Yaskawa Drive Fault

[craig_avanzar]

The true reason to only ground one end is to avoid "ground loops".

This is where a current is passed along the shield because it is connected at both ends and is therefore a complete circuit between two different sections of the machine that may have different potentials.

The magnetic field induced from the current flowing through the ground loop could cause more interference then using no shield at all.

Actually, having it grounded at only one end is WORSE for blocking RF, but the negative impact of potential ground loops is a bigger risk, so we only ground one end.

Here is a good explanation: http://www.epanorama.net/documents/groundloop/cable_shielding.html

Well I thought I knew to only ground one end, as it was explained to me, otherwise it would act as an antennae, BUT now I'm confused again. I suppose that there is more than one right way to do this.

 

[TheStarr]

I have heard the "Antenna" idea before too, but it doesn't pass my logic test.

Think about your truck/car, the radio antenna is only connected at one end (the other is sticking up in the air) and it works as a great antenna!

 

[DickDV]

Fellas, the reason instrumentation grounding is only to be grounded at one end is because ground potential differs from one location to another. Even 10 feet away, there can be measureable differences in voltage, both DC and AC.

If you connect your shielding at both ends, the shielding now tries to force both grounds to the same potential. As it tries to do this, significant AC and DC currents may flow, either exceeding the ampacity of the shield or at least inducing noise into the sensitive signals that were to be protected. Grounding at only one end keeps this from happening.

As to which end, I've always heard that the ground should be made at the sending or transmitting end. Actually, I've never seen any difference which end the attachment is made but, that doesn't mean that there is no difference. I cannot explain what the difference would be.

In power grounding, the above does not apply. Ground bonds are designed to carry current and are installed to force both ground locations to the same potential. This is necessary, for example, when grounding a motor and a frequency drive. Both must be grounded, and both must be tied together with a bond wire.

 

[keithkyll]

I have heard the "Antenna" idea before too, but it doesn't pass my logic test.

Think about your truck/car, the radio antenna is only connected at one end (the other is sticking up in the air) and it works as a great antenna!

One end of the car antenna is connected to the input, not ground. The center lead of the coax, not the shield.

 

[TheStarr]

Of course an antenna isn't connected to ground, it was an example, not a direct reference.

My point was that an antenna is not a complete circuit. It is a wire cut to a certain length and matched with the correct capacitance to resonate at the desired frequency.

Therefore, the idea that grounding the shield at only one end to prevent it from acting as an antenna is not correct.

DickDV is exactly correct, thank you for reinforcing my original message.

More importantly, PLC KKid, did you get the problem fixed?

 

[keithkyll]

You have DC, AC, and RF. The first 2 travel through conductors. The latter travels through the air. The shield on the wire is named appropriately. It shields the wires from outside noise. It blocks RF.
I don't disagree with DickDV. There can be a voltage difference between the 2 grounds, but the net result is a ground loop. A ground loop minimizes the effectiveness of the shield. The major source of noise is RF in the air. It does act as an antenna, and route the signal (noise) to ground.

 

[Protiusmime]

I'm enjoying this thread. It is interesting. So... Keithkyll, if I generate an RF sinusoidal wave at 100,000 Hz (near the low end of what is generally called "rf"), is this not Alternating Current? Or, ir RF an entity to itself and follows some other rules. It reminds me of the tank circuit at the Museum of Science and Industry in Chicago. The Galvanometer is located a few feet away, but low and behold, there actually is detectable energy coupled via "air" and inducing current in the detector. No, closed circuit as in DC, etc. Just curious.

 

[leitmotif]

I have thought about this for a couple days. I believe for the sake of this discussion we should be using term EMI instead of RF. EMI would include 60 Hz and associated harmonics which is probably more common than RF.

Dan Bentler

 

[keithkyll]

I'm enjoying this thread. It is interesting. So... Keithkyll, if I generate an RF sinusoidal wave at 100,000 Hz (near the low end of what is generally called "rf"), is this not Alternating Current? ...

Yes, RF is Radio Frequency, and it is AC. I used the term loosely to differenciate between electricity in a conductor verses electricity through the air.
Take an AM radio. Generate sparks, lightning, ignition noise from a car. The radio picks it up. What frequency is a spark?
Dan is correct - EMI is a better term.
At the input end, we use inductors and capacitors to filter out the noise. This cancels the AC component, leaving only the DC. That gets tricky when trying to read square waves from an encoder, because it is low frequency AC, increasing in frequency as speed increases. A high speed input has very little filtering, so shielding of the cable is critical.