Synchronous AC motor for leadscrew

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jesusfwrl
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Synchronous AC motor for leadscrew

Post: # 37037Unread post jesusfwrl
Wed Sep 02, 2015 6:00 am

Is there any reason why one shouldn't use a synchronous AC motor with a variable frequency drive unit, to drive a leadscrew on a lathe? The idea is to use a belt and pulleys to get a reduction so the motor can spin faster than the leadscrew.

Is there anything speaking against it?
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Re: Synchronous AC motor for leadscrew

Post: # 37039Unread post mossboss
Wed Sep 02, 2015 8:00 am

May be a DC permanent magnet motor or an ac motor with a built in reduction box
A straight out ac motor spinning at 1440 rpm you will need some serious size pulleys
Keep in mind that reduction of speed via a simple speed control reduces power output proportionally so it will have to de a reasonable size motor to start with if you are to reduce speed via an scr control
Why don't you try a simple spit motor there are easily obtainable particularly where you are they are cheap enough and quite powerful
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Chris

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rsimms3
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Re: Synchronous AC motor for leadscrew

Post: # 37040Unread post rsimms3
Wed Sep 02, 2015 8:07 am

Prestos all use synchronous AC motors. Did you see Brian's post on the subject? I actually just researched the topic last night because I bought a piece of medical equipment that is a feedscrew with rails attached to a set of reduction gears with various speeds. It's called an infusion withdrawal syringe pump. Seems to be used in experiments where animals are slowly infused with something and automates the process. For $20 it definitely look cool enough to buy at the local college surplus store. It has beveled stops that raise the sled off of the feedscrew to stop infusion. I'm looking forward to tinkering with it. Anyway, it uses a Bodine synchronous motor with essentially the same specs at the older Presto motors so if nothing else, I've got a potential back up motor. This piece of equipment is also wired for movement in either direction.

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Re: Synchronous AC motor for leadscrew

Post: # 37041Unread post markrob
Wed Sep 02, 2015 9:36 am

jesusfwrl wrote:Is there any reason why one shouldn't use a synchronous AC motor with a variable frequency drive unit, to drive a leadscrew on a lathe? The idea is to use a belt and pulleys to get a reduction so the motor can spin faster than the leadscrew.

Is there anything speaking against it?
Hi,

An AC induction motor with a VFD will work fine with the correct gear reduction on the shaft, of course. You can easily get a 20:1 speed range. I would not use a synchronous AC induction motor for the job. First off, you don't need super accurate speed control for this function. Secondly, these motors typically come with a capacitor matched to the expected line frequency to provide the needed phase shift. AFIK, they can't tolerate a large range of speed variation ( line frequency) from the design center. A standard AC induction motor can tolerate this as long as you operate it with a V/F style drive. That is, you keep the ratio of line frequency to line voltage constant over the speed range. In effect, you drop the driving voltage in proportion to the drop in line frequency. If you only vary the line frequency, you will see that the motor will draw increasing current at lower speeds. This will eventually burn up the motor.

The other issue is the required speed range if you want to run over the entire LPI range required in a cutting environment. When cutting, you need a range of something on the order of 75-400 LPI. This represents about a 5:1 speed range. No problem. Cutting lead in and lead out groove pitches opens up a can of worms as you need to get down into the 4 LPI range. That implies a speed range without gear change in the 100:1 range. That not a trivial thing at all. You could compromise on each end and limit the range to a more reasonable 25:1 if you limit to 10-250 LPI, for example. Otherwise , I suspect that you would need some sort of gear ratio change to cover that wide of range. In a hobby DIY systema 25:1 range with no gear change seems reasonable.

I don't deal with pro systems, so I don't know how this is done on these lathes. Maybe somebody that has such a system can let us know how this is accomplished. I understand they use a permanent magnet DC motor with tach motor feedback (no encoder). But I'm not sure they can attain a 100:1 speed range with no gear change.

