I have a data logger with AC current transducer(0-5A) and AC Voltage transducer(0-300V)connected to it. Both give 0-5Vdc output to the logger input. From the logger system, i can get the current and voltage values,but how to get the phase angle without knowing the true power,P or using wattmeter -power factor=true power/apparent power?

 

This depends on the sample rate of the transducers and data logger. If they are in the magnitude of a/few sample(s) per period, you CANNOT get phase angle nor power factor nor true power.
If it is many samples per period AND the logger can record current and voltage at (exactly enough) the same time, true power can be calculated as the sum of current+voltage samples. From this, power factor and phase angle can be calculated.

 

As you said - you can't without having phase angle related data. Meir

 

Following is an approximate method for determining pf.

Determine the motor's temperature, Tm. Calculate the temperature-rise above ambient, Tm - Ta. Ratio this to the rated temp-rise, Tr, found on
the nameplate. Determine pf as follows:

kWo = [(Tm - Ta) / Tr] x rated kWr], where,
kWo = approximate output load.
kWr = rated output in kW (note 1).
Tm = measured motor temperature.
Ta = ambient temperature.
Tr = rated temperature-rise above ambient.

Then, kWi = kWo / Eff, where,
kWi = input kW.
Eff = estimated efficiency as a decimal.
kVAi = input kVA = sqrt(3) x Amps x kV.

Finally,
pf = kWi / kVAi.

Note 1: if namplate output rating is in Hp, convert to kW by multiplying Hp by 0.746!

If you need greater accuracy, contact me regarding another field procedure for determining the motor's output.

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}

 

Exactly, one small misstake however, not the sum but the product of current and voltage averaged over one period (or more).



/Johan Bengtsson

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Pf = (746xHP*100/E x I x Eff x 1.732)*100

This works for a three phase motor

It can also be calculated by taking samples at atleast twice the frequency (or more for accuracy)

for example: if you know your frequency is 60 HZ then a sample of atleast 120 HZ would be required, now at this logging frequency you can determine the actual peak value of both current and voltage readings (you will actually have 120 readings over an entire cycle)

So, if your peak current value is six samples after your peak voltage value then it would be safe to assume that VOLTAGE leads CURRENT by 3 degrees. (your phase angle) or at sixty HZ your delta T would be 50 mS.

Hope this helps