Re: [CAD_CAM_EDM_DRO] Re: Members

Carol & Jerry Jankura 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.

In most cases, it is the current and the switching speed, not the voltage,
that produces the heat. varying the voltage will have a very small effect
on the heat produced. It is actually provable that in some designs, the
transistors will run cooler at higher voltage, because the pulse widths
will be smaller!

> 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?

Generally, yes. both with steppers and servos, if the driver has the highest

voltage available, it will allow the fastest current rise of stepper coil
current,
and the highest rapid traverse speed with a servo.

> And, BTW, how do you measure
> junction temperature anyway? :)

With great difficulty! There are outragous schemes that use some measurable
thermal effect of the transistor, like forward voltage drop at known current
to
determine Tj. In most cases, Tj can be inferred by knowing approximate
average power dissipation and baseplate temperature. this can be further
extrapolated from heatsink temperature if you know the performance of the
heat conducting materials used. Heatsink temperature is fairly easy to
measure,
although in a high electrical noise environment, it can still be tricky.

Jon