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NNTP-Posting-Date: Thu, 07 Mar 2024 15:27:30 +0000
From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: Motor Speed Control
Date: Thu, 07 Mar 2024 07:26:08 -0800
Organization: Highland Tech
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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?