Autotransformer output clipping

[gar]

131004-1937 EDT

GoldDigger:

Originally when I quickly read your 180 deg statement I misinterpreted it. Now as I closely read it there is no problem.

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[LMAO]

131003-2122 EDT

GoldDigger:

Yes. I was only correcting your comment of 180 deg.

Hysteresis is basically a power loss and therefore can be viewed as a resistive load, but not linear. Its current component will be in phase with the applied voltage.


steve66:

You said the output voltage was 90 deg shifted from the input voltage. This is just not true for a closely coupled transformer. The magnetizing flux is shifted approximately 90 deg from the applied voltage. This flux in turn induces a voltage in the primary coil that opposes the applied voltage, and in turn limits input current. Called a counter EMF. If a second coil, the secondary, is linked to the same flux and wound in the same direction, then the induced voltage is in phase with the primary voltage.


LMAO:

What is special in your autotransformer that you have not described? Is it really just an autotransformer? That is a single magnetic core with a single winding and one tap somewhere along the winding, where the flux density is relatively uniform throughout the core.

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there are two identical unloaded 600:690V, 777kVA autotransformers in parallel, fed off 600V side of a 7.5MVA, 12470:600V transformer. Nothing special. Secondary side of 7.5MVA transformer voltage looks fine.
I am about to isolate the autotransformers and measure each secondary to see if that makes any difference...

 

[LMAO]

OK, I did some more measurements and the problem turned out to be the scope probes. We use medium voltage probes that have a few buttons for 10X, bandwidth, etc... and after I changed some settings I was able to get good looking sine-wave waveforms.
Sorry about that! But reading your comments, I learned a lot about magnetization and transformers.
Thanks to anyone who contributed.

 

[gar]

131008-1247 EDT

LMAO:

Do you have any idea why a probe would have caused that waveform distortion?

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[ggunn]

131008-1247 EDT

LMAO:

Do you have any idea why a probe would have caused that waveform distortion?

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I read what he posted as the probe misreading the waveform, not distorting it.

 

[LMAO]

131008-1247 EDT

LMAO:

Do you have any idea why a probe would have caused that waveform distortion?

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I read what he posted as the probe misreading the waveform, not distorting it.

The probe was distorting it, not misreading it (clipping the top/button peaks at an angle, see pictures on first page). Not sure if the probe is bad or I am doing something wrong. One thing I know is that the transformers are OK.

 

[GoldDigger]

I read what he posted as the probe misreading the waveform, not distorting it.

Either way, the waveform as displayed on the scope is distorted.
It is hard to figure out any filtering or attenuation or even high frequency compensation setting on the probes that could cause that, especially given the low frequency involved. But maybe an issue with the ground reference on the scope input, combined with a messy voltage between the scope chassis ground and the transformer neutral could have that effect.

 

[ggunn]

The probe was distorting it, not misreading it (clipping the top/button peaks at an angle, see pictures on first page). Not sure if the probe is bad or I am doing something wrong. One thing I know is that the transformers are OK.

How do you know that? If with a set of probes the displayed waveform on the scope is distorted, how do you know whether the waveform is actually being distorted by the probe or the probe is giving you bad data?

 

[GoldDigger]

How do you know that? If with a set of probes the displayed waveform on the scope is distorted, how do you know whether the waveform is actually being distorted by the probe or the probe is giving you bad data?

Semantics!
The probe is causing a distorted waveform to be displayed. 100% guaranteed.
It is very unlikely that an independent observation would show that the actual waveform at the transformer was distorted simply by the act of connecting the probe to it. (I would expect a smoking probe in that case.)

You call that a probe error, I call that distortion in (i.e. passing through) the probe. Either way you are seeing bad data on the scope. Just using different terms to describe it.

It agree that it is important to keep the distinction in mind though, since connecting a large non-linear load instead of just a probe might distort the actual transformer output waveform.

 

[gar]

131009-0845 EDT

LMAO:

Is there anything special about the probe?

A normal attenuating voltage probe is basically a resistor and compensating capacitor to produce both a capacitive and resistive voltage divider in combination with the probe cable capacitance and the R and C scope input. This is simply a passive network. There is nothing in such a normal circuit to produce the waveform distortion shown in your post.

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[ATSman]

Ok, what am I missing here?
If the same probes and settings were used on the primary and secondary, why isn't the primary sine wave distorted?? :?

 

[SG-1]

Ok, what am I missing here?
If the same probes and settings were used on the primary and secondary, why isn't the primary sine wave distorted?? :?

The primary is 12470, so I imagine the probes were reading the output of a PT. Most likely 120 volts. Then they were moved to the 600 volt output with out changing the settings. I supect the scope input was overloaded. Maybe using 1X instead of the 10X.

 

[ATSman]

The primary is 12470, so I imagine the probes were reading the output of a PT. Most likely 120 volts. Then they were moved to the 600 volt output with out changing the settings. I supect the scope input was overloaded. Maybe using 1X instead of the 10X.

SG-1
The way I read the OP: he is measuring the pri & sec of two 600V to 690V autotransformers in parallel. The 7.5MVA transformer is only the supply to the 2 paralleled transformers. So why would he have to change the scale on the probes?

 

[SG-1]

SG-1
The way I read the OP: he is measuring the pri & sec of two 600V to 690V autotransformers in parallel. The 7.5MVA transformer is only the supply to the 2 paralleled transformers. So why would he have to change the scale on the probes?

You are correct, 600 to 690. If the probes or input were maxed out at 600 volts, then another 90 volts may cause some distortion. I am speculating, because we do not know the make or model of scope, or it's ranges.

 

[gar]

131009-2343 EDT

SG-1:

Any normal 10x probe itself is probably good for 1000 V or higher without damage. A typical analog Tektronix scope is 5 V/cm, and +/- 4 cm, or +/- 20 V full scale. But the amplifiers do not saturate at full scale but somewhat higher. Then there is an adjustment pot to lower the sensitivity. So one can display a waveform considerably above +/-200 V with a 10x probe, If there was voltage breakdown in the probe, then the probe would be destroyed.

The distorted waveform is not what is expected from amplifier saturation, and saturation is off screen anyway. It would likely be flat topped from amplifier saturation.

Whatever is the cause of the waveform distortion, it is not something normally expected. A mirror image in the time base and it would look like a bridge rectifier connected to a capacitor input filter with a resistive load.

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[ATSman]

Thanks SV-1 and Gar.
Maybe he should have used a Fluke 43B PQA to capture the waveform

 

[Besoeker]

How do you know that? If with a set of probes the displayed waveform on the scope is distorted, how do you know whether the waveform is actually being distorted by the probe or the probe is giving you bad data?

I think you can reasonably assume the latter. Scope proves have impedances of upwards of 1Mohm. A x100 at least tens of Megs as a rule.
The 777kVA transformer, I would think, is rather unlikely to be affected by that.

 

[ggunn]

I think you can reasonably assume the latter. Scope proves have impedances of upwards of 1Mohm. A x100 at least tens of Megs as a rule.
The 777kVA transformer, I would think, is rather unlikely to be affected by that.

That's what I meant as well. From the way it was worded I inferred that the poster thought the voltage waveform he was measuring was affected by the probe. I've seen that in the lab with very low power circuits, but as you say, it's understatedly unlikely here.