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DC motor that was exposed to foreign substance is surging 5

PaulKraemer

Electrical
Jan 13, 2012
151
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
 
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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.
 
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.
 
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.
 
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.
 
Thank you Gr8Blu, CompositePro, and Waross for your detailed replies -

This gives me things to investigate. To answer CompositePro's question, this motor is driving the whole Press. This is a very old press. I believe it was built in the 1980's or early 1990's. I suspect that the CMC drive is original, although it may have been refurbished at some point. I feel like it is a relatively simple drive system, it gets a start signal from a relay when the Press Start button is pressed and the Press Stop button takes out a stop signal. The speed reference just comes from a digital potentiometer. I think it looks more complicated than it is because it is an open design with more than one circuit board and wires connecting these circuit boards. I'd rather not do it unless necessary, but I don't think it would be terribly hard to replace it with a modern drive.

We had a problem with this drive in July 2024 where it would not run at all. At that time, we found the brushes to be very worn down, but we didn't notice the contaminant issue. We changed the brushes and it still wouldn't run. It turned out that there was a loose wire in the circuit that gives the drive its speed reference. Correcting that got us back up and running. We did not experience this surging issue until recently. After having changed the brushes in July, they still appear to be in good condition now. We didn't notice excessive powder, but I'll take a second look.

We will check all the things you suggested on the motor. Considering the possibility that it might be compromised, management may even agree to purchase a spare motor if an equivalent is available.

I appreciate your help!

Best regards,
Paul
 
First check that the speed reference is stable.
Second check that the supply voltage to the motor is stable.
Third do an insitu brush location check of the motor armature.
To do this, disconnect the field wiring from the motor and at least one armature connection.
Apply 110 V/50Hz or 120 V/60Hz to the field. Connect a voltmeter across the brushes.
Manually rotate the motor while watching the voltmeter.
The armature voltage should be very close to zero and should stay stable while the armature is rotated.
Any faults in the armature windings (shorts or opens) will show up as a voltage fluctuation.
On motors with fewer commutator segments there may be a slight voltage ripple as the brushes pass from one segment to another.
If any voltage ripple is uniform for a complete rotation it may be ignored.
An armature fault will be much more pronounced.
This test will find the same faults as will a growler test, but without removing the armature from the motor.
 

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