Electric Motor Noise

[Peter GainParticipant @petergain89847]

Alan, I have since been informed that the importer of my motor – he who claimed no noise complants – has supplied most of the units to fast food outlets. They are used for the ventilation of chip fryers & similar. No wonder he does not receive noise complaints.

Peter Gain.

[Billy MillsParticipant @billymills]

Well perhaps a small fryer would be handy for tempering……

Hope that your earplugs hold up until you can get a decent UK made motor.

 

Had a call the other day about my encounter with a motor with a static shaft and spinning rotor. Well I didn’t say that it was 3 phase! It was an ironless PM motor in a professional tape recorder ( no, not a Nagra but close). The rotor was an epoxy casting containing skewed copper windings with a multipole commutator on top. There were optical tacho markings on the top which was read through multiple sensors to average out errors. The shaft was about 1/2″ and was the direct drive for tape transport. It had a fast servo to set the variable speed with some precision, the opto tacho signal was limited then fed to a weise discriminator then to a DC power amplifier that drove the motor which was over 90% efficient. Very nice Swiss engineering, state of the art at the time.

 

It was a VERY expensive item 40 years ago . Just before letting a well known reviewer have this latest kit I did a quick test. The speed meter indicated that the motor was up to speed and locked but the tape did not move. The rotor was at speed but the epoxy casting and the shaft had divorced and were going their own ways. So even something as simple as a motor can sometimes do the unexpected.

Regards,

Alan.

[John OlsenParticipant @johnolsen79199]

I have a Uher tape recorder here somewhere from the 70’s that has a three phase synchronous motor, although the machine is battery powered. There is a transistor circuit to generate the threee phases, no brushes. Must have been quite advanced for its time.

 

regards

John

[Ian S CParticipant @iansc]

Hi Alan, I’v got a similar motor to yours, but the rotor is an aluminium die casting, don’t know its make, or where it came from. Ian S C

[Steve GarnettParticipant @stevegarnett62550]

Posted by Alan Gray 1 on 24/01/2011 17:44:52:

It was a VERY expensive item 40 years ago .

Sounds a bit like my Revox PR99… or the A77 (I have one of each!) If I recall correctly, the main idea behind using the outer rotor motors, often made by Papst, was that the radiated magnetic fields were somewhat lower.

Edited By Steve Garnett on 25/01/2011 10:42:12

[Billy MillsParticipant @billymills]

Steve, it was a bit more expensive than a Revox which was a domestic machine. That suddenly triggered an interesting memory. The Alistair Cooke “Letter from America” was sent over on 1/4″ tape that was then replayed on a rack mounted Revox which had been used for a very long time, the capstan had worn down so the program would run a bit slow and over-run it’s slot. So they told Cooke to slightly shorten the item as he was over-running, when the tape was replayed on the worn Revox it fitted fine.

 

The external rotor was good on radiated field but the main advantage was the flywheel effect from the largest possible diameter rotor (compaired with the internal rotor) to reduce flutter.

 

The mystery machine was a Stellavox Sp7 from Neuchatel. Like the Nagra 3 & 4 it was used for film sound recording and was also battery powered. The quality of the recordings was better than “Studio” recorders of that time because of clever design. These machines recorded a “pilot” tone from a generator attatched to the camera film transport. For 25 frames/sec it was 50Hz, for the US , 24 frames/sec and 60Hz. The pilot tone was recorded on two narrow tracks over the top of the full width audio track. One track was in antiphase and carefully amplitude balenced. So when you played back the recording with a full width head you only heard the program, the pilot balenced out completly . In the Transfer Suite the pilot head nulled out the program and replayed the pilot signal to lip sync the audio to the film. Simples! even a meercat could have done it!

 

The Stellavox had a very low rotor inertia because it was ironless- nearly all copper windings in epoxy. Unlike every other machine of that time you could run and turn corners whilst recording without any wow being recorded because the servo bandwidth was very high, the motor speed would vary to exactly compensate the motion of the machine around the capstan axis- but you can only do that when the tacho marks are extreemly accurate because the accuracy substitutes for the flywheel inertia that every other machine uses. If you ever use a magnetic recorder again try rotating the machine while replaying some music.

 

Sorry guys- way off three phase topic but hope you found it interesting as a different motor application!

 

Regards,

Alan.

[Steve GarnettParticipant @stevegarnett62550]

 

Posted by Alan Gray 1 on 27/01/2011 11:13:03:

Steve, it was a bit more expensive than a Revox which was a domestic machine.

 

Just as OT, I’m afraid…

 

Hmm… Studer-Revox introduced the PR99 as a professional version of the B77, but the A77 could certainly be described in its original form as domestic, yes. My A77 though has a flat-top conversion, and quite a few mods done to it, and although it may have had domestic origins, it isn’t really any more!

 

The Revox uses outer rotor motors for the reels, which is where the additional magnetic screening is useful, but as you say, the effectively larger flywheel effect comes into play on the capstan motor.

 

As for machines that you can swing around whilst playing with no ill-effects, I have a portable cassette recorder that you can do this with, but the effect isn’t achieved with complex servo electronics at all – it has dual contra-rotating flywheels joined by a thin belt drive, and I have to say that it works remarkably well.

Edited By Steve Garnett on 27/01/2011 16:38:19

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