Download Current wave shape of equipment in the home

INSTALLATIONS
& CONTRACTING
In view of the present Eskom power supply shortage and the change from filament lamps to compact
fluorescent lamps (CFLs), it was decided to study the electric current behaviour of such lamps and other
equipment used in the home. An electrically isolated digital oscilloscope was used for this purpose.
Current wave shape of
equipment in the home
The resulting figures all use the same
time scale of 2,5 ms per division, giving
a total time span of 25 ms. This shows
slightly more than one cycle of the
50 Hz mains supply, which has a
duration 20 ms. For the 10 horizontal
divisions, the change from the negative
half to the positive half occurs at the end
of the first division from the left, as shown
in Fig. 1a. The positive maximum occurs
at the end of division 3 with the change
from positive to negative at the end of
division 5 (the centre line). The negative
maximum occurs at the end of division 7
with the change from negative to positive
at the end of division 9 (similar to the
end of division 1.)
Lamps
The larger trace in Fig. 1a is the
mains voltage signal with a vertical
by J J Kritzinger
scale of 100 V per division. The
maximum positive or negative value
of 320 V corresponds to the normal
mains supply of 226 V RMS. The
smaller signal is proportional to
the current drawn by a nominal
60 W incandescent filament lamp.
It is measured as the voltage across
a 0,5 Ω resistor in series with the
neutral line. Thus the vertical scale
of 100 mV per division represents a
current scale of 200 mA per division.
The positive or negative peak value
of 400 mA gives an RMS value of
282 mA. When this is multiplied with
the 226 V RMS the power is given as
64 W. The current signal for this
Fig. 1a: Voltage: 100 V/div. with 60 W ref. current: 200 mA/div.
60 W filament lamp is used for
amplitude and time reference of Figs.
1b to 1g and Figs. 2a to 2d.
Figs. 1b, 1c and 1d show the current
wave shape for 11 W, 14 W and
22 W nominal power CFLs respectively.
The shape is similar in all three cases
with the amplitude increasing with
power rating. It starts with a fast rise to
maximum approximately 2,5 ms prior
to the maximum current point of the
filament lamp. Within the following
2 ms it decreases to about quarter of
the maximum as it passes the maximum
filament current point, to return to zero
1,5 ms later. The initial peak for the
14 and 22 W CFLs is comparable in
Fig. 1b: 11 W CFL with 60 W ref. current: 200 mA/div.
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Fig. 1c: 14 W CFL with 60 W ref. current: 200 mA/div.
Fig. 1d: 22 W CFL with 60 W ref. current: 200 mA/div.
Fig. 1e: 22 W CFL switching noise. Note the different time scale for
the top trace. Voltage: 2 V/div.
Fig. 1f: 36 W 1,2 m fluorescent tube with modern electronic control and
60 W ref. current: 200 mA/div.
Fig. 1g: 36 W 1,2 m fluorescent tube with old inductive ballast and
60 W ref. current: 200 mA/div.
amplitude to the maximum of the 60 W filament lamp and
obviously does not coincide with the supply voltage peak.
It is at least three times larger than that for a 15 W filament
lamp. Timing is similar for both positive and negative
parts of a cycle and for all the tested CFLs from different
manufacturers. The implication of this will be discussed in
the section for combined equipment.
Figs. 1b and 1c show the average value of 16 cycles but 1d
is a single record to show the higher frequency noise current
produced by the switching supply operation of the CFL. The
result in Fig. 1e is obtained when the oscilloscope probe is
placed next to the base of the CFL lamp. The top trace is at
a much shorter time scale of 25 μs per division, to indicate
the 35 kHz switching frequency. The bottom trace is at the
standard 2,5 ms per division reference, showing that the
switching operation is continuous but with higher intensity
when drawing current from the mains.
Fig. 1f shows the current of a1,2 m single 36 W fluorescent
tube fitting with modern electronic control. The current wave
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Fig. 2a: 1,4 GHz Celeron laptop with 60 W ref. current: 1A/div.
Fig. 2b: Pentium 3 desktop PC, with and without CRT monitor
with 60 W ref. current: 1 A/div.
Fig. 2c: 51 cm CRT television with 60 W ref. current: 1 A/div.
Fig. 2d: 93 cm LCD television, DSTV with 60 W ref. current: 1A/div.
shape is fairly symmetrical with that of the filament lamp
but of somewhat shorter duration and a faster rise at the
start. The current peak is substantially less than that of the
60 W lamp while the light output is much more than from
the filament lamp.
In Fig. 1g the current of a 1,2 m single 36 W fluorescent
tube fitting, with a very old inductive ballast, shows a very
high peak current in comparison. The peak occurs almost
at the time of minimum voltage, thus indicating a very poor
power factor, but with no rapid current changes.
