AC voltmeters and ammeters
AC electromechanical meter movements come
in two basic arrangements: those based on DC movement designs, and those
engineered specifically for AC use. Permanent-magnet moving coil (PMMC)
meter movements will not work correctly if directly connected to
alternating current, because the direction of needle movement will
change with each half-cycle of the AC. Permanent-magnet meter movements,
like permanent-magnet motors, are devices whose motion depends on the
polarity of the applied voltage (or, you can think of it in terms of the
direction of the current).
In order to use a DC-style meter movement
such as the D'Arsonval design, the alternating current must be
rectified into DC. This is most easily accomplished through the use
of devices called diodes. We saw diodes used in an example
circuit demonstrating the creation of harmonic frequencies from a
distorted (or rectified) sine wave. Without going into elaborate detail
over how and why diodes work as they do, just remember that they each
act like a one-way valve for electrons to flow: acting as a conductor
for one polarity and an insulator for another. Oddly enough, the
arrowhead in each diode symbol points against the permitted
direction of electron flow rather than with it as one might expect.
Arranged in a bridge, four diodes will serve to steer AC through the
meter movement in a constant direction throughout all portions of the AC
cycle:
Another strategy for a practical AC meter
movement is to redesign the movement without the inherent polarity
sensitivity of the DC types. This means avoiding the use of permanent
magnets. Probably the simplest design is to use a nonmagnetized iron
vane to move the needle against spring tension, the vane being attracted
toward a stationary coil of wire energized by the AC quantity to be
measured.
Electrostatic attraction between two
metal plates separated by an air gap is an alternative mechanism for
generating a needle-moving force proportional to applied voltage. This
works just as well for AC as it does for DC, or should I say, just as
poorly! The forces involved are very small, much smaller than the
magnetic attraction between an energized coil and an iron vane, and as
such these "electrostatic" meter movements tend to be fragile and easily
disturbed by physical movement. But, for some high-voltage AC
applications, the electrostatic movement is an elegant technology. If
nothing else, this technology possesses the advantage of extremely high
input impedance, meaning that no current need be drawn from the circuit
under test. Also, electrostatic meter movements are capable of measuring
very high voltages without need for range resistors or other, external
apparatus.
When a sensitive meter movement needs to
be re-ranged to function as an AC voltmeter, series-connected
"multiplier" resistors and/or resistive voltage dividers may be employed
just as in DC meter design:
Capacitors may be used instead of
resistors, though, to make voltmeter divider circuits. This strategy has
the advantage of being non-dissipative (no true power consumed and no
heat produced):
If the meter movement is electrostatic,
and thus inherently capacitive in nature, a single "multiplier"
capacitor may be connected in series to give it a greater voltage
measuring range, just as a series-connected multiplier resistor gives a
moving-coil (inherently resistive) meter movement a greater voltage
range:
The Cathode Ray Tube (CRT) mentioned in
the DC metering chapter is ideally suited for measuring AC voltages,
especially if the electron beam is swept side-to-side across the screen
of the tube while the measured AC voltage drives the beam up and down. A
graphical representation of the AC wave shape and not just a measurement
of magnitude can easily be had with such a device. However, CRT's have
the disadvantages of weight, size, significant power consumption, and
fragility (being made of evacuated glass) working against them. For
these reasons, electromechanical AC meter movements still have a place
in practical usage.
With some of the advantages and
disadvantages of these meter movement technologies having been discussed
already, there is another factor crucially important for the designer
and user of AC metering instruments to be aware of. This is the issue of
RMS measurement. As we already know, AC measurements are often cast in a
scale of DC power equivalence, called RMS (Root-Mean-Square)
for the sake of meaningful comparisons with DC and with other AC
waveforms of varying shape. None of the meter movement technologies so
far discussed inherently measure the RMS value of an AC quantity. Meter
movements relying on the motion of a mechanical needle ("rectified"
D'Arsonval, iron-vane, and electrostatic) all tend to mechanically
average the instantaneous values into an overall average value for the
waveform. This average value is not necessarily the same as RMS,
although many times it is mistaken as such. Average and RMS values rate
against each other as such for these three common waveform shapes:
Since RMS seems to be the kind of
measurement most people are interested in obtaining with an instrument,
and electromechanical meter movements naturally deliver average
measurements rather than RMS, what are AC meter designers to do? Cheat,
of course! Typically the assumption is made that the waveform shape to
be measured is going to be sine (by far the most common, especially for
power systems), and then the meter movement scale is altered by the
appropriate multiplication factor. For sine waves we see that RMS is
equal to 0.707 times the peak value while Average is 0.637 times the
peak, so we can divide one figure by the other to obtain an average-to-RMS
conversion factor of 1.109:
In other words, the meter movement will
be calibrated to indicate approximately 1.11 times higher than it would
ordinarily (naturally) indicate with no special accommodations. It must
be stressed that this "cheat" only works well when the meter is used to
measure pure sine wave sources. Note that for triangle waves, the ratio
between RMS and Average is not the same as for sine waves:
With square waves, the RMS and Average
values are identical! An AC meter calibrated to accurately read RMS
voltage or current on a pure sine wave will not give the proper
value while indicating the magnitude of anything other than a perfect
sine wave. This includes triangle waves, square waves, or any kind of
distorted sine wave. With harmonics becoming an ever-present phenomenon
in large AC power systems, this matter of accurate RMS measurement is no
small matter.
