Good morning Gary.

The difference in the diameters of your drive pulleys is 1.6% (assuming you are driving on the clean side of the belt in both case. This difference of 1.6% related to the slip of the motors will cause the load sharing to deteriorate dramatically.

We have come across bad load sharing before, and the casuses have been:

- Differing lagging thicknesses

- Differing FL motor speeds

- Different exact gear ratios

- Incorrect oil fills in fluid couplings

I have never really had ay success in trying to balance loads with fluid coupling fills as you only have about 3% slip in the coupling to play with. I have always had to ensure the motor speeds, gearbox ratios lagging thicknesses etc are identical to get the required load sharing. You don't even have fliud couplings so you will have to ensure everything is identical.

Just look at your motor torque vs motor slip curves Gary.. the line gets very steep as you approach synchronous.

Cheers

LSL Tekpro ■

Hi there Gary..

The following are pretty much the facts of the matter for your conveyor:

- For two drive pulleys of similar wrap, the primary drive pulley is capable of transmitting approximately 3 x that of the secondary one. This is because the tension T3 between the drive pulleys, which acts as T2 to the Primary, is higher than T2 for the Secondary. For equal wraps of theta radians, the actual ratio R of Primary to Secondary transmittable power is:

R=e^(mu x theta)

Therefore for mu=.35 and theta = PI radians (180 degrees) R= 3

- Your take up system has no bearing on the actual load sharing from two similar drive units

- If the smaller pulley is drawing more power than the larger one, I would first check that you have your Ampmeters connected to the correct drives.

- Failing that, check how many hunting teeth you have in each gearbox. In other words make sure that the exact ratios are identical. I have had this a few times before where seemingly identical boxes actually have different EXACT ratios. This occured with Flender as well as Hansen and BEW gearboxes. The last occasion was with Hansen, where their RDG 33 boxes for 2007 had different exact ratios for their RDG 33 boxes for the previous years.

- It is possible that motor on the secondary is defective, though I doubt this.

Let us know what you find Gary

Cheers

LSL Tekpro ■

Dear Gary,

Not being an expert, but is it doable to interchange the two drives as they are identical.

And then see whether the problem stays with the drives or with the drive position.

One would expect that the load should be shared, proportional to the mechanical differences and the droop of the elctric motors.

I do not know how the stretching behavior of the belt influences all this.

Interested

Teus ■

Hi Gary,Graham,

Still not an expert, but, could pretensioning be an issue?

All the drive capability is used by the primary drive, leaving not enough for the secondary drive.

( the 3 times drive capabilty of the primary drive compared to the secondary drive, as Graham indicated)

Slipping should be noticable when both drive pulleys are overloaded.

BR Teus ■

As I understand the posting, you are driving the dirty side on the primary pulley and the belt dirty side has a belt cover thickness (.25 in.) 0.125 inches more than the secondary which is drive on the clean side with a belt cover of 0.125 inches.

This is the equivalent of a 0.25 in. more in relative pulley diameter.

Stated in another way, the primary pulley has a larger diameter due to the increased radius when you understand the actual driven diameter is composed of:

1. pulley radius at shell before lagging +

2. pulley lagging +

3. belt cover +

4. distance to the belt tensile member's neutral axis above items 1-3.

This last point 4 is also important in that the skim coats between fabric layers may not be symmetrical, thus the tensile member neutral axis can ship in fabric belts. ■

Hi Gary..

Would I be correct in assuming that the effective diameters of each drive pulley, (taking into account the one worn lagging, combined with the differeing thicknesses of the belt covers on each pulley with one on dirty side and one on clean side) are the same?

If so, and if you can show the exact ratios to be identical, then the secondary motor could actually be the fault.

Also, in such a case of equal effective diameters, you could swop the motor cables over to prove it isn't the electricals.

Cheers

LSL Tekpro ■

Dear all,

look at the thread in this link:

https://forum.bulk-online.com/showth...=&postid=25924showthread.php?s=&postid=25924

Mr Nordell already mentioned there, that snub pulleys, when driven, can cause imbalances in motor load.

