single phase kw meter on a three phase motor?

[Slim shady]

i have a friend asking a question..

what i know is he has a three phase irrigation system being metered off a single phase kwh meter and what hes been doing is multipling that figure by three to get total power consumption

so he's been charging his neighbour this amount.
sooooooo i know this is wrong
for starters the kwh meter is for measuring 230 volts not 400 so how can it be giving a correct reading from the start let alone multipling this by three.

i recommended that he puts on a three phase kwh meter to get an accurate reading to work off .

can someone give me an answer to why what he's done is inaccurate
and weither he has been charging to much for his power usage or to little for his power usage
thanks guys
Slim

 

[mivey]

for starters the kwh meter is for measuring 230 volts not 400 so how can it be giving a correct reading from the start let alone multipling this by three.

The meter may be rated for higher voltages. The stator combines the flux from the voltage and current to get the energy. If the voltage & current are within the acceptable range of the meter, it will read fine (for single-phase loads).

i recommended that he puts on a three phase kWh meter to get an accurate reading to work off .

That would be the correct way. If you were absolutely positive the source & load were balanced, you could use the multiply-by-three method. The problem is that the voltages are probably not balanced. If decently balanced and he is reading the correct voltage and current, the error would be small and might be ok for estimating, but it is not technically accurate.

can someone give me an answer to why what he's done is inaccurate and weither he has been charging to much for his power usage or to little for his power usage

We could not tell if it has been too little or too much without some information on the balance, how the meter is connected, and specific details on the meter.

As for why it is wrong, refer to Blondel's Theorem (copied from the "Handbook for Electricity Metering"):

"If energy is supplied to any system of conductors through N wires, the total power in the system is given by the algebraic sum of the readings of N wattmeters, so arranged that each of the N wires contains one current coil, the corresponding voltage coil being connected between that wire and some common point. If this common point is on one of the N wires, the measurement may be made by the use of N-1 wattmeters."