Hi Barry,
After much thought I have compiled a list of things for you to check. I have probably forgotten something important but that's life.
1) All exposed metal parts forming part of a ‘mains operated machine’ must be connected to Earth. [this is more for your own personal safety than anything else].
2) All input devices such as limit switches, estop etc.should be connected with screened cable.
3) The screen of all screened cables must be connected to Earth and only at one end (preferably to one single Earthing point at the controller end).
4) The VFD frequency control input cable should preferably be screened or at least a ‘twisted pair’.
5) The VFD forward/reverse – on/off switching cables should preferably be screened or comprise of ‘twisted pairs’.
6) The cable between the VFD and the spindle motor should ideally be screened and not routed, for any great distance, alongside wiring for inputs (limit switches, estop etc.). [ideally the screen should be connected at the VFD earth terminal only but if the spindle motor is metal bodied the cable should contain a fourth wire for the earth connection to the motor body but the motor itself must then only be fitted to the machine using an insulated mounting to avoid creating an Earth loop].
7) There must be no Earth loops. [typical examples are where the machine frame is connected to Earth via the incoming supply and the VFD is also connected to Earth via the incoming supply and then these two items are fitted in electrical contact with each other thus an Earth loop is formed. The same thing applies with the PC which is also earthed via the incoming supply, if its metal casing parts or its cable screens/shields (LPT / USB etc) come into electrical contact with the machine frame an Earth loop is again formed].
Any forms of Radio Frequency Interference (RFI) or Electro Magnetic Interference (EMI) must be traced and eliminated by fitting screening, filters etc. [the EMI is relatively short range but the generation of RFI can be just as much of a problem for your systems as it is to your neighbors computer, TV and radio systems].
9) You should not make any direct electrical connection between ‘the computers Ground wires (pins 18 – 25) or the LPT cable screen/shield’ and ‘Earth’. [the computer ‘Ground’ and ‘Earth’ are to be considered as two separate circuits].
10) If anything here conflicts with any product manufacturer’s recommendations then their instructions take precedence. [they should be a lot smarter than I].
Hope this helps.
Tweakie.
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But seriously PCB's make the job look so much more professional and I think it is worth the learning experience.