I'm trying to design an EMI filter for a 3-phase induction motor driver. I've looked at some example filter designs but in one of them I couldn't understand the capacitors and resistors what they are stands for?

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There are 4 X-Y caps at the beginning and at the end, 1 common mode choke and DC bulk capacitors. Can you explain why the series resistors and other capacitors are used? I circled them in the picture.

Thank you.

  • \$\begingroup\$ the capacitors, plus the inductors, implement a standard PI filter, a low-pass filter. the capacitors connected to ground are there to shunt the EMI to ground (chassis). \$\endgroup\$ Sep 1 '19 at 23:04
  • \$\begingroup\$ The resistors are probably there to meet requirements to discharge the capacitors within a certain time period after power is removed. That way if someone is servicing the drive they won't get zapped by the charged capacitors. (Though the time constant seems long to me.) They are in series probably because the voltage rating on each resistor isn't enough to stand off the full voltage. \$\endgroup\$
    – John D
    Sep 1 '19 at 23:09

The resistors are to bleed off stored charge when input power is removed. This prevents possible dangerous voltages on terminals that are supposed to be "off". The R11 and R12 values are strange. They are not standard E96 or E48 values. Also, it is common to have two resistors in series so that a short in one does not compromise the circuit, but usually they are the same value. In higher-voltage circuits, multiple resistors in series prevent excessively high voltages on any one part.

  • \$\begingroup\$ R11 and R12 could be 1M ohm too, there was 2 designs, this could be old one. Thank you! \$\endgroup\$ Sep 1 '19 at 22:11

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