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Sorby lathe stutters at speed

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Hi, my Robert Sorby RS2 lathe stutters as it get above top 25% speed using the VFD. I thought it was the on-motor switch so bypassed it but wired it incorrectly, so it ran in reverse. It did this fine, no stuttering at any speeds. Having corrected the error (switched the live/ neutral wires back) it runs the right way but now stutters again in the top 25% speed range.
Any thoughts what this might be?
Thanks, Chris.
 
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Thanks Karl - I've checked the belt, seems fine.
I don't get it - why would the motor run smoothly turning in one direction, but stutter at higher speeds in the other?
I can't think of anything beyond the motor that would cause it to happen.
Attaching info plate from the motor in case it helps?!
 

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john lucas

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I would call.the company. VfD's are complicated electronic devices and a lot can go wrong with them. They are basically electronically disassembling and rebuilding the shape of the current waves to do what they do. A lot.of components to.do this. If any one of them fails you would get strange things happening to the motor.
 
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Thanks John - so you think it's likely the vfd, not the motor?
The vfd (made by Dart Controls) controls perfectly with the red and black wires swapped - doesn't that indicate that it is working properly?
Forgive me for questioning, I really don't understand electronics at all and am just trying to think it through.
 
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Thanks John - so you think it's likely the vfd, not the motor?
The vfd (made by Dart Controls) controls perfectly with the red and black wires swapped - doesn't that indicate that it is working properly?
Forgive me for questioning, I really don't understand electronics at all and am just trying to think it through.
not necessarily, if they (at that point) are using DC current, and have one way diodes , a diode failure could seem to be "fixed" swapping the wires (current direction) around .. transistors can get wonky depending on voltage polarity as well... so your issue might well be the controller portion (assuming you can get it separate from a motor) .. matter of fact I'd probably suspect a controller before a motor itself if motor seems running fine one way- motor would generally be a case of either it runs,in which case it might have obvious other issues indicating impending failure (burning, big sparks, bearing noises, getting real hot, etc) or it don't.
 
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Thanks Brian. In my ignorance I'm assuming that once the vfd has done its job, power is delivered at the set level to the motor - but in reality even if the motor is working right, something goes wrong in the vfd when the motor is set to work in one direction but not when the wires are swapped so it's working in the other direction?
Just trying to be clear before I send a part to be checked/ repaired.
Thanks, C
 
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The speed control for your motor is not a Variable Frequency Drive as those only work with 3 phase motors. The Dart controller on your lathe only has to rectify the incoming AC power to DC (direct current) and vary the voltage out put to the motor to control speed. The only thing I can think of is that it maybe the brushes in the motor are worn down and catching on the commutator causing the stutter, which due to the wear pattern of the brushes and the commutator would not happen in reverse. To solve the problem you will likely need to take the motor to a motor repair shop or check with Leeson.
 
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Thanks Don.

Well, this is turning out to be a steep learning curve for me! And I thought learning to turn was tricky.
So, I don't have a 3ph motor, which is what I just assumed, and therefore don't have a VFD. And whatever the motor, I just assumed it was brushless.
Instead, I have a single phase DC motor that's brushed, and a converter with speed control. (I'm writing that so I can be corrected if I have it wrong!)

Given that, and that the motor runs properly in reverse, can I now exclude the controller as being the issue?
I'm really sorry for being so slow - to get this fixed I'll need to package the motor up and send it off to be repaired and want to get as close to knowing the likely cause before I do so. Whether it's the brushes or not, if the fault's in the motor it needs sending off to be repaired.
 
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When you had the motor leads reversed did it shutter operating in reverse? If it did then you could figure that it is not a motor problem. It is most likely the speed controller anyway. I don't know that observing brush/commentator action with the brush covers removed would prove anything unless the brushes were severely worn. If there is a lot of carbon residue in this area it would be good to put a shop vacuum on one side and gently blow out the area with compressed air from the opposite side. How long have you had the lathe before this problem started?
 
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Well, it has been a while, and I am not an electrical engineer, but I think this used to be a problem with the old DC motors. It took some fiddling with the settings on them to get them to run smooth some times. I think it was with the early Nova lathes and before the 3 phase motors and converters became popular. I would call the manufacturer, or e-mail them.

robo hippy
 
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Joe - it works perfectly when wired to run in reverse. I've had the lathe about three years, bought it from an old fella who'd hardly put it to much use since he bought it new.
About a year ago I had problems with a connection to the switch, tripping the fuse when the spade connection shook looser than it should be, so thought the current problem was the same thing. But having bypassed the switch and seeing it run perfectly in reverse (a wiring error!), I can't see it's to do with those connections.

robo hippy - there's a repair place over here who I'll contact if I can determine if its the controller or motor.

Thanks for your efforts all!
 
