Just been dismantling an old and very dead Ferguson music centre, probably from the 70s. Nice little DC motor that powered the turntable, for which I have another use in mind.
Using a bench power supply, the motor spins well at 5v, but I don't know the voltage the motor was designed for, there being nothing stamped or marked on it. Were these things fairly standard, and is 12v likely to be the maximum voltage, or is that 'how long is a piece of string' ?
Some of these on both cassette and turntable had built in speed control. Very non sophisticated and seems mainly to be of two main types. The cheap and nasty ones use an old fashioned governor that disconnects power when a certain speed is reached. These tend to generate a lotof interference. The second sort have a little pcb in the back end of the motor that somehow monitors current anbut not actual speed. I did read these worked from some kind of clever back emf action, but they were not mega stable over time as no true servo control was used.
Brian
Incidentally, assuming it was a belt drive device, there is probably a little hole in the bottom of the motor to tweak the speed. a few made by Mitsubishi seemed to actually have some servo based on pulses fed from the turntable drive itself.
It is as you say going to be difficult without the circuit diagram to figure out what the voltage range was. Brian
In message , Brian Gaff writes
Thanks Brian. Yes, it is belt drive.
Thinking aloud, mainly because I didn't study physics at school, does reducing the voltage, and therefore the speed of the motor then result in a reduction of power, or torque?
Ah well, be prepar3ed for an education lesson.
Small DC magnetted Electric motors do not really have a 'voltage' rating.
Ther is a little motor that was/is used in electric tooth brushes on 1-3 V. Rebadged and sold with a gearbox this little motor was used to power model planes running of 7.2v or thereabouts packs. IT was a bit gutless.
I ran them on a much higher gear ratio on 11V, and got better life and power out of them. Simply because the higher voltage meant less current, and what broke them was brush burn.
If you delve into the theory of these motors, there are two basic groups of losses. Loses that represent a fixed current draw irrespective of the RPM or voltage, due to things like friction, and hysteresis losses. And looses that go up with the square of the load current which are basically resistive losses in the copper.
For a given heat dissipation, you can solve a slightly complex equation and get the best power and or the best efficiency of the motor. *If either of those is of interest* and trust me. largely they are NOT.
I.e. the motors in my car windows are DESIGNED TO STALL as that provides a way to tell when the window is fully open, or closed.
So a basic little battery motor can do massively more than you expect by putting it on a far higher voltage and letting the revs climb. For a given copper heating, you have a given current, and that means a given torque, available. If that torque is delivered at 50,000 RPM (by no means beyond reason for a small motor) rather than 5,000, you have 10 times the power for the same heating. Of course your FIXED current losses are ten times higher, but as long as these are not dominant, you have enormous potential.
IN model planes, we were always searching for performance, and what limited motors was mechanical issues by and large.
- Uber high RPM caused brush bounce and brush burnup due to excessive arcing. Solid state commutation (brushless motors) solved that.
- Uber high RPM caused physical disintegration of the rotors. Kevlar wrapping and gluing in place solved that to a degree.
- Uber high propellor RPM just makes noise, not thrust, so rather large reduction gearboxes were needed adding to cost weight and losing a bit of efficiency.
So there is no real concept of 'voltage rating' - it tends to be an arbitrary number stamped on the motor by the manufacturer, but in reality it means almost nothing.
Current rating is far more pertinent,. but no one ever quotes that.
In message , The Natural Philosopher writes
Thank you! Bottom line, apply my 5v as before, and gradually increase the voltage. My ears will tell me when I need to back off.
Actually, high speed is not important. Just the opposite, in fact. Vague plan is to use this record turntable as a toy train loco turntable, so slow speed is good. How quickly the TT will stop rotating once power is cut, I don't know yet, Assuming not terribly quickly, either some form of indexing will be required, or a lot of practice!
Next step is to try and find a replacement drive belt, which may not be easy.
6v, 9v or 12v. Try 6, if no go try 9, if no go try 12. Usually they were voltage marked, and usually one can get the psu working easily to check V.
NT
or physical stops.
Many stationery rubber bands last well, some don't. Superglue a few end to end to get a usable size.
NT
fact.
terribly
I think if you want to stop the turntable and drive a loco off without touching the turntable you'll need some form of indexing.
Moving ones so turntable can do 360 degrees plus. Hole in edge of turntable, lighly spring loaded to engaged position solenoid operated pin that is retracted when the motor is powered.
Or see what CPC have...
Ah, All sounds fun
try here for belts and gears
VCRs used to be a good source. Old ones, not so much the later ones.
NT
elastic also works.
NT
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