Re: Motor Speed Control

On 3/7/24 8:48 PM, Bill Sloman wrote:

On 8/03/2024 7:13 am, KevinJ93 wrote:

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Not in 1970. Even after that time they did not possess any advantage over DC motor drive with speed stabilization based on back-emf.

 Don't be silly. Back-emf depends on the strenght of the magnetic field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was adequate for the speed accuracy required under the required environmental conditions.

 Synchronous motors rotate at a rate that reflects the stability of the frequency source that determines the drive frequency, and reasonably stable frequency source - watch crystals have been around for ages.

Even for AC powered units where power was not an issue stepper motors were never used. Synchronous motors with synthesized drive were occasionally a feature but many/most used back-emf stabilization with DC motors.
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ICs were available to integrate that circuitry:
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eg https://www.precisionmicrodrives.com/ab-026
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Even implementing the discrete drive electronics would be more costly than necessary at a time where individual transistors were a significant cost; Philips' solution used two transistors - creating a divide by 4 plus driver transistors plus an oscillator would probably require about ten transistors plus numerous other components.

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Which you could could buy in an integrated circuit. Most of mine were in a chunk of PROM.

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Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all your power budget.

 It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple supplies required. The logic to drive them would have been TTL consuming significant amounts of power as well as expensive.
The first EPROMS that were easy to use, such as the 2708 weren't widely available till the late 70's.

If stepper motors would be such a great solution how come nobody has had your insight and used them in the past sixty years for tape drives?

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Beats me

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The permanent magnet DC motor with negative resistance driver worked perfectly well. It was low cost, used available technology, low power, was quiet and met the design requirements.

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The strength of the permanent magnet depends on the it's temperature, so the velocity feedback you get out of the motor coils does too.
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It might have been "adequate" but it wasn't all that good.

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There is little benefit to being more than adequate if it costs more and will not be perceived by the customer as being better.

 Tape recorder that didn't play back the recorded frequency weren't perceived to be "good" by their customers. That didn't worry the bottom end of the market.

Few customers had perfect pitch, an error of 1% was much preferable to high cost.
You may call them 'cheap and nasty' but the major portion of the market found this solution acceptable, only the high end went for more exotic approaches.
Wow and flutter performance was much more important and using a DC motor and belt drive with small capstans and a flywheel gave acceptable performance.
I see that tape decks available at Crutchfield currently have a pitch control so the speed can be varied anyway.

I'm afraid history is against you and regardless of your remonstrations stepper motors were never used significantly or at all for capstan motors.

 History doesn't make a cheap and nasty solution anything other than cheap and nasty. The thread is about what Cursitor Doom should do to get his antique tape recorder working again, and getting hold of the original motors used to drive it doesn't seem to be an option.
 

kw