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When the polarity was changed did the shutter shift to the opposite direction of operation or did the shutter disappear and the motor operated w/o the shutter in both directions of operation? I believe Wales power differs from US in that the line to ground is 230V. The US 240V power is 2 hot lines and each line to ground is 120V but opposite in polarity. I would assume that there is a difference in controllers and getting local help is the best bet.
 
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Joe - I'm not sure what a shutter is! Two nations divided by a common language... I'd definitely get local help if I could but where i am, I can't find any motor specialists. I did have a local household electrician look at the switch a while back, but beyond that he didn't dare venture.
Rusty - no, it's just one direction, reverse was a wiring error that, fwiw, turned up a bit of info - that it runs well in reverse.
 
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Chris,
Sorry, I meant to use your term stutter. Am I correct, the motor now stutters in the foreword (normal) rotation and not in the reverse rotation? If you reverse polarity of the wiring to the motor (live/ neutral wires) using the controller terminal output connections to the motor is probably the easiest. Does the motor now stutter in the reverse operation of the spindle and operate correctly in foreword rotation. In other words, does the problem shift with changing the wiring polarity to the motor or does it completely disappear? I was under the assumption that the problem disappears. I would expect the problem to shift with the wiring polarity change. Is the foreword / reverse switch a logic input to the controller (which I would expect) or actually a separate switch switching the polarity of the output from the controller to the motor?
 
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Thanks Don.

Well, this is turning out to be a steep learning curve for me! And I thought learning to turn was tricky.
So, I don't have a 3ph motor, which is what I just assumed, and therefore don't have a VFD. And whatever the motor, I just assumed it was brushless.
Instead, I have a single phase DC motor that's brushed, and a converter with speed control. (I'm writing that so I can be corrected if I have it wrong!)

Given that, and that the motor runs properly in reverse, can I now exclude the controller as being the issue?
I'm really sorry for being so slow - to get this fixed I'll need to package the motor up and send it off to be repaired and want to get as close to knowing the likely cause before I do so. Whether it's the brushes or not, if the fault's in the motor it needs sending off to be repaired.
The cover on the back end of the motor may be an access to the brush on that side and there should be a similar cover on the opposite side so with the power line off remove one of those covers and it may expose the brush mount. The brush probably could be removed for inspection or it may be visible and if in good condition it should be around 12 MM long.
The DC controller does not care what direction the motor is running all it does is provide a voltage to the motor so if the speed dial is set to 100% then 180 volts DC will go to the motor, if set to 50% it will be 90 VDC and if set to 25% it is 45VDC. The fact that the problem went away when the motor was reversed means it does not have anything to do with the controller.
 
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Thanks all.
Joe - ha, I even went searching on the net for US use of the word 'shutter' in relation to electronics! It is a one-direction lathe, no reverse switch. Runs fine in reverse (after rewiring) but stutters when turning in the correct direction.
Don - that's clear, thanks - there are two access points to the motor as you suggest, I'll take a look today. Having looked around the net I found the video below - my motor looks exactly the same, and it turns out that Dart Controls' motors (as in the vid) are in fact Leeson motors. Swapping the brushes looks a simple job (assuming I can get them here, no luck so far), as does inspection.
Gerald - I'll take a look at the belt again today. I'm surprised you have this lathe over there, in my mind it's a very British one (Sheffield), but as I've looked into this problem I've realised how American it is, with both Dart and Leeson doing all the hard work. Obviously, my surprise is based on complete lack of knowledge!
View: https://www.youtube.com/watch?v=U1bpQuOqrZ4
 
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I took the brushes out to inspect just now. One in particular has chips off the edge, is that potentially enough to cause the problem?
I've also included the best shot I could get of the inside, in case there's any info there - there's some discolouration, but I guess that's normal?
 

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I took the brushes out to inspect just now. One in particular has chips off the edge, is that potentially enough to cause the problem?
I've also included the best shot I could get of the inside, in case there's any info there - there's some discolouration, but I guess that's normal?
The chipping and burn on the contact edge of the brush usually indicates arcing when the brush passes a shorted armature coil and the discoloration on the commutator bars as seen in the lower portion of the photo 0216 also indicates the arcing. The contact face on the brush of a healthy motor should look like the bottom half without the burned & chipped area. That explains why it did not stutter in reverse. The final test is to reinstall the brush and with the covers off run the motor and you will likely see arcs tracing around the commutator. The bad news is it would cost more then the motor is worth to have the armature rewound even if you could find someone who could do it.
 
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Thank you, Don. Not good news, tho I just removed the cover as suggested to watch for arcing, and lo and behold it's running smoothly at all speeds (no arcing). It's been consistently stuttering at speed the past few days so I expect it to return, will report back!
I think a new motor is about £500 (I can't find exactly the same) - is rewinding really that expensive?
 
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Is it typical that the fault would be intermittent? It's running fine now, just did a quick bowl in apple, pic attached!

