[modeleng] Dimmers, motors, blisters, resistors.

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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