5 hp DC motor stall low rpm

I have a 1971 10ee that I have been working on for several months. I finally got the tube drive system working correctly and have been using the lathe a little bit. Today I was doing some work and was surprised to find that I was able to stall the motor out. I did it unintentionally but was surprised by it. My back gear is noisy so I was running in open belt, only at about 200-400 rpm parting a piece. I must admit that I didn't set up the tool 100% correctly as I was only parting 3/8" wall tubing 2 1/4" OD. The tool was a little cocked, more than I thought and ended up rubbing quite a bit. I guess it makes sense at low rpms that it would potentially stall, but I keep reading "constant torque" and I guess I thought it would have full power down to the lower rpms. I guess I should be using the back gear when doing moderate work at the lower rpms. Any comments or past experience with this?

It seems to have good power at higher rpms. The max cut I have taken is .050 off diameter, or .025 DOC. Seemed to be fine, I know that's not all that heavy. Could it be an issue with the compensation pot?

First and most important is what type motor shunt series compound etc. I would guess shunt since that is the most prevalent motor in "smaller" industry.

Yes a motor can deliver up to its full rated torque to meet torque demanded by the load at low speeds HOWEVER if your load is demanding too much torque the motor will become overloaded with possible (probable) overheating. slowing and eventually stall. This sounds like your situation.

Dan Bentler

Well I turned the compensation pot about 1/8 of a turn and it seemed better today.

I like what you are saying Thermite about the Series windings. In another post I mentioned that I had to swap the A1, A2 leads in order to get the correct motor rotation direction when the control lever was pulled toward the headstock. The motor was rebuilt and I thought perhaps since A1, A2 were backwards, that perhaps S1, S2 were backwards too. I called Monarch and talked to their tech, and he didn't seem to think the S1, S2 being wrong would really change anything. I swapped them and it ran about the same, I didn't take a cut.

I don't think the compensation pot is fighting the motor. After turning it a little today, it seemed much better. I took some decent cuts at about .005 feed and watched the tach and it never moved. I think perhaps it was adjusted a little low and my parting tool was so crooked that it was causing problems. Only time will tell.

The little things can bite you in the butt...possibly you are running the motor (wired for 440) on 220.

Lee (the saw guy)

thermite said:

The compensation pot is an ADJUSTMENT. The direction in which S1, S2 are connected w/r A1, A2, is quite literally 'hard wired'. Monarch tech may or may not be au fait with what that means inside the motor. The MOTOR makers will have guidance and tables of performance figures, even if now in their 'legacy' tech support docs.

If it was a boost/buck transformer winding feeding a VFD, you wouldn't correct bass-ackwards wiring that bucked when it should have boosted by changing the VFD's V/Hz settings.

You would have to correct the wiring.

One or both of you should research that.

"I got mine!"



Bill

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At this point in the game it is all about adjustments. The lathe works seemingly well, rpms just dropped off under moderate to heavy load. The compensation pot increases power under load. Over adjusted and rpms rise under load, under adjusted and rpms fall. That is why it is part of the system.

Much like a carburetor. Would you assume a faulty engine if it couldnt maintain idle? No, you would ADJUST a screw to raise or lower rpm.

The tech at Monarch is very knowledgeable and is their go to guy. I believe if the series was backwards either other problems would exist or it doesn't matter. I could wire a1/a2 correctly as named and swap the field leads to potentially correct any series/arm flip flop.

thermite said:

So you might. Unless I had MOVED that idle stop, I might first be checking FUBAR'ed PCV or EGR system, hose, body, or throttle shaft vacuum leaks, impaired linkage/cable, ignition timing, plug/ignitor and lead health, then burnt valves that leaked at low RPM.
All that before ODBC and even O2 sensors and more basic predecessor EC modules and throttle position sensors, idle speed recalibration at 11 MPH./.. or wotever...

... in general, 'coz conventional carburettors had largely left the room about the time the electrons entered.

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You would do all that prior to making an adjustment designed specifically for the problem I had?


