motor starting power factor profile reasonable? starts at pf ~ 0.33
motor starting power factor profile reasonable? starts at pf ~ 0.33
(OP)
Using recorded current and voltage data, I attempted to calculate motor power factor vs time during motor start for a 60hp 460vac KVA code G motor.
Results are shown in attached graph. Results are attached. It looks like power factor starts in the range of 0.31-0.35 at the moment of start and ends up 0.9 when running.
My questions:
1 - Does this data look reasonable to you?
2 - Particularly the starting power factor 0.31... 0.35 ? (seems high!)
3 - Do you think this would be reasonable power factor to use for calculating expected decay of the transient dc component?
4 - What about the strange wiggle in the power factor curve at 1.2 seconds in this graph (is it expected for some reason) ?
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fyi – Here is how I calculated power factor:
Started with recorded sampled data: Ia, Ib, Ic, Vac, Vbc
Solve 3 equations (below) in 3 unknowns (Van, Vbn, Vcn)
Equation 1: Vac = Van – Vcn
Equation 2: Vbc = Vbn – Vcn
Equation 3: Van +Vbn + Vcn = 0 (assumption).
Solution of 3 equations i 3 unknowns is:
Van = 2/3 Vac – 1/3 Vbc
Vbn = -1/3 Vac + 2/3 Vbc
Vcn = -1/3 Vac -+ 1/3 Vbc
Find dc components IAave, IBave, ICave as average of current over 1 cycle.
Find fundamental component of current as IA1 = IA – IAave
( IB1 and IC1 similar)
(based on the assumption that the current is sum of sinusoid and dc component)
Find powerfactor as
pfA = Sum (IA1 * Van) / [ sqrt( Sum(IA1^2) * Sum(VAn^2) ) ]
where the sums are done over one cycle
(pfB and pfC similar)
Does that sound reasonable? I can post the excel file if anyon is interested (large file).
Results are shown in attached graph. Results are attached. It looks like power factor starts in the range of 0.31-0.35 at the moment of start and ends up 0.9 when running.
My questions:
1 - Does this data look reasonable to you?
2 - Particularly the starting power factor 0.31... 0.35 ? (seems high!)
3 - Do you think this would be reasonable power factor to use for calculating expected decay of the transient dc component?
4 - What about the strange wiggle in the power factor curve at 1.2 seconds in this graph (is it expected for some reason) ?
============================
fyi – Here is how I calculated power factor:
Started with recorded sampled data: Ia, Ib, Ic, Vac, Vbc
Solve 3 equations (below) in 3 unknowns (Van, Vbn, Vcn)
Equation 1: Vac = Van – Vcn
Equation 2: Vbc = Vbn – Vcn
Equation 3: Van +Vbn + Vcn = 0 (assumption).
Solution of 3 equations i 3 unknowns is:
Van = 2/3 Vac – 1/3 Vbc
Vbn = -1/3 Vac + 2/3 Vbc
Vcn = -1/3 Vac -+ 1/3 Vbc
Find dc components IAave, IBave, ICave as average of current over 1 cycle.
Find fundamental component of current as IA1 = IA – IAave
( IB1 and IC1 similar)
(based on the assumption that the current is sum of sinusoid and dc component)
Find powerfactor as
pfA = Sum (IA1 * Van) / [ sqrt( Sum(IA1^2) * Sum(VAn^2) ) ]
where the sums are done over one cycle
(pfB and pfC similar)
Does that sound reasonable? I can post the excel file if anyon is interested (large file).
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RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
Bill
--------------------
"Why not the best?"
Jimmy Carter
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
But an intriguing idea anyway. At first thought, I wasn't sure that would be enough info to solve it. Then out of curiosity I drew a phasor diagram and saw it could be solved, but seems a little ugly. I attached my solution. Is that what you had in mind or is there a simpler way?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
Not quite the curve that I would have expected.
Most of the motors that I have checked have an initial power factor closer to 0.2 than 0.4, and the power factor has a more linear change with slip than indicated in your motor, but I have not checked any motors for a long time now. I wonder if this is one of the new high efficiency motors?
Best regards,
Mark Empson
L M Photonics Ltd
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
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RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
I would also expect that the maximum power factor should occur at a slip value equal to 2 - 3 times full load slip on the acceleration curve.
There is a peak on your curve but much lower than the full load pf. As the motor passes though the full load slip, the pf curve should show full load pf.
Best regards,
Mark Empson
L M Photonics Ltd
RE: motor starting power factor profile reasonable? starts at pf ~ 0.33
Maybe smaller motors are a little better.