3 phase 11 and 22kw - Current in Each Phase??

[CRMS]

I'm trying to get back to basic sine waves. How much current flows in each phase to produce 11kw and 22kw overall??

 

[mikeyd]

From quite a lot to a hell of a lot is the answer.

Why look at sine waves?

 

[AdrianUK]

22kW = 22/3 = 7.33kW per phase.

At 230V 7.33 kW requires 7330/230 = 31.88A.

So 22kW = 32A per phase.

 

[RandomGrinch]

Why look at sine waves?

Exam question?

 

[plugwash]

There are two models.

The star model as AdrianUK has stated.

The delta model.

22kW = 22/3 = 7.33kW per pair of phases.

At 400V 7.33 kW requires 7330/400 =18.325A

Total current per pase = 18.325A * 2 * cos(30°) = 31.73A

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.

 

[JohnW2]

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.

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:

.                            Source                         Current (A)

                         AdrianUK                        31.8840579710145
                         plugwash (using 400V)           31.7542648054294
                         plugwash (using 230V *sqrt(3)   31.8840579710145

Kind Regards, John

 

[AdrianUK]

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.

 

[Notch7]

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.

 

[CRMS]

Thanks for all the replies. It is all about EV charging. I posted in a bit of a rush before dinner. As I sat through the evening, I found the answer in plain sight. 11kw 16A, 22kw 32A.

 

[AdrianUK]

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.

For a 3 phase motor, started direct-on-line, you can expect the starting current to be in the region of 6 times the motor rating plate current (ie. 6 x full-load-current)

 

[fixitflav]

22kW = 22/3 = 7.33kW per phase.

At 230V 7.33 kW requires 7330/230 = 31.88A.

So 22kW = 32A per phase.

That's correct for supply at 400V phase-phase, which wasn't specified.

Also a motor rated power is the shaft output power. From a catalogue I have, for a 22kW, efficiency = 92.4%, PF = 0.83, giving rated current 41 amp.

 

[fixitflav]

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:

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.

 

[AdrianUK]

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.

Did you read post #7 ????

 

[plugwash]

When I heard 11KW and 22KW my first thought was EV charging and my understanding is that modern high power AC-DC supplies have a power factor extremely close to 1.

 

[fixitflav]

Did you read post #7 ????

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.
And while 400 volt is most likely, we haven't got a figure. If it's an EV charger in a domestic setting maybe it's 230 volt.