Re:Stepper Motor vs Servo Motor

Hi,

Step motor coils have inductance (L). Inductance has a property
called inductive reactance much like a resistor has a property called
resistance. Both are measured in Ohms.

Unlike resistance, inductive reactance increases proportionally with
frequency; if you double the frequency, inductive reactance doubles
as well.

Coil current follows Ohm's Law, where current I = V/R, R being
inductive reactance. This means coil current is the inverse of
frequency (step pulse rate).

Torque is proportional to ampere-turns. This means torque is
proportional to current multiplied by the number of turns of wire it
passes through. Because current is the inverse of the frequency,
torque is the inverse of speed as well.

There are several conclusions that can be drawn from this:

(1) A step motor is a "constant power" motor. Power (HP or Watts) is
equal to speed times torque. If torque is halved when speed doubles,
the product of the two stays constant. This means the motor's output
power is independent of its speed. You can think of the motor as an
infinitely variable ratio gearbox. The motor will stall at a speed
where available torque becomes less than the load.

(2) At near zero speed, the motor would produce near infinite torque,
consuming near infinite current while doing so. This cannot be
permitted, so the drive must limit current in some way. The simplest
way is to put a resistor in series with the motor coil which then
limits the current at low speeds. This is an "R over L" drive.

(3) To increase motor torque at speed you must increase voltage. This
comes from Ohm's Law mentioned before. This only works at higher
speeds because the drive limits motor current ( thus torque) at low
speeds or while stopped.

(4) To increase motor torque at speed, run the motor in parallel (or
half winding) instead of series (or full winding). Coil inductance
proportional to the square of the number of turns of wire. The
parallel connection has 1/4 the inductance because current now passes
thru half as many turns of wire. This means inductive reactance will
be 1/4 as well and coil current will be 4 times higher. The torque
will only increase 2 times because there are half as many turns of
wire now.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "John" <johnhe-uk@s...> wrote:
> > They have full torque at 0 speed but tend to drop as speed
increases.
> > Modern drivers have made these motors perform quite well. They are
> > easy to drive and maintain and are open loop capable. All the
> > positioning is done by the motor design you just tell it how many
> > steps to take by driving the coil sequence.
>
> An excellent post! I haven't taken the time to read through and
fully
> understand how steppers function in all the different modes yet but
I have a
> rough idea. I don't know how the actually stepper itself moves
though. You
> say they have less torque at full speed, I know that, but why do
they have
> any at all? Isn't there ever a point where there is no power to any
of the
> coils, or does the control fade between them? Do the coils active
and pull
> the magnets towards them or push them away?
>
> I bet I sound like I chew my ankles now. : )
> John