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

On Sun, 01 Jun 2003 16:08:40 -0000, you wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Harvey White <madyn@i...>
>wrote:
>> On Sat, 31 May 2003 20:24:53 -0000, you wrote:
>>
>> >
>> >Hi Lee,
>> > Won't series winding (full winding) RESISTANCE be TWICE the
>amount
>> >of the single coil (1/2 coil), while your series winding
>INDUCTANCE
>> >be FOUR times that of the single coil (1/2 coil)?
>> >
>> > With series winding (full winding), you have twice the number of
>> >turns of wire being energized as half winding, so you use half the
>> >amount of current to acheive the same holding torque.
>>
>> Seems to be two factors here. One is the maximum flux through the
>> motor, the other is the power dissipation of the motor. The
>> inductance does relate to the maximum supply voltage needed
>for "good"
>> performance.
>>
>> The motor is not rated (unless it's stated by the manufacturer) for
>> both halves of a 6 wire motor actuated at once (running unipolar as
>> bipolar). You have to derate the motor to get this to keep both the
>> flux (excessive can demagnetize the motor) and the power dissipation
>> in limits.)
>>
>> So running both halves of the motor windings (say 4 volts at 2 amps
>if
>> unipolar) as bipolar, full coil means that you have an 8 volt
>winding,
>> but at about 1.3 amps, assuming a derating factor of 0.66. Using
>one
>> coil of a unipolar stepper (ignoring the lead completely, using only
>> the CT and one wire) will get you the original nameplate ratings..
>say
>> that same 4 volts at 2 amps. You will need a bipolar driver to
>handle
>> it, and the easiest one of those is a chopper style driver...
>>
>> Further complications come when you use 8 wire steppers, which are
>> steppers with all 4 coils wired independently.
>>
>> (All the above is what I've managed to piece together, if it's
>> incorrect, please correct it, and blame it on the lateness of the
>> hour...)
>>
>> Harvey
>
>
>
>The thing that gets cloudy for me is that a 4 wire stepper has a
>single coil on a circuit. an 8 wire can be wired for single coil and
>just not using the remining two coils.
>
>yet a 6 wire is an 8 wire with a single center tap that connects 2
>coils. and all other things being equal, that style is often
>referred to as having one coil and being wired as half-coil.

I can see how this can be confusing. Let me see if I can set out the
options:

steppers can have (for 2 phase steppers...)

4 wires, 6 wires or 8 wires.

Neglecting the 3 phase steppers with a common center point (shows up
on old 8 inch floppy drives) we have

4 wires:

2 coils. always bipolar operation. all wires must be used.
Nameplate ratings apply. Needs H bridge driver or bipolar driver.

6 wires:

2 coils, each center tapped. Can be run unipolar with saturating
transistors to ground. All 6 wires must be used in this case.

can be run Bipolar by neglecting the center tap: Disadvantages: needs
higher voltage, current should be derated to avoid too much flux in
motor.

Can be run bipolar by neglecting one half of each winding, and using
the center tap and one end wire. Best compromise at using a
"unipolar" motor for bipolar. Use rated voltage and current. can be
referred to as "half-coil" hookup.

8 wires:

4 coils. May be connected in series to yield a 6 wire motor.
Advantages and disadvantages the same.

may be connected in parallel to yield a 2 coil motor. Disadvantages:
current goes up for driver, and motor must be derated because of too
much flux through core (which can demagnetize the core, and ruin the
motor). Minimal power/torque advantage for wiring up the motor this
way.

>
>As I have read, series will deliver full torque up to exaclty half
>speed of a single coil (on an 8 wire, or half-coil on a 6 wire)
>
>I am using series and using 1/2 nameplate as the setting for my
>Gecko's.
>
>so, the question how does one determine where the torque starts to
>drop off when on is not using a single coil (on an 8 wire) but is
>using less than nameplate on a series wired motor ?

I'd recommend using the 4 wire configuration, unless current was an
object. From what I understand, it's the most convenient, allows you
to run full current through the windings, and gives you the best high
speed torque for a given supply voltage. The remaining wires are not
connected, and are not allowed to short, of course.

The exact figures on what torque is available at what voltages and
what speeds is dependent on hookup and supply voltage. I don't have
them off the top of my head.


Harvey

>
>Dave