Escalators, ropeways and the bucket-wheel excavator

Continuous transport also carries people, and then the drive becomes subordinate to safety. At the other end of the scale, the largest machines in open-cast mining combine every mechanism of this course into one coordinated system. This lesson is course sections 5.3 and 5.4, with Tutorial 5.2.

Escalators and moving walkways

Escalators and moving walkways are continuous conveyors for people. The drive is a constant-torque induction motor, historically direct-on-line at fixed speed, now more and more often a VFD that drops to a slow standby speed when no passenger is present (detected by entry sensors) and accelerates smoothly to nominal speed on demand. In an intermittently used installation that saves a lot of energy.

Over the drive sits an extensive electrical safety chain, identical in philosophy to the elevator chain of Chapter 3: a series string of normally-closed contacts monitoring the handrail speed, the step band, the combplates, the drive-chain tension and the emergency-stop buttons. Any open contact de-energises the drive and applies the fail-safe brake, bringing the unit to a controlled stop. The step-band and handrail speeds are compared continuously, because a divergence signals a mechanical failure that must stop the machine.

Tutorial 5.2: tracing the emergency-stop circuit

The chain holds, in series and all normally closed: the two e-stops (top and bottom), the two combplate switches (entry and exit), the broken-chain switch, the handrail-speed monitor and the step-upthrust switch, all feeding the coil of the drive-enable safety relay.

Path. The enabling current flows from the control supply through every contact in turn and reaches the relay coil only if all are closed. The relay's contact then enables the drive and releases the brake.
Exit combplate opens because an object is trapped: the series circuit is broken, the relay drops out, the drive is disabled and the fail-safe brake applies. The escalator stops.
Why series and normally closed. It is a logical AND of "all safe" conditions in which any fault (an opened contact, a broken wire, a lost supply) removes the enable and stops the machine. A parallel, normally-open arrangement fails dangerously: a broken wire in one branch silently prevents that stop command from ever arriving, and the machine keeps running as if all were well.

Try it: the escalator safety chain (Tutorial 5.2)

Control supply on

Running: every safe condition holds.

Every contact must conduct for the current to reach the relay: a logical AND of safe conditions. An opened contact, a broken wire or a lost supply all remove the enable.

Drive: enabled · Brake: released

Predict first

Switch to parallel normally-open wiring. Cut the wire to the exit combplate, then trip the exit combplate. What does the escalator do?

It stops, like the series chainIt keeps runningIt stops, but only after a delay

Cableways and ropeways

Aerial ropeways and cable cars (téléphériques) carry the continuous-transport principle to passengers over difficult terrain, and here reliability and redundancy are paramount, because a stoppage can leave people suspended. The haul rope is driven by a large sheave at one terminal. Its tension is held by a hydraulic or counterweight tensioning system that absorbs thermal expansion and load changes while keeping the friction grip on the sheave secure (the capstan relation of lesson 1 again).

Because a failure is not tolerable, ropeway drives are redundant. A main electric drive is backed by a fully independent secondary drive, often diesel-hydraulic, able to bring every carrier to a station, and braking is duplicated, with independent service and emergency brakes on the drive sheave. The control system continuously supervises rope speed, tension and carrier spacing, and runs a controlled evacuation-speed run on the backup drive if the main drive fails.

The bucket-wheel excavator

The culmination of continuous-transport engineering is the bucket-wheel excavator, one of the largest mobile machines ever built, used in open-cast mining to win overburden and ore continuously. It brings together, in one machine under one coordinated control system, every mechanism of this course:

the bucket wheel turns at constant speed to cut the face: a high-inertia, constant-torque load whose drive must ride through the impact of each bucket biting into the ground;
the slewing mechanism swings the boom across the face: a large-inertia positioning drive that must accelerate and decelerate a structure hundreds of metres long smoothly and without overshoot;
the boom hoist sets the cutting height: an active load balanced by counterweights, controlled in all four quadrants exactly like a crane hoist;
the crawlers advance the whole machine;
a series of boom and discharge conveyors, themselves multi-motor load-shared belts, carry the won material continuously to the dump.

The supervisory control matches cutting rate, slew speed and conveyor speed: the flow the wheel wins must equal the flow the conveyors can carry, or the system chokes or starves.

FoundationStart here if this is new to you

A combine harvester in a wheat field cuts, threshes and unloads at the same time. Drive too fast and the thresher chokes; too slow and the machine runs half empty. The bucket-wheel excavator is the same idea on a scale of thousands of tonnes an hour, with the cutting wheel, the swinging boom and the conveyors all kept in step by one controller.

Chapter summary

Continuous transport replaces the single suspended mass with a steady flow, and the control problem becomes one of synchronisation. A conveyor is sized from its effective belt pull, the difference between the tight- and slack-side tensions, bounded by the capstan relation. Its constant-torque load makes power proportional to speed, a natural fit for the VFD. Several motors on one belt must be kept from fighting: master-slave gives stiff, precise sharing but depends on communication and a designated master; droop is simpler and inherently robust, at the cost of a small load-dependent speed error. In passenger transport the drive is subordinate to safety. The bucket-wheel excavator shows where the discipline ends: many high-power drives coordinated so that the material won matches the material carried.

ExplorerGo deeper: derivations and open questions

Handrail monitor. The handrail and the step band are driven from the same shaft through different mechanisms. Why compare their speeds, and what failure does a 15 % difference suggest? Where in the chain would you put that contact?

Ropeway evacuation. The backup drive of a ropeway is rated at a fraction of the main drive's power. Using $P = F v$ , explain why a lower evacuation speed lets a smaller drive bring every carrier home.