Nice discourse on current between stepper drives and stepper motors and
some of the theory of switching drives. I'm still left with the
statement made earlier in this thread by Rafael Skodlar,
"The reason I mentioned power supply is I'm not sure which way
to go, linear or switching. While switching PSU is good for
computers and electronics in general with almost constant power
use, I'm not sure about driving stepper motors with large
current surges during acceleration."
So let's pin you to the point. Do you see large current surges between
the power supply and drive during acceleration of a stepper motor?
Would the extent of those surges cause you to choose one kind of supply
over another?
Rayh
BTW I enjoy the 42 reference. I think I've figured out one possible
reason for his choice of that number. It's the age when far sighted
folk need glasses or eye surgery.
On Mon, 2008-07-21 at 20:50 +0100, Leslie Newell wrote:
> With a switching drive such as a Gecko the current through the motor is
> regulated by adjusting the output voltage. Below the corner frequency
> the current is fixed. Above the corner frequency the drive can no longer
> supply enough voltage to overcome the inductance of the motor so the
> current starts to fall.
>
> Now let's look at the voltage the drive supplies to the motor. While
> stationary the voltage is very low, probably only a few volts. As the
> speed increases up to the corner frequency the voltage rises. At the
> corner frequency the drive is supplying the full supply voltage to the
> motor.
>
> Now power = voltage x current so the maximum power into the motor is
> where the current and voltage are both at maximim. This occurs at the
> corner frequency. In general the risk of losing steps goes up fairly
> quickly once you hit the corner frequency so you don't want to go too
> much above it. Increasing the supply voltage increases the corner
> frequency which is why steppers tend to be run at quite high supply
> voltages.
>
> Switching stepper drives such as Geckos are pretty efficient so the draw
> from the supply is pretty close to the power into the motor. So how can
> the current drawn from the supply be low when the motor current is high?
> This is the clever bit. When you apply a voltage to an inductor, for
> instance a stepper coil, it stores up energy as a magnetic field. When
> you disconnect the supply that energy gets returned. This is
> demonstrated by the fat spark you get if you connect a coil to a DC
> supply then disconnect one of the wires. A switching stepper drive
> switches the supply to the motor on and off at high speed. The amount of
> time spent on effectively controls the voltage to the motor. When the
> drive turns off the stored energy is circulated back from the motor into
> the drive. The end result is that you get a high current circulating
> between the drive and the motor while only a small amount is drawn from
> the supply.
>
> There is another type of stepper drive. Linear drives behave more like
> you expect. In this case the supply current is equal to the current into
> the motor at all times. At low speed the voltage across the motor has to
> be limited otherwise it would self destruct. Linear drives therefore
> drop the voltage across a resistor or transistor. All of the excess
> energy is dissipated as heat in the drive. This is why linear drives are
> unpopular in medium to high power applications. They do have their uses
> in very cheap very low power applications such as low-end printers.
>
> As to the meaning of life - I think you will find the answer is 42 ;-)
>
> Les
>
> Ray Henry wrote:
> > I guess we must be looking at or thinking about different things.
> > Perhaps I did not say it well. The DRAW on the power supply is what I'm
> > saying is greatest at zero speed. After all the comment in the earlier
> > post was that the DRAW on the power supply would be greater during
> > acceleration. That statement would be true of a servo motor but is NOT
> > true of a stepper. At least that is my understanding of the meaning of
> > life.
> >
> > Rayh
> >
> >
>
>
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