Double power supply

Hi all, wondering if, in a pinch, it would work to connect two power
supplies in series to double the output voltage? Stupid question?
Thanks.
Nelson

--- In CAD_CAM_EDM_DRO@y..., mariss92705@y... wrote:
> Hi,
>
> The simplest way of building a variable voltage unregulated DC
supply
> is to build an unregulated DC supply (transformer, rectifier and
> filter cap) and drive the primary with a variable voltage
> autotransformer (variac). This is crude but effective.
>
> The larger question is why you would want to do that. As Jan
> correctly pointed out, the drive's function is to regulate current
to
> the motor, not the power supply voltage.
>
> Step motors are best thought of as current operated rather than
> voltage operated motors. Voltage has a secondary effect as it only
> determines how quickly current can be injected and evacuated from
the
> motor's winding. At rest or low speeds the motor's torque is
> independent of power supply voltage. The faster you go, the more
> rapidly this current has to be changed. Voltage is the only thing
> that can speed up the rate at which current changes.
>
> If you have lived long enough, you know there is no such thing as a
> free lunch, and that is the case here. Nothing comes for free. The
> price that a high supply voltage extracts is motor heating. Just
> think of it as nature's way of keeping you from getting a 1,000 Hp
> from a size 23 motor by using a 10,000V power supply. Motor output
> power (speed X torque) is proportional to voltage while iron losses
> (hysterisis and eddy current) goes up with the square of the
voltage.
> This simply means that heat losses quickly outrace power gains with
> increasing voltage until they become insupportable.
>
> The practical maximum supply voltage for a step motor is 20 to 25
> times the motor's rated voltage. The motor's maximum rated case
> temperature is reached at that point and you are getting all the
> motor has to give.
>
> Your choice of power supply voltage should be based on this limit
for
> a high speed application (3K to 10K full steps per second). For a
> medium speed application (<3K steps per second), use 10 times the
> motor's rated voltage for a supply. For low speed applications (<
1K
> full steps per second) use 4-5 times the motor voltage. You do that
> and you will be OK.
>
> The only reason for a variable voltage supply would be to slave it
to
> the motor speed; low voltage at low speeds and high voltage at high
> speeds. That would be marginally more efficient than a carefully
> picked fixed voltage but I think would not be worth the effort,
> complexity and expense.
>
> Mariss
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., JanRwl@A... wrote:
> > In a message dated 09-Oct-01 18:38:37 Central Daylight Time,
> > cadcamcenter@y... writes:
> >
> >
> > > Is it advisable to use variable voltage power supply to power
> these
> > > motor drives? Has anyone ever used variable voltage power
supply
> to
> > > power these drives? Anyone has any plan/diagram/source for a
> variable
> > > voltage power supply that can drive these with sufficient
> capacity
> > > for any motors likely to be used for a Sherline or equivalent
> system.?
> > >
> >
> > No! The very POINT of the Gecko drives is that THEY "regulate"
the
> power
> > flowing through the motor windings. Thus, "regulation" in the
> power-supply
> > would be pointless.
> >
> > You do NOT want to fiddle with limiting supplied voltage to limit
> the
> > motor-torque! That is the job of the Gecko drives, and if you
> adjust
> > supplied voltage, two things happen: The Gecko drive
> will "regulate" the
> > power to the motor UPWARDS, to maintain the set current, and it
> will write a
> > letter to the NSF, describing what has been DONE to it!
> >
> > Mariss, of course, has vastly more smarts on this topic than I.
I
> only know
> > enough to know one shouldn't even THINK of trying to vary DC-
"down"
> to a
> > stepper-drive!
> >
> > Regards! The Old Troll
> >
> >
> >
> > [Non-text portions of this message have been removed]