NCEES #135 and #522

[trying2pass]

Both of these questions ask for the internal voltage of a machine, but both uses different equations. Why? I know there is something I'm missing and I'm sure it's really simple.

For #135, I used E(internal)=E(terminal)+I(X'd) which I got correct.

For #522, I used the same equation as #135 and got it wrong. Looking at the solutions, they used this --> E(terminal)=X(I)+E(internal)

Can anyone help me on this?

Thanks!

 

[Flyer_PE]

The machine in #135 is a generator. The internal voltage will be greater than the terminal voltage.

The machine in #522 is a motor. The internal voltage will be less than the terminal voltage.

 

[yellowjacket03]

#135 refers to Transient Performance, and gives you the Transient Reactance X'd

#522 refers to Steady State Performance of the machine, and gives you the Synchronous Reactance Xs

Although the reactances are different for Steady State and Transient conditions. the equations to determine internal voltage are similar. I would recommend getting a hold of a good reference for machines. I would recommend Electric Machinery by A.E. Fitzgerald, Charles Kingsley, Jr., and Stephen D. Umans.

Both of these questions ask for the internal voltage of a machine, but both uses different equations. Why? I know there is something I'm missing and I'm sure it's really simple.
For #135, I used E(internal)=E(terminal)+I(X'd) which I got correct.

For #522, I used the same equation as #135 and got it wrong. Looking at the solutions, they used this --> E(terminal)=X(I)+E(internal)

Can anyone help me on this?

Thanks!

 

[Bluekayak]

#135 refers to Transient Performance, and gives you the Transient Reactance X'd
#522 refers to Steady State Performance of the machine, and gives you the Synchronous Reactance Xs

Although the reactances are different for Steady State and Transient conditions. the equations to determine internal voltage are similar. I would recommend getting a hold of a good reference for machines. I would recommend Electric Machinery by A.E. Fitzgerald, Charles Kingsley, Jr., and Stephen D. Umans.

Both of these questions ask for the internal voltage of a machine, but both uses different equations. Why? I know there is something I'm missing and I'm sure it's really simple.
For #135, I used E(internal)=E(terminal)+I(X'd) which I got correct.

For #522, I used the same equation as #135 and got it wrong. Looking at the solutions, they used this --> E(terminal)=X(I)+E(internal)

Can anyone help me on this?

Thanks!

For #135, the assumed terminal voltage is given as 1.0 pu. It then follows that because this is a transient stability study at rated MVA, rated voltage, and unity power factor, the current through the terminal leads is assumed to be (1.0 + j0) pu. Please advise if this assumption is valid.

 

[Flyer_PE]

For #135, the assumed terminal voltage is given as 1.0 pu. It then follows that because this is a transient stability study at rated MVA, rated voltage, and unity power factor, the current through the terminal leads is assumed to be (1.0 + j0) pu. Please advise if this assumption is valid.

It's valid.