Author Topic: Power Measurement  (Read 912 times)

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Offline Yokogawa TM UK

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Power Measurement
« on: June 21, 2017, 06:41:05 pm »
Hey Everyone,

Thought I would post about Power Measurement given that it is rarely mentioned on here yet an important factor in product development (these appliances don't rate themselves on efficiency ;) )

Ask most people today what is the unit of Power and you will usually find the answer is Watts. So it may come as a surprise that some people don't understand that there are four types of electrical power, namely DC, Apparent, Reactive and Active.

DC - Easily measured using multifunction testers, scopes and power meters too.
Unit Value: VA

Apparent - This is what is supplied to a building/system but is not what is necessarily used. Think about your home, a higher amount of electrical power is made available to the property so that you can use it when you need it. Again easily measured using multifunction testers, scopes and power meters too.
Unit Value: VA

Reactive - This is what is not used by a building/system and is returned to the grid/source. Expanding on the example above, you don't get charged by the utility company for what is supplied and then get credited for what you don't use. Like the others, it can be measured using multifunction testers, scopes and power meters too
Unit Value: VAR

Active - This is what is actually used by a building/system and is what we get charged for by the utility companies, we use it to determine how much power our systems/projects etc are using and can then use that information in a variety of ways e.g. Appliance energy efficiency ratings. Unlike the others though, only Power Meters can accurately measure Active Power. Using a multifunction tester or scope or another instrument will not provide a reliable result as the measurement uncertainty lies in the region of +/- 10%
Unit Value: W (watts)

The reasons why Power Meters are good at measuring Active Power is because they can simultaneously measure the phase shift which is very common in AC loads. In addition, scopes are further limited due to probe input impedance, probe measurement uncertainty, -3db and they do not simultaneously measure phase with the Voltage and Current.

An interesting observation in the Power Meter market though is the limited number of Power meters which are only specified for measuring DC, Apparent and Reactive Power only. Yes they can be called a Power Meter but not an Active Power Meter.

A common challenge in Power measurement is high frequency measurements and harmonic content and at times even a system with a fundamental 50Hz/60Hz can have impacting signals in the kHz range and beyond. Many power meters in the market place advertise ranges to 100kHz, 1MHz or more yet it is all well and good to advertise these but proving this capability appears to be more challenging for most manufacturers.

Another interesting observation with Power meters is the claims of being able to measure to high frequencies yet the inability to prove it. Commonly phrases such as typical or nominal serve to protect the manufacturer in the event of an error/fault etc.

My advice is if you are looking for the proven accuracy then take a closer look at both the specifications and the calibration capabilities of the manufacturer.

Any questions please ask

The following users thanked this post: Electro Detective

Offline Electro Detective

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Re: Power Measurement
« Reply #1 on: June 22, 2017, 12:14:23 am »
With all the stacked cheap dog box housing going up, during winter and summer demand extremes, the power quality in downtown kangaroo suburbia is garbage for want of a better term.

You don't need any fancy big dollar equipment to realize what's going when lights are dimmer and aircons and heaters just aren't cutting it,  their inards producing extra heat and noise, most likely because of the poor abused alternating current delivery.

You can do the usual tests with multimeters etc but they usually show normal 'voltage' readings (TRMS and averaging response), and oscilloscopes will show a flattened top and bottom of the sine wave, depending on what kind of hammering the abused street transformer is copping. 

The utility companies can't possibly cater for these easy profit dog box demands in the short term, filled with 'smart' technology that kicks reactive and capacitive crap back into the grid,
so established housing customers have to suffer AND pay ridiculous higher bills on top due to the snafu power factor going south..   |O 

That said, are there any poor mans power measurement meters available
or tedious methods to rig up existing test gear (clamps, CROs, DSOs, meters)  for say a one off job ?

My Fluke come LEM HEME LH1050 Clamp Meter still does the business afaict for W, VA, VAR and power factor even for distorted waveforms, and 3 phase balanced loads etc.
How well/accurate/precise I won't know till I compare with another similar or better? spec meter

Thanks for the post   :-+
« Last Edit: June 22, 2017, 12:50:58 am by Electro Detective »

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