Looking for advice on mechanical hum

[mansr]

Audioquest should make a Humbug to go with the Jitterbug.

[mansr]

21 minutes ago, One and a half said:

@extracampine the meter should be capable of 300V or better, it is a digital type?

For UK sockets the orientation of the earth , neutral and active are:

top earth, clockwise next is active, clockwise next is neutral. 

 

Use the black probe and plug it into the earth. Take the red probe and connect to active.

this should read 230V. 

 

Move the red probe and connect it to neutral. This should read very low, less than 2V, mV is even better.

 

take both probes out and measure across active and neutral, the value should be the same as active to earth, 230V.

Nominal voltage in the UK is 240 V.

 

21 minutes ago, One and a half said:

This is measured at the wall. Some outlets have a shutter that will prevent you sticking probes in and you won’t be able to make contact. In that case, a flying lead with bare ends needs to be fitted to a set of terminals to measure, a standard IEC 320 kettle lead would work as they have the inscription on the plug to tell you which is active, neutral and earth.

The shutter is released by inserting something in the ground (top) terminal. No need for dangerous flying leads. The kettle lead is a good option though.

 

[mansr]

7 minutes ago, One and a half said:

240V must be due to Brexit then 

It has always been 240 V. There was an EU-wide "harmonisation" back in the 90s, but the tolerances were set such that nobody actually had to change anything. Replacing all the affected grid equipment would have been far too costly.

[mansr]

6 minutes ago, extracampine said:

I do own a multimeter - or if I didn't, they're cheap enough to buy. I am going to test the mains with it to see what the AC voltage is doing; hopefully report back with this in the next few days.

Some multimeters can measure DC offset of an AC voltage. The procedure varies, so check the manual for yours.

[mansr]

DC offset has nothing to do with earth. An AC voltage between two terminals has a DC offset if the arithmetic mean over one period is non-zero. It can be measured with a multimeter or an oscilloscope.

 

With a multimeter, set it to DC voltage. Disable auto-ranging, if available, and select a range (typically 500 V) suitable for the AC voltage. Attach the probes to the live and neutral terminals of a power socket. The meter should now display the DC offset. Some multimeters, e.g. Fluke 287/289, can display both AC and DC components simultaneously.

 

Using a scope requires a little more care. First, make sure the scope input supports mains level voltages. Mine says 300 Vrms, so it's fine. Next, you must verify which terminal is the neutral, or you might get a short circuit as the "ground" on the scope input is typically connected to safety ground. A multimeter can be used for this. Connect the scope to the power socket observing the correct polarity. Adjust the time base and vertical scale such that a few periods of are visible and use the scope's measurement features.

 

Always use the utmost caution when working on mains power. A misplaced finger can kill you. Use only equipment rated CAT II or higher. If you feel unsure or uncomfortable about anything, do not proceed.

[mansr]

1 hour ago, PeterSt said:

Except that earth is the only absolute reference (and then still FWIW).

We're talking about the voltage between two wires. Their voltage relative to earth (or the moon, for that matter) is irrelevant to the functioning of a transformer connected to them.

[mansr]

8 minutes ago, rayl1234 said:

What PeterSt was trying to say is -- there is no such concept as an absolute voltage, only a relative voltage (as voltage is a potential, like height....).  i.e. 0 to 240 vs 5000 to 5240 vs -120 to +120....  those absolute numbers only have meaning if you define what 0 means, e.g. relative to earth.... or relative to the average.... And from there, DC offset only has meaning relative to an external reference (DC offset relative to XXXX).

We have two wires. The relevant voltage is that of one relative to the other. A DC offset exists if the average voltage between the wires is non-zero. An offset of a million volts relative to ground doesn't matter to something, such as a transformer, connected only to these two wires.

[mansr]

34 minutes ago, rayl1234 said:

HUH?

 

The "average voltage between the wires" is never going to be zero.... Rather than repeat the equations here, here is a quick tutorial:

 

http://www.electronics-tutorials.ws/accircuits/average-voltage.html

Did you even read that page?

 

"For a periodic waveform, the area above the horizontal axis is positive while the area below the horizontal axis is negative. The result is that the average or mean value of a symmetrical alternating quantity is zero because the area above the horizontal axis (the positive half cycle) is the same as the area below the axis (the negative half cycle) and cancel each other out in the sum of the two areas as a negative cancels a positive producing zero average voltage."

[mansr]

28 minutes ago, rayl1234 said:

Yes, but that is not the definition, which is stated further down:

 

"The average value of a whole sinusoidal waveform over one complete cycle is zero as the two halves cancel each other out, so the average value is taken over half a cycle"

The average over half a period is uninteresting in this context (DC offset), so that particular definition is irrelevant. Either way, it too doesn't refer to an external earth reference.

