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.