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07-19-2007, 02:37 PM #1
In the past couple nights I have gotten to know my 42 EE rather well. Had the wrong drive belts installed (B series, not A), feed belt idler bearing were dry, motor noisy. Got those problems fixed and noticed a rythmic clunking sound from the lathe with the spindle running. Traced it down to the gearbox. Ended up pulling the gearbox to find the idler gear that engages the stack of threading gears had at some time been jammed into gear and one of the teeth had been flattened a bit making the tooth wider. 6 hours later... All back together and running quietly and very smooth. Still need to replace the feed drive rod. Looks like it has a slight bend in it and it is galled a bit where it goes into its support block.
Yesterday I decided to tackle one of the remaining problems. Did an oil change. The thing runs faster in reverse than forward. Started out finding that they had replaced a screw in the AP relay jamming it closed . Fixed that and adjusted it as well as I could. Now will brake to slow before reversing.
Spent the next couple hours trying to figure out the speed problems. All voltage seem OK. Eventually it hit me, Timing!. I saw there was a bolt to clamp the brush adjustment. Went to loosen it and it was already loose. Argh! I adjusted it to a point where it seems to be equal in both directions.
Now for my remaining problem. Someone has screwed around with the FA relay and I dont know how to adjust it. Its taking forever to come up to speed with the FA relay kicking in at above base speed. There are three screws, One for spring tension, one for contact separation and one that controls travel. How should I set these? I had it working pretty well last night. (About 5 seconds to come up to full speed, 2500 RPM) and this morning it wouldnt come up the AF relay was coming in past base again. I do have a DC clamp type amp meter.
How fast should the spindle come up to speed? It will go from full speed to stop in about 3 seconds.
Last but not least my exciter voltage seems to be around 129v. Seems rather high. I think this is causing the machine to run faster than it should (It easily goes past to 2500 mark on the tach) Could this be causing some of my troubles? More voltage = more current = FA relay pulling in easier.
07-19-2007, 05:37 PM #2
The FA (field acceleration) relay is there to give you a full field while accelerating to a speed where the field would be weakened (accelerating with a weak field would take a *lot* longer and require a lot more power to the armature). It should be adjusted to close when the armature is taking 25A or more. If you have a good power supply and dummy load you could adjust it that way, another way is to sit in front of it and have someone else operate the lathe. It should be on about 1/2 the time it takes to accelerate from a dead stop to a full speed (that might be a bit short or a bit long, try it there and then see how it works at other speeds).
So far as I know it should never come on when the lathe is operating, only when starting the lathe or when making radical changes in speed.
07-20-2007, 01:30 AM #3
I think I got it all sorted out now. The resistors cam in from Surplus of Nebraska so I installed those. I wonder why a 2k resistor is in series with the AP realy?
Had to play with the motor timing a bit more. The relay kind of pulses on and off till it reaches it set speed where it will stay open. The lathe jumps right up to 1800 very quickly and takes a little bit longer to get up to the 2500 range.
07-20-2007, 04:48 AM #4
"I wonder why a 2k resistor is in series with the AP realy?"
Some machines have it, some don't.
Perhaps to fine tune the point at which the AP relay drops out.
07-20-2007, 05:10 AM #5
OOps, I typed that wrong. the 2K resistor is in parallel with the AP relays contacts.
OK, this drive is starting to annoy me. I just cannot get it running right. In one direction the motor will come up to speed and then suddenly it screeches to a stop accompanied by a big flash if the FA relay happens to be closed. Then the FA relay closes and it starts back up again. I monitor the armature voltage and it drops to nothing when it does this. I move the timing to favor the direction that is problematic and the problem goes away but reappears in the other direction.
Could there be something wrong in the generator and the armature is pulling too much current so it kills the output?
07-20-2007, 06:04 AM #6
I measured voltage across ga2 to gs1 while it does this. You hear the motor slow down a bit just as it starts to do this. As you hear it slow down you see the voltage start to spike to over 260v.
I need to check current flow through the generator control rheostat. Maybe it is flaking out.
Now one thing I have noticed is the small coil on the FA relay is not doing anything. There is current flowing through the coil and it does measure about 500 ohms.
07-20-2007, 07:46 PM #7
"I measured voltage across ga2 to gs1 while it does this. You hear the motor slow down a bit just as it starts to do this. As you hear it slow down you see the voltage start to spike to over 260v."
Check very carefully the series field of the generator.
If properly connected, as "cumulatively compounded", then the behavior is linear with speed pot position, and it properly compensates for applied load.
If improperly connected, as "differentially compounded", then the behavior is indeterminate, and it may start in reverse, it may slow down with applied load rather than staying the same, it may speed up with applied load rather than staying the same, and, in general, it may do things which are completely counter-intuitive.
07-20-2007, 10:17 PM #8
I checked and everything seems right.
I put my dc clamp meter on the armature leads nd found it pulling about 25 amps up to speed. When its doing its thing current is reversing into the generator! Saw peaks a high as 30 amps.
So I stuck a 60 amp, 600 v FRED diode I had laying around in the lead to the DC box. Now everything runs good. Forward and reverse run up to speed smooth. I know this solves the symptom and not the problem but it works for me.
Now to fix the noisy back gear.
07-20-2007, 10:32 PM #9
You installed a "free-wheeling" diode, and now it works ... very strange.
07-20-2007, 10:55 PM #10
Yep, pretty much. I just can understand how it is backfeeding current back through the generator. I monitored field current and voltage while it was doing this and it was stable. Brushes good all around. Strange. But if this is all it takes to fix it...
07-20-2007, 10:59 PM #11
"Yep, pretty much."
The only things between the generator and the armature are the F and R contactors.
At this point, I would measure the resistance of the functional contacts.
I would expect to fine that resistance in the sub-milliohm range.
07-20-2007, 11:05 PM #12
Contacts are good and clean. I measured across each contact for a voltage differential while they were running and only had a few millivolts so the contacts are fine. It takes a special meter to be able to measure anything below a few ohms accurately and I dont have that (4 wire meter). Looking for a voltage drop is the easiest way to find bad contacts. (I repair welders, plasma cutters, industrial electronics, etc for a living) Also checked the overload heater an no problems there as well.
07-21-2007, 08:10 PM #13
I think there is something screwy with my motor. Adding the diode has helped immensely but it is still not running right. All the voltages seem right. At minimum there is minimal arm voltage and 115v field, mid point 115v field and 240v arm, full position is 20v field and 240v arm.
Heres the even weirder thing. The FA relay keeps opening and closing in no matter what even at full speed. If I hold the FA relay open in full speed position you would think the motor would run at full rated speed. But it dosnt. It stops! Measuring arm and field voltages while it is doing this shows voltages that would equate to full speed (240v arm and 20v field) But the motor is slowing down! And it will trip the armature overload if left for too long.
I think maybe some of the windings on the armature are partially shorted drawing too much current. What do you think?