 

Earlier, I said this:

7 hours ago, mansr said:

An AC voltage between two terminals has a DC offset if the arithmetic mean over one period is non-zero.

 

[mansr]

44 minutes ago, rayl1234 said:

PS: To be clear, I did not want to derail the thread or challenge whether DC offset is or is not the issue.  Only to say that a DMM's reading of average voltage, if it offers such, is not the right number. Need a meter that reads out DC offset, which Fluke true RMS DMMs can do.....

Most multimeters will display the DC offset if simply set to measure DC voltage. I tested it myself before posting here using both a Fluke 289 and a cheap, nasty $10 no-name meter. Neither measures the half-cycle average you're going on about (because it's hardly ever interesting).

[mansr]

18 minutes ago, rayl1234 said:

I am sorry if offense is taken from my remarks.  I only mean to correct inaccuracies.

 

1. It is absolutely correct that DC offset is not measured relative to earth. That was someone else's statement that I was trying to clarify what I think he meant. Maybe that just added more confusion.

 

2. DC offset is the LPF of the AC signal.  (Though for higher power loads, Hall effect is used instead of trying to build an LPF.)

DC offset is the, uhm, DC component, also known as the average. Yes, a low-pass filter with a cutoff of a few Hz is one way of finding it.

 

18 minutes ago, rayl1234 said:

Thus any reference to average voltage, or zero, etc., is not really the best way of interpreting it.  If my being a stickler for precise definitions for engineering terms is offensive, then let me just retract the comment.

The standard definition of average is the integral divided by the range. Are you using some other meaning of the word?

 

18 minutes ago, rayl1234 said:

3. Not all DMMs will correctly measure DC offset if you just set it to DC mode.  You see crazy stuff like the OP in this: https://arstechnica.com/civis/viewtopic.php?f=11&t=100759  No, he did not really have a 20V DC offset....

Auto-ranging (or picking a bad range manually) can mess things up if the DC component is small compared to the AC voltage. That might be what happened there.

 

18 minutes ago, rayl1234 said:

Thus, I would only trust a DMM that is documented to measure it.  That's perhaps the non-academic comment I wish to make.

Using documented features is always best. The Fluke 87 manual suggests the method I mentioned earlier. There are of course other decent makes of multimeters, but I'm most familiar with Fluke.

[mansr]

8 hours ago, rayl1234 said:

I will agree that your use of the term "average" is of the typical mathematical text and I overreacted bec I read it in the AC electrical context, where it is 0.637 x peak voltage (hence will never be 0 for mains voltage).

That would be the average absolute voltage, which is but one of many averages one might compute.

 

8 hours ago, rayl1234 said:

However, I still urge caution on assuming the DC measurement from a DMM correctly measures the DC offset w/o your own LPF or a DMM explicitly engineered with an LPF.  Here is an article from Fluke urging caution on measuring DC when large AC component is present, which is the relevant use case for this thread.

 

http://download.flukecal.com/pub/literature/9080007_a_w.pdf

Interesting, thanks. Do note that this paper is mainly about precision measurements in the nanovolt range. Some of the effects discussed are not applicable here.

 

[mansr]

1 hour ago, extracampine said:

OK, further to the above - I tested the mains voltage at an outlet in my house with a multimeter and it was reading both 245 and 246 volts. 

That's normal for Britain.

[mansr]

20 minutes ago, extracampine said:

OK One and a half - I looked back at the post you mentioned. Here are my findings:

 

Black probe to earth, red probe to active: 246V

Black probe to earth, red probe to neutral: nothing registered on my multimeter - 0.00V (however the lowest "setting" for AC V is 200V)

Black probe to neutral, red probe to active: 246-247V

 

You initially said that it should read 230V. Does this therefore seem quite high?

230 V is normal for continental Europe. Nominal voltage in Britain is 240 V, and readings of 250 V are not uncommon.

 

If your multimeter has a DC range of 400 V or higher, you can try checking for DC offset using this. Some meter designs can apparently give false readings in this situation, but it could still provide some information.

 

As always, be extremely careful when handling mains power.

[mansr]

9 minutes ago, extracampine said:

I have the Sinometer 830B - this one:

 

http://jbryant.eu/pages/DMM.htm

 

I think it has a DC range of 600V, to the left of "OFF" if I am not mistaken?

That is correct.

 

A lot of cheap multimeters have exactly those settings. I suspect they all use the same circuit. I tested one, and it measures DC offset correctly, though that's of course no guarantee of anything.