Last time I looked my lawn mower, weedeater, chainsaw, snowmobile, atv.........all had carbs. I dont, and doubt most run a battery of tests.....they turn a screw. If that fixes the problem great. Move on. If not, dig deeper.

Did you miss my post where I mentioned the adjustment made a difference?

I think you were also the one who told me to rip out every last wire in the machine to find the previous problem. Turns out i just needed to swap two wires. I could still be doing a museum quality restoration......or I could be using a perfectly fine machine.

Monarch will sell me a known good motor and backgear for $500. If this one takes a dirt nap I will move on.

There is obviously more than one way to attack a problem. If I or anyone else knew exactly how to "fix" the machine there would have been no need for

"side-trips, guesses, experiments, half-measures and back-out and try something else outright reversals forward one day at a time."

That's how you diagnose and fix a problem. Especially when you know nothing , or very little about electronic drives. Which I claim to still know very little about.

I really do appreciate your comments and would like you to keep them coming, but bear in mind that sometimes good enough, really is good enough. At least for some people. I was able to, through lots of reading, and talking to the Monarch tech, "diagnose" what the problem was. Albeit through much trial and error, but that is how you learn. My goal all along was to learn about the DC drives, and I did, and in that I learned that polarity is definitely an issue, which led me to check the polarity of the transformer outputs. To say that I had gotten lucky in my fix is not quite accurate.

If you had just replaced everything with new would you not say you had gotten lucky in the fix because you would have had no idea what was wrong in the first place? And if it stopped working again, would you be more equipped to fix it or less?

Learning the drive and being able to diagnose and fix a specific problem was my goal in learning.

The series field polarity must be matched with the shunt field. That is why the shunt field and brush connections are brought out separately. You get the field polarities together and leave them that way, only reversing the armature, A1, A2. I haven't done it on a 10EE motor but have on another 5 hp DC motor that was originally driving the compressor on a locomotive air conditioner, consequently always going the same direction. The shunt field leads were separate but the armature and series field were connected inside and only two leads came out. Reversing either the field or armature/series field connections reversed the direction but the motor was useless because the shunt field compensation was backwards, reducing field strength with load. You probably have S1-S2 right if you can compensate that easily.

The compensation circuit uses the output of the current transformers to raise the armature voltage when it draws more current. Under some conditions you will be at the max armature voltage available and the compensation will stop. The series field compensation will continue though. If you can set the compensation to speed up under load, things are likely properly in line. Note that if you get the compensation too high, it will become unstable.

Bill

Just to chime in, if I run mine open belt at low rpm, its real easy to stall,( especially when threading) usually it knocks out the relay and shuts it down. I thought there was something wrong also, so I pulled out a " How to use the EE" treatise from some big university's engineering school, and they mentioned to always use the back gear under a certain rpm or it will knock out the relay and shut it down...doh!

daryl bane said:

Just to chime in, if I run mine open belt at low rpm, its real easy to stall,( especially when threading) usually it knocks out the relay and shuts it down. I thought there was something wrong also, so I pulled out a " How to use the EE" treatise from some big university's engineering school, and they mentioned to always use the back gear under a certain rpm or it will knock out the relay and shut it down...doh!

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Thanks for actual hands on, been there done that info. Exactly what I was looking for.

9100 said:

You probably have S1-S2 right if you can compensate that easily.

The compensation circuit uses the output of the current transformers to raise the armature voltage when it draws more current. Under some conditions you will be at the max armature voltage available and the compensation will stop. The series field compensation will continue though. If you can set the compensation to speed up under load, things are likely properly in line.

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That is kind of what I was thinking/hoping. I have not adjusted to speed up under load, just a small turn seemed to make it more stable under load though. I may not mess with it as it seems to be set about right.

I did call the company that rebuilt the motor in 2004. Kirby Risk Electric Motor Repair out of Lafayette, IN. I talked to their tech who wound motors for the prior 10 years. He indicated that the F1,F2, S1,S2 leads would have been checked for polarity while the armature was removed and labelled accordingly. He said the motors are put back exactly the way they are brought in.

