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Bit 10 of Ixx24
What does bit 10 of Ixx24 do on the Geo Brick LV?

In the Turbo SRM bit 10 is not described (page 81). However in the Firmware update listing it says (page 733) "5. Added new control bit (bit 10) to Ixx24 to permit direct-PWM control of two-phase motors."

In the Geo Brick LV manual (page 149), this bit is set in an example and described as "disable 3rd Harmonic."

"Third harmonic injection" is a commonly used technique for electronically controlled 3-phase motors that increases the effective supply voltage by about 15%. It adds a small component of a 3x frequency signal to the "base" sinusoidal voltage output of each phase. Because the phases are 1/3-cycle apart, all of these third harmonic additions are exactly the same for each phase, and the net phase-to-phase voltages do not change, but the use of the available voltage is better optimized.

However, with 2-phase motors, the phases are 1/4-cycle apart, so adding a 3rd-harmonic component would change the net voltages applied across the phases as a function of angle, introducing a lot of current and torque ripple. The Geo Brick LV was the first product that had a power stage to support 2-phase motors, so there was a late addition to the firmware of this control bit to inhibit the addition of 3rd harmonics.
Thank you very much for the quick response. It makes sense now.


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