NCEES 517

[Redskinsdb21]

Hello:

Im working through NCEES power practice exam....I have the following questions on problem 517:

1) THey use

P = V*I(conj) to get power for synchronous motor....the solution has 8k @ -45.6 degrees....

Could someone please show hot to get this.

2) When determining power for induction motor, they appear to use:

P = sqrt(3)*(14.43)*(480)*(invcos(0.6))....why do they use sqrt(3) here? why not calculate without sqrt(3).

 

[Flyer_PE]

1. For the synchronous motor, you are given the kVA value and the power factor and the fact that the power factor is leading. These three items give you everything you need to know about the synchronous motor.

2. For the induction motor, you are given the line current and the power factor. An induction machine will always have a lagging power factor. This is a 3-phase system so the apparent power (S) drawn by to the motor will be: S=sqrt(3)*VL-L*I*Line

 

[Redskinsdb21]

Do Synchronous motors always have leading power factors like induction motors have lagging power factors?

 

[Flyer_PE]

^No. Depending on the excitation level, a synchronous motor can have either a leading or lagging power factor.