UEi Test Instruments DL235 - Manual (Page 7)

DL235-MAN P. 6
To stop recording, use the “HOLD” push-button. Then, “RECORD” no
longer blinks, and the values of MIN, MAX, and AVG are frozen. Press
the “HOLD” push-button again to restart recording, when the values of
MIN, MAX, and AVG are not reset. The recording simply starts again
from where it left off.
Press and hold the “MIN/MAX” push-button for 2 seconds and then
release in oder to exit “RECORD”. This meter acknowledges with a
beep sound and “RECORD” is no longer displayed.
Using Record
After enabling “RECORD”, press the “MIN/MAX” push-button to cycle
through the MIN, MAX, and AVG readings held in memory. If this meter
is in “CREST” (that is, “PEAK”) function, only MAX is held in memory.
Battery life limits the duration of “RECORD”. The MAX reading is the
maximum value detected since “RECORD” started. The MIN reading is
the minimum value detected since “RECORD” started. The AVG reading
is calculated continuously from the start of “RECORD” (Fig 12).
To stop recording and to freeze the values of MIN, MAX, and AVG in
memory, press the “HOLD” push-button. Press the “MIN/MAX”
push-button to cycle through the readings, including a position where
the blinks. In this mode, readings may be taken without
disturbing the values held in memory. To restart recording, press the
“HOLD” push-button. Then, “RECORD” blinks.
When using “HOLD” and “RECORD”. Note that when “RECORD” is
blinking, this meter is recording values; when “RECORD” is not
blinking, this meter is not recording values. When is blinking,
the digital display is showing a real measurement; when is not
blinking, the digital display is showing a recorded measurement.
“HOLD” and “RECORD” apply to the digital display only. The bar
graph shows a real measurement only at all times.
If you press the “MIN/MAX” push-button to start recording while in
the “HOLD” mode, “RECORD” is not blinking because this meter is not
recording values. And if you cycle through the MIN, MAX, and AVG
readings, this meter displays just “- - - -”.
Press the “MIN/MAX” push-button for 2 seconds and then release
in order to exit “RECORD” (or “HOLD” and “RECORD” is selected).
This meter acknowledges with a beep sound and “RECORD” is no
longer displayed.
Applications of Measurement
Non-Linear Loads
True RMS current flow is very important because it directly relates to
the amount of heat dissipated in wiring, transformers, and system
connections. Most ammeters in the market measure average current
flow, not true rms current flow, even if this average current flow is
displayed on a scale calibrated in rms. These average-sensing ammeters
are accurate only for a pure sign-wave current.
All current waveforms are virtually distorted in some way. The most
common is harmonic distortion caused by non-linear loads such as
household electrical appliances, personal computers or speed controls
for motor drives. Harmonic distortion caused significant current flow at
frequencies that are at odd multiples of the power line frequency.
Harmonic current flow gives a substantial impact on the neutral wires
of star (wye)-connected power distribution systems.
In most countries a power distribution system uses commercial 3-phase
50 Hz/ 60 Hz power applied to a transformer with a delta-connected
primary, and a star (wye)-connected secondary. The secondary
generally provides 120 V AC from phase to neutral, and 208 V AC from
phase to phase. To balance the loads for each phase was a big
headache for the electrical system designer, historically.
The vector addition of the currents in the tranformer’s neutral wire was
zero or quite low (because perfect-balance was rarely achieved) in a
well-balanced system, devices connected to which were incandescent
lighting, small motors, and other devices that presented linear loading.
The result was an essentially sine-wave current flow in each phase and
a low neutral current flow at a frequency of 50 Hz/ 60 Hz.
But, devices such as TV sets, fluorescent lighting, video machines, and
microwave ovens are commonly drawing power line current for only
a fraction of each cycle so that they cause non-linear loading and
subsequent non-linear current flow. This generates odd harmonics
of the 50 Hz/ 60 Hz line frequency. Therefore, the current in the
transformer of today contains not only a 50 Hz (or 60 Hz) component,
but a 150 Hz (or 180 Hz) component, a 250 Hz (or 300 Hz) component,
and the other significant harmonic components up to a 750 Hz (or 900
Hz) component and beyond.
The vector addition in a properly-balanced power distribution system
feeding non-linear loads may still be quite low. But, the addition does
not cancel all the harmonic currents. The odd multiples of the 3rd
harmonic (called the “TRIPLENS”) are, particularly, added together
in the neutral. These harmonics can form a total rms current in the
transformer’s neutral wire that is normally 130% of the total rms
current measured in any individual phase, whose theoretical maximum
is 173%. For example, phase currents of 80 amperes may cause
harmonic current flow in the neutral of 104 amperes. The dominant
current flow in the neutral is most commonly the 3rd harmonic.
HOLD
HOLD
HOLD
(Fig 11)
(Fig 12)