Mark

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Re: Synchronous AC motor for leadscrew

Post: # 37043Unread post jesusfwrl
Wed Sep 02, 2015 12:12 pm

Mark, Neumann uses two motors to achieve this, one for the standard cutting LPI range and one for the faster speed for lead in, lead out and spirals. I don't have such a system but I'm very much interested in the technology behind them. I don't know what type of motors the Neumann lathes used. The Scully looks more complicated and I have no idea what they are doing on the motor side. What I have seen is that a shaft goes in at the bottom side under the carriage and it is belt driven by a motor, but there is several belts there and I haven't spent enough time on a Scully to find out what each one does.

Which Presto system are you referring to? The 6N and similar only have one motor that powers the turntable. The leadscrew is driven from the platter. The original Fairchild system has a fairly large motor driving the platter with a separate power take off shaft driving the leadscrew. LPI changes are effected by manually shifting gears. Spirals and lead outs are done manually. When I say manually, I really mean it: You quite literally grab the carriage with both hands and slowly and steadily move it at the desired speed immediately after disengaging the half nut. A bit primitive but it works. I usually wear my tarzan outfit while doing this for effect...

Chris, I was considering a synchronous motor with a fairly large number of poles. 12 or 24 poles, to be precise. This would already give me a rotational speed of 500 or 250 rpm at 50 Hz. Also, if a motor turning at say 500 rpm drives a shaft with a 10 to 1 reduction ratio, so that the shaft spins at 50 rpm, you are effectively reducing the speed while at the same time increasing the torque by a factor of 10, minus the losses due to friction and so on. So, theoretically you would need a much smaller motor with a reduction than you would need if you would be driving it in a direct drive configuration.

Also, I was thinking of the hysterisis type synchronous AC motors, which do not require capacitors. I have no idea what the speed range of such motors would be when driven by a VFD.

The main reason I am considering an AC motor with a variable frequency drive system is that I am thinking of designing and building the drive system myself so I can also integrate variable pitch functionality, while keeping the circuit simple and in the analog domain. This system would only be varying the pitch since my lathe would need a serious amount of modification to the suspension to implement variable groove depth. I'm not sure if it would be worth implementing variable groove depth, but it would definitely make things easier to be able to make spirals and lead out with the press of a button instead of doing it like a caveman...

I will look into spit motors.
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Re: Synchronous AC motor for leadscrew

Post: # 37044Unread post jesusfwrl
Wed Sep 02, 2015 12:35 pm

Chris, do you really mean the motors used for roasting lamb during Easter???

I didn't get it at first, I thought it was some kind of motor I don't know much about...

There are several varieties of these, but the common concept is that they have an integrated reduction gear system, which is usually so noisy that I can probably even taste the vibration in the lamb afterwards! I really can't see how this would be useful in a lathe. :lol:
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studiorp
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Re: Synchronous AC motor for leadscrew

Post: # 37045Unread post studiorp
Wed Sep 02, 2015 3:29 pm

I once I had tried to put a dc little gear motor but all vibrations goes to lathe and then on disc.
I think that the best solution could be have a motor with low number of rpm without gears, similar to quartz dd motor of a turntable, with proper electronic controller. These motor are super silent, no vibrations and precise, but as adapt a similar motor to a leadscrew system of a lathe ?

Otherwise use an external motor fixed out from your lathe and with a pulley-belt system you can solve; obvious that remain the choice of which motor utilize, but I think that a normal 1500-1800 rpm sync. hysteresis that has a good torque could be ok.

