Re: Stepper Motor Power Calculations

I don't completely agree with your conclusion.

You should pick your supply voltage to be just adequate to the task
and no more. Anything above that results in a hotter than necessary
motor with nothing to show for it.

What supply voltage is adequate? Empirically, run your mechanism with
a 24VDC supply initially. Note at what percentage of the desired
speed the motor stalls. Divide 24VDC by that percentage to arrive at
your optimal supply voltage.

Example: Say your mechanism reaches 70% of your design speed at
24VDC. Divide 24V by 0.70 and you get 34VDC for the optimal supply
voltage.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
<mattison20@c...> wrote:

>
> Nice tutorial. So this pretty much gets to the bottom of what this

was about

> to begin with. Torque decreases with speed due to inductive

reactance and

> iron losses. Torque could be maintained theoretically by increasing

voltage

> proportional to speed, but that produces diminishing returns

because power

> output increases linearly with voltage while iron losses increase
> exponentially. So there is a point at which iron losses exceed

power gains

> and any further voltage increase is dissipated as heat.
>
> Since chopper-type current regulators are designed to maintain a

constant

> current through the coils they will do this as speed increases up

to the

> point where the supply voltage becomes the limitation. Since

friction of

> lead screws and other mechanical stuff increases with speed, you

really need

> more power, not less, as speed increases. So I guess the bottom

line is that

> it makes sense to use a power supply as near as possible to the

limit of the

> motor controller's electronics to give it as much head-room as

possible.

> Beyond that there's not much you can do besides reducing power when

idle to

> minimize unnecessary heat accumulation, and tweaking your speed to

run as

> fast as possible without stalling.
>
> --Phil M.
>
> ----- Original Message -----
> From: "Mariss Freimanis" <mariss92705@y...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Monday, January 23, 2006 8:18 PM
> Subject: [CAD_CAM_EDM_DRO] Re: Stepper Motor Power Calculations
>
>
> > There are two causes of heat dissipation in a motor and inductance
> > stays constant.
> >
> > 1) Let's take care of inductance first. Inductance has a property
> > called inductive reactance. Inductive reactance is measured in

Ohms

> > and is proportional to frequency.
> >
> > To double the speed of a step motor you must double the driving
> > frequency of the winding currents. Inductive reactance follows

Ohm's

> > Law, so inductive current must be halved.
> >
> > Torque is proportional to ampere-turns, or simplified,

proportional

> > to current. This means if speed is doubled, torque is halved. This
> > means the motor speed-torque curve is the 1/x function with speed.
> >
> > Power is torque times speed. Speed doubles, torque is halved,

power

> > stays constant. This means the power output of a step motor is
> > independent of speed.
> >
> > 1a) Still following Ohm's Law from (1), inductive current is
> > proportional to supply voltage. This makes torque proportional to
> > supply voltage as well. By extension, output power is

proportional as

> > well, V/sqrt L specifically.
> >
> > 2) I^2*R losses have been covered previously except for the

motor's

> > rated current is not a thermal limit but rather a magnetic

saturation

> > limit.
> >
> > A 1A, 5V motor has 10W of I^2 dissipation (5W / winding), yet the
> > motor can output 80 Watts of mechanical power. Step motor

conversion

> > efficiency tops-out in the 70% range; 80 Watts output generates

34W

> > of heat at that efficiency.
> >
> > I^2*R losses drop very rapidly with increasing speed. Current

being

> > the inverse of speed makes copper losses drop as a 1/x^2 function

of

> > speed. They essentially become insignificant at higher speeds.
> >
> > 3) Iron losses (eddy-current and hysterisis) generates the

majority

> > of heat dissipated at high speeds. These losses are proportional

to

> > speed (frequency of flux-reversals per second)and are also
> > proportional to the supply voltage (inductive current amplitude).
> >
> > Conclusion: Motor output power increases proportional to supply
> > voltage while iron losses go up with the square of the supply

voltage.

> >
> > Please see "Losses and Power vs. Voltage.pdf" in the files

section of

> > this group, Circuits / Miscellaneous Circuits folder. This is some
> > dyno data I took a few years ago that deals with these subjects.
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
> > <mattison20@c...> wrote:
> > >
> > > Well then, that would be a good reason to ignore the advice I

saw

> > the other
> > > day on one of these boards, suggesting that it's ok to run them

hot

> > until
> > > they almost sizzle. I suspect that at high speeds the inductive
> > reactance
> > > gets so high that the voltage required to overheat the motor

would

> > be more
> > > than the switching components could handle anyway, which is
> > probably why we
> > > don't see motor drivers with better high speed performance. I

guess

> > this is
> > > one of those cases where there is a good reason nobody has done

it

> > already.
> > >
> > > --Phil M.
> > >
> > > ----- Original Message -----
> > > From: "Vlad Krupin" <vlad.cnc@g...>
> > > To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> > > Sent: Monday, January 23, 2006 5:32 PM
> > > Subject: Re: [CAD_CAM_EDM_DRO] Re: Stepper Motor Power

Calculations

> > >
> > >
> > > > On 1/23/06, Phil Mattison <mattison20@c...> wrote:
> > > > [snip]
> > > >
> > > > > 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.
> > > >
> > > >
> > > > It is, indeed the main limitation. Out of curiosity I did some
> > searching
> > > and
> > > > came up with this table: http://www.kjmagnetics.com/specs.asp
> > > >
> > > > Those are some seriously low temperatures! It shows that it

does

> > not take
> > > > much heat to permanently damage a neodymium magnet. Depending

on

> > the motor
> > > > design, I think, the magnets may have a hard time transferring
> > heat to the
> > > > body of the motor. You may not even realize that your motor

is at

> > risk
> > > until
> > > > it is too late.
> > > >
> > > > (probably 99% of you knew that already, but just how low the

safe

> > > operating
> > > > temperature can be was news to me. So I figured I'd share

what I

> > found and
> > > > maybe save somebody a bit of extra grief)
> > > >
> > > > Vlad
> > > >
> > > > [snip]
> > > >
> > > > --
> > > > Vlad's shop
> > > > http://www.krupin.net/serendipity/index.php?/categories/2-
> > metalworking
> > > >
> > > >
> > > > [Non-text portions of this message have been removed]
> > > >
> > > >
> > >
> >
> >
> >
> >
> >
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> > Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
> >
> > Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> > Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> > List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,

wanliker@a...,

> timg@k...
> > Moderator: pentam@c... indigo_red@q... davemucha@j...
> [Moderators]
> > URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
> >
> > OFF Topic POSTS: General Machining
> > If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to

reach it if

> you have trouble.
> > http://www.metalworking.com/news_servers.html
> >
> > http://groups.yahoo.com/group/jobshophomeshop I consider this

to be a

> sister site to the CCED group, as many of the same members are

there, for OT

> subjects, that are not allowed on the CCED list.
> >
> > NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY

POSTING THEM.

> DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> > bill
> > List Mom
> > List Owner
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
>