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EMI suppressor and protective earth
Chris Pearson 11001208764
1452 Posts
Question
I would be most grateful if the boffins in here could explain something please.

This suppressor was fitted to my lathe on the load side of the VSD. Something went wrong when I was parting off the other day - the tool dug in and the motor stalled - and now it is running roughly. Back to square 1!

In an attempt to determine whether the VSD has been damaged, I tried connecting another motor, but I was unable to demonstrate protective earth continuity. The cable from the green terminal on the suppressor goes to the motor's earth terminal. I expected that the case of the suppressor would be the supply side of the PE, but apparently not. So with the suppressor fitted there is no PE to the motor, which I find a little disconcerting.

FWIW, the capacitance between red and black is about 0.5 µF, the resistance is 470 kΩ, and the capacitance between each of red and black to green is 9 nF.

Would somebody kindly explain whether this is safe please?

9 Replies
Simon Barker 22060613
645 Posts
Looking at the little diagram on the filter, it looks like the green terminal should go to the motor's earth conductor.  But there's nothing to suggest it's the only thing that should go to the motor earth.  Why isn't the motor properly grounded through the supply cable?
gkenyon 82239607
1038 Posts
Difficult to answer without seeing the machine ... sometimes motors that are part of a machine or appliance are earthed by being bolted to the casing of the machine (as in my tumble dryer for example) rather than having a separate protective conductor.

A suppressor may well be earthed in the same way, usually through the can.

Having said that, is the motor (when mounted in the machine) intended to be double-insulated? If so, a suppressor can still be fitted, and connected to the metal parts of the motor (often arranged to connect the Class Y capacitors from L and N to the rotor through the bearings) - this is how some Class II vacuum cleaner motors have their suppressors connected. It still sorts out the EMC issue relating to the motor, but no connection with Earth is necessary. And yes, in such Class II appliances, the motor (or its rotor) may well sit above Neutral potential, which is no problem of course in machines where the user doesn't have access to the motor or any conductive parts connected to it.
Chris Pearson 11001208764
1452 Posts
Gentlemen, thank you. Ordinarily, the motor would be earthed by virtue of being bolted to the machine, but unless it is to do with suppression, the cable from the suppressor to the motor seems superfluous, which puzzles me.

At the moment, I shall be running the other motor clamped to a work bench so I shall have to devise an alternative PE. Thank you to Simon for pointing out that the cable from the green isn't the only thing which should go to the motor earth.
John Peckham 68348
395 Posts
Chris


I am thinking that the stud on the casing of your EMC filter needs to be earthed or bolted to an earthed part in addition to earthing the motor itself.


I this a Myford lathe? As I keep thinking that I might swap out my single phase motor on my Myford for matching VSD and new motor? Also another pipe dream is to fit glass slides and a digital read out display.
Simon Barker 22060613
645 Posts
The green terminal of the suppressor is there to aid in its suppressing function.  It's placing capacitors between each of the supply lines and ground.  These will present a low impedance to high frequency noise, effectively shorting it to ground.
gkenyon 82239607
1038 Posts
Simon Barker:
The green terminal of the suppressor is there to aid in its suppressing function.  It's placing capacitors between each of the supply lines and ground.  These will present a low impedance to high frequency noise, effectively shorting it to ground.

Usually, in Class I appliance, yes the Class Y capacitors go to PE. In Class II appliances, they can be connected to help ideal with eddy-type noise currents in the frame or rotor of the motor, and may not be connected to PE at all.

Chris Pearson 11001208764
1452 Posts
Gentlemen, thank you again for your responses. This rainy and windy afternoon, I have managed to get an additional bit of green and yellow down the flexible conduit to the supply side of the VSD - what a fiddle!

I have tried the machine with and without the suppressor and it makes no difference to how it runs so it will stay.

John, yes the suppressor is bolted to the cabinet when in situ, but the case is not continuous with the green terminal. I have very mixed feelings about VSD. Yes, they are great when they work, but in 30-odd years my single phase Super 7 never gave me any bother.
mapj1 80733779
2070 Posts
You have done the right thing. Without that CPC you would be creating a flying earth island and  as Graham alludes that is common inside a class 2 enclosure, but is quite dangerous for a metal bodied item as the two capacitors will float the case voltage to somewhere mid supply, not an issue inside a plastic box, less clever in a case like this.  Also where is the ADS if the motor faults to the case..
The aim of the caps is to make all 3 cores L/N/E equipotential to fast edged signals. (though still 230V between them at 50Hz and lower)

If you ever do have RFI problems, you may wish to RF choke the CPC, to raise the impedance to signals much higher than 50Hz. The usual then is a ferrite ring or beads
Chris Pearson 11001208764
1452 Posts
Mike thank you also. I think I now have a much better understanding of the situation. What a splendid forum this is! 😄

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