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.=20
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. (I=B2 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. =20
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?=20
(Lecture mode OFF)
Ross MacPherson=20
1947 MG-TC 3528
1966 MGB-GT =20
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