Conveyor load sharing using AC drive


Thread Starter


I used DC motors for load sharing conveyor applications. The motors are connected in parallel to a DC drive (AB 1395) and split the output current equally so they run at the same speed. I am now considering doing it with AC motors instead. The problem is that I will have to use a VFD for each motors and I do not know how to ensure that they will all run at the same speed. Does anybody has experience and/or advice in this field?

Paul Broughton

Have done this with an overhead chain conveyor, which we also needed to index. first time was just using separate Danfoss VFDs and relying on motor slip. second time using 2 positioning inverters (motors with encoders) from ABB (I think they were ACP600s) but after many weeks of ABB modifying their software to get the drives to lock together gave up. third time we used ABB ACS500s with one as master and the other as slave in tourque following mode. This worked well with the load being shared well. (I understand Danfoss 6000 series do the same) Final solution was to re-layout the factory and put in a shorter conveyor.

Steve Bailey

I've done it by putting encoders on each motor. I wired the encoders to High Speed Counter channels in the PLC and calculated the difference between the accumulators of the HSCs. I used the difference to calculate a trim value to be added to or subtracted from the analog speed reference for one of the motors.

I only had to synchronize two motors, but there's no reason why the technuque couldn't be applied to more.

Dean Kindrai, Neff Engineering

I suggest using the Yaskawa GPD515 drive with their electronic line shafting software. This allows any encoder signal to be the master signal (it could be off another drive) and the other drives will follow at a specific ratio.

Yaskawa has a unique algorithm for calculating the ratio that uses whole numbers only, eliminating accumulated rounding errors that others have. Even if the master changes speed suddenly and greatly (we were accelerating from 2Hz to 60Hz almost instantly) the slave never got more than 2 encoder counts off. Most of the time it was right on.

Butch Barker

You surely can connect multiple motors to one drive as long as they are identical motors.

How many do you need to connect?
I think the main problem is that each squirrel cage motor has the possibility of different slips. One way to get around this may be to go to synchronous motors. You will need excellent speed feed back also. If all the motors run at same speed and hp are same you could also possibly upsize your VFD for the total combination so they run on one VFD getting the same frequency. Again different motors have different slips so different speeds for same motor.

I have a friend at work who had extensive experience with VFD drives. I will ask him tomorrow.
Oh boy do we......

In load sharing, it is not the speed you want to remain constant so much as it is the load. You don't state the manufacturer of your VFDs, but you want to look for a drive feature that will be called something like "slip compensation". This feature allows the drive to look at the slip of its motor and adjust the output accordingly.
There several ways to distrinute one speed reference to many drives if you need adjustable speed systemwide. If the production speed does not change, the keypad setting method can be used.

[email protected]

Roland Lilja

Hi Jean-Yves

Yes in a aplication where i programmed the PLC we used VFD from the company DANFOSS ( Brand VLT5000 ) they were programmed to run in constant torq mode. This means that they output the same "power" to each motor thus making them run in the same speed. I don't know if this is aplicabel on your plant.

Roland Lilja

James Ingraham

Obviously, you can do this with any servo drive, though granted the cost will be considerably higher. A mid-range option might be vector drives instead of VFD's. I know that Rockwell synchronizes speeds on winders and presses with vector control. This is a good, "ask a vendor you trust" question. Just about any drive
manufacturer can do it, so work with someone you are comfortable with.

-James Ingraham
Sage Automation, Inc.
I tried load sharing on some conveyors here in Michigan with Chrysler and it was a pain in the a++! What i finially ended up doing was using a Master conveyor and synching the other (2) conveyors as slaves. We used 1395 A-B AC Drives. There (2) terminals above the regular 0-10V Speed reference and I think they were Trim settings. (Make sure you set up the Drive Parameters through the HIM module) Good Luck
You don't give details about the 2 dc motors with one drive. My guess is that one motor had speed feedback to the drive (speed master) and the other basically functions as a torque helper. You can parallel motors off VFDs too but in open-loop mode, because of slip differences, the actual
speed will be such that the individual torques (T1, T2 ...) add up to the required total. If you need greater accuracy, you will need to consider
high performance VFDs (vector) for either electronic speed ratio/synchronization or master speed with torque helper if the master drive can parallel the toque command to the slave (typical technique used by DC drives with cascade vel and current loops accessible by user).
For a customer of mine we tried the Danfoss as well using the software and motor slip settings, but never got it to work correctly. The lines
(overhead chair) were too long. We now use one large AC VFD running multiple (up to six) motors off the one drive. The motors are individually protected, we can go into bypass, and there is enough slip in the motors as well as give in the lines (though all gear boxes and rpm's match) not to give us any trouble. This setup has been very
successful with no problems in the past 5 years or so.

Good luck,

Kirk S. Hegwood
Signing for Hegwood Electric Service, Inc.
[email protected]
For those of you hellbent on connecting two or more induction motors in a "load-share" arrangement:

In most cases it can't be done. Please search Archives for previous threads which provide the theory and examples:

"Motors in Tandem"

"Motors Assisting One Another"

"Conveyor Belt Drive Transmission"

Unless some sort of slippage is applied there is the high probability of complete failure. The cause is related to the "torque-angle" produced by
each motor, and its position relative to another's.

Phil Corso, PE
(Boca Raton, FL)

Miles Kuperus

Our plant has 870 ft of overhead chain driven by 3-5 hp motors with gear reduction of 150:1. We snycronized these by having spring loaded take-ups after each drive. We mounted prox switches on the take-ups to monitor tension on the springs,and feed these signals back to the PLC,which in turn sends the info to the VFD. One drive is the master and the other two are the slaves. This setup has worked quite well for us as we are constantly changing line speed,and experiencing different line load.