RE Re: Variac vs. Fan spe

To: tesla-at-poodle.pupman-dot-com
Subject: RE Re: Variac vs. Fan spe
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 02 May 1997 23:50:13 -0500
Approved: tesla-at-stic-dot-net
Subject:       RE Re: Variac vs. Fan spe
       Date:   Thu, 01 May 1997 20:01:28 GMT
       From:   robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization:  Society of Manufacturing Engineers
         To:   tesla-at-pupman-dot-com


        You are =so= close to the truth - it's a pity you don't get
        the cigar:

        Variable-frequency drives (VFDs)  (more modern name: vector
        drives) are best used with (and intended for) 3-phase
        motors.  Such motors have no centrifugal switch and no
        special starting windings.  With a VFD their speed can
        typically be controlled from 0.1x to 2x the nameplate
        rating.   It's possible to run single-phase motors with
        a VFD but inadvisable for reasons stated.

                              - - - - - - -

        Stepping motors are quintessentially small devices.  A one
        hp stepping motor is already a monster for the genre.  And
        they require special solid-state drive circuitry.

                                - - - - - -

        The absolutely best motor for exquisitely-fine speed control
        is the dc or ac servo motor.  Servo motor speeds can be
        controlled from a few degrees of shaft revolution per minute
        to thousands of rpm with digital precision.  The motors and
        their drives and controllers ain't cheap, but the best rarely
        is.   Then, of course, there's always the usual surplus
        sources.

                                - - - - - -

        The best quick-and-dirty source for universal motors with
        good speed characteristics is to salvage one from an old
        vacuum cleaner.

                Such motors can also be begged from vacuum
                cleaner repair shops in the same manner as
                neon transformers.

        They can be controlled as-is by means of a Variac, or
        with a dc power supply + Variac a very fine degree of
        control may be realized.


                                Motoring on...


                                Robert Michaels - Detroit, USA


T>Variac's are great BUT they are EXPENSIVE! This is why I use a
T>home built saturable reactor to control the input power on my larger
T>coils and triac phase shift controls with universal motors on my
T>rotary spark gaps. BTW for any one monitoring this thread DON'T use a
T>triac phase shift control on an induction motor it will burn out the
T>windings as fast as you an through the switch. AC induction motors
T>should be controlled with a variable frequency control and then they
T>can only be controlled over a narrow range, because if they run too
T>slow the centrifical switch that activates the starter windings will
T>kick in and if the speed doesn't come up quickly again the starter
T>windings will burn out. The best motors to use for variable speed is
T>either D.C. or universal motors. Stepper motors will also work but
T>tend to be expensive.