Dear Jan,

I am breaking a promise not to engage in petty forum discussions, which I have unfortunately instigated with very late night dialog and confused brain.

I believe in your quest to be an energy steward and to understand the roll of power savings via Active Speed Control. Conveyor Dynamics, Inc. (CDI) has implemented this control scheme on a number of overland installations. Most recently, the Wesfarmers Curragh 20 km overland in Queensland Australia utilizes Active Speed Control.

Dr. Robin Steven, Goodyear Belt Conveyor and Rubber Technical Specialist, has written two topical papers on Curragh. The first was at the Newcastle University forum in Australia 2007. Your Prof. Lodewijks did attend and does have a copy of the proceedings. I believe BSH will publish the second of Dr. Steven's papers in the coming issue. He does note the variable speed control, but does not go into its detail. The paper does offer many interesting facts on Curragh, including high performance rubber rolling resistance, and the search process to define such rubber behavior. We were fortunate to partner in the search.

I will summarize the result with respect to your question:

Curragh is designed for 2500 t/h, 900 kg/cm coal, 1200 mm belt with a final max design speed = 7.5m/s.

Power savings, running at 1500 t/h and belt speed dropped to equal the crossection of full 2500 t/h design tonnage, with Active Speed Control, is >22% . This occurs when ambient temperature is set at 20 C. Any other temperature, speed, or tonnage will have differing savings.

The power savings is presented as a percent penalty, if you had to operate at full design speed, verses the necessary constant crossectional loading speed.

You cannot take this in absolute terms. There are many other factors that need consideration when finalizing the design. One such factor is that the belt may not be able to run at any speed below the ultimate design speed. There are speeds whiich should not be selected due to various detrimental vibration modes.

I do not wish to go into the design details, but to highlight the gain in Active Speed Control. I hope you do pursue this topic and publish your insight in a public forum such as this. ■

You are noting 10 year old technology. Electronics (Inverters) have a much faster cyclic improvement.

Modern inverters have a high efficiency over most of the speed curve by capturing the power factor and slip losses. Electrically, comparing 10% with 100% electrical losses are not significant. Inverters with transistor power with the use of flux-vector control, are needed to capture the old style designed inverter electrical losses which did become very inefficient in the lower speed range.

The Curragh savings are greater noted in the prior posting when you consider all tonnages and speed corrections. The belt is not continuously variable and does not operate to maintain a constant belt cross-sectional loading. Due to the higher than normal idler spacing, you must be cognicient of belt vibration and meter belt speed to tonnage ranges to minimize undesirable vibration. ■

Dear Jan,

Not knowing much about belt conveyors, I assume that the power demand versus tons/hr is not that complicated. (Otherwise there would be a lot of wrongly sized drives on newly built belt conveyors)

Additional aspects as vibration, acceleration, starting, stopping, etc. seems to me a case for experts with experience and mathematical capabilities.

The behaviour of an electric motor is a science in itself.

Squirrel cage motors with starting cages, running cages, magnetizing losses, impedance, Ohm resistance, Volt/Hertz ratios, various coil arrangements and on top of that the power electronics of a volt/hertz regulator or inverter, with its own efficiency and by-pass switches, are certainly the field of electrical engineers.

Also the temperature of a motor is important as it changes the resistance/impedance ratio, resulting in a better power factor and a higher torque.

Consult or cooperate with somebody from the electrical faculty of the Delft university and contact motor and inverter manufacturers.

Also try to get information from end users in the port of Rotterdam and Amsterdam and evaluate those designs against the operational and real performance.

Investigate both issues ( belt conveyor and electric drives) separately and combine them later.

A few years ago I made a spreadsheet to calculate acceleration times of a compressorset and also made a (very) simple calculation of an electric motor torque cuve. If you are interested, let me know.

Success ■

Another active speed control was implemented at Hamersley Iron's (Rio Tinto) Eastern Range installation. This preceded Curragh.

Curragh has a fixed set of speeds and a range of tonnages that operate in each speed band which were selected by the mechanical design. The system is automatically regulated by the receiving bin fill levels. The bin has 5 compartments with different demands that are automatically or apriori set by the different levels in the bin pockets.

Eastern Range is a less complex system, but also inverter driven.

Both systems are operator controlled. The operator can selected any speed. The control system blocks certain speed ranges and tonnages which are governed by the mechanical design.

There are categories of electrical losses. First, there is the electrical slip which is dependent on the motor rpm. As the inverter driving frequency is reduced, the motor slip characteristics stays more-or-less constant. Second, while old inverters suffer from power factor losses, modern well engineered systems do not. Thus, there rotor heating is dramatically reduced. Still special cooling is required at lower speeds that are not of concern at full design speed.

