measureing amperage in a 3phase circuit

[brutus1955]

ok fella's i am trying to get my head around this, and can't seem to grasp the concept, so i come to you with my hat in hand, hopeing to get some help.

i have a small 3 phase alternator, stator connected in Y configuration

i am measuring 80 volts phase to phase, and 18amps at load
with my clamp on meter across one leg

what i am trying to sort out is, how much power do i have in this circuit, if power factor is at unity (feeding a resistive load)?

do i calculate it as 80v x 18a =1440 watts, or do i calculate it as 3x that to account for all three legs?

or is there a 1.73 function i am missing?

i really appreciate you guys and all your help

TIA

bob g

http://www.microcogen.info

 

[racobb]

Depends on whether you have 18 amps in three legs or just one (line to neutral). Or is the load connected line to line with 18 amps in two legs?

If you have 18 amps in 3 legs then you need the 1.732 as well.

Alan

 

[desertfox]

Hi brutus1955

If you have the same amps in each phase then its a balanced load and your total power in a three phase balanced star connected system is:-

power = 3*Vp*Ip*cos(thi)

where Vp=phase voltage = Vl/1.73 Vl = line voltage =80v in your case

Ip=phase current =18amps in your case because Ip=Il
where Il=line current

cos(thi)= power factor (ie angle current lags voltage)

If you have different currents in each leg ie unbalanced the you need to calculate the power in each leg seperately

ie:- Vp*Ip*cos(thi) for each leg and just sum them at the end.

desertfox

 

[VTer]

Complex power (VA) = V(a-n)*Ia+V(b-n)*Ib+V(c-n)*Ic
For real power (W) you have to know the power factor (pf)
W=VA*pf

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[brutus1955]

let me add to this the following

i have no access to the neutral connection of the Y
so i am measuring the voltage across to legs, and it is a balanced load.

i am measuring the amperage with a clamp on meter on one leg
and getting an 18amp reading at full load,

the pf is unity

i assume that if i clip onto leg A for instance and get 18 amps, then there is also part of the 18amps flowing in leg
B and also leg C?

and to add to the confusion

the unit is running at 500hz, so maybe my meter is unable to read the amperage accurately or relatively so anyway?

so for the sake of discussion, lets assume the meter is correct, i am Y connected, no access to the neutral, 80vac
phase to phase, and 18amps as measured with a clamp on one line. and pf is unity.

therefore what is the power being delivered to my load?

bob g

http://www.microcogen.info

 

[jghrist]

80·18·sqrt(3) = 80·18·1.732 = 2494W
Note that this is the same as desert fox
3·[80/sqrt(3)]·18 = 2494W

 

[waross]

18 Amps x 80 Volts x 1.73 = 2491 Watts.
or
80 Volts / 1.73 = 46 Volts.
The wye voltage will be 46 Volts.
18 Amps x 46.24 Volts = 832.32 Watts
832.32 Watts per phase x 3 phases = 2496.96 Watts.
2496.96 Watts - 2491 Watts = 5.96 Watts. Rounding errors from 1.73

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brutus1955]

thanks one and all, you guys are the best

now for another twist, i was mistaken and forgot to consider
the three single phase transformers i am supplying with this power source,

i have them setup as delta on primary, and Y on secondary

the secondaries are rectified for battery charging.

so my pf is not unity! duh

if this were a 60hz project i could measure the pf, however
i have nothing to measure it with easily at 500hz.

also i am thinking maybe a good portion of my indicated 18amps on each leg, is reactive and not doing real work?

is there a way to calculate the pf of this system with
rough accuracy? i know that is subjective, but i just need a ball park number.

can i connect one of the transformers to a 60hz line current
and measure the pf, then use a formula to determine what the pf might be at 500hz?

sorry for the previous incomplete info

thanks
bob g

http://www.microcogen.info

 

[jghrist]

The rectifiers may also cause distortion which would contribute to a low power factor. Power factor can be low because of displacement of the current sinewave from the voltage by some angle or from distortion of the current.

To measure the total power factor, measure the real power output (watts) with a wattmeter and the apparent power (rms volts times rms amps - VA). Power factor is watts/VA.

 

[waross]

You can measure the displacement power factor with a capacitor but it won't help you with distortion power factor.
Connect a capacitor across one of the single phase transformers. Measure the current in the transformer the capacitor and the combined load and capacitor current.
Then scale the capacitor current as a vertical line on the drafting table. Use the other two current measurements scaled to construct a triangle. Scale the total current from the bottom of the vertical line and scale the transformer current from the top of the vertical line.
From the intersection of the two lines, construct a horizontal line back to the vertical line and extend the vertical line to intersect the horizontal line. Scale off your kW, KVA, KVAR and phase angle. Admittedly this works much better for motors than battery chargers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brutus1955]

thanks again,

one last question if i might

i am not convinced that my ampclamp meter is capable of reading amperage even approximately (+/- 5% would be nice)
at 500hz.

it appears to be able to read, and reads repeatedly with the same numbers, but i am not at all convinced i have 18amps
on a single leg,

if it is true, i would be about 52% efficient in the conversion and be dissipating approx 1450watts from the three 1.44kva transformers

seems like approx 500 watts dissipation would make some heat.

what kind of clamp on meter do i need to measure amps
on a line that is around 500hz?

bob g

http://www.microcogen.info

 

[jghrist]

You need a clamp on meter that is rated for 500 Hz. The 500A clamp on current probe for my Fluke meter has 2% accuracy between 45 Hz and 65 Hz (100-500A), but 7% error for frequencies between 65 Hz and 3 kHz.

You also need a true rms reading DMM because there is probably distortion from the rectifiers.