Well done, Ross. I think that we must have gone to different schools
together.
Bud Krueger
52TD
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 pedal!
> >
> > 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.
> >
>
> 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
> triac
> 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
> simply
> a switch that turns off for part of each cycle it doesn't need to
> dissipate
> a lot of power. The triac must be sized to pass the load current but
> since
> its resitance is very, very low it will not generate too much heat and
> the
> size of the unit can remain small.
>
> 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
> kind
> of useful current the resistor has to be able to dissipate a sizable
> amount
> of heat. (IČ R) One used to control a power tool would have to be
> nearly
> as big as the tools motor and would become very warm.
>
> 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
> because
> they'll run just as well on DC or AC, 50Hz or 60 Hz. They are easy to
> speed
> 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
> commutator
> (like your starter motor and generator) and a cooling fan pressed on
> to the
> 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
> speed
> isn't limited it'll self destruct, throwing bits of shrapnel at
> tremendous
> 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.
>
> Split phase and capacitor start motors are ususally used where high
> speed is
> not required but more power is. Table saws, compressors, grinders
> etc. use
> these types of motors. The speed of these motors depends on the
> frequency
> of the supply and speed controling these types of motors requires
> variable
> frequency drives. Any attempt to reduce the applied voltage, via
> dimmer OR
> rheostat, much below the designed range will damage them.
>
> 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.
>
> Any questions?
>
> (Lecture mode OFF)
>
> Ross MacPherson
> 1947 MG-TC 3528
> 1966 MGB-GT
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