Re: Motorized Variac question . . .

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Scott Hanson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

These motorized Staco variacs are a little unusual in that there are no
integral limit switches. The DC motor itself may be rated for 12V, but
because of the built-in reduction gearbox the motor must be run at much
lower voltage. 5-6 VDC gives a reasonably fast slewing speed from OFF to
MAXIMUM, but there is still the problem of knowing exactly when the motor
stalls at the ends of the variac travel.

One easy way to incorporate an indicator is to put a low voltage
incandescent bulb in series with the motor. I use a PR-6 bulb out of a
regular 2-D cell flashlight, and run the system on 6V.

This gives a good, smooth motion to the variac, and the bulb is completely
dark while the motor is running. The instant that the
variac hits the end stop, the motor stalls, the current increases and the
bulb illuminates at full brightness. Use a single three-position DPDT,
momentary contact, center-off toggle switch to control variac direction.
Push the handle up and the variac output increases, push the handle down,
the output decreases; release the handle at any point, and the output
remains fixed at that point. Mount the bulb next to the switch in a nice
panel-mount holder and you've got full remote control, plus an end-of-travel
indicator.

Scott Hanson


From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, September 17, 2002 3:54 PM
Subject: Re: Motorized Variac question . . .


> Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<davep-at-quik-dot-com>
>
> > I bought a used 120VAC, 20A motorized variac which I
>
> > plan on using in my power controller.  The motor runs
>
> > off of 12VDC.  Just had a few questions as
> > I have not dealt with these motorized variacs before:
>
> > 1.  I assume
>
> I wouldn't.  Test & find out.  cf below
>
> > if I put +12VDC/Gnd into the motor for one direction and
> > Gnd/+12VDC into the motor for the opposite direction?
>
> Maybe.
>
> >  Is this correct????
>   Maybe.  Some DC motors reverse when the voltage applied
>
> is reversed.  These are the ones with permanent magnet
> motors.  (eg as used in many model rr trains.)
> Others require reversing the connection of the field
> relative to the armature.  Given that this motor is
> designed to be reversed, if any of its control gear is
> presen, it should be able to be figured out.
> Alternately:
> If two leads (only) come out, probably reversible by
> polarity change, as you propose.
> If FOUR leads come out, probably two are armature, two
> are field.  Reverse EITHER with relation to the other.
>
> In any case, some testing with any old 'more or less 12v'
> supply should settle it.  (eg battery charger....)
>
>
> > 2.  Do these motorized variacs have mechanical limits on them,
>
> > or do I risk damaging the variac and/or motor if I continue to
>
> > run the motor once a variac limit has been met???
> Dunno.  Good thing to be sure of, inspect the insides.
>
>
> Another thing to think on:
> Assume the system is up and running.
> Assume the 12vdc supply fails.
> Is there a way to render safe the system with
> not 12v supply?
> (A manual variac can ALWAYS be spun down.  An motorized
> one...)
>
> best
> dwp
>