AC meters

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gar

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220305-2009 EST

Fundamentally what do these various meters read in AC mode? On a balanced AC signal, and with the addition of a DC component.

1. Simpson 260 or 270
2. Fluke 27
3. Fluke 87
4. An electrodynamometer type
 
220305-2414 EST

I am very glade to see Larry responding. But there should be many more responses. My other question was based on exactly the same question that A. D. Moore ask of students in a senior design class way back in the 1950s for me, and I suspect he had ask this question for many years before. It is surprising how many senior EE students failed this question.

A. D. Moore was an interesting person, and teacher. Usually students that wanted to avoid being questioned would sit in the back rows. Actually being in Moore's class and in the back of the room meant you were more likely to be ask to answer a question than if you sat up front. Actually Moore knew which students knew what was going on, and no matter where you sat made no difference. He would pick on the persons he knew wetre not really studying.

Moore was also a character around town. He was a city council member, and he rode his bicycle most places. He was also the youngest person to be president of the national Ta Beta Pi association.

The first half of our senior class with him was devoted to learning how to manually create field maps of various configurations. Those were the very early days of somewhat large digital computers. Very small by today's capability. So field mapping was not done by computer.
 
Thank you,and I didn't even notice the second question.

I believe an MC meter uses a rectifier for AC, so the DC offset would add to the AC, I think. :unsure:

I'm not familiar enough with the digital meters to answer about their responses.

A quick glance of what a electrodynamometer is tells me it should behave like an MC meter.
 
220306-1207 EST

Since Larry is the only one to respond so far I will provide the answers. The purpose of the question is to provide a learning experience.

But first I would like to know what an MC meter is?

Why should one know how an instrument works? To understand what a measurement means.


1. Simpson 260 or 270.

These are fundamentally DC meters based on the design principle of Weston and d'Arsonval (around 1880 ) movement. This concept was a moving coil in a constant uniform magnetic field with a linear spring restrainer. Thus, a constant magnetic field in which a variable magnetic field was generated and that force restrained by a linear spring. This was not working with the Earth's magnetic field as previous meters had. Rather a locally generated magnetic was used.

A meter of this type, compared to an oscilloscope, has mass in the mechanism, and a spring, thus a mechanical resonant frequency. There is also mechanical damping so not much mechanical oscillation occurs.

I used two sine wave frequencies 14 Hz and 100 Hz, and tested a Simpson 270 ( same as 260 except somewhat greater accuracy ) and a Fluke 27. The 14 Hz was selected because at this frequency there was no DC needle fluter on the Simpson. 100 Hz is high enough that there is not much error from the input series impedance of the input coupling capacitor. Sine wave peak voltage was set at 5 V.

Note that for a full wave rectified sine wave the ratio of RMS to average is 0.707 / 0.636 = 1.112 . The Simpson was on its 10 V range, and the Fluke on auto ranging. The Simpson in AC mode passes thru any DC component. With the Simpson you need to use Output mode to remove any DC component.

Simpson 270, 14 Hz
DC mode --- No needle motion
AC mode --- 3.8 V
AC Output __ 1.6 V

Simpson 270, 100 Hz
DC mode --- no needle motion ( with none at 14 Hz there should not be any at 100 Hz )
AC mode --- 3.8 V
AC Output --- 3.6 V

Note: 5 * 0.707 = 3.54 V, and 0.707 / 0.636 = 1.112, and that 3.54 * 1.112 = 3.94 . Thus, pretty good correlation with 3.8 .


Fluke 27, 14 Hz
DC mode --- Jumps around 0.007 thru 0.043
AC mode --- 3.94 V

Fluke 27, 100 Hz
DC mode --- 0.003 V, this is probably a DC component from the Fluke function generator, could not balance it any better
AC mode --- 3.94 V

This is ball for now.

.
 
220311-1728 EST

Continuing my experiments. Now the experiment is a balanced square wave of +/- 5 V, and the same waveform DC shifted to +10 V, and 0 V

Still using the 14 and 100 Hz waveforms even though there is slight flutter of the Simpson needle at 14 Hz. The flutter is visible but easily averaged. Probably in the 1% range.


Simpson 270, 14 Hz, square wave ( +/- 5 V )

DC mode --- No needle motion
AC mode --- 5.25 V
AC Output --- 1.6 V

Simpson 270, 100 Hz, square wave

DC mode --- no needle motion and none expected
AC mode --- 5.4 V
AC Output --- 5.0 V


Fluke 27, 14 Hz
DC mode --- Jumps around 0,003 thru 0.080
AC mode ---- 5.39 V

Fluke 27, 100 Hz
DC mode --- 0.024 V, this the lowest I could adjust with the function zero offset
AC mode --- 5.55 V

Because my attempt was to adjust the function generator to an average DC value of 0 the DC results were quite good.

Average reading AC meters based on a DC meter movement rectify the input AC with a full wave bridge rectifier, perform an averaging function, and then are calibrated to read RMS assuming the waveform is a sine wave. This means the AC scale calibration is 0.707 / 0.636 = 1.112 times the measure DC average value. Thus, a +/- 5 V square wave should read 1.112 * 5 = 5.558 . Thus, the Fluke is quite good at 100 Hz, and the Simpson is reasonably good at 100 Hz.


Later I will do the same for this same waveform, but with its mean position shifted to +5 V.

.
 
220311-2121 EST

Now the results for the same square waveform, but with an added DC component of +5 V. This means that what was previously the -5 V portion of the square is now at 0 V.

It is quite obvious that the average DC value of the waveform is +5 V, and thus a DC meter should read 5 V.


Simpson 270, 14 Hz, square wave ( +0 V and +10 V )

DC mode --- 5.05 V, expected value is 5 V, thus very good
AC mode --- 5.7 V, expected is 5.58, somewhat close
AC Output --- 1.6, expected to be low

Simpson 270, 100 Hz, square wave

DC mode --- same as 14 Hz
AC mode --- 5.75 V
AC Output --- 5.15 V


Fluke 27, 14 Hz square wave

DC mode --- 4.99 thru 5.02
AC mode --- 5.37 V, expected 5.58, somewhat close


Fluke 27, 100 Hz

DC mode ---- 5.07 V
AC mode --- 5.50 V, a little closer to expected

Pulse voltage levels set by scope calibration.

My next test I expect will be unbalance time periods for the two halves of the cycle.

.
 
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