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Re: Synchronous AC motor for leadscrew

Post: # 37050Unread post opcode66
Wed Sep 02, 2015 6:29 pm

Gear reduction on either the feed motor or the platter motor is unacceptable due to noise transmission. Unless you like to hear little clunking noises in unmodulated grooves. Markrob's standards might not be the same as a Neumann user, fyi.
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Re: Synchronous AC motor for leadscrew

Post: # 37053Unread post smithadamm
Wed Sep 02, 2015 6:43 pm

opcode66 wrote:Gear reduction on either the feed motor or the platter motor is unacceptable due to noise transmission. Unless you like to hear little clunking noises in unmodulated grooves. Markrob's standards might not be the same as a Neumann user, fyi.
I'm in agreement with most folks on here that there is a better motor for the job than a synchronous motor.

As far as the reduction gear issue, I've seen that solved two ways, the reduction gear on the feedscrew of my lathe is a 1/2" rubber pinch roller (capstan idler) from a tape machine. The rubber damps out any motor noise and it also makes for a smooth surface so there aren't clicks from a gear. It came that way stock. I had to make some modifications to be able to adjust the pressure of the idler against the motor shaft, but even without that, it worked well 99% of the time.

I've also seen the reduction done with a chain or belt, I assume that would damp out pitch motor noise and keep clicks and clunks from being transferred to the unmodulated grooves.

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Re: Synchronous AC motor for leadscrew

Post: # 37054Unread post opcode66
Wed Sep 02, 2015 6:49 pm

Rubber rollers aren't exactly perfectly quiet either. Though a descent solution. The best reduction scenario is in fact a belt.

I am working on a motor design. Once I can machine metal, these will be in production. 16 coils, 4 magnets, bldc, custom motor control logic with complex pulsing patterns, two forms of feedback (optical rotary and input from non-pulsing coils). This will be extremely high torque and extremely accurate direct drive.
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Re: Synchronous AC motor for leadscrew

Post: # 37061Unread post EmAtChapterV
Wed Sep 02, 2015 10:32 pm

jesusfwrl wrote:Mark, Neumann uses two motors to achieve this, one for the standard cutting LPI range and one for the faster speed for lead in, lead out and spirals. I don't have such a system but I'm very much interested in the technology behind them. I don't know what type of motors the Neumann lathes used. The Scully looks more complicated and I have no idea what they are doing on the motor side. What I have seen is that a shaft goes in at the bottom side under the carriage and it is belt driven by a motor, but there is several belts there and I haven't spent enough time on a Scully to find out what each one does.
Neumann pitch motors are a good size, about as big as a 50 or 75 pack spindle of DVDRs, and have a tachometer sensor attached to the opposite side of their driveshafts. They both feed into a gearbox that switches between the two - I've never had a look inside so I'm not sure how it does it but I'm assuming it's some kind of overrunning sprag clutch.

On non-LS76 Scullys, everything is driven off the one 1800 rpm synchronous motor, both platter and leadscrew. The fixed-pitch 501s have a series of gears arranged in a cone shape; the variable-pitch 601s have a servo-controlled idler running between two counter-rotating hemispherical pulleys. In both cases for lead-in and lead-out a relay triggers the sled to disengage from the feed nut and be driven forward by some other system I haven't quite figured out yet - I've seen the gear, screw and pulley layout in the lathe base, but not the underside of the sled.

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Re: Synchronous AC motor for leadscrew

Post: # 37063Unread post opcode66
Thu Sep 03, 2015 1:23 am

It is an oil filled clutch. It doesn't add any appreciable noise as a gear reduction does.
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Re: Synchronous AC motor for leadscrew

Post: # 37064Unread post Deke Dickerson
Thu Sep 03, 2015 1:27 am

I saw a 1940's Scully lathe in action in Minneapolis where they had adapted the leadscrew to be controlled by a variable servo motor. It worked quite well. The way the Scullys are designed originally, the leadscrew belt ran off the turntable shaft, which creates more drag on the turntable shaft and seems like a bad idea in design. The servo motor seemed to do the job well. They had a belt running between the servo motor and the leadscrew shaft.

