ever seen one of these cars and wondered why it has two pantographs, wonder no more! It doesn’t – one pantograph, one ice cutter.

View from above; the ice cutter is the one closer to the coupled end, the pantograph is the one in contact with the wire closer to the vantage point. Bonus cameo appearance by car 235

When good trains go bad: Pantograph edition

Working my way through a backlog of drafts as well as emailed and commented questions.. (and thanks to those who have pointed it out, I’m aware that some links in older posts are no longer working. At some point I’ll go through and see what I can do to fix them, but I think some of those news articles & blogs aren’t around anymore)

Today’s question is about pantographs.

токоприемник, a search which has brought a lot of people here,
presumably from Russia.

Why don’t pantographs wear out or break?

Oh they do.

Not my picture – this is the broken pantograph outside the tunnel by Goose Hollow that tied up the alignment for about 7 hours, January 31, 2009 

Pretty much everything on the train is breakable (this is not an invitation), though instances of something serious like a pantograph breaking are rare. Mechanics, operators, supervisors, and even the public tend to notice excessive arcing that often indicates something is wrong before a pantograph reaches a breaking point. Arcing when the lines are icy or where wires cross – such as around Pioneer Courthouse – is not unusual, but repeated arcing when it looks like the overhead is perfectly clear is not normal and could be indicative of something wrong with the pantograph.

As mentioned in the earlier pantograph post, the part of the pantograph that makes contact with the overhead wire is called the carbon shoe. The carbon on this is a lot like pencil lead – if you ran your finger over a carbon shoe, it’d leave a dusty black streak on your hand. This is the source of the gritty black dust on the trains which is most noticeable around the coupled cabs.

Carbon shoe dust

Also previously mentioned, the overhead wires are staggered so that they make a zigzag motion over the pantograph. This ensures that the carbon shoe wears down evenly across the length of its surface. Under normal wear and tear, a carbon shoe can last from 9 months to a year before it needs to be replaced.

It can happen sometimes that rather than sweeping back and forth over the carbon shoe, the catenary will instead wear a narrow groove into the carbon, causing the wire to become stuck in the groove and wear just that part of the carbon shoe down. Potentially the wire can saw down into the pantograph if the groove is not noticed and fixed – remember that the spring-loaded pantograph puts a considerable amount of upward pressure on the overhead wire. A groove in the carbon shoe will require the train be pulled out of service so that the carbon shoe can be replaced before the pantograph breaks. This is one potential cause of a pantograph breaking.

Another cause can be extreme heat, and we’re getting near that time again.. well, maybe, if we get any proper heat waves now that it’s summer. As I posted last year, hot weather causes the overhead wire to sag when the weights on the catenary poles hit bottom and can’t provide enough tension in the overhead wire.  When this happens, train speed is reduced to prevent the pantograph from getting caught in or pulling down the overhead wire, which would do significant damage to both.

It’s also possible that damage to the overhead wire can break a pan, such as intentional vandalism. This is part of the reason for sweep trains every morning as well as regular walking inspections of the overhead wires to check for any damage or anything else that looks questionable.

How can you tell something broke?

Aux Fail (the red light on the console), trailing Type 1 cab WB at Jeld Wen Field

In Type 1-3 cars, often the first visible indication that an operator sees that something went wrong with one of the pantographs is the “AUX FAIL” annunciator in the console lighting up (the reason why it’s lit in the above picture was actually for an HVAC fault in the Type 1, not anything with the pantographs, but I don’t personally have any pantograph problem photos. There are several different kinds of mechanical problems with the trains that will cause an aux fail). Type 4 consoles are different; the AUX FAIL annunciator reads AUX FAULT instead, there is also a MAJOR FAULT annunciator (though you can’t really see the annunciators in that linked picture), and there is also the TOD, or Train Operator Display screen next to the speedometer which displays mechanical problems with the train. And it goes without saying, but the train will also not operate properly if the pantograph is breaking or broken (moving sluggishly or not at all, lights going out, etc). Operators notify Control if there is any indication of a mechanical problem – an aux fail could be something benign like the HVAC blowers not working, but it could also be the first clue you have that your pantograph is currently being shredded.

So yes, pantographs can break, but it’s rare to have your trip disrupted because of a broken pantograph. The parts of them that are designed to wear out (such as the carbon shoe) are monitored and replaced when needed.

Of course, no post about broken pantographs would be complete without this (non-TriMet) video. Not really sure what the backstory of it is – some of the comments say it was done as a test but I don’t know if that’s true.

Washington Street

Washington Street in Hillsboro

Washington Street is the pre-empted area at the western end of the Blue Line. In this picture, you can see the pre-empts (all yellow horizontals since there are no trains moving through) on both sides of each intersection, though they’re a little easier to see on the left. Also in this picture, the way the overhead wire zigzags is clearly visible – remember this is done to evenly wear down the carbon shoe on the pantograph.

Another one for the electricity fans

I think I should just change the name of this blog to “Arcings of a TriMet Pantograph” since far and away that’s the sort of thing people are searching for when they end up here.

The other night I was on foot downtown; got this video.

And a still from the video:

Which looks neat and all in a special-effects kind of way, but it’s not really a best practice..

Stop (and go) – Part 1

Someone found my site by searching a few permutations of “How do the Portland MAX trains stop?“

I liked the question and so here’s your answer:

Multiple Brake Systems

Of course, it’s not going to be a simple answer. There are several brake systems on the MAX trains.

Dynamic braking

Dynamic braking is the primary method of braking used when the train is going faster than 3mph.

I’ve posted this picture before, but it’s helpful in this context so I’m posting it again – Reverser (on left) and Motoring Drum Handle (on right) of one of the low-floor cars. Click for larger version.

On a really, really basic level, you move the train forward by moving the motoring drum handle (MDH) into a propulsion mode, shown in green next to the MDH. This draws power from a substation through the overhead wires, which rotates the train’s motors and moves the train forward.

When you move the MDH into a braking mode (shown in red next to the MDH), that makes the motors stop acting like motors and start acting like generators. The function of a generator is to convert motion into electricity – think of wind or water generators that create electricity from the motion of the wind or water through them. So when the train is in a braking mode and the motors are behaving like generators, they take the forward motion of the train and convert it into electricity. This conversion slows the train down because the creation of electricity happens at the expense of that forward motion. This is a far more effective way to stop a train moving at high speeds than dragging something on the wheels of the train to slow it down would be.

But then what happens to that new electricity that was converted from the motion of the train?

In the Type 1s, that extra electricity is just given off as heat from the motors. It works, in that the train slows down which was your primary goal of braking, but it’s kind of a waste of that electricity.

The  Type 2s, 3s, and 4s, however use regenerative braking. I’ve mentioned circuits before in the sense of currents through the rails shunted by the axle of the train wheels. Now think bigger – a circuit going from a substation, along the catenary and down the pantograph to the train, out through the wheels and along the rails back to the substation.

More or less like that

When one of those low-floor cars slows down using dynamic braking, that created electricity isn’t just wasted as heat into the air. Those cars channel most of that energy back into that circuit, where another train can use it.

Think of it this way the next time you’re in a train braking to stop at a platform – if it’s not a Type 1, the extra electricity from your train slowing down is being sent back into the overhead wire. The next train can pick up that electricty created by your train and use it to propel forward instead of drawing all of its power from the substation. This is much more efficient, because the energy converted from your train isn’t completely wasted, and less power needs to be drawn from the substations when you take power from another train’s braking.

This blog discusses how regenerative braking works in detail with helpful diagrams if you are interested in a longer explanation.

More to come.