‘47 MG has exciter voltage but no generator output

usmachine · Mar 10, 2018

Just started poking around this old girl and had no voltage at E1. Put in new brushes and repaired broken wire to resistor in the exciter end bell and the exciter came back to life at around 125 Vdc
Motor contactors pull in now but spindle won’t go. Small arc from DB contactors when engaging fwd or rev contactors with drum switch.
No 230 Vdc coming from generator so spindle won’t go.
I have not looked at the generator brushes yet but that is on my list for tomorrow once the grandkids go home. Also plan to check out the generator field and armature while I’m at it tomorrow. The generator commutator looks good as does the motor so I dont suspect the brushes but I have them on hand just in case. Any thoughts?

usmachine · Mar 10, 2018

Also spotted the thread by gernoff and Cal Haines detailing how to check the generator wiring. That is now on my list as well.

thermite · Mar 11, 2018

usmachine said:
Also spotted the thread by gernoff and Cal Haines detailing how to check the generator wiring. That is now on my list as well.

Wire broken off at a termination, rheostat, switches, relays... but hung brushes the most likely.

There really isn't all that much to an MG, apparent complexity due to convoluted wire routing quite aside.

rke[pler · Mar 11, 2018

So E1-E2 is 125VDC? What do you get at EF1-EF2?

Cal Haines · Mar 12, 2018

We had a guy with a similar problem a few months back. Have a look at this thread:

http://www.practicalmachinist.com/vb/monarch-lathes/10ee-wont-start-344176/#post3095364

Cal

usmachine · Mar 12, 2018

Didn’t check EF1-2 but I went through the generator wiring tests in the gernoff/cal haines post and the gen armature showed open. Before I panicked I kept in mind the hack work on the brushes in the exciter and put the OEM brushes in the gen and ta-da the armature was not open anymore and the spindle commenced to rotate.
The gen voltage varied fully from around 20 right on up to over 210 as I increased the speed, success.

A new issue that has now surfaced is that the speed is very unstable in reverse with the AP and the FA relay pulling in and out. The tandem work perfectly in fwd with the FA relay causing the spindle to go from 0 to 1000 rpm in a couple noticible steps and leveling of nicely at the selected speed. In reverse the machine rapidly accelerates then the FA relay kicks in and the speed falls sufficiently to drop out he AP relay. That continues and the speed will not stabilize until I lower the speed significantly and then raise it very slowly up to the desired speed.

Could this be a problem with the resistor in the circuit? I have read of this symptom in other posts and have a friend that fixed his by changing the resistor. I suppose I will replace the motor brushes knowing the bad results from the other locations before I changed them there. BTW this is the control panel that has the three resistors in the upper right corner and the newer style contactors for fwd/rev

Cal Haines · Mar 13, 2018

usmachine said:
Didn’t check EF1-2 but I went through the generator wiring tests in the gernoff/cal haines post and the gen armature showed open. Before I panicked I kept in mind the hack work on the brushes in the exciter and put the OEM brushes in the gen and ta-da the armature was not open anymore and the spindle commenced to rotate.
The gen voltage varied fully from around 20 right on up to over 210 as I increased the speed, success.

A new issue that has now surfaced is that the speed is very unstable in reverse with the AP and the FA relay pulling in and out. The tandem work perfectly in fwd with the FA relay causing the spindle to go from 0 to 1000 rpm in a couple noticible steps and leveling of nicely at the selected speed. In reverse the machine rapidly accelerates then the FA relay kicks in and the speed falls sufficiently to drop out he AP relay. That continues and the speed will not stabilize until I lower the speed significantly and then raise it very slowly up to the desired speed.

Could this be a problem with the resistor in the circuit? I have read of this symptom in other posts and have a friend that fixed his by changing the resistor. I suppose I will replace the motor brushes knowing the bad results from the other locations before I changed them there. BTW this is the control panel that has the three resistors in the upper right corner and the newer style contactors for fwd/rev

The AP (Anti-Plugging) relay has nothing to do with your problem. It's supposed to open at about 300 RPM. It stays open as long as the armature voltage is above a certain voltage and it's there just to keep your from "plug reversing" the spindle motor.

The FA (Field Acceleration) relay is probably the culprit. Check the A-B resistor pair (two 1600 Ohm resistors in series), make sure they're not open. They should be the two on the top, sitting side-by-side. If they're OK, you might try adjusting the spring tension on the AP relay.

Check the voltage from A1 to A2 on the terminals at the bottom of the panel. Make sure you correctly identify the terminals, checking both the label above the terminal and the metal tags on the wires. A1 and A2 should be in the upper of the three rows of terminals. Check the voltage in both forward and reverse, at the same speed setting and above the speed where you are having the problem.

Cal

usmachine · Mar 13, 2018

Cal, I agree about the 2 relays, they are more like the canary in the coal mine of the system and the good thing is that both of them are functioning. Back early in my 10EE days I had an early round dial where the coil in the AP relay had failed and it was tough to find one out in the wild.
I have the part numbers for the resistors in hand and will head for GALCO in the morning.

Cal Haines · Mar 14, 2018

Was the A-B resistor pair open? It's a good idea to replace the B-E resistor as well.
Cal

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