Cost effectiveness depends on what application you will be using them for. Please tell us your application and we may be able to assist you further.

VFD's (Variable Frequency Drives) like Matt says have there place. They are primarily used to vary the speed of a motor by changing the Hertz (frequency) of the motor. In North America the hertz standard is 60 while in Europe it is 50.

The VFD allows you to adjust this number to change the speed of the motor. Most VFD's even allow you to go over 60 hertz to overspeed your motor for short duurations. VFD's also have the capability of providing soft starts on belt conveyor or in other words ramping up to full speed over a given time frame to eliminate across the line starting that can create loads on a conveyor system that may cause damage to the system. Most VFD's can provide dynamic braking as well. Dynamic braking puts a DC field into the windings of the motor and acts like a break to slow down the motor. This can be used in down hill conveyors but it does have its limitations.

VFD's are very common in bagging and packaging lines as well as palletizers.

If you are only interested in have a soft start for a conveyor then there is no need to go to the added expense of a VFD as a Softstart drive will give you the ramp start that you need.

You should speak to application engineers of drive suppliers about your application before you try one on your own. They are expensive and require parameters in the on board computer to be set up for your application.

Regards,

Gary Blenkhorn ■

VFD's break the soft start into two components: hertz = speed and voltage = torque. The VFD takes normal plant grid line voltage, which drives most motors(460-6600 volts) and frequency (50 or 60 hz). It first rectifies the voltage to DC then chops the rectified DC signal into controlled pulse steps as a psuedo sine wave mimicing the AC-line over the spectrum of frequencies.

This drives the typical standard AC electric motor with variable torque and variable speed. The process of electrical AC-DC-AC flip-flop is commonly refered to as an inverter. One of the tricky bits is that the above process has to be matched for each leg of the three electric phases simutaneously in nanoseconds.

There are two basic types of inverters: Variable Voltage Variable Frequency (VVVF) and Variable Current. THe VVVF is more commonly used because one inverter can drive many different motor power sizes. The VVVF cannot inherently regenerate back to the grid and therefore must have virtualy two inverters: one for the first two electric quadrants and one also for the last two commonly called four quadrant control. Variable current does inherently regenerate, but only for one motor size.

I designed the first inverters, installed for belt conveyors, in 1976. This was at the La Caridad mine in Mexico, as an employee for the Ralph M. Parsons Co.

Four Emerson Electric VVVF drives (4x600 hp=2400 hp)) were used to soft-start 12 conveyors with 21 motors totaling 9550 hp. The motors ranged in size from 50 hp to 600 hp. One conveyor had 4 x 600 hp for an uphill conveyor. One conveyor had 3 x 600 hp for a downhill regenerative conveyor.

The concept required that the inverters be programmed to soft-start the various drives with their individual respective start speed-time curves, then line-sync and phase lock at line frequency switching the conveyor's motors to the plant grid and disengage each inverter to start the next conveyor in the sequence. When finished the inverters were removed from the working circuit. They could be inspected and repaired off-line.

The VVVF inverters were over 1 million dollars cheaper than fluid couplings, DC or wound-rotor motor drives.

I published the concept in the SME 1977 meeting. I called it "multiplexing" conveyor drives. This concept had a number of advantages:

1. reuse of 4 inverters to start 21 motors on 12 conveyor

2. operate on powering and regenerative conveyors

3. inverters were switched out of the normal working circuit - idled

4. one universal VVVF unit could spare for any of the four

5. Mean-time-to-repair was less than 60 minutes for any VVVF

6. Reliablility is maximized: removes excess running components

7. spares stock is minimized

8. built-in belt inspection speed & belt threading for maintenance

9. inverter repair can be off-line increasing production availability

10. load sharing can be optimized during starting

Its still running today. ■

Dear Unclehan,

THe inverter will recover the electric power in proportion to the speed to differing degrees dependent on the inverter design. Over the 2:1 speed range they all will be quite efficient.

Your second posting was a little confusing. You stated that the conveyor had two operating conditions: two speeds with same torque and mass. Normally, when you double belt speed you cut the ore mass crossection by half. Thus the mass of the matrerai on the conveyor is inversely proportional to speed.

There is a third point in your scenario: parasitic losses. Parasitic losses do not change with speed change. Parasitic losses include idler and pulley columb seal drag, skirt and scraper drag. Parasitic power losses will increase in proportion to speed.

Inverters and DC drives, unlike fluid couplings, do recover power when speed is reduced. However, there different types of inverters: those that take advantage of efficient energy recovery over the full speed range and those that efficiently recover energy over about 50%-60% of the speed range.

THe inverter's fulll speed range (0-100%) efficient motor slip recovery uses a principle called "flux-vector" control. The electric motor power factor angle (cos(phi)) is corrected in the inverter over the speed range. This is a more expensive process. When you have significant changes in speed, for sustained operating periods, this can more than return the cost of the investment. ■

THee is another point: how often do you change speed or is the change in speed dependent on the expansion of production.

