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enter image description here

A PMSM motor is controlled by the inverter in the picture, which in turn is powered by a DC power supply that can not dissipate energy, only supply.

When the inverter trips, for example for over voltage protection, "the AC power stage is disabled (IGBTs are opened)". What happens then?

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  • \$\begingroup\$ You appear to have answered your own question here; what more do you want? \$\endgroup\$
    – Hearth
    Commented Mar 13 at 5:10
  • \$\begingroup\$ But just how does the machine accelerate such that the EMF is higher than the DC supply, and does it make any difference in this state whether the inverter is trying to operate normally as opposed to IGBTs are opened? \$\endgroup\$
    – greybeard
    Commented Mar 13 at 6:49
  • \$\begingroup\$ @Hearth I wanted a confirmation if I was right, or an explanation if I was wrong. \$\endgroup\$
    – K0ICHI
    Commented Mar 13 at 11:30
  • \$\begingroup\$ @greybeard It goes above nominal speed, where BEMF is higher than DC supply, by field weakening. While field weakening, everything is fine, voltages are kept under control. But as soon as the drive switches off, BEMF will jump up and potentially destroy components. \$\endgroup\$
    – K0ICHI
    Commented Mar 13 at 11:32
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    \$\begingroup\$ @greybeard Field weakening is used to increase the max speed of a PMSM; you just have to put a little d-phase current into the armature to counter the magnet. \$\endgroup\$
    – Hearth
    Commented Mar 13 at 13:48

1 Answer 1

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Assuming the motor rotates at a speed which BEMF is higher than the DC supply, current flows through the diodes in parallel with the IGBT's. This current produces a braking torque in the motor, slowing it down. The current also flows backwards through the inverter, charging the inverter's capacitors, as well as the capacitor in the power supply. If the motor was spinning fast enough, this could overcharge the capacitors.

Assuming the capacitors can handle the resulting voltage, when the BEMF matches the voltage on the capacitors, current is not able to flow through the diodes. With zero current through the armature, there is no braking torque on the motor, which then starts spinning freely.

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  • \$\begingroup\$ In the block diagram there are two components Passive Discharge and Active discharge. \$\endgroup\$
    – greybeard
    Commented Mar 15 at 6:59

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