dynamic vs static torque: is that something like when you start the motor
you can stop spinning it with an X power but you can't stop it with the same
once it spinning?

Tamas


On 23/10/06, Tony Smith <ajsmith@rivernet.com.au> wrote:
>
> > > Largish stepper motors can be had fairly cheaply.  At the
> > RPM you're
> > > after they'd work nicely.  Downside is you need to make drivers for
> > > them.  Not hard, but a bit of a nuisance and pushes the cost up
> > > somewhat.  The simplest driver is for unipolar motor (6
> > wire) and is 4
> > > fets and a bit of logic.
> >
> > I've trouble finding a stepper motor that can produce 50 Nm
> > (7000+ ozf.in).
> > The biggest I found have a holding torque of 40 Nm (5700
> > ozf.in), which is a bit more than what we have now (as stall torque).
> > http://www.anaheimautomation.com/hightorque.htm
> >
> > Are there stronger stepper motors?
> >
> > How does the holding torque of a stepper motor relate to the
> > stall torque of a DC motor? One site says that "dynamic
> > torque" is about 70% of holding torque. Is "dynamic torque"
> > what I have available to move something? (I know that the
> > available torque goes down with speed, but since I'm working
> > at around 1 rot/s, it seems that is very slow for a typical
> > stepper motor and it develops its maximum torque.)
>
>
> Ok, your definition of 'largish' is different to mine :)
>
> There are bigger steppers, the next size up from a NEMA42 is a NEMA66, but
> it starts getting a bit silly.  The body of that is bigger than a CD-ROM!
> You could bolt a gearhead to the stepper, at low RPM you tend not to loose
> too much torque.  Given the prices on this page
> <http://www.powerdrives.com/superior/order/nema42gearheads_rtplanetary.htm
> >,
> that's probably not an option.
>
> Not sure what they mean by dynamic torque, as you say torque varies with
> speed.  Most steppers have a torque curve that remains fairly flat up to a
> certain speed, and then falls off rapidly, a bit like a knee.  Maybe the
> 70%
> refers to that point.
>
> Looks like your stuck with trading speed for torque, get a motor and gear
> it
> down.  Servos on CNC machines are usually geared down (say 1:5), so
> pulleys
> & belts are common.
>
> 50Nm is a lot of torque, what are you making?
>
> Tony
>
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