Not specifically PLC

I was called to a machine several days ago because the cooling fan for an rewind motor alarmed and kicked out a couple of times. The filter was stopped up so I changed it but in the process of starting that rewinder we heard a loud bang, I lost power to the whole machine. It kicked the main 600 amp breaker feeding the 400amp main for the machine. I found a contactor fused so replaced it, then energized the system again, another loud bang. Closer inspection showed the Digital DC drive had all 6 SCR's blown apart.

Anyway I ordered a new drive and installed it, then did the setup and programming. I went thru a deluge of Alarms, from the drive and HMI/computer interface. The drive would not work. Fuses blew. Upon closer investigation I determine that of the 3 fuses on drive control board that were blowing were part of the Field circuit for the drive. I had Meggered the motor and could not detect a problem.

After talking to the Drive manufacturer I/we determined it had to be in the motor. So I started disconnecting the motor to send it out for checking/repair.

What I found amazed me, the cooling motor fan blades had separated from the squirrel cage assembly, on this system the cooling fan is directly above the terminals for the motor connections. I found half the blades from the fan inside the DC motor..specifically one laying right on top of the connector terminal and TONS of metal shavings all over everything.

This whole thing took several days and isnt resolved completely, the motor is out to shop now but the machine is running.

Guess I am stating this because anything can happen...and drive you nuts trying to figure the why. In this case the original problem was the cooling fan but more major problems happened and I didnt consider the original problem causing it. Ya never know.

Years ago, I got a small conveyor motor back from a rewind. When I turned it on it would not go, but I thought I saw a little blue spark from within it. I turned it on and off a few times and yes, I was sure there was a little blue spark eminating from the middle of the motor somewhere. My boss came by and saw that I was puzzled, I told him what was happening and so he decided to look at the motor while I switched it on. He got his head up close to see if he could see the little blue spark. This time the fault completely broke down, there was an almighty bang and blue flames surged out of every orifice of the motor. He jumped back in horror with singed eye brows and lashes. To this day he is sure I did it on purpose. I didnt of course, but it still makes me laugh. That was caused by a terminal nut left or dropped inside.

It's nice to know I'm not the only one to have survived a few "disasters"!

One of my most "interesting" occurred when the E-Stop was wired wrong at the factory on a 4160 Volt across the line starter tied to a 700 hp 3600 rpm motor direct coupled to a high inertia load. The circuit was supposed to be a standard three wire control. When I pressed the E-Stop the starter dropped out, and when I released the E-Stop after a second or so, it pulled in the starter and put the motor back across the line.

Did you know that when Grade 8 bolts on a 4" shaft coupling let go they sound exactly like an explosion? Do you know how much noise a gear coupling makes when it contacts the guard at 3600 rpm? Do you know how loud a contractor can squeal when he finds out you've just delayed his commissioning indefinitely?

Once, on our printing machine, we couldn't get the main motor to run more than 800fpm. (should go to 1200). The reference was there, and would climb past the 800 fpm point when the faster button was depressed, but the armature volts just flattened at the 800 fpm level. (I don't remember the actual voltage, it's been a long time ago.) But anyhow, what we found was that the motor peckerhead cover had been put on backwards, and the way the slant was built in to the cover, the inside of the thing was positioned dangerously close to the armature connection lugs inside the peckerhead. When we removed the cover, there was evidence of arcing on the inside of the cover and one of the armature connection lugs.

I never would of thunk it, but I suppose that when the armature volts rose to a high enough level, the arcing started, stole the current, and kept the motor from accelerating any further. At the lower speeds, and lower armature voltages, the thing ran perfectly.

We installed the peckerhead cover the right way and the thing was fine. What got me about the whole thing is that we never blew a fuse or saw any other symptoms--no current limit--no nuthin--other than the limited speed of the motor.

There's no tellin what will get you next.

I once had one of my "wiggy" leads on a 480vac terminal and scratched my head with the other lead while standing in water. I will never do that again......
OUCH!!!