Indramat Motor Cables: the First Spare You Should Buy 0

Posted on 28, May 2012

in Category Blog, Indramat Tips, Rexroth Service


Indramat Motor Cables: the First Spare You Should Buy/rexroth service indramat tips blog We get at least one call a week from some poor soul who has gotten a  Rexroth Indramat error code F822 (error code 22 on Diax02, and F8022 on Indradrive). Unfortunately, we often get the call after the client has run off in the wrong direction, sometimes disastrously so.

F822 is generated when the drive takes the information from the Heidenheim style feedback and does an overall summing of the signal. It’s a kind of first cut at seeing if the signal is good. If the signal summation comes in between 1v and 11.8 the drive calls it good and moves on to more precise measurements. If the signal is outside of these bounds, it triggers the F822 error code.

The manual lists three possible causes:

1. Feedback Cable defective
2. Transmission of the feedback signals disturbed
3. Feedback defective

For some reason, most people broad jump over the first two and go straight to the third. This is a basic mistake. Indramat troubleshooting guides have causes listed in the order of most probable to least, yet  the first thing many people do here is to replace the motor.

The single most common cause of F822 is a crack in the insulation of one of the signal wires in the feedback cable. This can happen when the cable is bent too tightly during installation, from repeating bending in a cable track, or from being squashed in shipping. The crack may be too small to notice at first, but it will eventually open up enough to bleed off voltage into the shield.  The cable may work for years before thermal cycling or vibration cause the split to open up enough to start bleeding voltage off, but once it opens, the problems begin.

Replacing the motor won’t solve this problem — but sometimes it seems to work… for a while.

A True Story (no names to protect the job of the client)

Maintenance gets a call that a machine is down, and they go out and find a drive faulted with F822. Since they have a spare motor, they change out the motor and the machine comes back up. They toss the old motor, order another spare and proceed with the week. Two weeks later, the same machine goes down.

They’ve seen this before. They rush a motor in and replace the motor once again, solving the problem. They toss the second motor, and with some muttering about how you can’t get good stuff these days, go on with their lives. Two more weeks pass and the machine goes down again. This time they fly a motor in, because they are in the middle of a time critical order, and on replacement of the motor the machine comes back up.

Two more weeks go past and the machine goes down again. We get a call complaining about the “durn two week lifespan on Indramat Motors”.

Okay, perhaps I paraphrased that a bit.

After we get the details, it is obvious what is going on, and we ship them a cable to fix the problem.

Why?

Because the problem was the feedback cable all along. Every time they replaced the motor, the machine came back up because they had moved the cable in the process of replacing the motor. The break was at the motor end of the cable, and the movement was enough to provide a temporary solution.

After about two weeks, the cable would move around enough to reopen the crack in the insulation. Replacing the motor again would clear the error — by moving the cable– and they were good until the next time the cable moved into its trained position. Ohm’ing out the cable wouldn’t detect the error, as they would have to move the cable from the motor to Ohm it out, clearing the problem. Mega Ohming the cable would find the problem, but you have to be careful about that — and it didn’t get done in this case.

The simplest way to check the cable when you get the F822 error message  is to have a new feedback cable on the shelf, and just lay it out on the floor between the motor and drive and plug it in. If the F822 clears, you know it’s the cable. If the F822 continues, you know it’s the motor.

This is the single fastest way to check a whole series of errors. EVERYTHING that looks like a feedback error to the drive can also be a cable problem, as can a thermal fault in the motor, a motor memory problem, a homing problem, a short in the stator, a velocity loop problem, and a bunch of other things. The very first spare you should have is a spare power and feedback cable as long as the longest run in the system. Call us at (479) 274-8422  to get your cable — or to get timely support that can save you money.

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