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From: KevinJ93 <kevin_es@whitedigs.com>
Newsgroups: sci.electronics.design
Subject: Re: Motor Speed Control
Date: Thu, 7 Mar 2024 02:14:49 -0800
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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.

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?

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.

kw