UEi Test Instruments DL235 - Manual (Page 8)

DL235-MAN P. 7
The electrical system designer must consider the following 3 issues when
he designs a power distribution system containing harmonic current flow :
1. The AC neutral wires must be of sufficient gauge to allow for
harmonic current flow.
2. The distribution transformer must have additional cooling to
continue operation at its rated capacity, if it is not harmonic-rated.
This is because the harmonic current flow in the secondary
neutral wire is circulating in the delta-connected primary winding,
after it is reflected to the primary winding. The circulating
harmonic current heats up the transformer.
3. Phase current harmonics are reflected to the primary winding
and they continue back towards the power source. This can
cause distortion
We can use this meter to analyze components such as power
distribution transformers and power factor correction.
General Applications
You can measure any conductor carrying AC unless this conductor is at
a potential above 600 V AC or at a frequency above 10 KHz. True RMS
measurement yields the effective current value.
1. Press the “ON/OFF” push-button to turn on the meter.
2. Clamp around the conductor to be measured. Be sure the clamp
jaws are securely closed, or measurements will not be accurate.
3. Observe the display for True RMS current flow.
4. Press the “Hz” push-button to measure frequency.
5. Press the “HOL D” push-button to freeze the digital display if necessary.
6. To soften readings, press the “SOFT CREST” push-button once
(“SOFT” displayed) or to measure half-cycle peak amperes, press
this push-button twice (“PEAK” displayed). A crest factor other
than 1.414 is an indication of non-linear current flow.
7. To record readings and to view readings, press the “MIN/MAX”
push-button.
8. To clear “RECORD”, press the “MIN/MAX” push-button for
2 seconds.
Motors
You can measure starting (inrush) current, running current, and current
imbalance. Inrush current is typically 6 times the value of running
current, depending on the motor type (Fig 13).
1. Press the “ON/OFF” push-button to turn on the meter.
2. Clamp around a motor phase conductor. Be sure the clamp jaws
are securely closed, or measurements will not be accurate.
3. While watching the bar graph, turn on the motor and observe the
level of inrush current. In this case, the bar graph is better than
the digital display because the bar graph response is faster than
the digital display.
4. When the motor reaches the desired speed, observe the
running current.
5. Repeat your measurement for each motor phase. Unbalanced
current may be caused by a voltage imbalance, or a shorted
motor winding.
6. Repeat steps 1 thru 3, then press the “MIN/MAX” push-button
to enable “RECORD”.
7. Turn on the motor. When the motor gets to the desired speed,
note the blinking upper bar on the bar graph (inrush RMS
current), and the displayed level (running RMS current).
8. To view the maximum inrush current, press the “MIN/MAX”
push-button once.
9. Repeat step 6, if necessary.
10. To clear “RECORD”, press the “MIN/MAX” push-button for
2 seconds.
(Fig 13)