Re: Members

Hi,

Actually a variable voltage is more for the motor's benefit than
anything else.

Step motor heating has two sources, one obvious, one not.

The obvious one is the heat generated anytime current passes thru a
resistance. The resistance here is the motor winding, and automatic
standby current schemes are used to reduce this heating when the
motor is stopped.

The other heating source is "iron losses" (eddy current and
hysterisis). You can think of it as a "friction" in the iron as the
magnetic dipoles switch back and forth. This heat quadruples when
voltage doubles.

A fun way of proving this exists is to chuck-up a step motor in a
drill press. Set it to high speed and make sure the motor leads are
not touching each other. The motor will be too hot to touch in about
five minutes, yet no current was flowing in the windings!

In theory a variable voltage supply might reduce motor heating but
the slight advantage is simply not worth the considerable circuit
complexity.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Carol & Jerry Jankura"
<jerry.jankura@s...> wrote:

> Jon -
>
> I can think of one reason why you might want a variable voltage

power

> supply - to limit the power dissipation in the transistors that

drive the

> motor coils. Often, these units can stand off a higher voltage than

is

> actually used because of power dissipation considerations. If you

really

> wanted to "tune" a system for performance, would it not be

reasonable to

> adjust the input voltage to as high a value as you could without

exceeding

> the temperature ratings of the drivers? And, BTW, how do you measure
> junction temperature anyway? :)
>
> -- Jerry Jankura
> Strongsville, Ohio
> So many toys, so little time.... (and just enough knowledge to be
> dangersous)