A sub note, as the torque-speed curve is shifted toward the y-axis (torque) with the reduction in frequency or speed shifted along the x-axis, eventually the breakdown torque (peak torque usually ~250% near 3-6% of full speed) engages the y-axis thereby enabling substantially more lock rotor (usually ~130% @ 50-60 hz) or initial torque than is provided by the 50-60 hz energized full frequency motor.

Rio Tinto has other installations that have the same capability.

I would not favor fully automatic closed loop speed vs tonnage. Go watch a conveyor that is poorly designed by not addressing conditions which amplify vibration. I have shown the volcanoes that can be produced, bridges that are put into large oscillation and structures that fail due to excessive oscillation. ■

Another point to consider:

The range of belt operating tensions which are controlled by belt speed and which can induce excessive and damaging vibration, are limited or may need to be adjusted, due to belt wear with use and variations in operating temperatures. ■

David,

I will be presenting a paper on the savings associated with Curragh's speed control and the use of the variable geometry chute and maybe a new belt feeder design which says substatial power. Maybe, you can use some of the information in your upcoming Vancouver Conveyor Seminar along with the significant power savings associated with modern low-rolling-resistance rubbers. Quid Pro Quo?

Go Green. ■

Hello,

Sorry for digging up an old thread.

I thought the real advantages of using a VSD on a Conveyor would be the energy savings. From reading the above posts, it seems that this is not necessarily the case.

Why, then, are so many conveyors now equipped with VSDs? Are there other advantages to using a VSD on a conveyor ie. during starting?

Thanks! ■

Dear SandS34,

There are many advantages of VSD of which most apply to overland/long belts:

1. As noted energy savings

2. Belt & idler life increase

3. Optimal selection of belt speed range, as already discussed vibration control, ...

4. Regulation of starting curve which is non-linear and has multiple attributes:

a) initial dwell period to remove undesirable slack along the strand from stopping

b) initial variable acceleration with "jerk" regulation - determined from dynamics

c) almost constant rate of mass acceleration after dwell and variable acceleration

d) final decreasing rate of variable acceleration to reach final selected speed

5. Variations to acceleration ramp to match variable speed range

6. Ability to share inverters between multiple conveyors such as at a stockyard stacking and reclaiming where variable speed is not as important

7. Adjust to variations in multiple drive starting strategies where load sharing is not possible and each motor may be controlled by special logic associated with the complexity of the conveyor

8. Variations in control of incline, horizontal, and decline geometries with tailored torque, speed, and tension regulation

I am sure this is not comprehensive and others will also contribute. ■

Hi all..

This is all very interesting as we are currently getting horribly bogged down in the great VSD debate here in South Africa.

It would appear that someone has done a very good marketting job on VSD,s here too, as quite a few of my clients are insisting that all conveyor drives are VSD on their projects.. irrespective.

Believe it or not, some even insist on VSD's even where we can easily operate at uniform throughput!

One of our clients even wants a normally constant capacity overland system to be fitted with VSD's and is putting in an unecessary huge silo to feed it as part of the control philosophy. The power just to put the material into the silo is almost more than the overland absorbs!

Such is the frightening extent to which the VSD's have infiltrated us here.

My comments are as follows:

- Keep it simple

- Design all your plant components to operate with nice fixed equilibrium where possible.

- Don't compromise the transfer chutes with varying trajectories unnecessarily.

- Don't compromise and complicate your control philosophy unecessarily either.

- Don't forget the heat loss in the MCC room where the VSD lives ... I measured 7% loss due to VSD on one of my larger installations what with the air-cons etc

- Go for saving where they can be realistically be made .. not for the run of the mill conveyors less than say 4.2m/sec (its an exponential type curve after this)

- The concept of always running with a full belt increases inprint drag and maximises idler loadings, causing maximum deflections at all times and causing them to fail quicker. The flingers fling better at the higher speeds too.

- Running always full is not good for housekeeping

- Always running full is always running at Tmax...ouch!

- VSD's are extremely expensive and as such need to be fully justified, and not just specified willy-nilly

- For high inertia installations where fluid couplings cannot take the heat, rather say go for a single drive with electrical soft start if you can. It can save you alot.

- Don't forget we got away with multiple vapourmatic starters for years!

If you have a major overland system and want a nice 300 second start time and have multiple drives, and occasionally want to overspeed a bit to "catch up" VSD's are just great. Lovely load sharing and so on.

Otherwise in my opinion, I reckon it is an over-kill, and you will not realise any savings in practice for the run-of-the-mill conveying plants. In fact it will more than likey cost you alot more in the long run.

"The human mind is easily led" as the wise man said... so as always.. keep it simple..

Cheers

LSL Tekpro ■