A B 503 will not sync. 1 inverter to another and keep time

[birdly]

I have an Allen Bradley 503 plc and it has an input from
1 inverter controlled motor (mechanically driven encoder)
and another inverter controlled motor that has a mechanically operated switch for an input to the plc.
It should compare the two inputs and sync. the two inverters
together with the first matching the second inverter.
But they run in and out of time sometimes.
Why does this happen?

 

[djs]

Hello;
Inverters are not servos. They will have slight variations in speed, and will not compensate for those differences. You will probably have to modify your PLC program to constantly adjust one or the other motor's speed to keep them in sync.

 

[birdly]

Thanks djs, this is one of two identical machines
running the same program and the other mach. runs fine.

 

[birdly]

I found the problems, 6 problems in all.
Thank you all very much for your help!