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I am using a BLDC motor with sinusoidal windings that I drive with FOC. The implementation uses a triple half-bridge and a DRV83 to drive the MOSFETs. I have 3 low-side amplifiers to measure the phase current Ia, Ib and Ic. The power is given by a DC power supply.

How can I estimate the instantaneous current Idc drawn from my power supply with my three phase currents Ia, Ib and Ic?

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  • \$\begingroup\$ I would expect it to be more promising to estimate \$average I_{DC}\$ from power than from phase current(s). \$I_{DC}\$ does not need to equal \$\sum I_{phase} \$ where the phase currents are controlled by, effectively, buck converters. \$\endgroup\$
    – greybeard
    Commented Oct 9, 2023 at 10:21
  • \$\begingroup\$ @greybeard I understand this would help to get a more direct and accurate estimation but I don't have access to Idc on my system \$\endgroup\$
    – Victo Rien
    Commented Oct 23, 2023 at 12:43

2 Answers 2

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Multiplying the phase currents by the cosine of the appropriate angles and summing them up with their respective weights (2/3) will give you an estimation of the DC current drawn from the power supply. Keep in mind that this formula assumes balanced three-phase currents. So, you get:

$$\text{I}_\text{dc}=\frac{2}{3}\cdot\left(\text{I}_\text{a}\cos\left(30^\circ\right)+\text{I}_\text{b}\cos\left(150^\circ\right)+\text{I}_\text{c}\cos\left(270^\circ\right)\right)\tag1$$

Which leads to:

$$\text{I}_\text{dc}=\frac{\sqrt{3}}{3}\cdot\left(\text{I}_\text{a}-\text{I}_\text{b}\right)\tag2$$

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  • \$\begingroup\$ Thanks ! it is a convenient way to compute the DC current. Do you have a link to literature ressource that derives those equations ? \$\endgroup\$
    – Victo Rien
    Commented Aug 21, 2023 at 8:31
  • \$\begingroup\$ After a second look, I realize that this equation cannot be true as Idc should always be the same sign but the sign of (Ia - Ib) is fluctuating between negative and positive \$\endgroup\$
    – Victo Rien
    Commented Aug 22, 2023 at 14:59
  • \$\begingroup\$ You need to go over the input power times efficency equals the output power, then you can estimate Idc pretty good. Pin = Vdc * Idc, Pout = sqrt((iqvq)^2 + (idvd)^2) \$\endgroup\$
    – HansPeterLoft
    Commented Sep 18 at 16:20
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Instantaneous \$I_{DC}\$ should be the sum of the phase currents
(positive currents from positive supply, negative currents to negative supply potential).
Otherwise, charges would need to accumulate somewhere.

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  • \$\begingroup\$ Sum of the phase currents is always equal to 0. Simple Kirchhoff law. So the answer is somewhere else. \$\endgroup\$
    – Victo Rien
    Commented Oct 27, 2023 at 9:09

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