Re: [CAD_CAM_EDM_DRO] 3 phase steppers (was: Motor current measurement)

Nicolas Benezan wrote:

>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.
>
>

Well, unless you need extreme smoothness, or high efficiency at high
speeds, a 6-step drive
will probably suffice. If you want the ultimate performance, then
sine-wave drive will do
just a little better. (The same is completely true for 2-phase
steppers, too.)

>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.
>
>

With a PWM amp, you CAN, indeed, feed sine wave currents to the
windings, setting the PWM
duty cycle to obtain the average current desired. This is exactly how a
2-phase microstepping drive
works. It is also how some VFDs work, although many of these are
programmed to produce 3-phase
voltages, not currents.

>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.
>
>

Yes, you still can use the free-wheeling diodes in the same way.

Jon