Protection and sequencing along the line
Pull cords, belt-drift, zero-speed and blockage detectors; starting a chain of conveyors from the discharge end and stopping it from the feed end; and the drive needs of screws, bucket elevators, chain conveyors and pneumatic conveying.
30 min
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A conveyor is a long, mostly unattended machine carrying a lot of stored kinetic and potential energy, often through areas people walk past. Its protection is therefore spread along its whole length rather than gathered in the control panel, and the control engineer specifies it as part of the drive design. This lesson is course sections 5.2.3 and 5.2.4.
Protection devices
Pull-cord (rip-cord) switches run a steel wire the full length of the conveyor on both sides, latching a stop from any point on the run. They are the main emergency stop, because an operator near a conveyor is by definition not near the panel. Once pulled they must be reset at the switch, not from the control room: the person who stopped the machine is the one who confirms it is safe to restart.
Belt-drift (misalignment) switches are pushed by the belt edge running against a paddle. They are fitted in pairs at each monitored station: an outer position that alarms and an inner one that trips. A belt running off-centre spills material, rubs its edge on the structure and, if left, destroys itself; the two stages give maintenance a chance to correct the tracking before production is lost.
Zero-speed (underspeed) sensors on a non-driven pulley detect the difference between what the drive believes and what the belt is doing. They catch belt slip on the drive pulley and a sheared drive shaft, two cases where the motor runs happily while the belt does not move. A slipping belt on a stalled load is also a fire risk, which is why this device is mandatory underground and in enclosed installations.
Chute-blockage detectors (capacitive, tilt or ultrasonic) stop the upstream conveyor before a blocked transfer point buries it. Belt-tear and rip detection, with conductive loops embedded in the belt, protects long, expensive belts against being split lengthwise by a trapped object.
Sequence interlocking
Where several conveyors feed one another, the order in which they start and stop is forced by one rule: material must never be delivered onto a stationary belt.
Starting runs from the discharge end backwards. The last conveyor in the chain starts first, and each upstream conveyor may start only once the one it discharges onto has proved it is running at speed, not merely energised. That is why the zero-speed sensor is part of the permissive. A short stagger between starts also limits the total inrush and starting torque drawn from the supply, a real benefit on a long chain of large drives.
A normal stop runs from the feed end forwards. The feeder stops first, and each conveyor keeps running after its feeder stops for long enough to clear what is on it: a run-on time equal to its transit length divided by its speed. The line empties itself and leaves no loaded belt to restart.
An emergency stop does the opposite: everything stops at once. The belts are left loaded and the next start will need the high breakaway torque of a full belt, because in an emergency the priority is removing energy from the machine, not protecting the production schedule. Telling these two cases apart correctly in the control logic is one of the recurring design decisions in bulk handling.
Try it with the interlocks off first: start C1 and the feeder, wait, and watch where the material goes. Then turn the interlocks on and use the sequencer.
Predict first
The line is running loaded. You press Emergency stop instead of Normal stop. What is different about the next start?
Other continuous-transport mechanisms
The belt conveyor dominates by tonnage, but other continuous mechanisms load their drives differently.
- Screw (auger) conveyors move material along a trough on a rotating helical flight. Compact and enclosed, they suit dusty or hazardous products, but their torque is high and highly variable: a screw left full and allowed to compact has a breakaway torque several times its running value. Size the drive on breakaway torque, and protect it with a shear pin or an electronic torque limit, since the flight fails before the motor stalls.
- Bucket elevators lift material vertically in buckets on a belt or chain. The load is almost purely gravitational: nearly constant torque, proportional to throughput. The critical failure is belt slip or a chain break with a full column of buckets above, hence a mandatory backstop and a zero-speed monitor, and, with grain or other combustible dust, bearing-temperature and alignment monitoring, because a rubbing elevator leg is a classic ignition source.
- Chain and slat conveyors carry heavy unit loads and hot material. Their load is dominated by the sliding friction of the chain in its guides, higher than rolling friction and dependent on lubrication and wear. They also show the polygon effect: a chain wraps its sprocket as straight links, not a circle, so its speed fluctuates at tooth-passing frequency, significantly with few teeth, and the drive cannot remove it.
- Pneumatic conveying moves powders in an air stream. For the control engineer it is not a conveyor problem but a compressor problem, sized and controlled with eq. 2.1 and the affinity laws of Chapter 2. It uses far more energy per tonne than a mechanical conveyor, accepted for complete containment and flexible routing.
FoundationStart here if this is new to you
Think of a line of people passing buckets. Before anyone passes a bucket, the person they pass it to must be ready: so the line gets ready from the far end back. When the work ends, the first person stops taking new buckets and everyone passes on what they are holding until their hands are empty. In an emergency everyone simply drops what they are doing, and the buckets stay where they are.
ExplorerGo deeper: derivations and open questions
Permissive. Why must the start permissive for C2 use C3's zero-speed sensor rather than the contactor feedback of C3's drive? Give a failure the contactor feedback would miss.
Stagger. Four 200 kW drives start with a 5 s stagger, each drawing 1.5 times rated current for 20 s. Sketch the total current against time, then again with no stagger. What does the stagger save?