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Join Date: Oct 2008
Location: Ohio USA
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# Grounding Transformers in Control Circuits

10/08/2008 11:56 AM

the equipment that my company manufacture are starting use plc and relay boards. it seems every once in awhile we boards that just go crazy and changing them does not fix the problem. swaping the secondary side of the transformer does. 208v to 120v step down transformer does. i have looking a wire diagrams and i have noticed that transformer X2( on the secondary side) is not grounded and diagrams on other equipment it is. i hope someone can explain why it should or it does not have to be grounded. thanks

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Guru

Join Date: Jan 2007
Posts: 1024
#1

### Re: Grounding Transformers in Control Circuits

10/08/2008 2:03 PM

It all depends on the circuit and its purpose.

The purpose of grounding the secondary side of a transformer is to give it a reference point. i.e. if the secondary of the transformer was used to develop both + and - DC voltages, you would ground the center tap of the secondary.

It can produce different AC voltages.

It could also be used to prevent the chassis from building up a potential dangerous voltages.

In multiple phase circuits, the ground path basically replaces one wire.

An isolated circuit will not be grounded.

We can only guess without a schematic.

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Active Contributor

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#5

### Re: Grounding Transformers in Control Circuits

10/09/2008 8:16 AM

this used for AC only. There is another board that has a transformer on it that take the 120v AC to 12v DC. we have isolation transformer on other equipment and they are grounded. our electrical engineer can not explain why they do and why they don't. the transformer is use to step down the high voltage (208/240/480 3 phase coming into the machine) into a control voltage.

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Join Date: Jan 2007
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#6

### Re: Grounding Transformers in Control Circuits

10/09/2008 9:55 AM

If that other board is in anyways tied to the problem board, it could cause problems. The problem board has no ground so DC voltages will build up on the secondary side. This is not a problem if it is all AC, however it will cause problems if tied to a DC circuit. Either a ground, or a capacitor isolation link to the DC circuit should fix the problem.

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Join Date: Sep 2007
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#2

### Re: Grounding Transformers in Control Circuits

10/08/2008 11:00 PM

do you use an EMC filter after the transformer? that could help

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Active Contributor

Join Date: Oct 2008
Location: Ohio USA
Posts: 14
#4

### Re: Grounding Transformers in Control Circuits

10/09/2008 8:09 AM

no, i don't. it is a standard step down transformer(208/240/480) down to 120 for controls in 3 phase machine.

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#7

### Re: Grounding Transformers in Control Circuits

10/09/2008 8:23 PM

Then you should check the noise on the voltages during operation

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Anonymous Poster
#3

### Re: Grounding Transformers in Control Circuits

10/08/2008 11:16 PM

Sounds like a classic ground loop problem. By changing the leads, or adding a transformer you are altering (or more importantly) breaking the ground loop path. You now have a magnetically coupled circuit (no galvanic connection) from the power source to the load. Ground currents cannot flow in this case and the problem goes away. You can search the internet for ground loops (and find much more information than I could put here). I personally fix this type of problem by using transformers all the time. As a matter of fact, I had to add one in my entertainment set on the S-video line to eliminate the 60 Hz hum from the amplifier, due to ground loops.

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