motor starting power factor profile reasonable? starts at pf ~ 0.33 1

[electricpete]

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).


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[waross]

I would connect a PF correction capacitor and record the line current, the motor current and the capacitor current. You know that the capacitor current is almost perfectly reactive. You can use the other two values to calculate the PF for any time group you wish.

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

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

 

[electricpete]

Thanks Bill. Our particular situation doesn't allow adding capacitors to a critical motor.

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?

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[electricpete]

Whooops. Is and Im are switched in the diagram so my solution is wrong.

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[electricpete]

A little simpler solution (attached) when the correct diagram is used.

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[electricpete]

Nope. Still wrong. Will post another correction.

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[electricpete]

This time it's gotta be right.

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[Marke]

Hi Pete

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

 

[electricpete]

Thanks Mark. Maybe I will try zooming in and estimating the time delay from voltage zero-crossing to current zero crossing and calculate pf angle and pf from there as a double-check

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[Marke]

Hi Pete

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

 

[dpc]

I just reviewed a bunch of motor data sheets for some new 4160 V motors - 250 hp up through 1500 hp. The starting power factor was generally less than 0.2.

Maybe smaller motors are a little better.