Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.design Subject: Re: Motor Speed Control Date: Thu, 07 Mar 2024 19:17:03 +0000 Organization: A noiseless patient Spider Lines: 72 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Injection-Info: dont-email.me; posting-host="ef59e564d14ec420f8670511c608bbd5"; logging-data="1283774"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19QZ7/2l75IxXC+ZHH29kQLLNBKnyYVcBQ=" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:QcOHxwqEzu5MpZi6LN+bXVjEqJw= Bytes: 4196 On Thu, 07 Mar 2024 07:26:08 -0800, John Larkin wrote: >On Thu, 7 Mar 2024 02:14:49 -0800, KevinJ93 >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