1942 10ee MG speed and voltage issues

[FortyFive]

I have a 1942 MG that won't go past about 975 rpm. Following some other threads, I checked the voltages at terminals E1 and E2 inside the DC control panel. With the spindle running it is about 128 dcv regardless of speed control knob position. Measuring terminal GS1 to GA2 varies with knob position from about 20 dcv to 350 dcv. I notice the top center ap relay functioning, but I don't ever see the top right or top left ones move. Thank you for any knowledge of what to check next.

 

[rakort]

I have a 1942 MG that won't go past about 975 rpm. Following some other threads, I checked the voltages at terminals E1 and E2 inside the DC control panel. With the spindle running it is about 128 dcv regardless of speed control knob position. Measuring terminal GS1 to GA2 varies with knob position from about 20 dcv to 350 dcv. I notice the top center ap relay functioning, but I don't ever see the top right or top left ones move. Thank you for any knowledge of what to check next.

A picture of the control panel will help others help you.

 

[FortyFive]

Here is a pic of the control panel. Thanks

 

[9100]

I overhauled a couple of those panels. Your field weakening must not be working. Normal operation is to adjust the armature voltage up to about 1000 RPM, then leave it constant and lower the field voltage to make the motor run faster. Measure the field voltage at different speed control settings.

I presume it is a round dial machine. I am not familiar with the speed rheostat because I only had the panels. The later square dials use a pair of rheostats about 6 in diameter with only the first half active on one and the last half active on the other.

bill

 

[Peter.]

I overhauled a couple of those panels. Your field weakening must not be working. Normal operation is to adjust the armature voltage up to about 1000 RPM, then leave it constant and lower the field voltage to make the motor run faster. Measure the field voltage at different speed control settings.

I presume it is a round dial machine. I am not familiar with the speed rheostat because I only had the panels. The later square dials use a pair of rheostats about 6 in diameter with only the first half active on one and the last half active on the other.

bill

The round dial is the same Bill, though I know that different models used different rheostat values.

 

[FortyFive]

Yes it is a round dial. I disconnected the 2-wire rheostat (I believe it is the one for field weakening) and measured its resistance. It is about 1.2 ohms constant for the first half of the knob rotation then it increases with rotation to a max of 270 ohms. Know of anything in the panel that would prevent the field voltage from changing despite the rheostat seeming to be functioning?

 

[Cal Haines]

I have a 1942 MG that won't go past about 975 rpm. Following some other threads, I checked the voltages at terminals E1 and E2 inside the DC control panel. With the spindle running it is about 128 dcv regardless of speed control knob position. Measuring terminal GS1 to GA2 varies with knob position from about 20 dcv to 350 dcv. I notice the top center ap relay functioning, but I don't ever see the top right or top left ones move. Thank you for any knowledge of what to check next.

It sounds like your Field Acceleration (FA) relay is stuck closed. The FA relay is the one in the upper-left corner of the DC control panel. It should only close briefly during heavy acceleration. When it's closed, it bypasses the field control potentiometer and keeps the field at full voltage. It may just be dirty or someone has dialed the spring tension up too high. With the machine powered off, press on the FA relay's armature (the movable part) and make sure it pops open when you release pressure.

Cal

 

[9100]

Seems reasonable. The relay is a Struthers Dunn. I have some that I could cannibalize if needed- that is if I can find one. That may sound silly but you haven't seen my shop.

The blue wires at the bottom of the panel don't look original, so it would be a good idea to make sure they are connected correctly.

Bill

 

[FortyFive]

It sounds like your Field Acceleration (FA) relay is stuck closed. The FA relay is the one in the upper-left corner of the DC control panel. It should only close briefly during heavy acceleration. When it's closed, it bypasses the field control potentiometer and keeps the field at full voltage. It may just be dirty or someone has dialed the spring tension up too high. With the machine powered off, press on the FA relay's armature (the movable part) and make sure it pops open when you release pressure.

Cal

Thanks. That relay pops back out after I push it in. While the machine is running, I have changed speed, started, stopped, changed directions, and I haven't seen that relay move. I carefully pushed it in while running and nothing happened either.

 

[FortyFive]

Seems reasonable. The relay is a Struthers Dunn. I have some that I could cannibalize if needed- that is if I can find one. That may sound silly but you haven't seen my shop.

The blue wires at the bottom of the panel don't look original, so it would be a good idea to make sure they are connected correctly.

Bill

The blue wires look newer since not cloth covered. Doesn't look amateur as they are neat with stamped metal tags crimped to them. Maybe something hooked up wrong though.

 

[Peter.]

