What's new
What's new

1947 mg problems

Mike Hill

Cast Iron
Joined
Aug 28, 2004
Location
upper michigan
Hi all, I purchased a mg 10ee last weekend, I bought it knowing that the drive had issues that were unsolved. The lathe had its exciter removed and replaced with a power supply, I hooked power up to the machine crossed my fingers hit start and guess what nothing happened. So I did some poking around and found problems with the main contactor hit the start ago and the mg roared to life.

Next I tried the forward lever, and nothing happened, did some checks on the the added on power supply and discovered a blown fuse, now I had dc power hit the forward the spindle turns, good sign right, turned the speed up slowly and its good till I hit about 400rpm on the tach then the spindle would momentarly shoot up in speed to about a 1000rpm then drop to almost a stop then back to a 1000 over and over again.

If I try to get it above this speed it just slowed down, looking at the big rheostats I could see some arching on the one that is farther away from the sprocket.

I removed the rheostats and found the one labled 375 to have some missing windings, if I am looking at this right that one controls the field voltage and the other controls the armature?

So in the end my question is there anything else that will burn out this rheostat or was it just old age, and would this cause the described symtoms ?
 
Poor brush contact in the generator can cause surging. The brushes should show a few small sparks across the width of the brush. If you see a single bright spot, check the brush for sticking in the holder or for being worn to the point that it does not have pressure across its width. If there is adequate brush length left, slide it up and down a few times to ensure that it is free. With the power off, of course.

The surging problem is normally not likely to be the control pots, but they may contribute.

Bill
 
I checked the mg and motor brushes before I fired it up and they have good length and move freely in their holders, one thing I didnt mention was the 375 ohmite shows open from one outside terminal to the other.
Mike
 
BTW I did remove the 375 ohmite, while looking at it with the coil side toward you and the terminals pointed at 6:00 the burned off windings are at 10:00, would this make the field voltage drop to zero after the wiper crosses this point dropping the speed to zero?
 
BTW I did remove the 375 ohmite, while looking at it with the coil side toward you and the terminals pointed at 6:00 the burned off windings are at 10:00, would this make the field voltage drop to zero after the wiper crosses this point dropping the speed to zero?

If you lose field voltage the motor would attempt to hit infinity as a speed (somewhere short of that the motor would birdcage). What's likely happening is that the field acceleration relay is kicking in and giving you full field in an attempt to lower the armature current and when that currend lessend the relay drops out slowly enough that the motor slows and the process repeats.

There's a few of those rheostats out there, get a replacement.
 
FWIW my rheostat has been repaired with normal copper wire in the same spot. It gives a jump in the motor RPM at about 400. I guess it must be the most frequently used zone on the thing.
 
thanks guys for the advice, the next thing I was thinking of trying is what Peter suggested, dont have much to lose.
I could use a few things for this lathe
1. compound, I think I have one lined up from a member
2. tailstock end leadscrew bracket, mine is cracked
3. 375 ohmlite
4. halfnut handle
5. dc panel reverse contact coil
6. main ac contactor for low voltage
thanks Mike
 
I have made a contactor coil. Easy to make and I have the right wire. If I can find the spool dimensions again I'll gladly wind one for you.

Now, IF your machine just happens to be a rare 240v DC panel machine I have a coil right here for it.

EDIT - Here is the info I used for the spool and the wire to wind on it. Post #8 links to my thread:

http://www.practicalmachinist.com/vb/monarch-lathes/dc-contactor-coil-winding-259308/
 
So on the parts I requested fellow pm members came through big, the only things I am still looking for are
1. half nut handle
2. dc panel reverse coil (I may try to wind one)
 








 
Back
Top