Starting motor makes profibus unstable ??

Hello,

This may be a well known problem for some of you,
As you all know profibus doesn't like EMC.

But, what if profibus falls out (not allways) when a motor is started with a contactor in the same closet...
Is it an earthfaulth, is it the motor...

We tried to change the motor, to seperate the cables, to make good earth connections... but still...

Any ideas ?

Thanks,
Gerry

Do you get the same results if you operate the contactor manually (i.e with a screw driver or the like).

How about if you disconnect the motor and just open close the contactor via the plc.

Varistor or R/C snubbers in place ?
Is the earth good enough in the entire plant ? This has burnt us once, you cant just assume that it is OK.

If you have done all the typical checks, and no improvement in sight, then consider to buy or rent a profibus tester. It can almost certainly pinpoint the culprit.

90% or more of profibus faults turn out to be cable related. You can check it manually or you can check it with a profibus tester but ensure your cabling and connections are in tip top condition. Each bus segment must have a terminator at each end and not anywhere else, limited spurs can be tolerated at lower speeds but should be avoided if at all possible, A B and Screen should be continuous without any connections between them.

Where the screens are made off in the plugs they should be free from "whiskers" that pick up EMC noise and also break off causing intermittent problems.

Is the motor connected to a VFD? If so check that the screen on the motor cable is continuous between the motor and the drive. A common mistake is to earth it at the gland plate in the panel.

Good Luck,

Nick

It is also not enough to rely on the screen to be terminated in the plug. You should have a fat clamp over the screen to an earth bar. Look in the profibus installation manual.
(we normally only do this of there are VFDs or the Profibus cable goes out from the cabinet, but if there are problems then it should definitely be done also).

L D[AR2 said:

Do you get the same results if you operate the contactor manually (i.e with a screw driver or the like).

How about if you disconnect the motor and just open close the contactor via the plc.

Click to expand...

If you try what LD suggested, you'll find out whether it is the contactor and/or the motor/power distribution that is the noise source.

Kalle

Make sure the profibus cable's sheild is terminated. We have had this problem before and it was caused by the sheild not being terminated.