Ground Reference

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Col.3:23

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Hello,
They say the best way to learn is to teach - I am finding that statement to be very true. While discussing system grounding with a group of 3rd year apprentices the topic of transformers with 120 volt 2 wire outputs came up. Having experienced connecting a 120volt isolation transformer that produced odd voltages until I grounded one side of secondary, I am still having trouble coming up with a good way to explaining why making this connection stabilized the output voltage of that transformer. I would appreciate any input that you would be willing to offer.

Matt
 
Col.3:23 said:
Hello,
They say the best way to learn is to teach - I am finding that statement to be very true. While discussing system grounding with a group of 3rd year apprentices the topic of transformers with 120 volt 2 wire outputs came up. Having experienced connecting a 120volt isolation transformer that produced odd voltages until I grounded one side of secondary, I am still having trouble coming up with a good way to explaining why making this connection stabilized the output voltage of that transformer. I would appreciate any input that you would be willing to offer.

Matt
Click to expand...

I believe because this is because by not grounding the secondary the nuetral is considered a floating nuetral. Although this nuetral will not have a direct connection to ground, it will still be capacitively coupled to ground and will develop a L-G voltage. However this natural capacitance to ground may not be exactly 120V to ground as would be with a solidly grounded connection and may be a value greater or less than 120V like say 90V. If that is the case then your L-N and L-G voltages will be incorrect and both output wires will have a different L-G. By grounding the secondary your transformer your establishing a solid connection to ground and stabilizing the voltage.

This is the way I understand it. If I am incorrect then I'm sure someone will chime in.
 
Col.3:23 said:
Hello,
They say the best way to learn is to teach - I am finding that statement to be very true. While discussing system grounding with a group of 3rd year apprentices the topic of transformers with 120 volt 2 wire outputs came up. Having experienced connecting a 120volt isolation transformer that produced odd voltages until I grounded one side of secondary, I am still having trouble coming up with a good way to explaining why making this connection stabilized the output voltage of that transformer. I would appreciate any input that you would be willing to offer.

Matt
Click to expand...

You should see a solid 120V across the secondary whether one secondary terminal is grounded or not. As a result of a voltage divider formed by stray capacitance, you would expect to see 60V to ground on each floating secondary terminal. You should be able to demonstrate this by using two identical low Z meters to simultaneously measure these line to ground voltages. The low Z meters form a more stable voltage divider than we get from stray capacitance alone.

By grounding either of the two wires, you force the other voltage to be 120V to ground, but the line to line voltage is unaffected.
 
I was thinking more of a 480-240/120 two winding transformer where typically the ground point is between the two windings. If this point in this type of transformer were floated what would the stray capacitance to ground be?
 
The voltage accross the secondary will be the same regardless if it is grounded or not. What will change is the voltage between either side and the seconday winding to ground. This is because of stray capacitance and can become a problem over time because of the inductance of the wire and ringing effect of a LC circuit.

When you ground one side of the secondary you short out the capacitance and establish a reference point. Ground is really the wrong term because it could be a car, plane, ship, or space craft which has no releation to earth or dirt.

So you got it half right IMO, referencing a system to a body like earth or ground does stablize a voltage with respect to the reference point. This is very useful and prolonges component and insulation life because you remove the resonance effect or ringing where the voltages can become quite high and breakdown insulation properites.

In addition to making a refernce point and stabilzing system voltages, it make over current protection very easy. Case in point take a ungrounded delta system vs a grounded system. In a delta system to protect from over current you have to install OCP devices on all circuit conductors. In a grounded system you only need to install OCPD on the hot or ungrouded circuit conductors.
 
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I started thinking about possible grounding scenarios on the secondary side of a 480-120/240 transformer and just wanted to clarify a few answers for myself.

In the attached sketch I have (3) secondary grounding scenarios.

In the first case I have the most common case which shows the secondary grounded between the two secondary windings. By doing this it will establish 120V L-G voltages on both of the secondary wires.

In the second case I have shown the grounding point on one of the secondary leads as I have seen done in the past. I'm guessing that the lead which is grounded will have a L-G voltage of 0V? What will the other lead have between L-G? I'm guessing that it would be 240V, and the center point would have a L-G reading of 120V? Is this correct?

In the third case the secondary is left ungrounded. What would the two L-G voltages be at both of the secondary leads? Would they be different values?

If in the third case a nuetral was connected to the center point what would the L-N voltage be. I would expected it to still be 120V even without a ground on the secondary due to the fact that the nuetral would simply be acting as a voltage divider.
 
mull982 said:
I started thinking about possible grounding scenarios on the secondary side of a 480-120/240 transformer and just wanted to clarify a few answers for myself.

