Motor speed control via back-EMF detection

[Joerg]

On Mon, 01 Sep 2008 17:54:08 -0700, Joerg
[snip]
Variable pulley ratio... you dig?

No grok here

Again:

Synchronous motor -> no slip -> always in sync with your utility's 60Hz.

Strobe disk -> lamp -> lamp also always in sync with utility.

So, where's the value in this measurement?

If I can change the pulley ratio I can adjust the PLATTER speed.

But what for? If the pulley diameters are fixed and there is a
synchronous motor the platter speed can never deviate from nominal. All
it can do with age is drop to zero upon a gummed up shaft, when the belt
snaps or something.

 

[Joerg]

The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

Ah, so Jim must have bought that thang from Walmart ...

<duck and run>

 

[Joerg]

Maybe. but maybe they never used them ;-)
From:
http://en.wikipedia.org/wiki/Synchronous_motor
<quote>
synchronous electric motor is an AC motor distinguished by a rotor
spinning with coils passing magnets at the same rate as the alternating
current and resulting magnetic field which drives it. Another way of
saying this is that it has zero slip under usual operating conditions.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<end quote>
I have underlined 'under usual operating conditions'.
When synchronous motors are used, with varying loads, they may very well slip,
and if in audio equipment cause horrible modulation of the sound pitch.
An example is the little 3 phase synchronous motor in the walkman I have (or had?)
If the tape sticks a bit, or the cassette mechanism is sluggish, then a singer
sounds like a highly stressed freaking out person... (as it jumps phases).
For the rest it will just keep running, dropping phases.

So, 'either in sync, or stalled' is not correct, no matter what university wrote it.
No matter where in the universe, and no matter when.

Nah, that's like a slipping stepper. Unless it's in acceleration a slip
means it's on the verge of stalling. And even in acceleration we avoid
slips in steppers like the plague. Not a useful operating point.

 

[Jan Panteltje]

Nah, that's like a slipping stepper. Unless it's in acceleration a slip
means it's on the verge of stalling. And even in acceleration we avoid
slips in steppers like the plague. Not a useful operating point.

Oh I agree with that.
But the point is that the 'slipping' happens, and in case of a player
of sorts, like difficult to move tape, will result is an average slower speed,
with severe speed modulation.
Of course it should have been designed to be able to pull the heaviest loads
expected without slipping.
I have had cassette tapes I could hardly move by hand though

 

[Joerg]

John Larkin wrote: [snip]

The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

Ah, so Jim must have bought that thang from Walmart ...

<duck and run>

I bought that "thang" around 1958-59. I don't think Walmart even
existed then.

Hmm, sounds like a genuine Muntz design then. Maybe he walked around the
lab, replaced the nice synchronous motor that should have been in there
with the motor of a cheap bathroom fan and said "See, works! Why spend
the extra pennies on a friggin' synchronous motor"

 

[Joerg]

Oh I agree with that.
But the point is that the 'slipping' happens, and in case of a player
of sorts, like difficult to move tape, will result is an average slower speed,
with severe speed modulation.
Of course it should have been designed to be able to pull the heaviest loads
expected without slipping.
I have had cassette tapes I could hardly move by hand though

Well, the point is that adjusting a gear ratio would never be the answer
to a slippage problem in a synchronous motor. Taking things apart,
cleaning, lucricating, that would be the answer.

 

[Joerg]

I worked on everything from the $10 British 'Bull Shit Reserve' crap,
to cast aluminum broadcast turntables with AC motors, and all developed
speed problems as they aged. The motor may not change speed, but the
platter will. Dried out rubber idlers or belts, or bad lube would slow
them down. Even direct drive turntables developed speed problems, if
used long enough. Have you ever moved a 100+ pound turntable? The
platter was over 45 pounds of cast & machined aluminum on some broadcast
turntables. They used a 1/8 HP motor, and still developed speed
problems. In the '60s & '70s some shops had one or two techs who worked
full time on turntables. Speed problems, bent linkages, and even seized
platter bearings were common. Some replacement motors ran 2% fast, so
emery paper was used to take a little off the motor shaft at a time,
till it ran the right speed. Customers with 'perfect pitch' were a pain
in the ass. You could run a lab grade test record with a 1000 Hz tone,
and display it to three decimal places and they would claim the speed
was wrong. The platter speed is controlled by the ratio of diameters,
not the fact that the motor was synchronous. Somewhere, I have a '50s
or '60s manual from H. W. Sams describing every motor used in
turntables, wire & tape recorders to that time.

Ok, I grew up in Europe and the turntables I fixed were different. Some
were direct-drive and as a kid it always puzzled me how they did the
33-1/3 to 45 shift electrically. Some had meshed gearboxes that under no
circumstances you were supposed to shift while it was turning (but
people did ...). Then the cheap ones with belts. Only the ones without
toothed belts could theoretically be adjusted but nobody ever did. I
also remember some where the motor would lean against an extruded and
polished ring in the table but those wouldn't last too long.

 

[Joerg]

True.

