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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Motor Speed Control
Date: Thu, 07 Mar 2024 19:17:03 +0000
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On Thu, 07 Mar 2024 07:26:08 -0800, John Larkin <jl@997PotHill.com>
wrote:

>On Thu, 7 Mar 2024 02:14:49 -0800, KevinJ93 <kevin_es@whitedigs.com>
>wrote:
>
>>On 3/6/24 8:05 PM, Bill Sloman wrote:
>>> On 7/03/2024 5:36 am, KJW93 wrote:
>>>> On 3/5/24 5:51 PM, Bill Sloman wrote:
>>>>> On 6/03/2024 5:57 am, KevinJ93 wrote:
>>>> ...
>>>>>>>
>>>>>>> That's not all that "old school" - Philips got a patent on it 
>>>>>>> around the 1970's. It wasn't remotely good enough for audio work, 
>>>>>>> and neither were centrifugal governors. Synchronous motors with 
>>>>>>> stable frequency drives was what the old school relied on
>>>>>>
>>>>>> Philips used the negative resistance approach for speed control in 
>>>>>> their portable cassette players - so it wasn't too bad. 
>>>>>
>>>>> The feedback from a DC motor depends on the strength of the permanent 
>>>>> magnets in the motor being regulated, and that is temperature 
>>>>> dependent. Philips may have relied on it, but it was still ghastly.
>>>>
>>>> Obviously Philips didn't agree with you.  For a consumer product used 
>>>> over a benign temperature range it was fine.
>>>>
>>>> The temperature coefficient was low enough to keep the tape speed 
>>>> within 1% or so.
>>>>
>>>>>> Synchronous AC motors  weren't an option in a portable unit.
>>>>>
>>>>> Watches are portable, and electronic watches rely on a 32,768 Hz 
>>>>> watch crystal as the frequency reference. Some of them included 
>>>>> stepper motors to drive a mechanical display.
>>>>>
>>>>> Synchronous motors obviously are a practical option in a portable 
>>>>> unit, though perhaps not in a really cheap one.
>>>>
>>>> At the time these devices were first designed (mid-late 60's) 
>>>> synchronous motors weren't a practical option for a consumer item.
>>> 
>>> 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.
>>> 
>>
>>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.
>
>Efficiency wouldn't matter for a capstain motor (they may well absorb
>power!) and microstepping is easy and smooth.
>
>
>
>>
>>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.
>>
>>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?
>
>Does anybody still make audio tape drives?

Prepare to be shocked!
https://www.youtube.com/watch?v=38_SVIa8BDQ