Rexroth third party repair shops don’t really exist. But third party repair shops that claim they can repair your Rexroth industrial motion control are legion. We know, because we get calls from them all the time. They want parts and manuals, most of the time, but sometimes they just want our expert advice.
Today I got a call from a guy who says his motor won’t generate torque, and is making a whining noise. The first question I ask is easy: “Who repaired it?”
From years of field service experience, I know that these symptoms are usually signs of third party repair. Sometimes I can tell plants have used 3rd party houses simply by walking through the plant on the way to a down machine. The telltale whine of an improperly commutated motor is unmistakable.
“I don’t know,” the guy tells me. “I’m just trying to make it work.”
“I don’t know” is not a plausible answer to “Who repaired it.” But I go ahead and help him out. I tell him that most likely, this motor has been 3rd party repaired, and that the information contained in the feedback (improperly in this case referred to as an encoder) doesn’t match the actual commutation of the motor.
“It does,” he assures me, “because I am checking it with a Mitchell system and the commutation that the motor says matches what’s in the motor”.
Let’s look at just a couple of the mistaken assumptions revealed here.
The commutation in the feedback matches that of the motor.
The commutation of the motor and its relationship to the feedback is set at the factory. Once you separate them, it is impossible to get them reassembled correctly. That’s what causes the whine (and also generates heat and uses more energy). A factory repair rewrites or replaces the feedback information once the unit is reassembled. The feedback will give a commutation value if queried while lying on the bench, unattached to a motor.
Obviously, this is false. Because, well, there’s no motor there.
The drive commutation is determined separately from the motor.
The drive reads the commutation value from the feedback along with about 20 other parameters, depending on the motor. If the drive was connected to our feedback lying on a bench unconnected to a motor, it would still read the commutation value of the motor that it had been attached to at the factory.
The guy goes on to say, “I know it’s the motor that’s the problem because I have tried it with another feedback.” He also finally admits that he is the 3rd party house that has tried to repair the motor before for the hapless client. He still has the same two misconceptions described above, and also makes a third, much more dangerous mistake.
By randomly changing the feedback with one off of another motor (or bought from a salvage company), he is telling the drive that the motor is something different than it really is. Remember, the motor feedback tells the drive a number of pieces of information about the motor, including the motor type, serial number, commutation value and MAXIMUM AMPS. The latter is the dangerous mistake and we have seen it more than a few times.
Lets take a really small motor, say an MKD025A, and then put the feedback from a MKD112D on it. Plus, since the thermistor in Indramat motors is in the stator windings themselves for minimum detection time of an overheat condition, lets allow Bob’s BBQ and Servo Repair Shoppe (Would you like fries with that?) to bypass the thermistor. Lest you think we have overheated imaginations, this is a situation we have seen more than a couple of times in the repair lab.
So now, the drive thinks this is a big honkin’ motor, that can take a ton of amperage because that’s what the feedback is telling it. The MKD112d=142.2Amps. The system takes off, and because the motor is improperly commutated, it is having trouble generating torque, so the drive just throws more amps at it, not realizing that this is actually a MKD025A (=59 amps peak). The drive does shutdown when it detects that the motor windings have melted together, causing a short, but by this time, the motor may simply be slumping into a puddle.
In short, someone actually paid for a motor repair, and got this guy instead. The motor won’t work, no matter how many times they send it back. This money has been wasted, time has been wasted, salaries and man hours have been wasted.
And the client still has to get those repairs done, properly this time, by sending it to the factory.
Sometimes by this point a factory repair won’t be possible (see the puddle above) and the client will just have to buy a reman. Fortunately, factory reman is just as good as new — sometimes better.
How much of a deal was this repair again??
We don’t share these stories to embarrass the guys at Bob’s BBQ and Servo Repair Shoppe. We know that guy who called us is in a fix. We’ve used up our compassion for Rexroth third party repair shops, though, because we have more compassion for their clients, who tried to get a deal and instead got worse problems.
We share these stories because we don’t want this to happen to you. Rexroth third party repair shops don’t exist. They can’t buy original parts (we can only imagine where they get the parts they use). They don’t have the training or the information for factory repairs. Call us first.