Re: variable V power supply

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]