Exam question?Why look at sine waves?
Indeed, and if one uses 230V and (230 x sqrt(3) V (about 398.3717) and round to a large number of decimal places, then the two methods produce identical results:Ideally the two models should give the same result, the slight inconsistensy is due to rounding errors including the fact that 230V and 400V don't actually correspond precisely.
. Source Current (A)
AdrianUK 31.8840579710145
plugwash (using 400V) 31.7542648054294
plugwash (using 230V *sqrt(3) 31.8840579710145
I’ve been thinking that whilst reading this thread.22kW & 11kW are standard IEC motor sizes.
If the OP is trying to determine the current for such motors then our methods above are too simplistic.
The formulae that we have all used assumes a balanced resistive three phase load (ie. pf = unity hence kW=KVA & efficiency = 100%). This method will NOT work for a motor - that is a far more complex equation.
I’ve been thinking that whilst reading this thread.
I used to run a joinery shop and some of the machines had an amp meter on the control panel, on a wide belt sander for example the start up load was massive, like 60 amps, but once upto speed the current was small and only increased when the machine was being used.
Sometimes the machine would trip the DB breakers - it used to be on very cold days, I don’t know if that was due to electrical or mechanical reasons.
That's correct for supply at 400V phase-phase, which wasn't specified.22kW = 22/3 = 7.33kW per phase.
At 230V 7.33 kW requires 7330/230 = 31.88A.
So 22kW = 32A per phase.
And both are wrong, because they ignore efficiency and power factorIndeed, and if one uses 230V and (230 x sqrt(3) V (about 398.3717) and round to a large number of decimal places, then the two methods produce identical results:
Did you read post #7 ????And both are wrong, because they ignore efficiency and power factor
I thought I'd edited this but perhaps I forgot to hit Save. To say (before somebody tells me) the OP didn't say it's an electric motor, though the 11 and 22kW figures might give that impression.
Yes, that was one reason for my initial assumption that we were talking about motors. Calculating motor current isn't much more complicated, just needs efficiency and power factor. I know it may not be the whole story, I worked on a job in the 1980s where there was a big variation of current between the phases. Our client tried to make a fuss, but when we looked into it there seemed to be no standard allowable figure. Don't know whether that's still the case.Did you read post #7 ????
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