Wouter,

On Sat, 22 Apr 2006 16:45:38 +0200, Wouter van Ooijen wrote:

> > But lamps are a really bad representation of a motor!  They 
> > don't have the inductance, they don't generate 
> > back-EMF, and when their power is shut off they don't turn 
> > into generators and send voltage the wrong way into 
> > the electronics (think of Fleming's two hands...).
> 
> No, but the question here was about the switching speed of the FETs, for
> which inductive or reactive load will not make that much of a
> difference. And to be on the safe side I used a much heavier load than
> expected. (That is: at that moment :) specifications will always
> change...)

I think there may be more than just the load - the *reaction* of the load, especially to changes of drive, 
edges and so on, could be your problem, and lamps don't do that.
 
> > I think you need to set up a test rig with actual motors, 
> > even if they don't have the actual live mechanism 
> > connected, but some sort of dummy load such as a flywheel.
> 
> A flywheel is not exactly a dummy load, it does not dissipate. These
> motors are geared, and the gearbox does not like begin (back-) driven.

It's a load as its speed changes, but that will fall off as it stabilises.  Perhaps if you added a fan, or 
gears in an oil bath, or a water pump, or something like that, it would provide a load under stable speed 
conditions.

Can you tell us what the motor will be driving in the live system, so we can see what the problem is?

Cheers,


Howard Winter
St.Albans, England


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