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.