Is it just me or does the spin-up book incorrectly show how to sum three phase currents?
This problem (4-62) is the second time I have noticed this, and I want to know if the way I'm thinking about it is incorrect, or if the book is incorrect.
To find the solution to this problem, you have to find the current in the neutral. Neutral current is In = Ia + Ib + Ic.
You have to find current Ia, which is a single phase motor between A and B with a power factor of .75 lagging, 9.6kVA.
Current Ib is given at .25 < 10 degrees.
Current Ic must be calcuated. The load is a single phase motor with a power factor of .8 lagging, 30kVA.
The solution shows ((9,600/240) < -36.89 deg) + (.25 < -10 deg) + ((30000/240) < -41.41 deg)
The magnitudes are correct, but I do not understand why the angle of the Ib and Ic currents are not being adjusted based on the fact that voltages B and C are 120 and 240 degrees out of phase with A. The angle determined by the power factor is the angle between the current and the voltage, not just the absolute angle of the current, right?
Let me pose another situation to better explain my point: Same wye system, with (3) 10kVA, 240V single phase motors (connected AB, BC, CA), all having a power factor of .8. The neutral current should be 0, since the system is balanced right? If we were to solve this problem using the spin-up method, you would end up with this: ((10000/240) < -36.9 deg) + ((10000/240) < -36.9 deg) + ((10000/240) < -36.9 deg), which makes no sense at all, we know the neutral current is 0.
If anyone could shed some light on this it would be appreciated, thanks!
This problem (4-62) is the second time I have noticed this, and I want to know if the way I'm thinking about it is incorrect, or if the book is incorrect.
To find the solution to this problem, you have to find the current in the neutral. Neutral current is In = Ia + Ib + Ic.
You have to find current Ia, which is a single phase motor between A and B with a power factor of .75 lagging, 9.6kVA.
Current Ib is given at .25 < 10 degrees.
Current Ic must be calcuated. The load is a single phase motor with a power factor of .8 lagging, 30kVA.
The solution shows ((9,600/240) < -36.89 deg) + (.25 < -10 deg) + ((30000/240) < -41.41 deg)
The magnitudes are correct, but I do not understand why the angle of the Ib and Ic currents are not being adjusted based on the fact that voltages B and C are 120 and 240 degrees out of phase with A. The angle determined by the power factor is the angle between the current and the voltage, not just the absolute angle of the current, right?
Let me pose another situation to better explain my point: Same wye system, with (3) 10kVA, 240V single phase motors (connected AB, BC, CA), all having a power factor of .8. The neutral current should be 0, since the system is balanced right? If we were to solve this problem using the spin-up method, you would end up with this: ((10000/240) < -36.9 deg) + ((10000/240) < -36.9 deg) + ((10000/240) < -36.9 deg), which makes no sense at all, we know the neutral current is 0.
If anyone could shed some light on this it would be appreciated, thanks!