What is considered starting power factor?

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t_van

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HI,

I has a 3-phase, 480V, 75Hp, PF=0.77, NEMA code letter K motor. I have the power factor values for when the motor is 1/4 loaded, 1/2 loaded, 3/4 loaded and full loaded. My question is, what is considered the starting power factor? Is this when the motor is 1/4 loaded? Is it at t=0 when the motor first has voltage applied to its windings? Thanks in advance.
 
t_van said:
HI,

I has a 3-phase, 480V, 75Hp, PF=0.77, NEMA code letter K motor. I have the power factor values for when the motor is 1/4 loaded, 1/2 loaded, 3/4 loaded and full loaded. My question is, what is considered the starting power factor? Is this when the motor is 1/4 loaded? Is it at t=0 when the motor first has voltage applied to its windings? Thanks in advance.
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IMHO the starting power factor would be the PF associated with the locked rotor current (LRA) at the time voltage is first applied and for the next few cycles.
The current surge will be many times the normal Full Load Amps (FLA) and I do not think that the PF at that point would be particularly important except when considering starting the motor from a generator or inverter or if checking to see whether a parallel capacitor switched in only for starting would be any help in reducing voltage drop.
 
GoldDigger said:
IMHO the starting power factor would be the PF associated with the locked rotor current (LRA) at the time voltage is first applied and for the next few cycles.
The current surge will be many times the normal Full Load Amps (FLA) and I do not think that the PF at that point would be particularly important except when considering starting the motor from a generator or inverter or if checking to see whether a parallel capacitor switched in only for starting would be any help in reducing voltage drop.
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I agree. Starting PF is at t= 0
 
Jraef said:
I agree. Starting PF is at t= 0
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IMO it has to be at least from t=0 to t=1/120sec (first half of a voltage cycle).

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Image courtesy of Carultch.
 
t_van said:
Thanks for the quick replies. I am sizing a generator to start three of the below mentioned motors. I appreciate the input!
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pf is not that critical
as long as the motor Code starting kva < alternator rated motor starting kva

you may want to consider staggering the starting of the motors

most genset mfgs will offer a sizing program or size it for you

eg Code K 8-8.99 kva/HP
75 HP motor and using 9 kva/HP gives 675 kva or 850 A at 460/3 required to start per motor
running kva 76.5 kva (based on table 430.250)

starting in sequence 76.5 + 76.5 + 675 = 828 kva
starting simultaneously 3 x 675 = 2025 kva

much smaller set required when sequenced
 
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