I did some experiments with VCR drum motors. The ones I tested used a commodity BLDC motor driver, which generates very uneven torque at low speeds. I believe this was the fault of the driver, rather than the motor. In about a month, I expect to try again with a linear driver and pure sine waves, but I have other things to get out of the way first.
In my experiments, I was trying to use it as a DD motor, but couldn't get it to rotate slowly enough. I did get quite smooth rotation at speeds that would work for belt drive, and that was with the commodity driver chip.
The VCR motors are not like regular stepper motors. They have no iron in them, so they spin freely even though they have permanent magnets. The stator coils are usually trapezoidal in shape, spread out flat under the rotor. In a stepper, the stator surrounds the rotor.
In my experiments, I was trying to use it as a DD motor, but couldn't get it to rotate slowly enough. I did get quite smooth rotation at speeds that would work for belt drive, and that was with the commodity driver chip.
The VCR motors are not like regular stepper motors. They have no iron in them, so they spin freely even though they have permanent magnets. The stator coils are usually trapezoidal in shape, spread out flat under the rotor. In a stepper, the stator surrounds the rotor.
Right. They have their own category. It's called BLDC.
Yes, and no.
My intuition is that they will make excellent belt drive motors. But I don't want to make any definite claims yet. I'm thinking of using a method similar to yours (using the dremel to grind the pulley) to machine the drum of the VCR motor so it can drive a platter. They go to great lengths to make the drum as slippery as possible, for obvious reasons, but methinks slipperiness can be overcome. Probably with sorbothane. Or Corian.
What does that stand for?
Mine as well..truth be told, I was originally going to use the head motor out of a 3/4" commercial VTR that had been promised to me. Turned out the donor was in no hurry to get it to me, and while waiting, I convinced myself that setting up a more conventional motor would be easier..yea..that worked out
I quess it depends on how slowly you can run the motor smoothly, and not loose it's torque. All the heads I've seen only had about 3/8" of meat around the periphery, the rest is a void for the motor hub.. if you can get it down to 200 rpm or so, you should be ok...if not, email me, I may be able to help out.
Probably with sorbothane. Or Corian.
O.K....I deserved that
-Casey
Ah..I'm familiar enough with Brushless DC, just wasn't connecting it to the acronym. I'm not sure the fact that it's BLDC (see, using it already
) has much to do with the pole/coil shape in the drum motor. BLDC is used to describe a family of motors that are, I believe, synchronous with an integrated DC-AC converter. I always thought some marketing guy came up with the name.-Casey
Tht's right. EC8010 above noted that steppers had pole shapes that tend to produce cogging, and thought that VCR drum motors were steppers. I'm just explaining that they are quite different beasts really, and that there are good reasons to believe that VCR motors would be OK where steppers would not.
That's interesting to hear about drum motors. But what about the capstan motor? That's designed to rotate at lower speed and without a hint of cogging, otherwise the heads wouldn't be able to lay video tracks adjacent to one another. And it has a nice small shaft (designed to take a sideways load) onto which you can fit a pulley . Or, as you're using video tape as your belt, perhaps you just need to press a pinch roller up to it?
Nice idea, but you do need a larger pulley if you want to use the standard control circuits.
WARNING: MATHS AHEAD
PAL VHS moves at 142cm/ min. If you run the tape around a 30cm platter (circumfernce= 94.24 cm), the platter will turn at 142/92.24= 1.5 rpm.
Therefore, you need a pulley that it (33 1/3 / 1.5)= 22.2* original diameter for 33 1/3 rpm, and (45/1.5)=30 * original diameter.
Or run the motor at 22.2* and 30* original speed with new circuitry.
I like the idea of this though.
James
p.s. these are back-of-fag-packet type calculations- don't complain to me if you do this and your TT runs fast or slow.
p.p.s. What type of bearings does the head run on? I am guessing miniture ball races.
I've noticed a bit of variation in the capstan motors. They all seem to have a large (i.e. 2 to 3 inches) diameter rotor, with the magnet around the rim. I think this is to take advantage of the flywheel effect to help maintain constant speed. They also have high torque (relative to the head motors). Those I looked at all had a special housing / motor mount that partially encloses the shaft and privides a small bearing at the far end. This allows it to take radial loads. If you wanted it to drive a TT, you would have to attach a pulley, and that would mean removing the housing. It would be possible, but you would need to provide a bearing for the far end of the shaft. In some cases it would be possible to glue a pulley to the back of the rotor, and keep the shaft housing/bearing intact.
The head motors use ball bearings. Very high quality, very quiet, but nevertheless potentially subject to bearing chatter. What effect that would have on the sound (given that we're talking belt drive) I don't know.
