Isolation Transformer
- Thread starter awc
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ELA
Senior Member
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- Electrical Test Engineer
dereckbc said:
What if it is differential mode noise :smile:
An isolation transformer can help clean up the line up to a point. The higher the offending frequency the less attenuation the transformer will provide.
If the isolation transformer has a electrostatic shield between windings that helps it work to higher frequencies.
As far as the ground it will not help clean up ground noise if the ground is bonded to the secondary.
L
Lxnxjxhx
Guest
Here's a link but I couldn't easily get a data sheet for one of these. Having numbers is better than saying "superior" and "noise reduction".
http://www.tripplite.com/products/conditioners/transformers.cfm
Their sales pitch does say:
"Superior line noise reduction and spike suppression
Faraday Shield offers 100% isolation from the input AC line
Secondary neutral to ground bonding eliminates common mode noise
Removes EMI/RFI noise, utility switching transients and background spikes"
I would say "removes" should be "weakens."
http://www.tripplite.com/products/conditioners/transformers.cfm
Their sales pitch does say:
"Superior line noise reduction and spike suppression
Faraday Shield offers 100% isolation from the input AC line
Secondary neutral to ground bonding eliminates common mode noise
Removes EMI/RFI noise, utility switching transients and background spikes"
I would say "removes" should be "weakens."
It may or may not and depends on what you are talking about. For example if you are talking high frequency noise using a shields and ferroresonant tunning it will virtually eliminate all HF noise, but for common mode noise, there is nothing better than a transformer for the money. Otherwise you would use a dual conversion UPS to remove both common and normal modes.ELA said:
Sorry but that is inaccurate. The main function of an isolation transformer is establishing a new power ground reference point close to the point of use. This greatly reduces (up to 140 db) the problem of common-mode noise induced through "ground-loops" or multiple-current paths in the ground circuit upstream of the newly established reference ground point.ELA said:
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ELA
Senior Member
- Occupation
- Electrical Test Engineer
dereckbc said:
Read closely. I said it would not clean up the "ground noise". Not that it cannot remove common mode noise from the other two conductors with respect to ground.
An isolation transformer can provide a new reference point. It can also be clean if the incoming ground is isolated from the outlet ground.
However if the inlet ground is bonded to the secondary, and the ground has noise on it .... it will not magically remove that ground noise .
ELA it is all in respect to the power source. An isolation transformer forms a new ground point, what ever is upstram is never seen by the equipment with respect to ground once the new reference point is established. So yes it magically makes the ground noise dissapear. 
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- retired electrician
There is no code compliant way to accomplish that. The primary and secondary equipment grounds must both be connected to the transformer enclosure.ELA said:
Ok, I believe my question is now answered. The bonding point on the isolation transformer will form a new grounding point and help reduce some of the specific noises? :-? So now would you bond to building steel as you would any other transformer. Sorry guys but i'm new to all this so I apologize if some of my questions are a bit annoying.
ELA
Senior Member
- Occupation
- Electrical Test Engineer
don_resqcapt19 said:
I know this. I was pointing out that is the only way to get true isolation.
If the ground is noisy, which it often is. Ground is often refered to as the sewer system of electronics.
I don't believe in magic.dereckbc said:
If there is a large amount of noise on the ground it passes directly across to the load side of the transformer. Then due to either leakage or EMI protective capacitors on a piece of equipment downstream these currents can flow into the equipment intended to be protected by the transformer and upset it.
In addition it may be the case that the equipment downstream of the transformer has a reference to ground though its I/O or communications ports (could be a distance away). The impedance of this ground may be different than the ground at the transformer secondary.
This allows noise currents to continue to flow.
This can be demonstrated by performing EMC tests.
It would be great if you could totally isolate the ground lead as well. NEC required electrical safety (bonding) often conflicts with a desire to have a noise free electrical environment.
I am not sayinig the isolation transformer is not a great tool. Just that it cannot perform magic (When the secondary is bonded to the primary ground).
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AWC you are asking good question and no reason to apologize. Before you read my answer do yourself a favor and read 250.30(4) as it will tell you your options.awc said:
OK did you read?
The GE SHALL BE as near as practical to and preferable in the same area as the GEC connection to the system The GE SHALL BE the nearest one of the following:
1 An effectively grounded structural metal member of the structure. (that would be your building steel and IMO the best there is and does not get any better).
