Saturation test of a 1500/5 multi-ratio CT

[Tseppish]

During a saturation test of a 1500/5 multi-ratio CT, 400 volts is applied to the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?

Correct answer is 0 Volts.

Can we have clarification on what the question is suggesting? Are you applying 440 V directly on the taps? Aren't the X4 and X5 taps the same point, electrically, and would therefore have the same potential voltage?

[nscdrgs]

During a saturation test of a 1500/5 multi-ratio CT, 400 volts is applied to the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?

Correct answer is 0 Volts.

Can we have clarification on what the question is suggesting? Are you applying 440 V directly on the taps? Aren't the X4 and X5 taps the same point, electrically, and would therefore have the same potential voltage?

The 0V is the correct answer.
The CT is not like an "autotransformer". The circuit is closing thru x1-x4. There is no current flowing on X5. So zero current = zero voltage.

[SecondGen]

The 0V is the correct answer.
The CT is not like an "autotransformer". The circuit is closing thru x1-x4. There is no current flowing on X5. So zero current = zero voltage.

Agreed. This question has kind of tricky wording.

[GPerske]

Agreed. This question has kind of tricky wording.

The voltage across X1-X5 in the condition described will be 500V. 1500/1200 X 400 = 500

[Skyehaven40]

During a saturation test of a 1500/5 multi-ratio CT, 400 volts is applied to the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?

Correct answer is 0 Volts.

Can we have clarification on what the question is suggesting? Are you applying 440 V directly on the taps? Aren't the X4 and X5 taps the same point, electrically, and would therefore have the same potential voltage?

[BigJohn]

The 0V is the correct answer.
The CT is not like an "autotransformer". The circuit is closing thru x1-x4. There is no current flowing on X5. So zero current = zero voltage.
I don't follow? If you have voltage applied across terminals 1 and 4 of an autotransformer winding, even if there is no current flowing at terminal 5 you would still measure a boosted voltage.

What is different about a CT winding?

[BigJohn]

I actually tried this because it didn't make sense to me:

200/400/600:5 CT.

I applied 5VAC to X1 and X2 and got 15VAC between X1 and X4, just like you would expect on any multitap winding.

Maybe I'm missing something, but it appears that test answer is incorrect.

[Eiltsman]

GPerske and BigJohn are correct.
The entire length of the coil is one continuous wire. Therefore regardless of where you stab into the taps with your test voltage, voltage will exist on the entire coil and will be boosted beyond the test points, all the way to each end of the coil.
1500/1200x400=500.

[bertywho105]

I'm helping a friend study for an introduction to motor controls class. They have been really confused about calculating multi-tap on a CT. If a test voltage is applied X1 and X2 what would be the test on the subsequent x4 and x5 taps? Any help would be greatly appreciated!

[Realtime]

I'm helping a friend study for an introduction to motor controls class. They have been really confused about calculating multi-tap on a CT. If a test voltage is applied X1 and X2 what would be the test on the subsequent x4 and x5 taps? Any help would be greatly appreciated!

You need the ANSI "C" class number (C800, C400, etc) to determine. You can google ANSI C400 and get a graph showing the kneepoint voltages for all tap combinations.

[bob152637485]

During a saturation test of a 1500/5 multi-ratio CT, 400 volts is applied to the X1 to X4 tap. The X1 to X4 tap is the 1200/5 ratio. What is the expected voltage across the X1 to X5 tap?

Correct answer is 0 Volts.

Can we have clarification on what the question is suggesting? Are you applying 440 V directly on the taps? Aren't the X4 and X5 taps the same point, electrically, and would therefore have the same potential voltage?

My initial reaction was that the answer would be correct, but after drawing it out, now I am not so sure. I would definitely not say that it's the same point electrically, because we are dealing with AC as oppose to DC, so the fact that the coil of wire acts similar to a resistor with AC, it would be reasonable to have a difference in voltage of some kind. If they were the same point electrically, then you could say the same about X1 and X4, really.

Now, I am actually surprised that the voltage would not be higher than what was injected, because I would assume the CT to essentially act as an auto transformer, since it is one continuous piece of wire. Also, some other comments mentioned that they even tried it, and was able to read a voltage, so my instinct says the answer is wrong, but I am willing to be convinced otherwise.

[RabbleRabble]

The 0V is the correct answer.
The CT is not like an "autotransformer". The circuit is closing thru x1-x4. There is no current flowing on X5. So zero current = zero voltage.

If you take a voltage measurement across an open switch you will read line voltage. There isn’t any current flowing in an open switch. Current is dependent on voltage. Voltage is not dependent on current. The excited core of the CT induces a voltage in the windings between 4 and 5 that would be additive in this situation.