Cram for PE Exam Vol 2 Q 30 - Capacitor-Start Motor

[akyip]

Hey guys,

I have yet another question, this one on capacitor start motor, from Cram for Exam Volume 2 question 30.

This question gives a starting impedance Zmain for the main winding, and a starting impedance Zaux for the auxiliary winding.

The question asks to find the starting capacitance needed to place the main and auxiliary winding currents 89 degrees apart at starting.

Reading the solution, I am confused at one thing. The impedance angle of the auxiliary winding and starting capacitor is calculated to be the main impedance angle minus 89°... Why is it that the main impedance angle is subtracted instead of the auxiliary winding angle to get the angle of the auxiliary winding and capacitor?

I was trying to upload photos of this problem and the solution, but my photos are apparently always too big to upload...

Thanks to anyone who can help explain this!

 

[Cram For The PE]

Hey guys,

I have yet another question, this one on capacitor start motor, from Cram for Exam Volume 2 question 30.

This question gives a starting impedance Zmain for the main winding, and a starting impedance Zaux for the auxiliary winding.

The question asks to find the starting capacitance needed to place the main and auxiliary winding currents 89 degrees apart at starting.

Reading the solution, I am confused at one thing. The impedance angle of the auxiliary winding and starting capacitor is calculated to be the main impedance angle minus 89°... Why is it that the main impedance angle is subtracted instead of the auxiliary winding angle to get the angle of the auxiliary winding and capacitor?

I was trying to upload photos of this problem and the solution, but my photos are apparently always too big to upload...

Thanks to anyone who can help explain this!

See image below of capacitor start motor with main, aux and capacitor. I am trying to get them 89° apart. My main is sitting at 31°. In order to put them out by 89° the aux winding w/cap has to be -58°. To move the aux (which is 10+j2) with the capacitor it is going to be minus. Remember inductance is +j and capacitance is -j.

 

[Dude99]

~ 147.4 uF

 

[Cram For The PE]

~ 147.4 uF

View attachment 16724

that's the answer!

 

[Dude99]

Do I get a cookie!?!