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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Motor Speed Control
Date: Sat, 9 Mar 2024 15:42:32 +1100
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On 9/03/2024 5:49 am, KevinJ93 wrote:
> On 3/7/24 8:48 PM, Bill Sloman wrote:
>> On 8/03/2024 7:13 am, KevinJ93 wrote:
> ...
>>>
>>> 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.

Motors run hotter than their environment

>> 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.
>>>>
>>>> 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.
> 
> MOS EPROMS such as the 1702 were cumbersome to use with multiple 
> supplies required. 

It was one-time programmable, not an EPROM.

>The logic to drive them would have been TTL consuming 
> significant amounts of power as well as expensive.

CMOS was around and cheap. I'd first used it around 1975, and the price 
fell by a factor of three as I was developing the 1975 circuit.

> The first EPROMS that were easy to use, such as the 2708 weren't widely 
> available till the late 70's.

The stepper motor circuit that I worked on was developed in 1978.

<snip>

-- 
Bill Sloman, Sydney