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I need to measure the power consumption of a large (11kW) electrical motor during various stages of operation. I have a few interlinked questions:

  1. I'm interested in the power the operator of that motor would be charged for. I'm not sure if that is real, apparent, both?

  2. Can I use a clamp on ammeter and measure the current of just one phase, or do I need to measure all three?

  3. Given I cannot measure the 400V supply, is it reasonable to trust that it is in fact 400V, or what voltage do I need to use for a three phase power supply? Or do I need to hire a qualified electrician (Health ans Safety...)?

A certain scientific rigour for this is required as I intend to model the power consumption of the motor, but with a linearized system, to be presented at a civil engineering conference. (So accurate, but I and the audience are not interested in the details too much)

I intend to use a clamp on ammeter, preferably with logging (PC hookup), capability. Any recommendations (I'm UK based)?

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  • \$\begingroup\$ Don't assume a nominal 400V supply is actually exactly 400V. More likely, it's 400V ±10% or perhaps ±6%. \$\endgroup\$
    – Simon B
    Oct 25, 2015 at 19:37

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  1. It depend on contract, usualy only real power is charged, the instrumen shall count real, reactive, apparent.
  2. 3 current transformers and 3 volatages is the exact measuring, but you can use one current and one voltage with asumption that network and motor are balanced (that would be rathere an estimation than measuring).
  3. Any power meter for rated voltage for your country is ok, let's say 400VAC nominal.

Industrial power meters, there are a lot of them. I can just give you one example: https://www.youtube.com/watch?v=ggDtJWUbkJ4

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  • \$\begingroup\$ Thanks, I've since found power meters, that should be up for the job. eg: test-meter.co.uk/… \$\endgroup\$
    – ic_fl2
    Oct 25, 2015 at 17:00
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Speaking on a conference, it would be much entertainment if you can show the signal(s) acting on a wire. A meter cannot provide you with that ability, but a stand alone merging unit (aka SAMU, AMU, MU), like this or this, can.

Having current(s) and voltage(s) signals represented as sampled values over real time (this is what an MU does), you may reconstruct any derivatives, such as instant/averaged power, harmonics/spectra, etc.

Comparing with an MU, a meter is very inertial instrument not intended to facing transitional and non-periodic signals, it is oriented to only slowly changing, periodic signals near around mains frequency (generally, 50/60 Hz ± 5 Hz).

Also, if your motor is powered thru an inverter, a typical meter will lie.

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  • \$\begingroup\$ Thanks for the input, I'll look into using a merging unit. What do you mean, the meter will lie? Not measure the reactive or complex power? \$\endgroup\$
    – ic_fl2
    Oct 26, 2015 at 19:05
  • \$\begingroup\$ Typically, a meter is calibrated at the mains frequency (50 or 60 Hz depending on the country it is for) and has the measuring error bounded (by design, to be cheaper) in a very narrow frequency range, for example, mains ± 5 Hz. Out of this band, the measuring error is unpredictable and may be any. \$\endgroup\$
    – asndre
    Oct 27, 2015 at 15:40
  • \$\begingroup\$ When you connect a motor directly to the network, the mains "rotates" it directly, i.e. on "self", therefore there is one power frequency --- the mains frequency and a meter before the motor will work in its specified freq range with known measuring error. \$\endgroup\$
    – asndre
    Oct 27, 2015 at 15:42
  • \$\begingroup\$ When you connect a motor to the network indirectly, using an inverter, the mains supplies the inverter, and the inverter --- in its turn --- forms a signal to "rotate" the motor. Typically, an inverter does not apply a mains freq signal when it need to start up the motor, it generates a signal with growing up frequency, from 0 Hz to the frequency you set (mains or other). Therefore, a meter between the inverter and the motor will work in the unspecified freq range and its measuring error will unknown. \$\endgroup\$
    – asndre
    Oct 27, 2015 at 15:47
  • \$\begingroup\$ Also, you need to remember that a meter gets the power to work from the network (lines) it measures on, too. Its internal power supply is oriented to work on the mains frequency and its behavior is unpredictable outside this range. \$\endgroup\$
    – asndre
    Oct 27, 2015 at 15:50

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