BR

Teus ■

Gary,

You are getting the insight right. Each cover contributes to the distance from pulley center to belt tensile member neutral axis.

The belt's neutral axis defines the spin radius from which the motor demand or slip speed is determined. The belt comes into the primary drive at a unknown belt speed, which is one set of the equilibrium and compatibility equations that need to be solved simultaneously, that also include:

1. belt stretch from loss of its length across the primary drive due to its drop in tension from T1 to T2

2. drive motor electric slip loss and resulting RPM

3. demand power of each drive compatible with the equilibrium that = total power draw and the electrical slip losses of the two drives that result from the differential speeds of the two drives

4. deal with reducer ratios that set compatibility between motor speeds and belt speeds.

Note: the belt will leave the primary drive slower than when it entered due to loss of length.

Generally, the larger pulley (neutral axis position) hogs the power, as it runs slower along the AC synchronous electrical slip curve of the motor, all else being equal, which in your case it is not. If pulleys are equal, then secondary will pull more power, given the secondary is fed a slower belt. There is more to this than the obvious, but I get to prattling and lose focus. Need more help write a love letter. ■

Dear Gary,

Another thought,

Replace the Softstart for two Volt/Hertz regulators in a master and slave setup, wherby the frequency of the secondary motor is regulated until the Amps of that motor equal the amps of the primary motor.

Also it must be possible to put all the preveous mentioned effects into a calculation scheme and investigate the respective influences on the power sharing.Should be not such a big problem for conveyor belt builders.

Success

Teus ■

Gary..

Do you have a system where you split the output of the soft start between the two motors so they get the same, or is it possible to programme the soft start to proportion power between the motors to different ratios?

What soft start system do you have?

Cheers

LSL Tekpro ■

Gary,

To further the finality of the thread, Conveyor Dynamics, Inc. has designed and installed hundreds of multiple driven pulley systems since 1974, including inverter, hydraulic, DC-SCR and wound rotor motors with varying load share trim methods. To date, not one has given any problem and has been predictable.

I note 1974 is when I designed the world's first inverter drive for a belt conveyor system for the La Caridad mine in Mexico. Four inverters started 20 drives with up to 2400 hp, with steel cord and fabric belts, many different power ratios, et al.

Our power range is to 10,000 kW per conveyor, head and tail remote by up to 16 km, load cell (tension), velocity and torque controls

Good engineering makes good projects, lack of good engineering is Las Vegas. ■

Oh dear Gary...

Your thread reminds me of a who-dunnit book, that has the last page missing!

I have always been wary of a dual drive operating off a single soft start, and hoped to learn from your tribulations. Especially as I am considereing it for one of my applications at present.

So I was wondering if anyone out there has actually had any success using a single soft start on a conveyor fitted with a dual pulley drive?

Cheers

LSL Tekpro ■

Dear Graham,

They have done you one better. They connected two driven pulleys through a common gearbox. Not done today, but you might recall the period when this applied to underground coal mines. They did so with clean side dirty side drives as well.

If they can make that work, then one inverter powering two pulleys is of the same mold.

However, it was not a good idea as they found heavy wear and lack of load sharing as you might expect. It just tears and wears the belt and lagging as you probably know, or I hope you know, and they cannot get full motor capacity delivered to the two motors.

For giggles, why do you ask? ■

Hi Larry..

We had this guy from a VSD company to give us a speel on what they can and what they cannot do with electrically controlled starts.

At that time he said soft starts are for single drives only.

Since then, a company from down under told us they could do a dual drive no problem from a single soft starter.

The sad experiences of poor old Gary have cast doubt that the blokes from down under are fully up to speed.

The best way I though to really find out is to ask if anyone has proved the concept in practice.

If it works it would be great, but does it??

Giggle on young man...it's Friday and the fridge is full of cold beer and biltong.

Cheers

LSL Tekpro ■