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In my CNC machine, I have a small air compressor and a spindle motor VFD connected in same 240 VAC outlet.

Whenever the compressor switches on or off, the VFD reports "OC-o" error. According to manual this code means "Overcurrent when motor off" and is due to external interference.

Is there a way to reduce the interference from the compressor?

jpa
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  • While self-answered Q&A is fine in itself, your question still needs to hold the same standards as any other question here. That means that details such as schematics should be added to the question, not just to the answer. As this stands, the question seems to be an off-topic "use of electronics question" or "repair question without details". – Lundin Feb 06 '23 at 10:28
  • @Lundin I tried to write the question with the level of detail I had before deciding to modify the compressor itself. Maybe it is not a good question for the site format. Closely related to https://electronics.stackexchange.com/questions/42131/how-to-design-an-rc-snubber-for-a-solenoid-relay-driving-an-inductive-load?noredirect=1&lq=1 and https://electronics.stackexchange.com/questions/554040/do-switches-induce-voltage-spikes-in-the-ac-power-grid but the answers there weren't directly useful to me, so I decided to add this one. – jpa Feb 06 '23 at 11:27

1 Answers1

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I opened up the compressor. It has a mechanical pressure switch, which apparently arcs a bit when it connects or opens. The typical solution to arc noise from mechanical switches is to add an RC snubber.

The RC snubber is a capacitor and resistor in series, connected across the switch:

schematic

simulate this circuit – Schematic created using CircuitLab

The capacitor slows down the voltage change over the switch, to avoid arcing when switch opens or the contacts bounce. The resistor limits current spike when the switch closes and dampens oscillators with the motor inductance.

When choosing the parts, a few things must be considered:

  • For mains voltage motor, the capacitor must be rated X- or Y-class.
  • Larger capacitance will let more current through and heat up. Value of 1 nF to 47 nF is usually suitable.
  • Larger resistance will dampen oscillations more and reduce current spike on connection, but increases the maximum peak voltage on disconnection. A value of 5 to 500 ohms is usually suitable.

For me, I happened to have 33 nF X2 capacitor and 10 ohm 1 watt resistor available. Adding them across the switch helped enough that the VFD no longer errors out.

jpa
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