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Noisy Tranformer - Bad core laminations?
4

Noisy Tranformer - Bad core laminations?

Noisy Tranformer - Bad core laminations?

(OP)
Hey all,
I have a 300kVA Marcus brand transformer with an excessively high sound output. The sound output has been metered at ~85dB, which is well over the rated 65dB. The transformer was only purchased and install a little over 1 year ago. It was physically inspected and is clean as a whistle. Runs very cool.

Here are the tranformer specs:
Brand            Marcus
Model            MK300A7-13-F
Power Rating        300kVA
High Voltage        600?347V
Low Voltage        416Y240V
Impedance        3.3%
Temp. Rise        115ºC
K-Factor        K13

We had some test equipment install for approx 24 hours, which showed low power factor ~0.5, and elevated current harmonics %THD consistantly over 25% (between 27.5% and 30%). What should I make of these results?? I was told it could be bad core lamination. Any advice/comments are appreciated!

RE: Noisy Tranformer - Bad core laminations?

It sounds like you have an elevated primary voltage. Not much that can be done (add more turns to primary winding or lower primary voltage is all that I can think of). I do not think that the laminations are loose.

K13 is not a very high distortion factor. Do you have much non-linear loads? (Like VFDs and SCRs).

Gunnar Englund
www.gke.org

RE: Noisy Tranformer - Bad core laminations?

(OP)
The primary voltage readings stayed below 347V for the duration of the 24 hour test. At some points the voltage dipped to as low as 338V, but only for short time periods. The voltage %THD stayed between 1.0% and 1.7%. So I don't believe the voltage is the problem.

The loading on the transformer is part of a plastic extrusion line: an extruder motor (150kW DC), a melt pump (small), and heating elements.

RE: Noisy Tranformer - Bad core laminations?

I assume those  voltage THD numbers came from the line side.

Were the current harmonics on the load side or the line side?  Did you check the opposite side?  If current THD higher on the load side than I tend to think the load created the harmonics, if higher on the line side I would think lean more towards overexcitation.

Don’t rule out overexcitation based on a voltage measurement without looking at the tap settings.  With the fairly low transformation ratio, is this possibly an autotrasnformer?

=====================================
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RE: Noisy Tranformer - Bad core laminations?

(OP)
The testing was performed on the primary side only.

It's not an autotransformer. It's a K-Factor 13 as described at http://www.marcustransformer.com/vw/fs/p017.htm

The Marcus site claims its design is aimed at non-linear loads. And it's 'features' are listed as:

§ Specifically designed copper windings with multiple conductors reduce harmonic high frequency 'skin effect' and increase efficiency
§ Low flux density wound cores reduce saturation
§ Oversized neutral conductors eliminate overheating
§ Double grounded electrostatic shielding minimizes electrical noise

I just don't know what to think about all this.

RE: Noisy Tranformer - Bad core laminations?

If it has been noisy its entire life, the first thing I would check are the internal bolts.  Usually the core is firmly bolted into place for shipment and then once installed the bolts are supposed to be backed off so that there is some give in the rubber mounts.

RE: Noisy Tranformer - Bad core laminations?

(OP)
Hmm that's an interesting thought. We had some professional maintenance guys do a visual inspection when the plant was shut down last month. They said everything was tight (probably referring to the connections), but as you say, perhaps something is too tight!?

I still think it must be something electrically related because it buzzes loudly when under load, but not when the machinery is off-line.

RE: Noisy Tranformer - Bad core laminations?

Your 150 kW DC motor is probably (I'm sure) thyristor controlled. SCRs produce lots of harmonics.

Is the sound level the same if the extruder is lightly or heavily loaded?

Is the sound a typical "electric hum" or is it a "yelling" sound?

Are there line reactors for the thyristors? Or does the transformer serve as a combined line reactor/transformer?

In the latter case, sound is inevitable.

Gunnar Englund
www.gke.org

RE: Noisy Tranformer - Bad core laminations?

(OP)
The sound level isn't really variable. It's either on or off: loud or quiet. If's definitely more than a hum, like a loud buzzing. We have a larger 750kVA in the same electrical room which makes half the noise but runs way hotter.

As for reactors or thyristors, my knowledge of the internals of the control cabinet is limited. I'll check in to it if I can.

RE: Noisy Tranformer - Bad core laminations?

Loud sound can be relative.  If you can somehow check the frequency of the sound and distinguish it from 60 cycle hum, you may find that it is indeed a result of nonlinear loads.  Sounding louder when the load is on is a sign it may be caused by the nonlinear loads.

