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.