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TMS320F28027: Understanding the Space Vector Modulation for the InstaSpin Software

Peter Davis1
Intellectual 440 points

Part Number: TMS320F28027

Hi I have a couple questions regarding the SVM portion of the instaSpin Software:


1. In the Instaspin labs Vsref is set as 0.8*USER_MAX_VS_MAG_PU, why is this level chosen, and can it be increased to 100% of the USER_MAX_VS_MAG_PU to maximize speed before entering the fieldweakening region. Currently at 0.8, field-weakening begins at a significantly lower than expected speed, causing significant current to be sunk into the direct axis current Id. Is there any risk to increasing Vsref to 100%?

2. Regarding, choosing between the three modes when setting the USER_MAX_VS_MAG_PU, I would like some clarity on what setting these actually achieves. My understanding thus far is that the first level (0.5 ) which sets a peak sinewave at 86.6% dutycycle would still have overmodulation through injecting third harmonic oscillation on on all three phases, but that this allows the ESC to function without requiring three current sensors, and no current reconstruction is required because of a too small of sampling window on one of the phases. Whereas, setting it to 0.557 (1/sqrt(3)) allows the code to fully utilize the duty-cycle up to 100% of the pwm but this requires three current sensors. My questions are:
 a. Is that a correct understanding
 b. Are there any other risks or concerns we should be aware of when using the higher duty cycle
 c. Can you set a value such that the peak sine wave occurs at a duty cycle between 86.6% and 100% pwm to adjust how much into overmodulation you go

Where Peak of sinewave shown in this image changes depending on duty cycle set

Thank you,

  • 0
    TI__Guru** 117270 points

    1. You can set 100% to the VsRef that is for field tweaking control as the threshold.

    2. Right. You need to use the three-shunt and over-modulation as lab10a if you set the USER_MAX_VS_MAG_PU over 0.5. The higher USER_MAX_VS_MAG_PU needs good current sensing on the board since the sampling time window will be very small. The peak value of the phase voltage should be equal to the dc bus voltage, you can't limit it. You just adjust the RMS value of phase voltage by setting the modulation index/duty.