In the attached sketch I have (3) secondary grounding scenarios.

In the first case I have the most common case which shows the secondary grounded between the two secondary windings. By doing this it will establish 120V L-G voltages on both of the secondary wires.

In the second case I have shown the grounding point on one of the secondary leads as I have seen done in the past. I'm guessing that the lead which is grounded will have a L-G voltage of 0V? What will the other lead have between L-G? I'm guessing that it would be 240V, and the center point would have a L-G reading of 120V? Is this correct?

In the third case the secondary is left ungrounded. What would the two L-G voltages be at both of the secondary leads? Would they be different values?

If in the third case a nuetral was connected to the center point what would the L-N voltage be. I would expected it to still be 120V even without a ground on the secondary due to the fact that the nuetral would simply be acting as a voltage divider.
Click to expand...

What sketch? Just guessing without seeing the drawing:

1. Sounds like a standard single phase 240/120 and L-G voltage is 120 and L-L is 240

2. Sounds like another single phase 240/120 again with a centered tap secondary just no ground? L1-L2 = 240, L1 or L2 - Xo = 120. L1 or L2 -G = unknown, X0 - G = unknown. No reference to ground.

3. Voltage cannot be determined as there is no reference. Voltage would be determined by whatever stray capcitance coupling there is to ground. In theory they should be roughly equal whatever that voltage may be. Same as an ungrounded delta service/circuit. In otherwords if you had a real circuit like this like a delta, and measured the voltage from one phase to ground, you could get anything and what ever reading you get is pretty much meaningless unless it is 0, or all three read the same which would indicate a ground reference somewhere.
 
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dereckbc said:
What sketch? Just guessing without seeing the drawing:

1. Sounds like a standard single phase 240/120 and L-G voltage is 120 and L-L is 240

2. Sounds like another single phase 240/120 again with a centered tap secondary just no ground? L1-L2 = 240, L1 or L2 - Xo = 120. L1 or L2 -G = unknown, X0 - G = unknown. No reference to ground.

3. Voltage cannot be determined as there is no reference. Voltage would be determined by whatever stray capcitance coupling there is to ground. In theory they should be roughly equal whatever that voltage may be. Same as an ungrounded delta service/circuit. In otherwords if you had a real circuit like this like a delta, and measured the voltage from one phase to ground, you could get anything and what ever reading you get is pretty much meaningless unless it is 0, or all three read the same which would indicate a ground reference somewhere.
Click to expand...

My apoligies I forgot the sketch. Here it is.
 
081210-1936 EST

mull982:

The many different questions you ask in different threads at this site indicates that you need to get some basic electrical education. There is nothing wrong with you asking the questions here, but I think you need to find a junior college and take some basic courses. This might start with various math courses, physics, maybe chemistry, and then basic electrical theory courses.

I do not want to discourage you from asking the questions you do here, and you should continue, but I believe you will advance faster in your understanding of electrical circuit theory by taking some formal courses.

There are three fundamental passive electrical components. These are resistors, capacitors, and inductors. Resistors dissipate energy, ideal capacitors and inductors do not dissipate energy. There is a new fundamental component called a memistor.

All real world components have characteristics of resistance, capacitance and inductance. Every piece of wire is a resistor, inductor, and capacitor. At DC frequency in the steady state it is a resistance. For a very rapid change of voltage applied across it it is an inductor and resists the instantaneous change of current thru it. At DC the wire has a steady state magnetic field around it. At DC and AC frequencies it has an electric field around it thus forms a capacitor.

A carbon composition resistor has wires to it that have a significant inductive effect in the 100 mHz range, and significant capacitance across it.

When ever you measure a voltage it has to be between two points. and a current has to be at a point. If a current is dispersed over an area, then you have to define its distribution over that area or assume it is uniform.

At high frequencies current primarly flows near the surface of an otherwise uniform conductor. Thus, the conductors apparent resistance is higher than its DC resistance.

To your specific question on the various grounding and ungrounded circuits the answer has previously been provided. However to expand. Two wires or other shaped conductors insulated from one another form a capacitor. The windings of a transformer are insulated from the core and maybe some other windings. These all have capacitance between them. Besides this there is leakage resistance between the windings and to the core. Thus, you have a combination of resistive and capacitive coupling to different places. Suppose the transformer core is mounted on large conductive plate. If you measure the voltage with a high impedance meter compared with the impedance of the coupling elements you will get readings that depend upon the relative values of the various leakage elements and the secondary voltage of the transformer. Use a very low impedance voltmeter and your reading will be near zero.

Directly connect any one point on the transformer to the reference plate and the voltage between the plate and any other point on the transformer secondary is that which you would read if the same two transformer points were measured in free space without the reference plate.

.
 
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