Those blue wires are exactly like mine. I am sure they are standard for the machine. Steel core BTW, not copper.

 

[Cal Haines]

Thanks. That relay pops back out after I push it in. While the machine is running, I have changed speed, started, stopped, changed directions, and I haven't seen that relay move. I carefully pushed it in while running and nothing happened either.

OK. Probably a bad coil or stuck contacts on the Dynamic Braking (DB) relay, the one in the upper right corner. It should operate every time you start/stop the machine. If it's contacts are closed, it's bypassing the field rheostat and preventing field weakening. It's function is to apply full field while the spindle motor is slowing. It should open as soon as either the forward (F) or reverse (R) contactor closes. It should be closed only when the F and R contactors are open AND the anti-plugging (AP) relay is operating. You can put card stock between the the DB relay's contacts to allow field weakening.

I agree that the baby blue wiring is original. Just about every panel that I've seen has that color wire on the bottom terminals.

Cal

 

[FortyFive]

OK. Probably a bad coil or stuck contacts on the Dynamic Braking (DB) relay, the one in the upper right corner. It should operate every time you start/stop the machine. If it's contacts are closed, it's bypassing the field rheostat and preventing field weakening. It's function is to apply full field while the spindle motor is slowing. It should open as soon as either the forward (F) or reverse (R) contactor closes. It should be closed only when the F and R contactors are open AND the anti-plugging (AP) relay is operating. You can put card stock between the the DB relay's contacts to allow field weakening.

I agree that the baby blue wiring is original. Just about every panel that I've seen has that color wire on the bottom terminals.

Cal

I will try that card stock test since it was not cycling as you described.

Do you know what the two components are in the center (above the fwd/rev contactors and below the AP relay)? They each have a plunger. I don't see them in pictures of other control panels I've seen on here. Overload?

Thank you

 

[Cal Haines]

...

Do you know what the two components are in the center (above the fwd/rev contactors and below the AP relay)? They each have a plunger. I don't see them in pictures of other control panels I've seen on here. Overload?

Those are thermal overloads. They're standard on all round-dial piggyback exciter DC panels. The one on the left protects the coils on the F and R contactors. The one on the right protects the entire panel. Inline exciter DC panels only have one overload which protects the F and R contactors.

Cal

 

[FortyFive]

OK. Probably a bad coil or stuck contacts on the Dynamic Braking (DB) relay, the one in the upper right corner. It should operate every time you start/stop the machine. If it's contacts are closed, it's bypassing the field rheostat and preventing field weakening. It's function is to apply full field while the spindle motor is slowing. It should open as soon as either the forward (F) or reverse (R) contactor closes. It should be closed only when the F and R contactors are open AND the anti-plugging (AP) relay is operating. You can put card stock between the the DB relay's contacts to allow field weakening.

I agree that the baby blue wiring is original. Just about every panel that I've seen has that color wire on the bottom terminals.

Cal

Well, that was it. With cardboard between the DB contacts, it goes to full speed. I'll check into it's coil next.

Is the 350 armature max voltage a concern? Other posts indicate 240 being the max.

Thank you for the help diagnosing this. You and others seem to donate quite a bit of time helping people keep these old machines in service.

 

[rke[pler]

Is the 350 armature max voltage a concern? Other posts indicate 240 being the max.

What are you measuring with? You could be seeing transients. If you're reading with anything that's averaging things (analog) then 350 would be very high and might be from someone trying to tweak the top speed up.

 

[FortyFive]

What are you measuring with? You could be seeing transients. If you're reading with anything that's averaging things (analog) then 350 would be very high and might be from someone trying to tweak the top speed up.

Measured with digital multimeter on terminals GS1 and GA2 inside control panel with machine running and speed control at max speed. Maybe someone did something since that relay wasn't working...

 

[rke[pler]

Measured with digital multimeter on terminals GS1 and GA2 inside control panel with machine running and speed control at max speed. Maybe someone did something since that relay wasn't working...

A Fluke or other reputable brand or a Harbor Freight special? I have to say that there's a big difference. Any access to something like a Simpson 260?

Also - check the voltage at the DC motor across A1 and A2, could well be a difference.

 

[FortyFive]

A Fluke or other reputable brand or a Harbor Freight special? I have to say that there's a big difference. Any access to something like a Simpson 260?

Also - check the voltage at the DC motor across A1 and A2, could well be a difference.

No- not a Fluke meter. Radio Shack. I'll check there. Thanks

 

[RC99]

Mine would not do that either. The dynamic braking relay coil was fried. I diagnosed the issue by putting a slip of paper under the dynamic brake relay contacts, then it went above ~900rpm, so the issue was found.