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Re: Synchronous AC motor for leadscrew

Post: # 37065Unread post markrob
Thu Sep 03, 2015 8:30 am

EmAtChapterV wrote:
jesusfwrl wrote:Mark, Neumann uses two motors to achieve this, one for the standard cutting LPI range and one for the faster speed for lead in, lead out and spirals. I don't have such a system but I'm very much interested in the technology behind them. I don't know what type of motors the Neumann lathes used. The Scully looks more complicated and I have no idea what they are doing on the motor side. What I have seen is that a shaft goes in at the bottom side under the carriage and it is belt driven by a motor, but there is several belts there and I haven't spent enough time on a Scully to find out what each one does.
Neumann pitch motors are a good size, about as big as a 50 or 75 pack spindle of DVDRs, and have a tachometer sensor attached to the opposite side of their driveshafts. They both feed into a gearbox that switches between the two - I've never had a look inside so I'm not sure how it does it but I'm assuming it's some kind of overrunning sprag clutch.

On non-LS76 Scullys, everything is driven off the one 1800 rpm synchronous motor, both platter and leadscrew. The fixed-pitch 501s have a series of gears arranged in a cone shape; the variable-pitch 601s have a servo-controlled idler running between two counter-rotating hemispherical pulleys. In both cases for lead-in and lead-out a relay triggers the sled to disengage from the feed nut and be driven forward by some other system I haven't quite figured out yet - I've seen the gear, screw and pulley layout in the lathe base, but not the underside of the sled.
Thanks for the explanation. Todd indicates that there is no gear reduction on the motor shaft or anywhere in the path from motor to driveshaft. If this is true, what type of motor is used and what is the shaft speed? Is the reduction done entirely via belts and pulleys? What is the pitch of the feedscrew used?

Mark

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Re: Synchronous AC motor for leadscrew

Post: # 37066Unread post mischmerz
Thu Sep 03, 2015 10:32 am

jesusfwrl wrote:Is there any reason why one shouldn't use a synchronous AC motor with a variable frequency drive unit, to drive a leadscrew on a lathe? The idea is to use a belt and pulleys to get a reduction so the motor can spin faster than the leadscrew.

Is there anything speaking against it?
Why not just take a stepper motor?

mm.

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Re: Synchronous AC motor for leadscrew

Post: # 37067Unread post gold
Thu Sep 03, 2015 10:33 am

The Neumann gear reduction is in a sealed gear box that also has the differential (i think that's the right term) that sums the pitch motor and the fast run motor.

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Re: Synchronous AC motor for leadscrew

Post: # 37068Unread post jesusfwrl
Thu Sep 03, 2015 10:36 am

Let's get more specific:
the feedscrew in the fairchild has a pitch of 35 tpi. This means that you need 11.67 rpm of the leadscrew for 100 LPI, or 5.83 rpm for 200LPI. This would be reduced to 2.33 rpm at 500LPI and 233.3 rpm for a lead out pitch of 5 LPI.

So, if I was going to add a motor in a direct drive configuration to the leadscrew this motor would need to be able to spin within a range of 2.33 rpm to 250 rpm or so. With the construction of the overhead the biggest reduction ratio that can be achieved with pulleys is approx. 5:1., of only 2 pulleys are to be used. This would mean that I could then use a motor with a range of about 12 to 1250 rpm. If this would not be practical to achieve with a single motor, then there would be a need for two motors. There is also the option of adding four pulleys instead of two to get a greater reduction ratio, but this seems like overkill for the application.

I see that a synchronous AC motor would actually not be very practical for this range unless there is a complicated gear reduction system for it. A 24 pole motor that would run at 250 rpm at 50 Hz would actually need to be run at 0.5 Hz to spin at 2.33 rpm, which I doubt would work. Or would it? Even if I would limit my LPI range to 300 LPI it would still need an AC motor that would be able to run with a line frequency of 0.8 Hz or so. If I would use a DC motor instead I could only achieve this range using a gear head on the motor or a large reduction ratio using multiple belts. I have not yet found a motor that would spin at 2 rpm with enough torque and would also be able to spin at 250 rpm with no problems. If anybody is using a motor with a similar range, would you like to share with me some details on what you are using?