Electronic equipment
In this series of Figs, 2a to 2d, the current sensitivity is 1 A
per vertical division and the 60 W filament lamp is used for
comparison in each case.
Fig. 2a shows the current wave shape of a Celeron
1,4 GHz laptop computer. The current is peaked with a width
of approximately 2,5 ms but fairly symmetrical with respect
to the supply voltage peak.
Fig. 2b indicates conditions for a 533 MHz Pentium 3 desktop
computer, with the 43 cm (17 inch) CRT monitor on and
switched off. The current width and time of occurrence is similar
to that of the laptop but with double the peak value.
Fig. 2c, for an old 51 cm CRT television is similar when
compared to the CRT monitor of the desktop computer.
Fig. 2d is for a modern 93 cm LCD television. The shape is
a reasonable approximation to a sine wave and occurs in
phase with the supply voltage. The third trace, showing a
small peak similar to Figs. 2a to 2c, is for a DSTV decoder.
The peak value is similar to the peak value of the 60 W
filament lamp. This is representative of other equipment such
as a VCR, DVD or audio player.
In the kitchen
In this series, Figs. 3a to 3c, the current sensitivity is approximately
2,5 A per vertical division and a 700 W resistive load (a coffee
machine) is used for comparison. These results were taken with a
current transformer in the mains supply at the distribution board.
Fig. 3a is for a refrigerator in comparison with the 700 W
load. It does not have fast changes but is symmetrically later
in time than the mains voltage peak by about 2 ms. This is
owing to the inductance of the electric motor.
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Fig. 3a: Refrigerator with 700 W ref. current: 2,5 A/div.
Fig. 3b is for a microwave oven with rated output of 500 W. The
graph is fairly smooth but with symmetry slightly earlier than the mains
voltage peak. This is caused by the capacitor of the voltage doubling
circuit, used to generate the 4 kV required by the magnetron.
Combined equipment
When different pieces of equipment are combined interesting
results are obtained.
As a starting point the current of two desktop computers is
shown in Fig. 4a with the 700 W resistive load as comparison
at current sensitivity of 2,5 A per division.
Fig. 3b: Microwave oven with 700 W ref. current: 2,5 A/div.
When 12 CFLs are switched on in addition, Fig. 4b is the result.
This indicates that the two peaks are adjacent, with the overlap
increasing the current of the two computers by only 10% - 15%.
If the currents were coincident the peak value would be almost
double.
For Figs. 4c to 4e a current scale of approximately
5 A/div is used with a reference resistive load of
1800 W of an electric kettle.
Fig. 4c shows the current for a 12 000 BTU air conditioner
compared to that of the kettle. The slight variation from
sinusoidal shape comes from the standby currents of
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Fig. 4a: Two desktop PCs with 700 W ref. current: 2,5 A/div.
Fig. 4b: Two desktop PCs and 12 CFLs with 700 W ref. current:
2,5 A/div.
the electronic equipment. The current symmetry of the airconditioner lags that of the kettle by approximately 1 ms, caused
by the motor inductance (similar to that for the refrigerator.)
Fig. 4d shows the result when the conditions of 4b is added
to 4c. It shows clearly that the current of the CFLs brings the
resulting wave shape closer to that of the ideal, represented by
that of the kettle. It thus performs a kind of time multiplexing
and compensates somewhat for the current lag of the air
conditioner.
Fig. 4c: Air conditioner with 1800 W ref. current: 5 A/div.
Fig. 4d: Air conditioner and two desktop PCs and then with
12 CFLs added, in comparison with the 1800 W ref.
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Fig. 4e: Refrigerator, two desktop PCs and 12 CFLs and then with a
2500 W resistive load added, in comparison with the 1800 W ref.
When the current of a large resistive load of 2500 W is added
to the currents of a refrigerator, two laptop computers and
12 CFLs, Fig. 4e results. The addition of the currents can
be seen, but the deviation from the ideal sine wave is not as
significant as it is at lower resistive loads. Resistive loads of
stoves, kettles, dishwashers and hot water cylinders are usually
in the range of 2000 to 3000 W.
In conclusion it appears that, until the peaked current of
electronic equipment can be made more sinusoidal, the
early peaked current of the CFLs perform a useful function to
compensate for the lagging symmetry of electric motors, without
significantly adding to the current peak. A penalty is the higher
frequency harmonics introduced.
If solar hot water systems become common, large resistive
loads will diminish in residential areas so that the peaked
current of electronic equipment may become significant at
substations. It will be interesting to monitor the current wave
shape at residential substations and to consider what influence
this will have on fast acting current protection.
Contact Johan Kritzinger, Tel 021 852-3592,
[email protected]
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