RE: Noisy Tranformer - Bad core laminations?

(OP)
It was proposed by the company that did the testing that the source of the bad current harmonic distortion could be a result of bad iron core laminations. From the email I recieved:
-------
It is possible that these harmonics are at least partly responsible for the noise level in the transformer. However, I have spoken to several people about your problem and the general consenus is that the most likely culprit is the laminations in the transformer. Although there is likely nothing "wrong" with the transformer, our suspicion is that the iron in the laminations is simply not top quality - which is relatively indicative of a low cost Marcus transformer.
-------
It kind of makes sense that bad iron would result in greater hysteresis losses which in turn increases the THD.
Would anyone agree with this evaluation?

[I'll post the graphics they sent me so you can take a closer look]

RE: Noisy Tranformer - Bad core laminations?


Core lamination noise (called magnetostriction) is not load dependent since the flux remains almost constant from no-load to full load.

Did you check whether the frequency is as per name-plate  (50 or 60 Hz) ?

* Algebra - The weapon of math destruction *

RE: Noisy Tranformer - Bad core laminations?

The most common thought is that the harmonics results from the load connected to the transformer.  However, I can agree that the iron lamination condition can contribute to excessive noise.

RE: Noisy Tranformer - Bad core laminations?


of course, my comment is valid only for linear loads. Non-linear loads may id=nduce higher harmonic noise from the laminations.

* Algebra - The weapon of math destruction *

RE: Noisy Tranformer - Bad core laminations?

The current waveform (3rd graph from end - I assume that the graph Y axis is labeled incorrectly) is typical of 3 phase bridge rectifier current upstream of a delta-wye transformer.  This is apparently the source of the current harmonics.  The source must be pretty stiff or you would have voltage notching at the current discontinuities.

RE: Noisy Tranformer - Bad core laminations?

(OP)
The transformer is fed from a large piece of switchgear (in the same room). The fuses are Ferraz Shawmut A6D300R. And the switchgear is fed from a 5MVA (25kV/600V) transformer. I'm not sure what you mean by an upstream rectifier? There's nothing else inline with the tranformer.

RE: Noisy Tranformer - Bad core laminations?

What I meant was that the current was measured on the source side of a delta-wye transformer.  By stiff source, I meant that the source impedance is low.  If it weren't, then the step increases in current (high di/dt) would result in sharp voltage increases or decreases (notching) at those points in the waveform.  V=L·di/dt

Considering that the 300 kVA transformer has only about 70 kVA of load and that it has a K-factor of 13, there does not appear to be a loading problem with the transformer.  There is no voltage harmonic problem.  The only problem appears to be the noise.

RE: Noisy Tranformer - Bad core laminations?

I agree with jghrist's observations. What you labeled as V is actually current drawn by a three phase full wave rectifier.

In most cases I believe transformer noise is electromagnetic in origin and unrelated to any looseness. Certainly it applies to motors (like fractional slot motors which are rich in harmonics and very loud).

It was suggested this is not electromagnetic since the flux does not change with load much.  That may be true for sinusoidal current but I don't think it's true with so much harmonic content.  

My vote (guess) is that it is most likely electromagnetic noise related to the load harmonic current.  There isn't much you can do about that... unless maybe someone sells a filter for this situation?

=====================================
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RE: Noisy Tranformer - Bad core laminations?

I think that the labelling is correct. There are two sets of vertical scales,volts to the left and amperes to the right.

The current waveform is not typical for a bridge rectifier input current, there are usually two equally high current pulses. But given that there are also linear loads and that the transformer winding also influences the total curve form - current pulses added - I think that this is a very normal look for a transformer primary when a thyristor controlled DC motor is connected.

What I do not like at all is the phase relationship between the currents. There must bee a mistake when connecting the voltage CTs. Phase A current should be 180 degrees shifted.

The interesting thing is that the voltage waveforms are also shifted. Again, the phase A is 180 degrees wrong. I really hope that it is a measurement connection error and not a condition that the transformer is working under. That would certainly produce lots of interesting effects.

It is interesting that PF seems to be about right (assuming that the extruder was run at low speed). So the connection U vs I is probably right.

Anyhow. With that primary current, there is bound to be a lot of noise from the transformer. The thyristor commutation seems to take place in about 3 degrees electric so it looks like the transformer is also doing duty as a commutation reactor. That is probably why you get so high a noise level.