Todd, I am very much interested in seeing your progress regarding the motor for the leadscrew.

Regarding torque, I've no idea how much torque is enough torque and how this could be easily translated to motor ratings in Watts but I'm trying to figure this out. On the Fairchild, there is really low resistance when turning the feedscrew by hand with the half nut engaged. I do not have any torque measuring tools that would register such low values. It is definitely below 1 Nm.
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Re: Synchronous AC motor for leadscrew

Post: # 37070Unread post markrob
Thu Sep 03, 2015 11:26 am

gold wrote:The Neumann gear reduction is in a sealed gear box that also has the differential (i think that's the right term) that sums the pitch motor and the fast run motor.

Thanks! So there is a gear reduction present. That makes sense. There are high quality, low noise DC servo gear motors available, but they are not cheap (you get what you pay for). I don't know if Neumann rolled their own gear box or if they used something off the shelf, but either way I'm sure if was not low cost.

Some here have been using a stepper as a poor man's 2 phase sync motor (coils driven by phased sine waves). That has promise, and might just work at the low shaft speeds you calculate are needed.

As far as HP goes, it gets interesting if you intend to try a 100:1 range. HP is proportion to torque x rpm. So at ultra low shaft speeds, the HP is very low. However if you keep the torque the same at your high speed HP will be up 100X over the cutting speed. That sounds bad, but you don't spend much time at the lead-in lead-out function so you can under size quite a bit on the rating.

To measure the required torque just hang an a weight tied to a string from a pulley of known diameter and find the required weight needed to make the shaft move. Torque is just the radius of the pulley x mass of the weight (watch the units e.g oz-in). From this double the measured torque just to have some safety margin. Figure your max shaft speed and use:

Horsepower = Torque (oz-in) X RPM / 1008406

to estimate the worse case HP. If you go with a DC PM motor, torque is directly proportional to motor current and heating is based mostly on I^2 R loss in the windings. If you are using a gear reduction, the torque at the motor shaft is reduced by the gear ratio and the shaft speed is increased by the same amount (so HP is constant).

Direct drive with a brushless DC or 3 phase AC induction motor sounds difficult to pull off, but might just work. I would scale is based on the top speed running 10:1 over its 60 Hz rating (600 hz) and 1:10 on the low side. That would give you a better low frequency drive option 6 Hz - 600 Hz. Not sure if that is realistic either.

Mark

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Re: Synchronous AC motor for leadscrew

Post: # 37072Unread post EmAtChapterV
Thu Sep 03, 2015 12:41 pm

markrob wrote:
Thanks for the explanation. Todd indicates that there is no gear reduction on the motor shaft or anywhere in the path from motor to driveshaft. If this is true, what type of motor is used and what is the shaft speed? Is the reduction done entirely via belts and pulleys? What is the pitch of the feedscrew used?

Mark

IIRC the Neumann feedscrew is 26 threads per inch. The output of the gearbox tops out at around 300 to 315 rpm for the coarsest available leadout pitch of 2.9 lpi at 33 1/3 and 6.4 lpi at 78. The fast pitch motor is directly coupled to the gearbox on the opposite side from the output shaft; the recording pitch motor below it via a belt at a 90 degree angle at the back. That way they both fit inside the "doghouse" with the pitch meter and controls.

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Re: Synchronous AC motor for leadscrew

Post: # 37079Unread post opcode66
Thu Sep 03, 2015 3:58 pm

Hmmm. I was unaware that there was a gear reduction in the clutch box as well. Interesting. I was wrong. Al described it to me as simply a two gear clutch.

The gear reduction must be extremely precision made. It imparts no noise. I didn't even know it was in there... LOL It is nothing like the gear reductions you can commonly purchase for a standard motor. Those are always noisy in my experience.
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