I may try to check the polarity with a needle type voltmeter and a couple batteries per a test I read online. Hook the voltmeter up to f1,f2 and apply voltage to s1,s2 and watch which way the needle dips. Sounds tedious but i read it in a couple of different places, one "official manual".

I realize I could be speaking out of school here as I'm a spring chicken in comparison to the 10EE and it's vacuum tube drive. What I'm about to say applies to later DC drive technology which may or may not help with setting the compensation control. I know this as IR Compensation. IIRC - I always set these controls with no load. Motor connected (mechanically) to the driven equipment but without material, no cutting, etc. and at the lowest speed command permitted. Then increase IR comp until the motor speed begins to "hunt" or pulsate. Then back it off until smooth rotation occurs and then just a hair more.

Also, IIRC constant torque refers to a range of RPM that doesn't actually correlate to 0 - max RPM. It's usually some min RPM - max RPM. The motor mfg will be the best source for that information. It's also been my experience that a rewound motor rarely matches the characteristics of the original so whatever data you get from the motor mfg will only be for reference.

As always YMMV...

"The series field polarity must be matched with the shunt field."

If the polarities match, this is called "cumulative compounding", and is perfectly stable.

If the polarities do not match, this is called "differential compounding", and may or may not be stable; in fact it may be so unstable that a command to rotate the spindle in one direction may actually result in the spindle rotating opposite to that which was commanded.

Hence, the series field (S1-S2) must be cumulatively compounded with the field (F1-F2) for the drive to work at all.

The two regulators, the field regulator and the armature regulator, are "open loop" in their basic design. It is the addition of the compensation system which attempts to close the loop of the drive system.

The series field, if "cumulatively compounded", does most of the heavy lifting, while the compensation system does the fine-tuning.

The M-G system did not have a series field in the spindle motor for the very simple reason that the main generator was provided with a series field.

Indeed, the exciter also was a compounded dc machine (although self-excited), so that the exciter terminal voltage would be very stable (115 volts for most examples, 230 volts for a very few examples) over all of its load conditions.

I may ruffle a few feathers with this question, but I'll risk it anyway. Bear in mind I've only seen pictures of the 10EE and most certainly never ran one. But, it seems the subject of drive issues pops up a lot here on PM and there's usually some whining and consternation about the price/availability of replacement tubes.

So, here's the query: Why not rip that old drive out by the roots and replace it with a later, more up to date DC drive? I see SECO drives headed to the scrap heap coming out of industrial equipment very frequently and figure someone could pick one up for the trouble of pulling it out of the cabinet. I know there are other drive mfgs out there, but SECO is prevalent in these parts and I just happen to know them like the back of my hand having repaired them for several years. And, it readily comes to mind. Just seems like a 'no brainer'.

Or, is it a hi-fidelity thing where the warm, soft sound of the chips being cut just can't be matched by solid-state?

Take me to school, will ya?

WayneC369 said:

So, here's the query: Why not rip that old drive out by the roots and replace it with a later, more up to date DC drive? I see SECO drives headed to the scrap heap coming out of industrial equipment very frequently and figure someone could pick one up for the trouble of pulling it out of the cabinet. I know there are other drive mfgs out there, but SECO is prevalent in these parts and I just happen to know them like the back of my hand having repaired them for several years. And, it readily comes to mind. Just seems like a 'no brainer'.

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That is the knee jerk response of most people. When it gives trouble, why not rip the engine out of your Ford and put in a Chevy? Because it just isn't that simple. Pasting in something that makes the spindle go round is easy, but getting all the features back in service and making it do everything it is supposed to do is not. For example, you have an anti plugging relay that allows the operator to throw the switch from full forward to full reverse without stressing anything. It holds the motor in braking mode until it is almost stopped and only then closes the reverse contactor to make a normal start without slamming the motor with a serious overcurrent. When you switch the spindle off, the field voltage goes to max for braking, then a timer shuts it down, avoiding heating.

