Re: 8-wire stepper: series is 1/2 the amps.Strange but True

Hi,

This stuff is not really hard to understand.

1)Torque is proportional to ampere-turns.
All this means is 1A of current passing thru 20 turns of wire (full-
coil) produces the same torque as 2A passing thru 10 turns of wire,
(half-coil). If you have a 6-wire motor, it means you have a motor
with 2 center-tapped coils. If the coil has 20 turns of wire, then
there will be 10 turns of wire on either side of the center-tap.

2)Inductance goes up with the square of the number of turns of wire.
If 10 turns of wire (half-coil) has an inductance of 1 millihenry
then 20 turns of wire (full-coil) will have an inductance of 4
millihenries.

3) The effects of inductance come into play only when a motor turns.
Inductance has a property called "inductive reactance" the same way a
resistor has a property called "resistance". Both are measured in
Ohms. The difference is inductive reactance "Ohms" double when you
double the frequency (step rate) across it.

According to Ohm's law, current is voltage divided by resistance. The
current that can flow thru a 4mH inductor (full-coil) will be 1/4 as
much as can flow thru a 1mH (half-coil) inductor at a given
frequency. However, the full-coil current passes thru twice as many
turns of wire as the half-coil current. That means the full-coil
motor will have 1/2 as much torque at high speed as the half-coil one.

Still according to Ohm's law, if you were to double the voltage,
twice as much current would flow. Now the full-coil motor torque at
high speed would be identical to the half-coil torque at the original
voltage. The performance would be identical; only the voltage would
be different.

I hope this is not as clear as mud.

Mariss







--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lee Studley"
<indigo_red@q...> wrote:

> Hi,
> I would in my early days have agreed until Mariss gave me new
> insights into motor inductance:
>
> When wiring stepper coils in series, or full winding, the amperage
> is 1/2 the plate rated( I think this plate rating is for a single
> coil energized), this is due to the inductance being twice the
> amount than that of a single or 1/2 coil. Using V=L di/dt and stick
> a fixed dt=1 (1sec) relation in there to simplify the thought
> process, and setting V equal to the fixed motor drive voltage.
> V is proportional to L * di where di is a change in current. So if
> you double the L value( 2coil in series) then di ( or I) had to
> decrease by 1/2 if V is fixed. This relation shows the ability of

an

> inductor to oppose a change in current.
>
> So 1 amp full series, or 2amp single or 1/2 coil.
> Do the experts agree?
> -Lee
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "forumtvm" <forumtvm@y...>
> wrote:
> > "WUKSTA" <gerry@z...> wrote:
> >
> > > After reading all the posts on this subject I am a little
> > confused.
> > > I have three Vexta (Oriental Motor) PK268M-02A steppers that
> will be
> > > driven by Gecko 201 drives. These steppers are 187 in/oz.;
> 2A/phase;
> > > 4.5V; 2.25 ohms/phase; .9 deg; 6 wire motors. If I run these
> motors
> > > "Half Coil" would I run them @ 2amps or 4amps? With my cheap

volt

> > > meter, I get a resistance of 4.8 ohms full coil & 2.5 ohms half
> > coil.
> > >
> > Hi Gerry,
> >
> > I believe it should be 2Amp. Your cheap voltmeter I believe has
> > confirmed that - 4.5V / 2.5 ohm = ~2A
> >
> > Peter