Gunnar Englund
www.gke.org

RE: Noisy Tranformer - Bad core laminations?

"I think that the labelling is correct. There are two sets of vertical scales,volts to the left and amperes to the right."
But that particular graph only shows current, not voltage.

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RE: Noisy Tranformer - Bad core laminations?

OK, I see the colors are supposed to tell us that we're looking at current.  And the graph a few above has the voltages.  

I also see what you are saying the current peaks do not appear in the same order as the voltage peaks which seems like just a measurement polarity.

You are right - this looks like a transformer input waveforms resulting from add together two phase currents of the FWR input such as would appear on the input of a delta-wye transformer feeding a FWR.  See last waveform on page 2 here
http://home.houston.rr.com/electricpete/FWR.pdf

Why do you think it has to be a thyristor-commutated  rather than a line commutated (diode bridge) rectifier?

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RE: Noisy Tranformer - Bad core laminations?

(OP)
Thanks for your input everyone!

I'll try to collect a few more load specifications which might help us find a solution.

The puzzling thing is that we have 8 other similar extruders, a few with dedicated 175kVA tranformers, and they don't seem to make nearly as much noise. Of course, those ones aren't likely K13 rated. Is it possible the higher K-rating is trying to 'fight' (damp) the harmonics generated by the DC motor and is therefore working harder?

Addressing the graphical issues, I don't see the problem. It goes Ch.2-Ch.1-Ch.3 (Yellow-Red-Blue) in both cases; voltage and current. Do you think Ch.1 and Ch.2 should have been reversed? Does it really make a difference?

RE: Noisy Tranformer - Bad core laminations?

You're right: 2, 1, 3 for both current and voltage.

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RE: Noisy Tranformer - Bad core laminations?

Re. voltages and currents:

The vector sum of the three phases should be zero. The red phase signal is definitely reversed on both the voltage and current traces. It is inverted in polarity on the image. The other two phases look normal. If the transformer really is wired up like this, and it isn't a measuring instrument error, then expect wierd things to happen. You can verify this by looking at the time axis on the images: there should be 5.56ms between each successive crest for a 60Hz system, where your sequence shows periods of 2.78 ms / 2.78 ms / 11.11 ms between crests.

Transformer must be a delta-star (or star-delta) type as noted above to produce the current waveform displayed. Not much inductance in the load either.

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Noisy Tranformer - Bad core laminations?

(OP)
I will contact the person that performed the testing and point out the waveform issues mentioned. I agree that the vector sum of the waves should be zero, and clearly this is not the case.

RE: Noisy Tranformer - Bad core laminations?

Sorry to be late with these comments to the posts above. Tried earlier, but the "<." stopped me.


First, if it is a DC motor for an extruder, it has to be a speed controlled one. And that means thyristor control. The commutation is when phase B takes over from phase A etc. That means a momentary short between the phases and that's what normally produces the sound, either in the line reactors or in the transformer - as seems to be the case here.

Second, the angle between the phases shall be 120 degrees. You have 60, 60, 240 degrees in the recordings. That's why I think there is a polarity error in phase A (the one that sits between B and C).

Gunnar Englund
www.gke.org

RE: Noisy Tranformer - Bad core laminations?

(OP)
I just collected the info off the DC motor:

Brand: Thrige-Scott Ltd.
Type: Shunt Motor
Power: 150kW
RPM: 2000
Armiture: 460V - 350A
Exciter: 360V - 6.1/4.85A

Again, the control cabinet also powers some small melt-pumps and heats.

I checked the power factor stated by the switchgear and it said 0.77 (with the extruder off-line). How is this related to the ~0.5pf displayed in the graphs? I didn't think the pf on the switchgear bus and the transformer primary would be different considering they're directly connected.

RE: Noisy Tranformer - Bad core laminations?

The phase error would be in the voltages (you said current before). I agree with you know one phase voltage is flip-flopped  ( I guess I've flip-floppwed a few times on that subject... better keep my mouth shut).

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RE: Noisy Tranformer - Bad core laminations?

What do you mean by "power factor stated by the switchgear"?  Is this power factor measured on the 600V side or the 416V side?  If it is with the extruder off-line and the pf with the extruder on-line is 0.5, then the extruder has a lower power factor than the rest of the load.  How much depends on how much of the total load is made up by the extruder.

RE: Noisy Tranformer - Bad core laminations?

