[modeleng] Re: Dimmers, motors, blisters, resistors.

R.L. Roebuck wrote:
> On Wed, 17 Sep 2008, Shep wrote:
>
>   
>> I have been slowly progressing with the construction of my little lathe 
>> - finished the leadscrew and the clasp-nut mechanism.  I am now rigging 
>> up a car wiper motor at the tailstock end of the lathe, to drive the 
>> leadscrew via a belt.  The idea is to be able to vary the speed of the 
>> leadscrew, whilst actually turning.  I am using a 12V DC battery charger 
>> as a power source, but found it difficult and expensive to locate a 
>> suitable robust speed control - eg a wire-bound rheostat.  I then 
>> attacked the problem in a different way, and interposed a mains lamp 
>> dimmer which I had in stock, between the mains power source and the 
>> battery charger.  This works a treat, and gives a smooth variable-speed 
>> drive.  Problem solved at no cost!
>>     
>
> Hi there Hubert,
>
> This sounds like a neat idea - I might give it a try on the hobbymat (you 
> can't dissengage from the leadscrew, and it is a mighty fine pitch, and 
> the handle for turning it is quite small - on the rare occasions these 
> days when I manage to have a few days of serious loco building on the 
> trot, I always end up with a blister from working the lead screw handle!).
>
> Not knowing how much or not you know: on the subject of 
> the lamp dimmer (I dabble btw - but there may be more experienced than 
> me in the group):-
>
> * As I understand it, the issue here is that a lamp is very much like a 
> electrical resistance - you flow current through it and heat comes off, 
> and some light as well in the case of the lamp. The motor though is quite 
> inductive, which in this case means that when you try to turn is off, it 
> uses energy stored within it to try and keep the current flowing, even if 
> you have switched it off (sometimes seen as a spark across the switch 
> contacts).
>
> * As far as your dimmer is concerned, when it is dimming a lamp, 
> it sees either maximum voltage across itself when no current is flowing, 
> or visa versa, or some compromise between the two; with the motor, there 
> is likely compromise, it sees full current you're supplying and full 
> voltage you're using many times a second.
>
> * So the fact you've got it working is a good sign: things I'd be looking 
> for is the main switching component (transistor, thristor, etc) 
> overheating, or having reduced life-span.
>
> * This may likely not be a problem - you aren't using the motor to do 
> anything arduous, and you may not be concerned about having to replace the 
> dimmer say once ever couple of years.
>
> * What I would say though is when these switching components fail, I've 
> always had them fail in the fully on state (ie the motor goes to the speed 
> it would be at if wired directly to the power supply). If this means that 
> you might run the topslide directly into the chuck, or write off a 7.25" 
> gauge cylinder or wheel casting, then it may be worth pondering over some 
> kind of cutout.
>
> * A fuse in the line might do the trick, but remember that they don't blow 
> that fast (a 13A mains fuse will take roughly 40 seconds to blow with 40A 
> flowing through it).
>
> Anyway, hope that's of some use - I will still be trying it on the 
> Hobbymat though - so thanks for the tip. Good luck with the geared 
> screw-cutting mechanism.
>
>
> Yours,
>
>
> Rich.
>
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>   
Rich,
What about one of those sewing machine motors?
You know, the ones that had the foot pedal, which was basically a 
potentiometer?
Patrick



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