Electronic – I am measuring a 9W LED with a clamp on ammeter. Why does it only draw 7.62W

clampmultimeter

I bought a UNI T UT210E true rms multimeter.

I measured the current of 9 W LED lamp. It shows 0.033A. For the power I get \$0.033 A \cdot 220 V = 7.26 W\$ only.

But bulb was 9W. Why this difference comes? Actually am new to electric.

I measured also a running 5hp water pump 3 phase with 240 VAC it gives 

I1 = 7.35 A
I2 = 6.75 A
I3 = 6.15 A

Best Answer

That user manual (which you should link to in your question) shows the following:

enter image description here

Figure 1. Just because it's digital, doesn't mean it's accurate.

  1. You are measuring at the bottom end of the range and if it were an analog meter you would be squinting at it to try to make out the reading.

Figure 2. Reading position on an analog scale.

0        0.4       0.8       1.2       1.6       2.0 A
|---------|---------|---------|---------|---------|
 ^-- 0.033 A
  1. The manual shows that that accuracy is only for readings > 5% of full scale, 100 mA on the 2 A range. I have no idea what is meant by "<20 residue readings".
  2. The manual doesn't make any claims about true RMS.

enter image description here

Figure 3. The crest factor of an AC current waveform is the ratio of waveform's peak value to its rms value. Source: Ametek.

  1. Your LED lamp will probably have a high crest factor (peak current to RMS value due to the rectification action of the diodes. The meter doesn't handle this well with a further 7% error possible.

Multiplying VRMS by IRMS gives you the VA and not the watts. To calculate the power consumed is more difficult and involves integration of the power curve. Digital power meters sample the voltage and current waveform many times per cycle, multiply the instantaneous readings together to get the instantaneous power, sum them (integration) and average the readings to give the average power.

In short, it's the wrong meter for a true power calculation.

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