Many of the things can be programmed into a modern drive, but you better know what you are doing. What we really need is a drop in solid state replacement for the thyratron tubes. The main problem with thyratrons is that they are no longer made and replacements from the stocks that are left are expensive. The second problem is that people are not familiar with them and are uncomfortable working with them. I just did some maintenance on a thyratron controlled 10EE that I originally got working for the customer over 10 years ago. I practically had to beat the owner severely about the head and shoulders to keep him from taking the motor and all out and installing an AC one with a VFD. He finally gave in when I promised a ceiling on my charge. It has been running in a job shop ever since with no more maintenance it until now and is the favorite lathe for small work.

I made a saturable reactor drive for my 10EE that would give full power and was intrinsically current limited, protecting the motor. It worked well, but had the usual issues that people not working in R&D rarely understand. Development of a product is not mostly making something that meets the general specs but tying up all the loose ends to make a marketable unit. My standard speech to someone who wants a new machine built is to add up the design time, making the parts and assembly and getting it running, then add at least that much time again to make it marketable and you will still be short of the final cost.

Bill

Re tach generators, put a magnetic pickup on the notched wheel of the spindle lock and feed it to a circuit I found in a circuit designer's handbook. It uses standard digital components and has the signal from the pickup set one flipflop and the signal from your speed control sets another. Each pulse resets the opposite flop. If a flop receives two set signals before it gets a reset, the signal to that one is higher frequency. That information goes to an integrator that raises or lowers the control voltage accordingly. It is completely consistent and you can't fool it. Since it is an integrator rather than a normal closed loop, it will not oscillate and variations like belt slap will only cause minor variations that should be unnoticeable in this service.

I'm at home and the book is at the shop so I can't send it now, but I can tomorrow.

Bill

Monarch EE lathe stalling at aprox. 200 rpms is somewhat normal. Having run Monarch EE about as much as anyone.That is why the back gears are there

"Jonathan's D510 is not "independently" POWERING those S leads."

What some drive manufacturers do to accommodate the series field is place the series field within a full-wave bridge rectifier.

In this way, the polarity of the series field will always follow the polarity of the field, thereby enforcing "cumulative compounding".

I found this, and other interesting tidbits within the schematic diagram of a Reliance dc drive, one which was rated 7.5 HP if operated from a 277/480 system, with 277 into the drive (single-phase).

Thanks, Thermite and 9100 (Bill & Bill) for those fine explanations. And, my apologies to the OP for hi-jacking your thread. Another thread on the replacement (or not) of the D510 (or other) with later tech would be simply awesome! Nothing like soaking up new knowledge. At least for myself anyways.

9100, I'm coming from an industrial electrical background and I simply assumed one would install the required controls to perform those tasks you mentioned. I'm reaching back quite a few years and remember SECO building what we called a regen-drive in which the reversing task was handled just as you described. However, the field volts were constant on all these drives IIRC. And, the field remained hot to keep the motor warm for better speed regulation. In many situations we installed current relays that monitored the field current and were wired into the e-stop circuit in case a motor lost those precious field volts during operation.

Thermite, I never knew the motors were 230 vdc. I remember seeing it mentioned but assumed the guy writing the post was confusing the armature volts with incoming supply volts.

"What some drive manufacturers do to accommodate the series field is place the series field within a full-wave bridge rectifier.

"In this way, the polarity of the series field will always follow the polarity of the field, thereby enforcing "cumulative compounding".

The Monarch dc drives always reverse the armature, never the field.

Therefore, if the spindle motor is compounded, some way of simultaneously reversing the armature and the series field is required.

On a Monarch dc drive, this is already being done.

On a re-purposed dc drive, this must be accommodated, somehow.

Take the Reliance dc drive, for example. It employs one set of "power blocks" (integrated SCRs with associated protective diodes) for FWD operation, and another set of "power blocks" for REV operation.

On such a drive, provision for the series field must be made after the "power blocks".

If the S1 terminal of the series field is connected to the "+" terminal of the full-wave bridge, and the S2 terminal is connected to the "-" terminal of the bridge, and the "~" terminals of the bridge are interposed between the drive's armature supply source and the motor's armature, the required polarity will thereby be maintained.