More real world power measurements

In a previous Blog entry we looked at the current transformer. As some more real world examples let's use the model LCTC-0250 clamp-on current sensor and the M1k to measure the current waveform of some different kinds of lights. The LCTC probe has a built-in current to voltage (burden) resistor so the output voltage is specified as 15mV/A.


Figure 1, LCTC-0250 clamp-on current sensor

For a refresher the data from an LED Holiday light string from the previous Blog is repeated here. The string consists of 35 white LEDs in series. About a foot of the wire was untwisted and one leg was wrapped about 5 times around the clamp. The sensitivity should be about 75 mV/A (5 * 15 mV/A).

The probe is connected directly to the input of an M1k without any additional amplification or filtering. As you can see in figure 2 the current is simple 1/2 wave rectification and the peak current is between 35 and 45 mA. Hard to see exactly with the noise and the signal is too small to trigger on properly and apply trace averaging.


Figure 2, Current waveform without any signal processing

By applying some mathematical wizardry we are able to clean up the noise and make the "signal" big enough (by 10X) to trigger on and use trace averaging. A simple 20 tap box car digital filter with an overall gain of 10 is applied to the captured waveform trace and trace averaging is used (set to average 8). The waveform is nice and clean now and the p-p current is 42 mA.


Figure 3, filtered current waveform

To measure lights with a standard screw in Edison base, a socket was connected to a plug and a few feet of zip cord. About a foot of the wire was un-zipped and one leg was wrapped 6 times around the clamp. The sensitivity should be about 90 mV/A (6 * 15 mV/A). Six different light bulbs were measured. In all the following waveform screens shots the channel A user measurement formula was used to display the calculated peak-to-peak current (in Amps) and the channel B user measurement formula was used to display the calculated (true) rms current (in Amps). Ignore the channel B voltage trace. It is only there to enable the channel B user measurement display.

As a first experiment let's measure an incandescent light bulb. The spec for this bulb is, 390 lumens and 40 W. The packaging for these bulbs says "Double Life" so the light output is probably lower than from other similar 40 W bulbs.

The measured rms current is 0.33 A. Assuming 120 V rms mains voltage results in an rms power of 39.6 W. The measured power consumption is almost exactly what it should be. As the filament in an incandescent lamp is almost purely resistive the current wave form, shown in figure 4, is a nice clean sinewave. The vertical voltage scale is 20 mV/div (20/90 or 0.222 A/div).


Figure 4, Current waveform for 40 W Incandescent lamp

A second incandescent bulb is tested next with 770 lumens and 60 W. The measured rms current is 0.498 A. Assuming 120 V rms mains voltage results in an rms power of 59.7 W. The measured power consumption is almost exactly what it should be. Again, the purely resistive current wave form, as shown in figure 5, is a nice clean sinewave.


Figure 5, Current waveform for 60 W Incandescent lamp

We can now assume that our test setup is probably giving us accurate results.

Next let's measure a compact florescent lamp, CFL, that is marketed as a replacement for a 40 W incandescent lamp. The specs listed are 700 lumens, 11 W and 140 mA. The 11 W and 140 mA seem inconsistent? The measured rms current is 0.130 A. Assuming 120 V rms mains voltage results in an rms power of 15.6 W. The measured power is 4.6 W or 42% higher than the specification. The electronic ballast used in the CLF results in the rather nasty looking current waveform shown in figure 6. It draws current on the positive and negative halves of the AC voltage but has a huge crest factor.


Figure 6, Current waveform for first CFL lamp

Next let's measure another CFL, that is marketed as a replacement for a 60 W incandescent lamp. The specs listed are 15 W and 230 mA. The 15 W and 230 mA also seem inconsistent? The measured rms current is 0.175 A. Assuming 120 V rms mains voltage results in an rms power of 21 W. The measured power is 6 W or 40% higher than the specification. The current waveforms are consistent between the two CLF examples if we compare figures 6 and 7. The higher power lamp having larger peak and rms currents of course.


Figure 7, Current waveform for second CFL lamp

Next we measure an LED lamp marketed as a replacement for a 60 W incandescent lamp. The specs listed are 800 lumens, 9 W and 90 mA. The measured rms current is 0.077 A. Assuming 120 V rms mains voltage results in an rms power of 9.24 W. The measured power is much closer to the listed value and only 2.5% higher. As we see in the much smoother current waveform in figure 8, it draws current on the positive and negative halves of the AC voltage with none of the nasty spikes of the CFL. Note that the vertical scale at 10 mV/div is half that in the previous waveform screen shots.


Figure 8, Current waveform for LED lamp

As another LED example, a 40 degree LED flood light, was measured. The specs listed are 840 lumens, 12 W and 130 mA. The measured rms current is 0.108 A. Assuming 120 V rms mains voltage results in an rms power of 12.96 W. The measured power is much closer to the listed value and only 8% higher. As we see from the current waveform in figure 9, it draws current on the positive and negative halves of the AC voltage but with very square looking pulses at what might be two distinct levels as compared to the more rounded waveform in figure 8. Also none of the nasty spikes of the CFL are present. Note that the vertical scale is 10 mV/div as in the previous waveform screen shot.


Figure 9, Current waveform for LED flood light

The lower crest factor of the LED waveforms probably accounts for the measured power being closer to the listed power. The CFL manufacturers did not probably use high bandwidth hardware like the M1k to measure the true RMS current of their lamps and missed the high crest factor of the current drawn. It is probably important to note at this point that the measurement results shown here do not take into account any phase difference between the voltage and current waveforms (power factor).

To also display the AC line voltage waveform a voltage step down transformer was used to both isolate and reduce the voltage before measuring it with the M1k channel B input. The step down ratio for the transformer was measured first by connecting the channel A and B AWG outputs of the M1k to the primary side of the transformer. The two AWG channels were configured as 60 Hz complementary sinewaves from 0 to 5 V to produce a differential 10 V peak to peak voltage at the primary. The secondary of the transformer was connected through a 5K / 2K resistor voltage divider to BIN (Split I/O mode) and the 2.5 V mid rail. The channel B gain and offset were adjusted such that the CHB voltage waveform was also 10 V p-p. Now with the primary connected to the AC power line we can display the line voltage while displaying the current on channel A as before (channel B vertical scale set to 50 V/div).

To check out the setup for proper phase between voltage and current the 40 W incandescent bulb was used. Figure 10 shows that the two waveforms are in phase as they should be for a resistive load. Note that the RMS voltage for the AC line measures at 117.75 Volts which seems right on the money.


Figure 10, Current and voltage waveforms for 40 W Incandescent lamp

Now, in figure 11 we check the relative phase between current and voltage for the second CFL bulb.


Figure 11, Current and voltage waveforms for second CFL


I'll leave it up to the reader to interpret what that means. Things like the instantaneous point by point power could be calculated from the two waveforms and then the RMS value of that calculated for example.

Conclusion

For measuring AC power line current the current sense transformer can safely isolate the high line voltages from the measurement circuitry. The same is true for using a voltage step-down transformer to measure the line voltage. This is a very important safety consideration. The wide dynamic range and relatively high sample rate / bandwidth of the 16 bit ADC in the ALM1000 allows the use of current probes like the LCTC to measure detailed current waveforms directly without any signal processing amplification or filtering circuitry.

Leave your comments and suggestions below as usual.

Doug

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