(OP)
We have a piece of metering equipment installed in the switchgear which shows the instantaneous kVA, kVAr, kW, pf, etc. of the SG bus. It claims a pf of 0.77, which is okay. Why would the pf on the transformer primary be so bad (0.5pf)? I'm starting to get the feeling that the test equipment must have been set up incorrectly.

RE: Noisy Tranformer - Bad core laminations?

"I didn't think the pf on the switchgear bus and the transformer primary would be different considering they're directly connected."

I'm not surprised by the difference even if the two metering points are being monitored simultaneously. Your PQ metering is looking at true rms values that include higher order harmonics. Your switchgear metering is probably only looking at fundamental quantities.

RE: Noisy Tranformer - Bad core laminations?

If your instrument had the red phase inputs reversed, you could not really expect to get any meaningful pf information from it. Was it a different instrument to the one which captured the waveforms?

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Noisy Tranformer - Bad core laminations?

(OP)
Well the fact that the waveforms are not as expected, and the powerfactor is significantly lower than I think they should be, I'd say the leads were hooked up wrong.

The thing is, I remember when he hooked them up, he said "the tester won't allow them to be backwards, it'll beep or something". Haha, as a skeptic I believe everything can be hooked up wrong!

I've already emailed the guy, so I'm hoping he'll get back to me soon, maybe tomorrow.

RE: Noisy Tranformer - Bad core laminations?

Calculating the power factor from the measured kW and kVAR gives around 0.47 pf.  At about noon, total kW is around 90, total kvar is around 167, giving total kVA of around 190 and pf = 90/190 = 0.47.  This would include the distortion factor.  

Calculating the power factor from the phase displacement also gives around 0.42 pf.  Yellow phase voltage peaks at about 5.6 ms, yellow phase current peaks at about 8.6 ms, 3 ms lag or 65°, cos(65°) = 0.42.

These two measurements are within the margin of error of picking up points from the graphs.  It wouldn't appear that the distortion contributes significantly to the low power factor.  This is to be expected with very low voltage distortion.

The transformer var loss would not account for the difference.  It is only 60% loaded and the impedance is only 3.3%.  I still think the difference can be explained by the fact that the switchgear bus measurement was with the extruder off-line.

RE: Noisy Tranformer - Bad core laminations?

I agree the red phase leads, both V and I are reversed. As Skogurra noted, this placed the red phase pf right in line with the other two phases. Watts, VARS, rms values, THD, pf; all measures the same. Just the two waveforms are inverted.

RE: Noisy Tranformer - Bad core laminations?

(OP)
OKAY, here's what the "technical representative" said in his reply:

"Yes, it looks like the PT may have been reversed. Hard to say for sure, but it is either that or you have a more serious power problem that we first thought! <G>"

I'm not sure what the "<G>" means.. but it looks like I might not get much help out of this guy.

So you think besides the waveforms that the other data is correct? I find this a bit hard to believe.

RE: Noisy Tranformer - Bad core laminations?

Hello Chap,

The <G> probably means "Grin" - a way of saying "Sorry, but what can I do about it?"

I have studied your waveforms in detail. Except the phase error in A voltage and current, I think that the data are correct. The low PF is typical for a thyristor controlled DC motor running at low speed. In fact, there is an almost linear connection between percent rated speed and power factor.

As I said earlier, the commutation between phases seem to take about 3 electrical degrees. That is a rather typical value. Perhaps on the low side. And that indicates that the transformer with its low impedance is the only impedance in the thyristor/motor path. That makes the transformer noisy - nothing to do about it. It won't fall apart.

Gunnar Englund
www.gke.org

RE: Noisy Tranformer - Bad core laminations?

Well alot good info has been discussed and without an electrical solution I suggest. Placing rubber under the transformer mounts to reduce the noise transfer to the floor that will help reduce the total amount of noise in the room.

RE: Noisy Tranformer - Bad core laminations?

(OP)
That's a good idea. I'll discuss it with my superior. I'm going to recommend more testing (ensuring the leads are attached properly), and maybe an analysis of the motor control cabinet.

Thanks again to everyone who offered info and advice.

If I see any further conclusion to these issues I'll be sure to post back with the results. THANKS!

RE: Noisy Tranformer - Bad core laminations?

Quote:

"I didn't think the pf on the switchgear bus and the transformer primary would be different considering they're directly connected."
Another point;
The power factor is a phase shift between the voltage and the current. If there are any other loads on the switchgear bus, then the phase angle of the switchgear current will be the vector sum of all the load currents. This current and the resulting phase angle and power factor may easily be different than the current, current phase angle and the power factor in the transformer.
respectfully

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