RE: [CAD_CAM_EDM_DRO] Re: stepper driver help

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. 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.

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!

John Dammeyer




Automation Artisans Inc.
http://www.autoartisans.com
Ph. 1 250 544 4950

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Paul Kelly
> Sent: Thursday, September 27, 2007 3:45 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] Re: stepper driver help
>
>
>
> The L298 can do 2-3 A (depending on duty cycle) *or* 46 V,
> but not at the
> same time. It's limited to 25 Watts, ie 3 A @ 8.3 V or 0.5 A at 46 V.
> And that's *total* power dissipation, the L298 has two
> internal H-bridges,
> so if you use both you're limited to 12.5 Watts per H-bridge.
>
> [Paul Kelly] Ahhh,, no. Those equations only come into play when it's
> switching.
> The dissipation is Volts (across the device) * Amps. As each
> leg of the
> bridges turns on and off (which they do a lot for current
> control) there is
> a time when the voltage and current are both high, heat is
> generated then.
> When the 298 is conducting, you might see 1 or 2 volts across it.
> With a small heatsink I run 298's at 2A and 38V in my lathe all day.
> I highly recommend implementing idle current reduction. It
> slashes the heat
> dissipation requirements of any stepper driver.
>
> PK
>
> Source: the ST Microelectronics L298 Data Sheet
> <http://www.st.com/stonline/products/literature/ds/1773/l298.pdf>
>
> If anyone knows how to push the L298 design to 138 Watt (3 A @ 46 V),
> I'd love to hear it.
>
>
> --
> Sebastian Kuzminsky
>
>
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