[CAD_CAM_EDM_DRO] Re: Stepper Motor Power Calculations

I understand. There are a variety of factors that impose practical limits on
the voltage. When I said "highest supply voltage possible" I was assuming
within the limits of whatever controller components you were using, though I
can image hitting the limits imposed by the motor first. The dynamic
response of a stepper motor is probably too complex to represent with any
general mathematical model, and so probably the best anyone can do is to
come up with a set of parameters that have been shown to work reasonably
well within a small group of motors from a specific vendor, or with similar
characteristics. To really optimize performance you would have to develop a
3-dimensional graph relating torque to voltage and speed, which probably
would be more effort than it's worth unless you were a high-volume
motor/controller manufacturer. Most of them only offer charts of torque vs.
speed at the rated current. I guess they don't want to take chances on
someone misinterpreting a chart and burning up a motor.
--Phil M.

----- Original Message -----
From: "caudlet" <thom@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, January 23, 2006 10:08 AM
Subject: [CAD_CAM_EDM_DRO] Re: Stepper Motor Power Calculations


> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
> <mattison20@c...> wrote:
> >
> > Excellent answer, thanks. So the short story is that overkill is not
> a bad
> > idea unless you are designing a commercial power supply for volume
> > production and need to squeeze every penny. I was thinking about the
> dynamic
> > effects and it makes sense to use the highest supply voltage
> possible for
> > the best high-speed performance, since torque drops off at higher
> RPM and
> > inductive reactance increases at higher frequencies.
>
> There is a point of diminishing returns. After you get past the
> reluctance/impedence of the drive added voltage just goes up as heat
> and adds nothing to the torque curve. The magic number may not be the
> most voltage you can use with a given drive.
>
> If you operate the motor below the impedence "knee" then it sould not
> ned heatsinks and fans.
>
> The concept that if 48V is good 65 is better and 80 is great should be
> tempered with the knowledge that every motor design is different and
> you should only use the voltage that gives the best performance with
> acceptable heat losses. Bottom line you can only stuff current into a
> given coil using more voltage to a given point. It is a non-linear
> function so that at a point adding more voltage does not result in the
> same porportion of increased turn on time.
>
>
>
> I presume most
> > solid-state motor-control current regulators dynamically adjust the
> PWM duty
> > cycle to compensate for changes in power consumption at different step
> > frequencies, but have not seen data sheets state that specifically.
> If not,
> > I wonder if it makes sense to have an external microcontroller that does
> > that. I have observed that stepper motors dissipate a lot more heat when
> > holding than when spinning, if holding current is not reduced. I
> would think
> > that the motor current ratings are for holding current, and could be
> > substantially exceeded when stepping at high speed without damaging the
> > motor, since heat dissipation is the main reason for current limitation.
> > That would also make a case for heat sinks and fans on stepper motors.
> > Wouldn't it be funny if people started juicing up their stepper
> motors the
> > way people juice up their Pentiums now. Maybe empirical observation
> is the
> > only way to find out, as you suggested. The thing is, I've
> re-invented so
> > many wheels in my life time that I'm a little gun-shy now. It's worth
> > spending some time to find out if someone has already done it.
> > >
>
> You are over-thinking the problems. All of this has been taken into
> consideration by the motor drive designers. Most higher current
> drives like the Gecko have automatic idle current foldback so that
> power dissipation while locked (not moving) is reduced.
>
> You COULD "juice up" the steppers by running them at higher then their
> rated current and at higher voltage then use fans, heatsinks,
> heatexchangers, etc to take away all of the wasted energy. For that
> you might gain another 10% in torque and speed for an added 80% more
> power loss. Add in the shortened life of components and it's not in
> the realm of practicalibility.
>
>
>
>
>
>
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