DC motor that was exposed to foreign substance is surging 1

[PaulKraemer]

Hi,

I am trying to troubleshoot a DC Motor and Drive Control System on a Printing Press. This is a 10 HP DC Motor with a field winding. It is controlled by a Cleveland MachineControls (CMC) PM3 drive. The speed reference to the drive comes from a digital potentiometer. Control is open-loop (no feedback from a speed sensor). Throughout my troubleshooting session today, I observed that the speed reference from the digital potentiometer is rock solid at 4.18 VDC.

This Printing Press can handle multiple webs that get laminated together in our process. Operators have brought it to my attention that this motor has been “surging” at times during production at our normal operating speed. In a test trial, I found the motor ran smoothly when we were only pulling a single web at a low speed. A clip-on ammeter indicated at this time the motor was drawing 6 amps. After this, we introduced two additional webs. This increased the motor current to 9 amps, but the motor still ran smoothly. After this, we increased to our normal (higher) operating speed, and the motor started surging. This forced us to stop quickly to avoid making a mess and wasting material.

We had changed the brushes in this motor about a year ago. We checked them again and they appeared to be in good condition. When we opened the motor to look at the brushes, we found that there was an oily substance inside the motor body. Investigation led us to the conclusion that this is coming from a glue melter positioned near the press. Every time the lid of the melter is opened, fumes escape into the area of the motor. These fumes get pulled in by the motor fan and pass through / around the motor windings before settling / condensing in the chamber where the brushes are accessible. We are going to take measures to reposition the melter to mitigate this issue.

I am not sure whether to suspect the motor, the drive, or something that might be making the load "irregular".

I am curious if anyone here has experience with a motor being subjected to a foreign substance like this, and whether it might lead to surging at higher loads / faster speeds as opposed to lighter loads / slower speeds.

Any suggestions or advice will be greatly appreciated.

Thanks in advance,
Paul

 

[Gr8blu]

If the brush-to-commutator contact is not consistent, the voltage drop across the interface is not consistent.
If enough contaminant gets caught between the brush and holder, the brush can get stuck and (perhaps) not be making contact with the commutator at all.

If the brush has poor contact, the ability to carry current is compromised. If the current flow is compromised, the ability to produce useful torque gets compromised.

If the contaminant is conductive, it can get stuck between commutator bars and/or other conductors in the machine. If that happens, it's the equivalent of introducing a short circuit. If enough of the conductors are shorted together, the ability to produce torque gets compromised.

The other thing that will happen is that the spring that pushes the brush against the commutator may be enough to overcome the "stickiness" caused by the contaminant. The brush may break contact, but because you're not turning all that fast, you don't miss many bars and the torque "ripple" isn't as big a deal. Compare this with a high-speed surface - the possibility of skipping a long span is considerably higher. This is the same concept as riding a bike slowly over the curb, vs hitting it at high speed.

The other thing that can be happening is that the fumes from the glue melter might also attack the resin that holds the brush material together. If this is happening, there should also be excessive brush wear (they'll get shorter faster than normal) and - probably - an increase in dust accumulation inside the enclosure. Note here that the dust will be conductive, which brings you right back to having conductive contamination between commutator segments.

 

[waross]

I agree with Great Blue.
I was going to post, inspect and clean the brushes and brush holders thoroughly.
After reading Great Blue's post I'll add, inspect and clean the commutator also.

 

[Compositepro]

Is there only one motor driving the whole press? CMC drives are usually multi-axis servo drive systems. There should be some speed and/or tension sensors somewhere.

 

[waross]

The next thing to check is all of the wire connections.
I serviced a DC motor on a press that would not run in reverse.
The cause was a loose connection that had overheated and part of the copper terminal had oxidized.
This formed a copper oxide rectifier that would pass enough current to run forward but blocked the current when the polarity was reversed.
To check, used an ohmmeter to check the resistance across each connection. Then reverse your meter leads and check again. The readings should match. If not suspect a bad connection.
You may check several connections in series. If you get a bad reading then check individually.