3 phase steppers (was: Motor current measurement)

nf1z wrote:

>
> If you want to see what's going on, you need to use the scope. The
> current waveform will be a sawtooth as the current ramps up linearly
> to the set value, which turns off the L298. The current then decays
> exponentially for a fixed time (set by the L297 - or equivalent -
> oscillator frequency), when the L298 is turned on again. That is of
> course, if you are using the L298 in chopped mode, not DC.

Btw: Can anybody explain to me how 3 phase stepper motors are
controlled. I know that from the electrical point of view they can be
handled like 3 phase synchronous AC motors with a big number of poles.
But how are the currents through the windings controlled for a 3 phase
motor.

At 2 phase (bipolar) stepper motors, things are less complicated because
the windings are connected separately from each other. Currents can be
controlled by one current sensor (resistor) for each winding.
Microstepping can be done by modulating the current of the first winding
with an approximated sine wave and the second with cosine.

But how is this done with 3 phase steppers? The three windings are
connected together in a star or triangle configuration. Theoretically
you need only two current sonsors because the current at the 3rd
terminal is always the (inverted) sum of the other two. But is this
actually done this way? With a linear amplifier we could replace sine
and cosine by sin(x), sin(x+120°) and sin(x+240°) for the 3 phase motor.
But with chopper/switch mode it's not that easy. If you switch one path
off when the nominal current is reached that will affect the other two
windings.

Also, you can't use free wheeling diodes for 3 phase motors because the
current of one winding will always have to "push" against the voltage
applied to the other two windings. Or am I missing something.

Regards

Bene

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