Camara Practice problems for Chap 40

[nmjwolf]

Hi,

Perhaps I am missing something (or perhaps I am dense)...but can someone shed some light on when to divide by the sqrt of three on AC motor problems when the connection type isn't given?

For example in practice problem 3 they say calculate the full load phase current drawn by 440V, 20hp (per phase)...now i suppose you could make the assumption that (per phase) must apply to both the HP and the V?

#4 assume delta-wired?

and so on.

All the book says that it is common to refer to line-to-line as terminal voltage.

Thanks!

 

[Flyer_PE]

Hi,
Perhaps I am missing something (or perhaps I am dense)...but can someone shed some light on when to divide by the sqrt of three on AC motor problems when the connection type isn't given?

For example in practice problem 3 they say calculate the full load phase current drawn by 440V, 20hp (per phase)...now i suppose you could make the assumption that (per phase) must apply to both the HP and the V?

#4 assume delta-wired?

and so on.

All the book says that it is common to refer to line-to-line as terminal voltage.

Thanks!

Unless they specifically state they are giving you a phase voltage value (VLN) the voltage rating of a 3-phase machine will be the line voltage (VLL). The motor doesn't care if the system supplying it is wye or delta connected.

Practice problem 3 is worded badly. To me, "(per phase)" implies that it is a 3-phase motor. However, their solution indicates that it is actually a single phase motor. The sqrt of three does not apply to a single phase device.

For practice problem 4, it doesn't matter if the motor is wye or delta. The apparent power (S) in a 3-phase machine will always be:

S=sqrt3*VLLIL

For this problem you can determine S from the output power, efficiency, and power factor. VLL is given.