Vfd Speed Following

Could anyone share their experiences of matching the speeds of two conveyors by using the "analog out" of a vfd (master) as a speed reference to the "analog in" of the (slave) vfd? Running both motors off of one vfd is not an option because the drive ratios of the gearboxes are different.

Have worked on various systems using multiple vfd units, never had much problems, only when working with old vfd lack of info on the drive. Same old story different bit of kit.

What are you worried about?

What do you want to know?

What drives you looking to use ?

Most of the time i have come accross this application, it is always been done with Danfoss VLT

Funny you should mention it, at a factory i have just taken a job as a Process Engineer, there are 16 Danfoss VLT 5000 running together, I'am just getting to know the system.

If there is anything i can help with just let me know.......

You may want to program the second analog input of the slave unit as a "TRIM". There will be a slight difference in actual speeds.

You can usually do some trimming in the VFD parameters to better match the speeds. If the one can run slightly faster, like the down stream one, you could get away with out the external speed trim pot.

If they run at constant speeds you would be OK once the initial speeds are set. If they change speeds routinely, the addition of the trim pot will help keep things running smoothly.

Hi, John! You have a couple of choices available to do this. One way is as you mention---use the Lead drive analog output to drive the Follower drive analog input. You can use the Min Speed and Max Speed Parameters on the follower drive to produce offset speed. Sounds like you need it if the gear ratios are different.

Another choice is to generate a single analog speed signal and connect it to the analog speed input on both drives. If the drive inputs are optically isolated, this is easy. You will also need to connect the negative or ground side to both drives so there is a common return path.

There is a significant and maybe important difference between the two schemes. In the first scheme, where the Lead analog output is connected to the Follower analog input, the whole system depends on the Lead drive. If it fails or faults, the Follower drive cannot be run independently.

On the other hand, taking the same speed signal to both drives allows either to fail and the remaining unit will continue to run.

This might or might not be important in your application.

There are numerous imbellishments to this that I have seen. Most have been with V/Hz drives and trying to compensate for motor slip error. This error, of course, is greatly minimized in sensorless or vector drives.

I've seen some usually clumsy attempts to duplicate slip between drives by setting the Lead drive's analog output to track Lead motor torque and then using this as a negative trim on the Follower drive's main speed signal but, today, using a precision speed regulator for both drives is far simpler and works much better.

As I sit here, it occurs to me that a third approach to this is to put an encoder on the Lead conveyor and use this as a speed reference for the Follower conveyor drive. This is expensive and largely unneccessary today. And, it also suffers from the Follower drive not being operable when the Lead drive has failed or faulted.

Hope this helps.

I go with DickDV on this, use a seperate speed signal to each drive and adjust the drive speed using the "min speed" and "max speed" parameters. If you use 0-10V for the speed signal then a trim pot can be fitted to each drive speed signal and the speeds can be trimmed as required without having to make constant changes to the drive parameters. I have used this system and it works fine. If you need more accuracy say, for tensioning of a strip of product then use a dancer arm with a trim pot.