=09Question would one of these dimmers work in place of the headlight
dimmer? If so they would be a lot cheaper, I've been planning on going to
Radio Shack to see what they have but have not done so yet.
=2E..Art
On Sun, 26 Oct 1997, Ross MacPherson wrote:
> At 06:20 PM 10/25/97 -0400, you wrote:
> >Carol wrote:
> >> - Rheostat: use a lamp dimmer or (my favorite) a sewing machine foot p=
edal!
> >
> > I don't know if I agree with this from a health standpoint. You
> >had better be sure that the style of power-reduction employed does
> >not burn out the motor. Without sticking my neck out to say what
> >works and what doesn't, there are right and wrong ways to do this.
> >
> > The two general methods are voltage reduction and duty cycle
> >reduction. I don't know which if either are safe for tools. HOWEVER,
> >I do know from experience that a poor extension cord can ruin
> >an electric motor if the voltage is allowed to drop too low.
> >
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> Before this thread leads to some damage let me clear a couple things up.
> First and foremost a lamp dimmer and a rheostat are NOT the same thing. =
A
> lamp dimmer uses a timing circuit based on the applied frequency in our
> household AC (60 Hz in NA, 50 Hz in the rest of the world) to switch a tr=
iac
> on for part of each cycle. If the triac is on for all of each cycle the
> load sees full power. As the triac is turned on later in each cycle the
> load will see less of the voltage. Because the triac in the dimmer is sim=
ply
> a switch that turns off for part of each cycle it doesn't need to dissipa=
te
> a lot of power. The triac must be sized to pass the load current but sin=
ce
> its resitance is very, very low it will not generate too much heat and th=
e
> size of the unit can remain small.=20
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> A rheostat, on the other hand, is simply a variable resistor and any
> current applied to the load also passes through the resistor. For any ki=
nd
> of useful current the resistor has to be able to dissipate a sizable amou=
nt
> of heat. (I=B2 R) One used to control a power tool would have to be nea=
rly
> as big as the tools motor and would become very warm.
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> There are several types of AC motors used in tools. The Dremel, and lots=
of
> other high speed tools are powered by a universal motor, so called becaus=
e
> they'll run just as well on DC or AC, 50Hz or 60 Hz. They are easy to sp=
eed
> control because the speed of the motor is a direct result of the applied
> voltage and the load. A universal motor always has brushes and a commuta=
tor
> (like your starter motor and generator) and a cooling fan pressed on to t=
he
> rotor of the motor. The fan serves also to provide load to the motor
> because universal motors have this interesting ability to accelerate to
> infinity if there is no load. Naturally, at some point if the motor spee=
d
> isn't limited it'll self destruct, throwing bits of shrapnel at tremendou=
s
> velocity. In most circles this is frowned upon. The fan on a universal
> motor is carefully designed to apply sufficient load to limit speed to a
> safe level. =20
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> Split phase and capacitor start motors are ususally used where high speed=
is
> not required but more power is. Table saws, compressors, grinders etc. u=
se
> these types of motors. The speed of these motors depends on the frequenc=
y
> of the supply and speed controling these types of motors requires variabl=
e
> frequency drives. Any attempt to reduce the applied voltage, via dimmer O=
R
> rheostat, much below the designed range will damage them.
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> I don't recommend using the lamp dimmer on anything much bigger than a
> dremel because the dimmer is designed to be used in a purely resistive
> circuit and the inductive characteristics of motors drasticly reduces the
> safe capacity of the dimmer.
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> Any questions?=20
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> (Lecture mode OFF)
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> Ross MacPherson=20
> 1947 MG-TC 3528
> 1966 MGB-GT =20
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