It is normal to have some difference in the amperage however you need to explain, are both motors couple to the same pulley or are they on a twin drive arrangement. If it is a twin drive there is a much bigger issue as you must set the drives up to load share in a manner that does not see any "hunting" between the drives. We need much more detail of what your problem is to really make any comments

Cheers

Col Benjamin

Gulf Conveyor Systems P/L

www.conveyorsystemstechnology.com ■

Originally Posted by sganesh

Dear Experts,

Please assume that there are two VVVF ( Variable Voltage and Variable Frequency ) drives are installed in a single conveyor having specification of ST1000. What should be the maximum ampere differences can be allowed between the two motors? Because, more the difference will affect the belt & it's joints.

Thank you all & Regards,

There is elasticity in the belt, however should you be monitoring something other than motor amps to ensure that the two drives are synchronised? Is not the drive speed more important than the motor amps? After all, you are always going to get difference in motor amps because you have mechanical components that are NEVER going to be equal. No matter how "identical" you might want the drives to be, you have clearances and alignment tolerances that will see difference in the internal load on the drives and therefore motor amps. These differences will change throughout the life of the drives.

Just recently the contract I am working on had a fan with one bearing running at around 50 deg C with the other bearing running at over 100 deg C. No matter what the fan supplier did, one bearing would run art more than double the temperature of the the other. In the end, the fan supplier agreed to replace the bearings if they failed, and so far, after some 6 months of fairly heavy use, there has been no issue with the bearings. ■

To answer your question, there should be very little difference between the amps drawn between the drives... however..

The trouble is that in practice it depends on a number of things:

- The skill of the chap that sets it up

- Which system you buy, and..

- Does it have a tail drive a long way away??

I have a 7km conveyor with VSD c/w tail drive, and to this day the VSD supplier cannot balance the load sharing, ESPECIALLY DURING START UP.

For conveyors with head drive only, load sharing is relatively simple and to be within 5% is quite achieveable.

Cheers

LSL Tekpro ■

Graham,

We have many head and tail drives working with high satisfaction. The drives cannot load share during starting, unless you have a very rigid or frigid belt. But, I believe this would be evident with your dynamic analysis program. (Written by you?).

Our head and tail drives load share within 1-2% of demand summary. We write our our control codes. Works for fluid couplings, wound rotor drives, and VFD when in velocity control and load sharing algorithm.

Howver, there are conveyors that must be designed with tail drive that does not load share. This is the nature of what and why the designer wishes to do and knows how to do it. ■

Slight clarification:

The drives can be made to load share during steady-state running, but, not during starting and stopping. Think elastic belt and elastic waves.

They can be made to load share during starting and stopping at great increase in start-stop timing.

Control algorithms come in many varieties, which are the IP of the experts. Usually, there is an Objective Function such as:

1. controlled belt stress with respect to belt rating

2. minimum starting torque together with minimum belt stress - motor is capped separate from belt tension limit.

3. horizontal curve transverse motion control

4. incline and decling product stability

5. motor torque limit

6. brake regulation and control schemes

7. drive slip

8. take-up stability and regulation of oscillation

There are many other control algorithms, but, the discussion becomes more complex and off track.

Hire an expert to solve your problems. Problem is - who is an expert? Ask and you will find many with raised hands. Hopefully, this forum gives some clues. ■

WAI Engineering:

Dual or more drive pulley speeds, located in near proximitiy, cannot synchronize without serious problems. Since the belt length contracts across a positively powered drive pulley (T1 into T2), a following secondary drive will always run slower than the primary, given the local belt speed is judged by the belt speed entering "the" drive pulley. Reality is a little different, but the concept is the same.

You may wish to load share on amps or regulate on volts (torque) with most VFD's. Most VFD's (Variable Frequency Drives) applied to conveyors use voltage (torque) and frequency (speed) for regulation. In fact the regulator is: Volts/Hz. Current regulation can be used, but, is typically a different type of inverter: VFVV (Very-Frequency-Very-Voltage) drive vs. VC (Very-Current) drive. ■

SGANESH:

To you original question: What is an allowable difference in amperage, with dual drives on dual pulleys for a ST-1000 N/mm rated belt? Like all things in life, more needs to be known such as the belt width, belt speed, drive efficiencies, .......

