Re: Motor Speed Control

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

On 3/7/24 6:07 AM, Bill Sloman wrote:

On 7/03/2024 9:14 pm, KevinJ93 wrote:

...

Back then they were called "stepper motors" and would have been entirely practical. Admittedly, I didn't get to design one into what would have been a cheap product until 1978 (and at EMI Central Research) but they were pretty cheap.

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Stepper motors are much too inefficient and have too much torque ripple for capstan drive - not at all suitable for a battery powered device, they also tend to be noisy.

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Twaddle. A stepper motor is a synchronous motor, and if you are careful how you drive it, it doesn't have any torque ripple, and it isn't any less efficient than any other synchronous motor.

 Stepper motors are invariably of the reluctance type. With simple drivers they have a great deal of cogging, which is undesirable in a capstan drive motor.
 

ESCAP did do a range of small stepper motors where a sine wave drive did give a uniform rate of rotation - with others you had to massage the waveform a bit to get uniform rotation.

 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.
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.
 ICs were available to integrate that circuitry:
 eg https://www.precisionmicrodrives.com/ab-026
 

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.

 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.

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.

 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.

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.
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Bill Sloman, Sydney