With the aid of a cheap laser diode, a phototransistor and an
oscilloscope, I've measured the motor speed to be around 3000RPM (+/-
about 100RPM as the motor load changes). Now I just need to track down a
MOSFET and a microcontroller of some description, and rig up some form
of speed controller.

With a Mabuchi consumer grade motor? Wow. That sounds like Beluga caviar
on a slice of Wonderbread ;-)

 

[Joerg]

On Tue, 02 Sep 2008 08:39:07 -0700, Joerg

John Larkin wrote:
[snip]
The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

Ah, so Jim must have bought that thang from Walmart ...

<duck and run>
I bought that "thang" around 1958-59. I don't think Walmart even
existed then.

Hmm, sounds like a genuine Muntz design then. Maybe he walked around the
lab, replaced the nice synchronous motor that should have been in there
with the motor of a cheap bathroom fan and said "See, works! Why spend
the extra pennies on a friggin' synchronous motor"

You're being dense this morning, Joerg! It _is_ synchronous, but the
belt-drive pulley ratio is so large to get down to 33-1/3 RPM that
tweaking is needed to get it exact.

Why didn't they just use a toothed belt?

 

[Joerg]

On Tue, 02 Sep 2008 09:00:05 -0700, Joerg

Jim Thompson wrote:
On Tue, 02 Sep 2008 08:39:07 -0700, Joerg

John Larkin wrote:
[snip]
The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

Ah, so Jim must have bought that thang from Walmart ...

<duck and run>
I bought that "thang" around 1958-59. I don't think Walmart even
existed then.

Hmm, sounds like a genuine Muntz design then. Maybe he walked around the
lab, replaced the nice synchronous motor that should have been in there
with the motor of a cheap bathroom fan and said "See, works! Why spend
the extra pennies on a friggin' synchronous motor"
You're being dense this morning, Joerg! It _is_ synchronous, but the
belt-drive pulley ratio is so large to get down to 33-1/3 RPM that
tweaking is needed to get it exact.

Why didn't they just use a toothed belt?

Bump... bump... bump... ;-)

Nope, I've seen a fancy Japanese turntable with such a belt. High end. A
classmate absolutely had to have it and it blew my mind when he really
plunked down those 300 Deutschmarks. That was a fortune back then. He
asked me to come along so nobody would hold him up and snatch his wallet.

 

[Glen Walpert]

Well, the point is that adjusting a gear ratio would never be the answer
to a slippage problem in a synchronous motor. Taking things apart,
cleaning, lucricating, that would be the answer.

Well, no, the point is that high end turntables like the Rek-O-Kut did
not use gear drive or toothed belt drive because of the tooth passing
noise these produce; they used flat belt drive and the seldom needed
adjustment was to compensate for wear - or allow slight pitch
adjustment for those with perfect pitch . (I might have one of
those stored in my barn somewhere too, but I think I threw it out.)

BTW synchronous motors not run from variable frequency drives have a
second rotor winding for starting purposes (amortisseur winding), so
the motor starts as an induction motor with slip then locks in as
synchronous.

 

[Philip Pemberton]

With a Mabuchi consumer grade motor? Wow. That sounds like Beluga caviar
on a slice of Wonderbread ;-)

LOL!

The only reason I'm using a PIC micro and a FET is because both of those items
happen to be sitting in my junkbox gathering dust. The 8-pin PIC12F615 is 50p
in one-off for DIL, or 45p if you prefer the SOIC package. Total parts cost
including a small FET is probably about 75p (or US $1.50)... hardly worth
worrying about. The PIC has an internal oscillator, PWM, A/D and a 20mA output
stage -- if you disable MCLR, you don't even need the external reset pullup
resistor.

But yes, it's overkill... Not on a Heath Robinson or Rube Goldberg scale, but
still overkill.

Thanks,

 

[Joerg]

Well, no, the point is that high end turntables like the Rek-O-Kut did
not use gear drive or toothed belt drive because of the tooth passing
noise these produce; they used flat belt drive and the seldom needed
adjustment was to compensate for wear - or allow slight pitch
adjustment for those with perfect pitch . (I might have one of
those stored in my barn somewhere too, but I think I threw it out.)

BTW synchronous motors not run from variable frequency drives have a
second rotor winding for starting purposes (amortisseur winding), so
the motor starts as an induction motor with slip then locks in as
synchronous.

Except in the Hammond organ here. You push in one switch, helper motor
spools up, then throw main motor switch, clutch engages, wait three
seconds, hope for the best, disengage starter motor switch, clutch
disengages. When all the tubes have heated up the organ runs. Or not.

 

[Joerg]

Well! I climbed up in the storage closet. ...

Easy on your new hip there ...

... The turntable I saved is a
JVC JL-A40, looks to be direct drive PLL controlled.

That's how it always should have been! Now you can listen to them old
bluegrass records. For youngsters: These are those huge black CDs.

 

[Joerg]

LOL!

