Belt Conveyor Protection

To detect anything other than a break in the belt, you would need to monitor the belt optically so that the defect could be detected.

Forget the MPCB path to a solution, they are only there to prevent a fire when the motor is overloaded, they will always suffer from nuisance tripping as a "load monitor".

I have done this many times, but have proposed it many more and had it turned down because of cost, so start there. What's your budget?

Redfred's suggestion #2 does in fact work, because when there is a lengthwise tear in the belt, there is still a sudden change in the load profile, one way or the other, which lasts longer than a typical transient load spike. I use kW sensors (now kW sensing solid state OL relays for DOL starters, or kW capable Soft Starters or VFDs), because sensing current alone is too susceptible to nuisance actions if there is any variability in the supply voltage (and there usually is in material handling conveyor applications in my experience). The Active Power monitors do the same thing, but typically only look at one of the 3 phases, so are also more prone to variability.

What I have done, successfully, with a kW monitor is to use a PLC to monitor a kW RANGE of operation for the equipment working at normal capacity, use it to establish a model of predictive behavior, then constantly monitor the kW in operation and compare it to that model. A small horizontal tear in the belt results in an increase in the kW on a repeating basis as the tear passes over rollers and belt tensioners, so whenever there is a spike in kW, you trigger a time / load comparator and look for it to repeat at regular intervals predicted by known belt speed. If kW spikes 3 times within the same narrow time windows, that indicates a tear going over a roller. If there is a large open tear and product is falling through, it reports as a load loss. I often also used a belt weighing system to know how much product was being put on the belt at any given time so that the load profile could be adjusted accordingly to filter out normal production changes.

This was NOT a cheap solution, even if you toy with and create the software yourself, because the hardware to implement it is not cheap. Hence my first statement; how bad is the problem and how willing is the owner to solve the problem? If you are only using MPCBs to control 55kW conveyors, and not VFDs (many of which by the way could easily be made to accomplish this on their own), that says to me that the owner of your facility is cheap, and will only want a cheap solution. THAT does not exist.

The protection arrangement for the motors is wrong, for starters (pun intended). Proper motor circuit protection depends on having co-ordinated devices, as advocated in standards such as British Standard 7671. One device, usually a bank of fuses, protects the cable; the other device, usually a motor overload unit, protects the motor. In a BS7671-compliant installation, the motor overload device can then be set to the full load current of the motor.

Solutions for detecting non-moving and/or broken belts are given above. A further one is detecting the rate of change of weight at one or both ends of the conveyor with suitable weighing devices.

"Check this up" as some of my Swedish co-workers would say:

http://www.cbmi.com.au/

Measure the current consumed with unloaded belt and set undercurrent relay.

About 30 years ago I managed an electric department [high voltage] of a steel complex.

Hundreds belt conveyors was installed there- for raw material mostly-the longest of 2 km.

Never we used D.O.L. starting. Since VFD was not available then the wounded rotor medium voltage induction motor was most of them. For low voltage- short length- double cage or deep bar rotor induction motor was provided.

The belt break or tear was controlled by means of speed switches. See[for instance]:

http://www.electro-sensors.com/applications/belt-conveyors/

http://www.electro-sensors.com/products/shaft-speed-switches/

Keep the motor protection according to manufacturer specifications.

The use of proximity detectors for the chain moving elements could be a solution, the number and the positions should be decided based on chain characteristics.

Complementary, use a row material flow detection (limit switch) on the charged side or at the discharge end.

These signals shall be correlated to running status of the motor.