I like the idea of using the pinch roller, and video tape as a belt. It's usually not too difficult to control the speed of the capstan motor, although 30x the original speed might be asking a bit much.
Casey, are you using video tape as a belt, or some other tape?
The head motors use ball bearings. Very high quality, very quiet, but nevertheless potentially subject to bearing chatter. What effect that would have on the sound (given that we're talking belt drive) I don't know.
I like the idea of using the pinch roller, and video tape as a belt. It's usually not too difficult to control the speed of the capstan motor, although 30x the original speed might be asking a bit much.
Casey, are you using video tape as a belt, or some other tape?
Just one problem here..the splice is thicker than the rest of the belt. It will make noise as it passes through. How much, and your tolerance for it are the variables.
My first experimental belts were made out of VHS tape. It was too butt-ugly. I found that this stuff worked as well (it should, as it's essentially he same thing), and looks very nice. I bought a roll of the "silver" and "fire red" to try..I'll be using the "silver".
-Casey
All this fighting over belts...
Why not be idle, or do idle..? The drum would take a nice smooth nitrile O-ring of an appropriate diameter, and not slip much to noted. If the drum was mounted ala Lenco, you could vary the position from the centre of the platter to change the platter speed
Sure it wouldnt be as silent as a belt drive motor, but a light axial load will nicely stop chatter on the bearing
Just a few thought-ettes
Owen
Why not be idle, or do idle..? The drum would take a nice smooth nitrile O-ring of an appropriate diameter, and not slip much to noted. If the drum was mounted ala Lenco, you could vary the position from the centre of the platter to change the platter speed
Sure it wouldnt be as silent as a belt drive motor, but a light axial load will nicely stop chatter on the bearing
Just a few thought-ettes
Owen
It's the end of the weekend, and this is when I would normally post a series of photos/descriptions of the progress made over the last couple of days..fate, I'm afraid, had other plans.
Wednesday evening, I was in a bit of a hurry when I got home from work, and while walking from the car on the glacier that is our driveway, I lost my footing and went down. Unfortunately for me, I flipped sideways and landed on the Leatherman on my belt, which in turn drove the ball of my hip into the socket, smooshing the cartilage lining it. The prognosis is optimistic, and I should be ok..in time. Meanwhile, I get around on crutches. Needless to say, progress will slow down, but not stop, for awhile.
I spent Saturday evaluating how best to proceed. I have been focused on the motor for so long, I needed to review the project as a whole, and lay out the order of the remaining tasks. Naturally I'm dieing to see the new motor spinning my platter, but realized I needed to finish the platter work first. I never surfaced the bottom of the platter, but after I cut the strobe pattern, it became obvious that I needed to..it has a about a .02” variance in thickness, and as it spins around with the pattern, it really sticks out. Cutting the bottom flat will increase the gap between it and the plinth, requiring me to lower the platter, which will change the height of the belt groove. I can't finish the motor mount until this is done.
Enough time has lapsed since I spent a bagillion hours cutting the strobe pattern, that I can be honest with myself...I can't live with the slight wobble it has. In addition, I found that getting my motor to spin up fast enough for 78 rpm, requires more voltage than I'm comfortable feeding the motor, so I decided to go with just 33/45 ( I can always cut a pulley for 78 later). Troubleshooting my dividing setup, I determined the wobble came from the alignment (or lack there of), rather than the printed pattern I used on the dividing circle.
During the time I wasn't working on the table this last year, I pretty much finished a dividing head project I have been working on, off and on, for years. Today I drilled the dividing plate I need for the strobe divisions. This is what I will use to cut my new pattern...
... I still need to fabricate the arbor, and tail stock, but this will be cake compared to making this head.
I should be healed up enough by next weekend to dive back in, in earnest.
-Casey
Wednesday evening, I was in a bit of a hurry when I got home from work, and while walking from the car on the glacier that is our driveway, I lost my footing and went down. Unfortunately for me, I flipped sideways and landed on the Leatherman on my belt, which in turn drove the ball of my hip into the socket, smooshing the cartilage lining it. The prognosis is optimistic, and I should be ok..in time. Meanwhile, I get around on crutches. Needless to say, progress will slow down, but not stop, for awhile.
I spent Saturday evaluating how best to proceed. I have been focused on the motor for so long, I needed to review the project as a whole, and lay out the order of the remaining tasks. Naturally I'm dieing to see the new motor spinning my platter, but realized I needed to finish the platter work first. I never surfaced the bottom of the platter, but after I cut the strobe pattern, it became obvious that I needed to..it has a about a .02” variance in thickness, and as it spins around with the pattern, it really sticks out. Cutting the bottom flat will increase the gap between it and the plinth, requiring me to lower the platter, which will change the height of the belt groove. I can't finish the motor mount until this is done.