2. An effectively grounded water pipe within 5-feet from point of entry to the building.
3. Other Electrodes. Go read I am tired of typing. :wink:
One more note I design a lot of data centers and we use isolation transformer out the wahzoo. Typically we install what we call RFB's (raised floor busses) under the raised floor. The RFB has a dedicated 4/0 or large conductor ran back to the building ground electrode system and bonding jumpers to the nearest building columns, and connected to the SRG, raised floor pedestals, conduits, pipes etc?
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
080423-1405 EST USA
Broad comments:
An electrostatic shield between primary and secondary and appropriately connected to "something" only shields capacitively coupled signals from the primary to secondary. That something is meant to be vague, but it is not the hot side of the primary. The electrostatic shield has negligible effect on magnetically coupled noise.
Inherently a transformer is a low pass filter because leakage inductance is a series inductor between primary and secondary. This is excluding capacitive coupling.
In the true sense an isolation transformer has no DC path from primary to secondary. In reality with no electrostatic shield there will be some leakage resistance from primary to secondary, but this generally small.
In my opinion once you connect an isolation transformer so that there is a connection between the primary and the secondary it should not be called an isolation transformer.
Without an electrostatic shield more noise is coupled from primary to secondary thru primary to secondary capacitance as frequency increases. This is because capacitive reactance decreases with frequency.
If you connect the primary ground conductor to the secondary ground conductor you do not break the ground current path thru these conductors. Again I am being sort of vague. If by ground is meant earth, then it is not generally a very good conductor, but substantial current can still flow thru earth. I can have an voltage gradient in the earth from one end of a building to another whether a true isolation transformer is used or not.
If I have a steel structure building all bonded together with a concrete floor and all reinforcing rods bonded to the building I can have a fairly good equipotential structure even with the earth having a potential gradient. It is a matter of the relative values of resistance.
If I have a sensitive electronic system battery powered inside a magnetically and electrostatically shielded box I can subject it to most normal magnetic and electric fields with little internal effect. If I use a fiber optic communication cable to communicate with this box I can raise the box to a 1,000,000 V above ground and probably not affect its operation. The key point here is that if I want to solve many electrical noise problems I should use electrical isolation of the gaging and communications areas and not worry too much about uncontrollable ground current problems.
I have to assume the above mentioned Tripplite transformer also includes filter circuits.
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Broad comments:
An electrostatic shield between primary and secondary and appropriately connected to "something" only shields capacitively coupled signals from the primary to secondary. That something is meant to be vague, but it is not the hot side of the primary. The electrostatic shield has negligible effect on magnetically coupled noise.
Inherently a transformer is a low pass filter because leakage inductance is a series inductor between primary and secondary. This is excluding capacitive coupling.
In the true sense an isolation transformer has no DC path from primary to secondary. In reality with no electrostatic shield there will be some leakage resistance from primary to secondary, but this generally small.
In my opinion once you connect an isolation transformer so that there is a connection between the primary and the secondary it should not be called an isolation transformer.
Without an electrostatic shield more noise is coupled from primary to secondary thru primary to secondary capacitance as frequency increases. This is because capacitive reactance decreases with frequency.
If you connect the primary ground conductor to the secondary ground conductor you do not break the ground current path thru these conductors. Again I am being sort of vague. If by ground is meant earth, then it is not generally a very good conductor, but substantial current can still flow thru earth. I can have an voltage gradient in the earth from one end of a building to another whether a true isolation transformer is used or not.
If I have a steel structure building all bonded together with a concrete floor and all reinforcing rods bonded to the building I can have a fairly good equipotential structure even with the earth having a potential gradient. It is a matter of the relative values of resistance.
If I have a sensitive electronic system battery powered inside a magnetically and electrostatically shielded box I can subject it to most normal magnetic and electric fields with little internal effect. If I use a fiber optic communication cable to communicate with this box I can raise the box to a 1,000,000 V above ground and probably not affect its operation. The key point here is that if I want to solve many electrical noise problems I should use electrical isolation of the gaging and communications areas and not worry too much about uncontrollable ground current problems.
I have to assume the above mentioned Tripplite transformer also includes filter circuits.
.
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johnny watt
Member
Just guessing here?
But if you connect any primary wire to the same point as any secondary wire
ya don?t got an ?isolation transformer?
ya just got a transformer.
But if you connect any primary wire to the same point as any secondary wire
ya don?t got an ?isolation transformer?
ya just got a transformer.
jrdsg
Senior Member
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- Vancouver, Canada
signals and cctv
signals and cctv
this is a significant problem for those of us installing camera wiring, since we are dealing with only 1V Pp signals to start with.
signals and cctv
this is a significant problem for those of us installing camera wiring, since we are dealing with only 1V Pp signals to start with.
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