This is worthwhile. If there is going to be enough regeneration it is only necessary to have a little diddy motor for starting up the empty conveyor. Once loaded running you should be able to cut out the motor & cog in a generator at the tail end. To minimse the electrical investment the recipient equipment should have a power demand that closely matches the downhill conveyor operation.eg an overband magnet & dust extraction fan. ■

Hello Adrian..

Re-gen conveyors are common place and not that difficult if not too big, and you can do a complete dynamic analysis.

We have had success with them, and are currently designing one that will put about 350kW back into the grid. This is part of a mine backfill system here in South Africa

The biggest I have seen is the huge 9km 6800t/h downhill curved one that MAN Takraf did at El Abra in Chile. It produces plenty of re-gen power as the lift is minus half a kilometre. When stopping it can only reduce to about 30% of the 4.8m/second via the a-synchronous slip ring motors. The last 30% is done by huge disc brakes.

There I noticed that all is hunky-dory until they have a power failure, especially when the belt is empty (oops... now where did that counterweight go?)

Regards

LSL Tekpro ■

Dear Adrian,

The inverter configuration you describe is a two quadrant control. It only has the ability to produce positive power to the motor. A four quadrant inverter is needed to electric push power back into the grid to recover power from a downhill conveyor.

The same can be said for a fluid coupling or wound rotor motor type motor control.

Mr. Sprigg apartly does not understand the design principle of the 9km x 7200 kW EL Abra downhill conveyor nor its action during a stop. I was the client's design auditor on the project and have first hand knowledge of its design. Your noted tonnage is low, your speed specification is low, and your knowledge of the brake function during a power outage is incorrect. There is a batttery-backup for the wound rotor that uses a DC injection dynamic braking circuit. The hydrualic brake system has a UPS battery back-up to powerl the PLC and hydraulic valve action during a power outage. Your inference that the counterweight caused destruction of some kind sounds that you know something that the client does not know. Please tell us about the OOPS of the counterweight. CDI assisted in the commissioning and have no field measurement records of the OOPS of the head station TUP.

A larger system is the Los Pelambres copper mine in Chile. It is designed for -10000 kW on each of two downhill conveyors. At present, it is operating at -7500 kW while transporting over 8000 t/h at 6 m/s. The upgrade to 10000 kW will occur later this year. Los Pelambres is 4 quadrant inverter driven, while El Abra is wound rotor controlled with a DC injected dynamic braking.

The 12.2 km ( 3 flights) Los Pelambres downhill was designed by Krupp Canada and CDI. CDI designed the mechanical hydraulic braking circuit and control logic for starting, running and stopping. A normal stop uses the four quadrant control of the inverter on a prescribed braking velocity verses time ramp. The same ramp is applied by the mechanical brake if the inverter malfunctions.

A third large downhill regenerative system is Colluhuasi not far from El Abra and Los Pelambres. The longest flight is over 6 km, is designed to carry over 10,000 t/h, runs at 6 m/s and has 8000 kW with regenerative inverter control. CDI worked with Krupp Canada on the design and provided the PLC control logic to start and stop the system.

I designed for the first inverter drive applied to conveyors at the La Caridad mine in Sonora,Mexico in 1977. This was a two quadrant controlled system as described Adrian. Here four inverters start 20 drives in a selected pattern. The inverter is removed after starting each of 13 conveyors. The regenerative action of the 1800 hp downhill and load sharing regeneration between drives on the same conveyor is dumped to a fan cooled resistor bank. This is a short duration before the conveyor reaches full speed. ■

Do we assume you know how to calculate the belt tensions? Remember, the forum is for advice not engineering.

Assuming you know about the tension calculation and you are on an electric grid, then if the conveyor does regenerate power you should capture and use the power to lower your overall plant power costs.

A simple test for regeneration is to look at the overall slope of the conveyor path. If the total drop is greater than 2-3%, then you may regenerate power. More downhill slope = more regen power.

Your conveyor is very small, as such, I would recommend, a simple starting control such as a fixed-fill fluid coupling that regulates the acceleration ramp per good engineering practice. This means that you will also need a mechanical "fail-safe" brake to hold the downhill load when stopping and at rest. There is no need for the expensive inverter, WRM, or DC options. In fact, the mechanical brake can be used to both stop and start with the simplest fluid coupling. This means the control of acceleration and deceleration is done by the mechanical brake. When at full speed, the AC motor will inherently feed power back to the grid without need for further control. The fluid coupling will still transmit the conveyors power, but is passive in nature. ■

Hi Larry..

Feedback to you about the post commissioning experiences at El Abra downhill.

I was quoting the engineers on site who's words I noted pretty much verbatim.....

Quote:

" The conveyor blew up quite alot of electricals on commissioning, but was then sorted out.... Now we can only drive the conveyor down to 30% of the 4.8m/sec....the last 30% has to be done with mechanical brakes. Power failures are a major problem as you cannot re-generate. This is worst when the conveyor is empty as the take up at the head end takes an "almighty leap upwards"....One of the head end pulleys has recently failed..." One engineer then told me a joke he said "There was this man who asked a lady if they could have sex for $250.

She said "certainly not!" to which the man replied..

"Oh please lady - I really need the money!""

End of quote

I trust you find the feedback of some value, even if it is only the one about the 250 bucks!

Regards

LSL Tekpro ■

This post is going downhill, thank goodness. ■

Hello

I'm a student from Portugal, and I'm finishing my masters degree and preparing my final thesis.

I'm studying the energetic efficiency on systems that have regenerative breaking. On my google search I saw this forum and I I'm writing to you all, because I think you may give me some important knowledge about this matter! Do you know where I can find some material that explains with detail, the regenerative process on electric motors? And also the conversion of kinetic energy into electric energy.

I'm having some difficulties to find it! Can you help me?

Thank you so much for your attention!

Best regards! ■

Dear Faraday,

Normal AC-synchronous motors operate in the first quadrant of a rectilinear plot of torque vs rpm.

Regeneration takes place in the fourth quadrant ie. negative torque vs rpm.

Try Googling the details for a multitude of information such as:

http://cosmos.phy.tufts.edu/mhonarc/.../msg00253.html ■