The only reason I'm using a PIC micro and a FET is because both of those
items happen to be sitting in my junkbox gathering dust. The 8-pin
PIC12F615 is 50p in one-off for DIL, or 45p if you prefer the SOIC
package. Total parts cost including a small FET is probably about 75p
(or US $1.50)... hardly worth worrying about. The PIC has an internal
oscillator, PWM, A/D and a 20mA output stage -- if you disable MCLR, you
don't even need the external reset pullup resistor.

I wonder if TI ever reaches down into those price categories. They'll
have to, since Atmel does it as well.

But yes, it's overkill... Not on a Heath Robinson or Rube Goldberg
scale, but still overkill.

Still, I'd have done it sans micro. Just for kicks.

 

[Jamie]

No they can slip.

Then they are not synchronous.

Maybe Asynchronous is what you're thinking of?



http://webpages.charter.net/jamie_5"

 

[whit3rd]

No grok here

Again:

Synchronous motor -> no slip -> always in sync with your utility's 60Hz.

Strobe disk -> lamp -> lamp also always in sync with utility.

So, where's the value in this measurement?

OK, so it's a shaded-pole motor, and those (because of rotor
magnetization hysteresis) are sometimes synchronous. But, the
ones used for phonographs weren't synchronous, because that
would make line fluctuations telegraph to the platter. The core
material was soft, and slippage happened, so that the platter
inertia kept d(theta)/dt constant-ish. It took a few turns before
the platter got up to full speed.

More important, it was the PLATTER that the strobe disk mounted to,
and not the motor. The speed control was via the idler wheel.
Change the idler height and it rubbed the conical motor shaft at
a different diameter.

If the bearings really gummed up, the motor would slip badly, and
your strobe disk would tell you whether the idler adjust was
sufficient.

 

[JosephKK]

Jim Thompson wrote:
On Mon, 01 Sep 2008 17:54:08 -0700, Joerg
[snip]
Variable pulley ratio... you dig?

No grok here

Again:

Synchronous motor -> no slip -> always in sync with your utility's 60Hz.

Strobe disk -> lamp -> lamp also always in sync with utility.

So, where's the value in this measurement?

If I can change the pulley ratio I can adjust the PLATTER speed.

But what for? If the pulley diameters are fixed and there is a
synchronous motor the platter speed can never deviate from nominal. All
it can do with age is drop to zero upon a gummed up shaft, when the belt
snaps or something.

What part of variable diameter (tapered) do you still not read? I am
not used to you being obtuse like this, please quit it.

 

[JosephKK]

Jim Thompson wrote:
On Tue, 02 Sep 2008 09:00:05 -0700, Joerg

Jim Thompson wrote:
On Tue, 02 Sep 2008 08:39:07 -0700, Joerg

John Larkin wrote:
[snip]
The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

Ah, so Jim must have bought that thang from Walmart ...

<duck and run>
I bought that "thang" around 1958-59. I don't think Walmart even
existed then.

Hmm, sounds like a genuine Muntz design then. Maybe he walked around the
lab, replaced the nice synchronous motor that should have been in there
with the motor of a cheap bathroom fan and said "See, works! Why spend
the extra pennies on a friggin' synchronous motor"
You're being dense this morning, Joerg! It _is_ synchronous, but the
belt-drive pulley ratio is so large to get down to 33-1/3 RPM that
tweaking is needed to get it exact.

Why didn't they just use a toothed belt?

Bump... bump... bump... ;-)

Nope, I've seen a fancy Japanese turntable with such a belt. High end. A
classmate absolutely had to have it and it blew my mind when he really
plunked down those 300 Deutschmarks. That was a fortune back then. He
asked me to come along so nobody would hold him up and snatch his wallet.

Yes, we still call them audiophools. Complete nutter.

 

[JosephKK]

Maybe. but maybe they never used them ;-)
From:
http://en.wikipedia.org/wiki/Synchronous_motor
<quote>
synchronous electric motor is an AC motor distinguished by a rotor
spinning with coils passing magnets at the same rate as the alternating
current and resulting magnetic field which drives it. Another way of
saying this is that it has zero slip under usual operating conditions.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<end quote>
I have underlined 'under usual operating conditions'.
When synchronous motors are used, with varying loads, they may very well slip,
and if in audio equipment cause horrible modulation of the sound pitch.
An example is the little 3 phase synchronous motor in the walkman I have (or had?)
If the tape sticks a bit, or the cassette mechanism is sluggish, then a singer
sounds like a highly stressed freaking out person... (as it jumps phases).
For the rest it will just keep running, dropping phases.

So, 'either in sync, or stalled' is not correct, no matter what university wrote it.
No matter where in the universe, and no matter when.

Jan, you have a "terminology" issue. Synchronous motors do not "slip"
ever. They can and do "cog" at near stall conditions. All of the
drops are full electrical / mechanical cycle steps, not loosing a
fraction of an mechanical rotation with each electrical cycle. That
is why the terminology is different. If your little tape player was
cogging it needed a serious tear down, cleaning, and re-lubrication.
Unless you are interested in what you might learn from such work it is
far cheaper to replace.