Power Factor

[FilipRST]

At one of the pumping stations PQM shows when there is no pump running:
Vab = 614 V
Vbc = 617 V
Vca = 614 V
Ia = 6 A
Ib = 7 A
Ic = 7 A
P = 2 kW
A = 8 kVA
PF = 0.27
When one pump is running the PQM shows:
Vab = 618 V
Vbc = 616 V
Vca = 612 V
Ia = 35 A
Ib = 35 A
Ic = 34 A
P = 27 kW
A = 37 kVA
PF = 0.72
What would be the reason for such power factor values?
Best,
Filip

 

[bacon4life]

Transformer no-load (excitation) losses are mostly reactive power. There may be either stepdown transformer to serve the motor, or a control power transformer.

 

[che12345]

I have the following opinion for your consideration.
1. No pump is running P = 2 kW, P =0.27
Note: a) with 2 kW shows that there are some other loads, (e.g. lightings) with low PF. Power transformers lightly loaded or some (smaller?) motors which are lightly loaded etc.
b) Many lighting fittings on the market are operating on low PF. Manufacturers omitted the PF capacitor to cut down cost.
2. One pump running P = 27 kW, PF = 0.72
Note: a) The pump (27 - 2 = 25 kW) is the major load. Together with the other low PF loads the PF improved from 0.27 to 0.72.
3. If possible, check the pump PF alone (excluding all other loads), at full load should say PF > 0.8 .
4. In general, all motors run on low PF when lightly loaded. As an indication, the PF improves when the loading > 0.75 power rating.
Che Kuan Yau (Singapore)

 

[crshears]

In general, all motors run on low PF when lightly loaded. As an indication, the PF improves when the loading > 0.75 power rating.

Beginning of rant:

I have heard this stated many times, and it's always made me scratch my head in puzzlement . . . and I've finally put two and two together about why.

If one uses the classic real power / reactive power / apparent power triangle, and uses the decimal fraction ratio of reactive power over real power to define power factor, it becomes apparent that the amount of reactive power consumed is not changing at all, or if it is, it isn't by much; all that's happening is that the amount of real power consumed is rising as the motor is loaded.

In my view, this suggests an "improvement" is occurring when there isn't actually much improvement at all, if any, but in many cases users learn to live with this, mostly because of the cost of doing anything about it such as installing local shunt capacitors. Having to pay a power factor penalty is generally, if not exclusively, the only thing that incentivizes the taking of any mitigative action in this regard.

End of rant.

Thanks for reading all the way through.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Hi CR
Looking at it from the field point of view, I have done quite a bit of PF correction.
The payback time was generally in the range of 6 months.
The reactive does change slightly under load due to distortion of the flux in the air gap.
However, back in the day of bulk PF correction I always ignored that factor in my calculations and I was always successful.
If someone wants to explain how much the respective powerful changes under load it will be appreciated.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

I have the following opinion.
1. In general, motor PF (kW/kVA) is ALWAYS low say (< 0.5) at low (< 0.4) loading. PF improves from say (> 0.70) say (> 0.8) loading. After that PF improves not very much from 0.8 to 1.00 loading.
2. Most LV supply from utility say < 500 A, do NOT penalize consumers on low PF. The charge is kWh , irrespective of PF. In this case, consumers ignore installation of capacitor to improve the PF, as it does not pay for the cost of the Cap banak. ON the other hand, where utility penalizes on low PF, it is worthwhile to study the investment of Cap bank seriously.
Che Kuan Yau (Singapore)

 

[davidbeach]

uses the decimal fraction ratio of reactive power over real power to define power factor

Er, no. The definition of power factor is real power (W) divided by apparent power (S). There is a companion reactive factor which is Q/S, but it's much less commonly used.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

Respectgilly, David, for many years, utilities measured KVARHrs and KWHrs and derived the aversge PF from that.

And as far as voltage and PF correction, I have done PF correction in the Yukon Territory in northern Canada and in Honduras in Central America and many points in between.
The most common voltages were 480 Volts, 208 Volts and 240 Volts.
Yes, in thearger world many utilities do penalize low PF at lower voltages.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Er, no. The definition of power factor is real power (W) divided by apparent power (S). There is a companion reactive factor which is Q/S, but it's much less commonly used.

Doh!

Sorry David, I don't know what I was thinking, and make no excuses.

for many years, utilities measured KVAHrs and KWHrs and derived the average PF from that.

Thanks for that, Bill.

Hmmm, I wonder if they used Q/S to derive W to get apparent power so they could bill for power factor itself, or if the calculation was performed some other way.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[stevenal]

Waross,

My local utility still bases its billing on "average" power factor. kVAh is not and never was directly measured. Some trig is applied to the kWh and kvarh to derive the average pf.

Traditionally, two kWh electro-mechanical meters were used, one being fed from a reactiformer that shifted the voltage by 90 degrees to measure kvarh.

 

[waross]

Thanks for the correction, Stevenal.
I am on the road and on my phone.
KVAHrs should have been KVARHrs.
I missed the R.
Yes, I remember those phase shifters.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Thanks Mr stevenal
Let us get it right. Following are examples on some common items:
1. KVAHrs should be kVAh. Attention: NOT KVAh,
2. KVARHrs should be kvarh. Attention: NOT kVArh,
3. KV should be kV . Attention: NOT kv,
4. CB 'trip time' 0.1 s, Attention: NOT 0.1 S,
5. BB 'contact resistance' 0.5 mOhm. Attention: NOT 0.5 MOhm,
6. 'Insulation resistance' 100 MOhm. Attention: NOT 100 mOhm,
7. Temperature rise from 40 to 100 deg C is 60 K. Attention NOT 60 deg K.
8. etc...
Che Kuan Yau (Singapore)

 

[che12345]

@ Mr FilipRST (Electrical)
I notice some unusual data in your measurement. Please advise/clarify.
1. a) When no pump is running with 2 kW loading, the average voltage is 615 V, 6.67 A, pf 0.27...,
b) When one pump is running with 27 kW loading, the average voltage is 617 V, 35 A, pf 0.72...
2. Observation:
a) Current increases from 6.67 to 35 A is OK, but the voltage also increases from 615 to 617 V. This is unusual?. Noted pf increases from 0.27 to 0.72,
b) With 2 kW or 27 kW load, the voltage is around 616 V. What is the load rated voltage. If the loads are rated say IEC std 690 V 50 Hz, the measured voltage 615 to 617 V is far too low ! Or are you in the US ...V 60 Hz location?
Che Kuan Yau (Singapore)

 

[waross]

Hello mr. Che.
FilipRST appears to be posting from Ontario Canada.
The standard there is 600 V 60 Hz.
The over voltage is acceptable.
The variation is probably time of day and not load related.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FilipRST]

Thanks waross.
You are correct.
Just to note: pumps are associated with VFD motors.
Best,
Filip