Re: [CAD_CAM_EDM_DRO] Re: stepper driver help

John Dammeyer wrote:

>Paul is dead on correct here!!! Even the Gecko's don't run 7A at 80V
>which would be 560W. (ouch)
>
>It's easiest to think of things happening in stages.
>
>The device turns on and puts say 50V across the winding which draws no
>current initially, so there's no current through the driver and no power
>turned into heat.
>
>Next the current flows over time into the motor until it reaches the
>current rating of the motor.
>

Just so people understand this, the current increases until it hits the
limit of the driver - there's no other magic that limits current to the
rating of the motor. If you set it wrong, you may burn up the motor.

> At that point the voltage across the
>winding may well still be as high as 45V. This is totally dependant on
>the winding inductance and resistance. And so the drop across the
>driver might be 5V at 3A for 15W dissipation.
>
>

The voltage should all be across the motor, except for a few tenths of a
volt across each FET. The gecko should only be dissipating a couple of
watts, like 3A * 0.5 V = 1.5W or thereabouts (that's an estimate - I
haven't looked up the on resistance of the FETs).

>Then the driver switches off because max current in the winding has been
>reached (3A). With the drive off, when the current decays to somewhat
>less than the 3A setpoint the whole sequence starts again.
>
>Note that if the 50V was left on eventually the only thing limiting the
>current through the motor would be the winding resistance and then it
>would be like having a 50V supply across a 1 ohm winding (for example)
>with 50W dissipation. Smell that magic smoke!
>
>

Even better, it would be 2500W! :)
50V across a 1-ohm load gives 50A, and 50V * 50A = 2500W. (P=V^2 / R)
In practice, the magic smoke would prevent this much dissipation well
before that was reached.

- Steve

[big snip]