Enough time has lapsed since I spent a bagillion hours cutting the strobe pattern, that I can be honest with myself...I can't live with the slight wobble it has. In addition, I found that getting my motor to spin up fast enough for 78 rpm, requires more voltage than I'm comfortable feeding the motor, so I decided to go with just 33/45 ( I can always cut a pulley for 78 later). Troubleshooting my dividing setup, I determined the wobble came from the alignment (or lack there of), rather than the printed pattern I used on the dividing circle.
During the time I wasn't working on the table this last year, I pretty much finished a dividing head project I have been working on, off and on, for years. Today I drilled the dividing plate I need for the strobe divisions. This is what I will use to cut my new pattern...
An externally hosted image should be here but it was not working when we last tested it.
... I still need to fabricate the arbor, and tail stock, but this will be cake compared to making this head.
I should be healed up enough by next weekend to dive back in, in earnest.
-Casey
Hi Aengus,
Thanx for the sentiment. According to my 'Doc', I'm actually lucky. Turns out, this sort of injury is fairly common...except it's usually a pager on the belt providing the impact point. He informs me that he has seen shatterered joints from a plastic pager, the fact that I landed on a, more or less, solid hunk of steel, and got away with just some damaged cartlidge is somewhat of a miracle. Being 6'2" (making the fall further than average), and 240 lbs. (making the impact force greater than average), makes it all the more amazing that I got away as little damage as I did.
I'm mending quickly..with any luck I should be able to limp around without a crutch in a few more days.
-Casey
Thanx for the sentiment. According to my 'Doc', I'm actually lucky
. Turns out, this sort of injury is fairly common...except it's usually a pager on the belt providing the impact point. He informs me that he has seen shatterered joints from a plastic pager, the fact that I landed on a, more or less, solid hunk of steel, and got away with just some damaged cartlidge is somewhat of a miracle. Being 6'2" (making the fall further than average), and 240 lbs. (making the impact force greater than average), makes it all the more amazing that I got away as little damage as I did.I'm mending quickly..with any luck I should be able to limp around without a crutch in a few more days.
-Casey
Hi James,
Thanx for the well wishes.
Thanx..It was started over ten years ago! I didn't have enough sense at the time to realize it was over my head (then). Over the years, when I came to a point where I needed one, I would dig it out and do some more work (re-do some more work usually) on it. My strobe pattern being the last big push to finish it up.
-Casey
Thanx for the well wishes.
Thanx..It was started over ten years ago! I didn't have enough sense at the time to realize it was over my head (then). Over the years, when I came to a point where I needed one, I would dig it out and do some more work (re-do some more work usually) on it. My strobe pattern being the last big push to finish it up.
-Casey
Hi Casey - sorry to hear about your adventure with the driveway - it is so easy to do harm to one's self... and, it gets in the way of our hobbies!
The dividing head is a marvel. I have just enough knowledge about the challenges in making something like this to know how demanding it is. Had it been me, I would have bought one - mostly out of abject fear at the scope of the job of making one. Bravo!!!
By all means turn your platter to a uniform thickness. You should seriously consider balancing it IMO. For years I have used the trick of applying a coat of Duct Seal mastic to the underside of thin, resonant factory tt platers to deaden them. If the mastic puts the platter even slightly out of balance (+/-1-2 grams), the result can easily be heard as wow in the output.
Hope you are off the crutches soon
Jess
The dividing head is a marvel. I have just enough knowledge about the challenges in making something like this to know how demanding it is. Had it been me, I would have bought one - mostly out of abject fear at the scope of the job of making one. Bravo!!!
By all means turn your platter to a uniform thickness. You should seriously consider balancing it IMO. For years I have used the trick of applying a coat of Duct Seal mastic to the underside of thin, resonant factory tt platers to deaden them. If the mastic puts the platter even slightly out of balance (+/-1-2 grams), the result can easily be heard as wow in the output.
Hope you are off the crutches soon
Jess
I'll try to learn from your mistake...
Casey, sorry to hear about your fall and hope you get well soon. Noting your damage, I have just removed the large Swiss army knife that has graced my belt for the last 24 years. We don't have a lot of ice here, but I've come off my bike twice in the last two years and a third fall might be the one to hurt my hip.
Nice dividing head. Reminds me that it really is time that I cleaned the protective grease off my (commercial) dividing head and made it usable.
Casey, sorry to hear about your fall and hope you get well soon. Noting your damage, I have just removed the large Swiss army knife that has graced my belt for the last 24 years. We don't have a lot of ice here, but I've come off my bike twice in the last two years and a third fall might be the one to hurt my hip.
Nice dividing head. Reminds me that it really is time that I cleaned the protective grease off my (commercial) dividing head and made it usable.
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