Power Factor and Calc Current on Motor

[hanksmith]

When I am calculating the current a motor is going to use for a three phase load what power factor should I use?

The power factor from the manufacturer of the motor or the power factor of the system the motor is connected to?

Formula i am using:

A= (HP*746)/(V*sqrt(3)*PF*Eff)

Thanks

 

[waross]

Use the nameplate current or use a current value from the tables in the code book.
But to answer your question, use the motor power factor. The system power factor is a weighted average of the power factor of all connected loads.

Bill
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"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

A good "rule of thumb" value for rough calculations for induction motors is 0.8 power factor.

It'll get you in the ballpark.



old field guy

 

[Marke]

The power factor and the efficiency are very dependent on the size, poles and design of the motor.
The power factor of a three phase motor can be as low as 0.6 and as high as 0.92.
Larger motors tend to have higher power factors, slower motors tend to have lower power factors.
The only correct way is to use the actual motor data in which case you would use the nameplate current rating.

Best regards,


Mark Empson
L M Photonics Ltd

 

[ScottyUK]

Other thing to consider is that the magnetising current is, to a first approximation, independent of load. The reactive load of the motor is therefore largely constant assuming the voltage is constant, and this will give a lousy power factor if the motor is running open shaft because the active power is very low. Power factor improves as the motor load increases.

We're also assuming this is an induction machine; if it's synchronous then the full load PF could be anywhere from about 0.8 leading to about 0.9 lagging.


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