Inverters can provide two functions: 1) soft-start the conveyor by allowing a programmed acceleration from zero to full speed and 2) can operate at a range of speeds as noted above.

You have not provided sufficient information to give a responsible comment on the need for these two points.

Soft-starts are applied when the starting torque needs to be controlled to limit belt stress, belt components stress, ore acceleration stability on inclines, vertical curves dynamics, shock wave controls, horizontal curves, and structural tensile stresses.

If the change in speed is soley to increase the production, with a constant speed operation, and conveyor power is below 75 kW at the lower speed, there may be more cost effective means such as your comment on a reducer gear change together with a simple soft-start mechanism. ■

Has anyone had any experience with using VFD's on belt winders for belt changeouts

Shane ■

Dear sir

Generally vfd are not provided on belt conveyors and same should not be used as gearbox as at reduced peed there efficiency is low

Inverter or VFD can be provided on belt conveyors for following reason

1.0 soft start and stop of conveyor however same can be achieved by fluid coupling

2.0 In case of multiple drives syncronising speed of various drives

3.0 load sharing can be done in case of multiple drives

4.0 speed variation in case of different material conveying and by reducing speed when ever desired ,

By operating conveyor at optimum speed one can control power consumption by using vfd drive

A R SINGH ■

As the requirement is to provide two different but constant speeds I believe that dual polarity motors (eg 8pole/4pole) could be a possibility.

Regards,

Adi Frittella ■

Dear Adi

yes we were achieving different speed by dual pole motors in 1990 .By using dual pole motor chice of speed was restricted to to speeds based on motor and additional cost involvement was there in mcc to select the desired pole

today VFD is not too expensive and we can select any speed in addition to other benifits indicted in earlier posts

A R SINGH ■

Hello Unclehan..

VSD's are of great benefit when you need a long start up time due to high inertia as you get in todays very long overland conveyors for example where 4 minute start times are common.

Inclined conveyors have a lot of power but relatively little inertia, and you can often get away with reasonably short start times and so you don't need anything fancy. Often TSS fluid couplings suffice.

In the old days for long start times we used scoop controlled fluid couplings with large heat exchangers to burn of the excess due to coupling slip during starting. There then followed the drain couplings which did the same job but were cheaper.

Electrical VSDs are getting cheaper and more versatile by the week and are catching on well now.

The concept of electrical soft start however hasn't caught on despite being very economical. The reasons are that you can only really have one Soft start per conveyor, and that there is poor control of the rate of application of torque before break away, and you won't meet the Funke requirements.

Regards

LSL Tekpro ■

Dear Graham,

The high inclination may lead to unacceptable shock wave action using a fluid coupling that can declutch the motor inertia duning its stopping cycle per your noted selection. Not so with the inverter. This degree of shock amplitude depends on: loading, the incline slope, belt length, belt rating, and modulus, take-up location and type, motor power, drive inertia and its coupling to the fluid coupling, etc.

THe reader should take all comments with some reservation given no one has the design factos on which to render an accurate opinion. ■

Morning Larry..

I reported above with regards to the drive selections for overland as well as inclined conveyors, based largely on on practical experience over the years.

As such, (and sheltered though I must surely have been), I personally have had tremendous success using the good old TSS fluid couplings on quite major inclined conveyors.

I even used them successfully on six declined man-riding conveyors, and we are about to install 3 x 710kW drives with TSS 750's on them on a major incline to replace some old scoops.

I am glad you actually seem to agree with me on the other thread though.. The one about the short reversible conveyor.

Regards

LSL Tekpro ■

Hi Graham,

I would about your comment on elec. soft starters.

We have one with a pole changing inclined conveyor.

The motor size is just over the demand power.

Needless to say the torque depends upon the power and speed.

The SS works but the issue is the initial design, the deisgner did not take into account the required starting time and torque available and the motor thermal capacity.

Bascially, the high speed mode would start very slowlyl fully loaded and the motor would trip out on thermal limits before even getting to 50% speed.

This is from a cold start, a trip when running is worse as the motor is warm and has a lower thermal capacity left.

We had to use the pole changing to run off in low speed and then switch over to high speed to fix this.

The lesson is that just becuase a selected motor power is above the calculated design power does not mean that the selection is OK.

Cheers

James ■

Hello James..

Thanks for your comment.

Tell us though, what was the reason for the SS drive selection you describe? What size drive is it, and why does it not have something maybe more conventional?

Regards

LSL Tekpro ■

Hi Graham,

Because the conveyor is dual speed - with pole changing motor.

Designer decided to attack the lower start up tension with a SS.

Other option could have been a VFD but more $$$.

Conveyor runs with coal and limestone at same tonnages about, so needs different speeds. The conveyor is used for both circuits and then splits to each material stream there after.

Cheers

James ■