The astute reader will note that I have
omitted the CRT "movement" from the RMS/Average discussion. This is
because a CRT with its practically weightless electron beam "movement"
displays the Peak (or Peak-to-Peak if you wish) of an AC waveform rather
than Average or RMS. Still, a similar problem arises: how do you
determine the RMS value of a waveform from it? Conversion factors
between Peak and RMS only hold so long as the waveform falls neatly into
a known category of shape (sine, triangle, and square are the only
examples with Peak/RMS/Average conversion factors given here!).
One answer is to design the meter
movement around the very definition of RMS: the effective heating value
of an AC voltage/current as it powers a resistive load. Suppose that the
AC source to be measured is connected across a resistor of known value,
and the heat output of that resistor is measured with a device like a
thermocouple. This would provide a far more direct measurement means of
RMS than any conversion factor could, for it will work with ANY waveform
shape whatsoever:
While the device shown above is somewhat
crude and would suffer from unique engineering problems of its own, the
concept illustrated is very sound. The resistor converts the AC voltage
or current quantity into a thermal (heat) quantity, effectively squaring
the values in real-time. The system's mass works to average these values
by the principle of thermal inertia, and then the meter scale itself is
calibrated to give an indication based on the square-root of the thermal
measurement: perfect Root-Mean-Square indication all in one device! In
fact, one major instrument manufacturer has implemented this technique
into its high-end line of handheld electronic multimeters for "true-RMS"
capability.
Calibrating AC voltmeters and ammeters
for different full-scale ranges of operation is much the same as with DC
instruments: series "multiplier" resistors are used to give voltmeter
movements higher range, and parallel "shunt" resistors are used to allow
ammeter movements to measure currents beyond their natural range.
However, we are not limited to these techniques as we were with DC:
because we can to use transformers with AC, meter ranges can be
electromagnetically rather than resistively "stepped up" or "stepped
down," sometimes far beyond what resistors would have practically
allowed for. Potential Transformers (PT's) and Current Transformers
(CT's) are precision instrument devices manufactured to produce very
precise ratios of transformation between primary and secondary windings.
They can allow small, simple AC meter movements to indicate extremely
high voltages and currents in power systems with accuracy and complete
electrical isolation (something multiplier and shunt resistors could
never do):
Shown here is a voltage and current meter
panel from a three-phase AC system. The three "donut" current
transformers (CTs) can be seen in the rear of the panel. Three AC
ammeters (rated 5 amps full-scale deflection each) on the front of the
panel indicate current through each conductor going through a CT. As
this panel has been removed from service, there are no current-carrying
conductors threaded through the center of the CT "donuts" anymore:
Because of the expense (and often large
size) of instrument transformers, they are not used to scale AC meters
for any applications other than high voltage and high current. For
scaling a milliamp or microamp movement to a range of 120 volts or 5
amps, normal precision resistors (multipliers and shunts) are used, just
as with DC.
- REVIEW:
- Polarized (DC) meter movements must
use devices called diodes to be able to indicate AC quantities.
- Electromechanical meter movements,
whether electromagnetic or electrostatic, naturally provide the
average value of a measured AC quantity. These instruments may be
ranged to indicate RMS value, but only if the shape of the AC waveform
is precisely known beforehand!
- So-called true RMS meters use
different technology to provide indications representing the actual
RMS (rather than skewed average or peak) of an AC waveform.
|