In simple terms, assuming the amps = tension, then a 10% variation is acceptable, a 20% is probably not if you must then factor in: starting, stopping, dead-band regulation, slew rate latency, age of belt, prior history of abuse (cruelty), irregularity of pulley geometries, proximity to idler transition distance, proximity to dirty side pulleys and badly loacated snubs, belt tracking error anticipated or observed, perfection of splice construction, belt safety factor, ....... shucks, and a whole lot more. ■

Larry

I have had no problem load sharing between head and tail drives during on anything other than variable frequency, variable speed drives.

Even up to 12.8km long horizontally curved conveyor, using good old Scoop Control couplings.. no problem.

Even 4km using TSS traction couplings on the head drives and tail drives .. no problem as long as the drives and pulleys are identical and the sequencing is correct.

But the VSD boffins don't seem to get the hang of it during starting for some reason.

I hope they crack it before we commission the 21km overland conveyor system I am currently designing.

(And indeed, my dynamic analysis programme was written by your favourite old South African conveyor designer with the white hair, at some time, during his sheltered existence.)

Cheers

LSL Tekpro ■

Originally Posted by sganesh

Dear all,

Our electrical engineers created to get graph on RPM of individual motors, installed approx 100 meters apart. Now we are analysing the RPM difference , which can make the pulling or slipping of the belt which is not advisible. But I do not know how much difference can be allowed for pulling?

Belt is ST 1000 which is going under pulling stress and both drive pulley diameters are 810 mm.

Thanks & regards,

Sorry Sganesh..

...but I don't understand what you are trying to say.

LSL Tekpro ■

Hi Sganesh

Not really better to measure the pulley rpm, as the outside diameters of the pulleys are never identical, and I have often come across supposedly identical gearboxes, but have found that their exact ratios sometimes actually differ.

The Amps measured however, relates directly to the load on each drive, and so you can directly check the load sharing.

I normally ask for a print out from the control room, of a 24 hour trace for the Amps. This enables me to immediately see the accuracy of the load sharing during starting as well as running for the various loading conditions during the day. I can then make the necessary adjustments as required.

Good luck ■

Dear sganesh,

Low frequency resonance?

Have a nice day

Teus ■

Dear sganesh,

I assume that a belt conveyor with drums, drives and load, together with a flexible belt forms a mass-spring system that has its own resonance frequency.

In case there is a master and slave control, set with a too high magnification (strong reaction on small disturbances), another resonance can occur (hunting)

If the 2 resonance frequencies coincide, you have a real problem.

May be a case for a DEM calculation?

Have a nice day

Teus ■

Dear sganesh,

What you did in your test was tuning a 3 mass/spring system.

The system is buildup from the moment of inertia of the master drive – the inertia of the counter weight – the moment of inertia of the slave drive.

By increasing the counterweight’s mass, the resonance increased, proving that, under those conditions, the system was running close to the resonance frequency.

By setting the drive rpm’s equal, with the result that the drive motor took the complete load, you virtually uncoupled the slave drive. (actually virtually decreasing its moment of inertia to zero)

Here, it is proven that the setting of the VSD’s are also influencing the system.

As you did the trials under no belt load (idle running), the situation can (will) be totally different under situations where the belt is loaded with material.

Undoubtedly a mass-stiffness resonance problem.

Your only option is to call in an expert (Lawrence Nordell Conveyor Dynamics, Inc.) in resonance dynamics and let him calculate the critical frequencies under all conditions.

From these calculations it must be possible to extract a solution.

Success

Teus ■

Could you please specify what you mean by fluctuations?

Frequency, amplitude; FFT pattern? ■

I Thinrk I have said this before:

Pulley lagging or diamter irregularity of 1% can result in a 100% drive fluctuation with a 1% slip efficient motor.

Same can be said for near proximity snub pulleys as they tug and loosen the belt strand captured between drive and drive or drive and snubs. These conditions are made worse by material build up or when material falls into the interface between belt and pulley.

VFD drives are complex devices that need to be detuned for belt conveyors to allow for small (< 0.1% dimensional repeated changes in the pulley diameter. Diameter changes are measured from the belt's neutral tensile axis. Thus, belt anomalies can contribute to aberrations in drive flux.

You can improve the local aberrations by adjusting the slewing rate of current and/or voltage changes that are allowed in the load regulating algorithm within the VFD. Talk to your provider. VFD's are usually adjusted to change at a faster rate than necessary or desirable for conveyor applications.

Get an expert to assess your problem. Fish for carp means you like carp. ■