Compensating Train Performance with Automode – Electric Part

An important detail of the metro train is its ability to compensate its performance based on amount of passengers in the train. The Newton’s first law shows that acceleration is equal to force divided by mass (F = m*a or A = F/m), so if the engine power stays the same, increasing number of passengers (and therefore the total mass of the train) would result in train acceleration and deceleration decreasing. The end result is that it takes considerably longer to reach the target speed and the braking distance grows very considerably.

To compensate for its weight, the train uses a device called an automode (automatic mode). It’s an electropneumatic device which adjusts both electric and pneumatic performance of the train and its operation is fully recreated in Subtransit. This post only explores the electric part of the automode.

Normally, the acceleration of the 81-717 train is controlled by an electric relay called RUT (the name translates as “Current Control Relay”). This electric relay prevents train from increasing the current through the engines beyond a set threshold. The threshold is reached when total magnetic flux in the power coils of the RUT relay exceeds magnetic flux of the control coils – this closes the relay and stops the rheostat. For more information on how the rheostat is used to control electric current and start the train, see this earlier post:

How Does a Rheostat Work?

This is the rough schematic for the RUT relay with all the coils, as well as the direction of mechanical force produced by magnetic flux of each coil:

When RUT relay is closed (on), the rheostat rotation will be inhibited. Therefore when the electric current reaches the threshold specified by the automode coil and the return spring, the relay will close and stop its further rotation. The electric current through automode coil works against the flux of power coils, causing extra force which keeps the relay open (off) for longer, allowing higher current threshold.

The other RUT coils are the lifting coil which forcefully closes RUT relay (used to ensure that rheostat stops at the given position even when electric current is below the threshold) and regulating coil which provides a slight adjusment to braking current on 81-717.5 trains.

So to provide automatic adjustment functionality, the automode current must depend on train weight and increase the current setpoint for the train. Higher current will create higher torque in the motors and compensate for the weight.

A basic automode device looks like this (only the electric side is shown), though this is actually an older type of automode, lacking the pneumatic adjustment unit:

For now, ignore the RKTT relay. This electric relay is very similar to RUT, it is used to detect when electric power circuit is unable to attain the required braking effort. A higher braking current threshold is used when train is heavier – so the control system will always sense low acceleration regardless of weight.

Mechanically, the automode consists of a long rod that is leveraged between the automode device and the bogey suspension. As train becomes heavier, the springs in suspension will shrink and the bogey suspension will press against the automode lever. This motion is mechanically transmitted to the automode itself, causing the central rod of the automode to move down.

Movement of central rod transfers motion to the sliding contact through a worm gear – the sliding contact sets one of 8 possible resistances for the automode electric part:

This is how the automode is connected into the train control circuits:

The same schematic, but slightly simplified:

As should be pretty obvious, the automode creates a shunt for the RUT and RKTT relay automode coils. As weight of train increases, the shunting of RUT relay decreases (increasing automode current through RUT) and shunting of RKTT relay increases (decreasing automode current through RKTT – the automode coil in RKTT has inverse polarity). So as train becomes heavier, both relays will end up with a higher setpoint for the electric current.

In other words, here’s how the current flow looks like with train partially loaded:

More current is going through RUT and less current going through RKTT. If the train was fully unloaded, instead most current would go through RKTT and none would go through RUT.

The additional connection towards node 6I is used by thyristor braking controller to adjust its own setpoint based on the train weight (the braking controller does not use RUT for current threshold, instead it has an active feedback loop that tries to adjust thyristor output to maintain the given current setpoint).

The automode has 8 weight positions, covering eight steps from 0 kg to 16,000 kg. The latter weight corresponds to about 200 people. During peak rush hours, these trains may transport more than 200 people (the maximum capacity is around 250-300 people) – this is well known to make trains very sluggish. Even with the automatic mode, the train under full capacity feels really sluggish as the traction system is simply unable to safely output more current and torque to the motors.

You’ll be able to experience this yourself in Subtransit – the automode and all the dynamic effects of increased train mass are simulated in the game. When the train is at its heaviest, beyond the control limit of the automode, the braking distance starts to sharply increase when electric braking is used.

Still, even with a full load of 300 passengers (24,000 kg), the automode provides safe (even if sluggish) train performance.

It is worth noting an interesting design choice with the RUT relay. The typical failure scenarios (return spring failure, etc) would virtually always result in decreasing the electric current setpoint for the train, preventing overload of the traction system. And since there’s no compensation for the battery voltage, if train battery is discharged, the electric current setpoint will also decrease as automode will not be able to provide the same counter-flux as with the fully charged battery.