The lathe's been in storage since the start of Covid as I couldn't get into the workshop and lost the plot somewhat - if nothing else, turning that bowl has reminded me of how wonderful turning is. I'll get it fixed, even if it's a new motor. :)
 

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OK, I'm on the same page now, no reversing switch. About the only further diagnostic that I can think of that you can do is to load the motor by forcing a board against a cylinder of wood to load the motor and observe the commutation or arcing of the brushes.

If you ask a motor repair shop if they have an armature growler and you get a questionable or no response you are probably wasting your time. An old established auto repair shop may still have one. They were often used in automotive generator servicing until the mid 1960's when alternators replaced generators. With the growler and a little experience it is easy to determine if the armature of the motor or generator has a open or shorted windings. The bad news is, with either defect you are basically out of luck. Rewinding and associated rebalancing is much more difficult than with AC motor stator coils.

Growler example: https://www.indiamart.com/gemotors/growler-tester.html
 

Dave Landers

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Thank you, Don. Not good news, tho I just removed the cover as suggested to watch for arcing, and lo and behold it's running smoothly at all speeds (no arcing). It's been consistently stuttering at speed the past few days so I expect it to return, will report back!
In my mind, that does seem to point at either the brushes or maybe the commutators. Taking them out and re-installing them may have caused them to re-seat and behave (at least for a while).
 
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I'd wonder if, when the brushes were taken out to look at them, did they come out freely, or did one or both seem a little bit tight or sticky? Old motors that sat for long time, especially in an environment where they can pick up moisture, the brush holders can get a little corrosion (or corrosion causing them to deform a little), or old lubrication dries out, among other little things that can happen to motor brushes.. anything like that , that might have been "fixed" simply by removing and re-installing the brushes, could have been the cause of the problem...
 
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Chris the one we had the owner had modified it. I do not know what the reason was but probably motor failure. He had replaced the motor with a very heavy 220v motor and a VFD. That may be more than you want to spend, but just to show that you can adapt a different motor.IMG_3557.JPG
 
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From the pictures there seems to be a bit of graphite dust around.

If it's working leave it alone.

If it starts again blow the dust out and try again, but note the direction of the brushes on the commutator. they wear in with time and do not like to be rotated 180 deg.

As noted in the pictures there are wear tracks on the commutator and it looks like some areas may be close to being shorted due to this wear. A motor shop can remove the armature and turn the commutator and clean the insulated spaces. This is after tests for shorting prove negative. New brushes will then be required.

Good luck,

Stu
 
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From the pictures there seems to be a bit of graphite dust around.

If it's working leave it alone.

If it starts again blow the dust out and try again, but note the direction of the brushes on the commutator. they wear in with time and do not like to be rotated 180 deg.

As noted in the pictures there are wear tracks on the commutator and it looks like some areas may be close to being shorted due to this wear. A motor shop can remove the armature and turn the commutator and clean the insulated spaces. This is after tests for shorting prove negative. New brushes will then be required.

Good luck,

Stu
I had thought about asking him if he had maybe rotated the brush 180 degrees which would probably have the same effect as reversing the motor.
 
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Thanks all.

The brushes went back in the same way, Don.
Stu - yes, I cleaned some of the dust out but, not knowing what I'm doing, didn't want to probe too far inside. I don't have a blower but have a vac so gave it a go with that after a cloth.
Brian - one was a bit tighter than the other to remove, here's hoping! Tho I'm not holding my breath.
Joe - the more I think about it, the more it's probably a question of waiting to see now - that definitely sounds expensive (there's nobody near here to do the work, either).
I'll let it run and see if it starts to happen again and if it does (and if I can then see shorting inside), I'll buy the same motor. I guess the one you have in the club is better specced, Gerald, but I really like the lathe as it is and can't help but see it as a beautiful object as Sorby made it! I've written to a Leeson supplier over here to ask for a price, we'll see how bad it looks when they get back to me.

I really appreciate all the help you've offered, everyone.
 
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Common practice for a DC motor with brushes is to clean and file the brushes flat and then clean the commutator on the rotor. If the commutator bars are rolled over and shorting out between the separate bars you will need to resurface the commutator. Common practice for this repair is to use an emery board and clean up the rolled over edges and clean the metal contamination between the commutator bars, some motor you can remove the rotor assembly chuck it up on a lathe and use a cutter and take several thousandths off of the commutator until they are flat and then clean between the commutator bars and the motor winding armatures to make sure they are not shorting out from conductive filings and carbon. Re-assemble and it should run properly. You can also measure the insulation values on your windings to determine if they are failing.
 
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Thanks Mike. Oddly, the motor continues to run without issues for the moment - I've turned four bowls in the last few days, including half logs (so bumpy), no stuttering. Maybe just removing the brushes and refitting, as suggested above, has cleared the problem.
If/ when the problem returns, I've now been recommended a reliable repair place not far away that I can easily drive to - tbh, I don't trust myself to take the motor apart, always best to recognise your limits in these matters! That said, I've learned a lot about the setup on my lathe in particular and motors in general from this thread and my reading around, trying to understand what people are saying. All good.
 
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