Re: Electrical wizards

>I have a really, really dumb question. A friend of mine asked why he
>could not "Dial down" a variac and use it to supply power for a stepper
>driver system. I could only conclude that there is the chance it could
>not maintain the voltage accurately, in fact even fail miserably,
>because it is nothing like having Primarys and Secondarys of a
>transformer. true ??

DUMB DUMB DUMB idea !

But not for any of the reasons listed.

The reason is that a Variac has one side connected directly to the AC line,
the other side more or less connected to the AC line.

GUARANTEED BIG sparks will fly sooner or later, with electrocution a
distinct possibility somewhere along the line.

Strangley, that's about all that's wrong with the idea, but THAT'S ENOUGH.
In fact, the voltage will be reasonably well regulated, but in fact, that's
irrelevant. For modern controllers, a regulated supply is of no use and
even has a few potential drawbacks.

>Then I was wondering what actually happens if Variacs are used to supply
>a transformers primary winding. If it was all setup, and running some
>stepper drivers and you dialed down the voltage, What do the Stepper
>motors do ?
>
>Slow Down ? get weak ? What ?

At some point in the reduced voltage ( and assuming a modern controller ),
the motor's top speed is reduced. As the voltage is further dialed down,
the motor torque is reduced.

With a modern controller, the actual supply voltage is not that relevant, at
least on the high side.

It can't be so high that it exceeds the insulation resistence of the motor
coil windings, but that would be a value of at least 300 volts. And it
can't be higher than the breakdown voltage of the transistors in the
controller, which is more likely to be the actual top limit.

But there is, in most cases, a pretty wide range over which things will
work just fine.

For example, in a Sherline sized unit, with appropriately sized
steppers, I doubt that there is much meaningful difference between 18 volts
and 36 volts.


Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta