Good Afternoon,

Within SKM, Xd" and X/R values utilized for motor contribution calculations can be defined in a few different ways. These values can be calculated utilizing user defined parameters consisting of LRA/FLA and the starting PF. Additionally, typical values can be utilized from sources including IEEE 399, IEEE 141 and Conrad St. Pierre's Short Circuit Calculation Guide.

For those of you who utilize SKM, what approach do you usually utilize? Exact parameters required are not always obtainable, especially for existing installations.

Thanks.

I use the guidance in the Red Book.

Mike

If it is a existing installation, you can find the impedance of the motor by looking at the NEMA starting code on the motor nameplate. This is a letter ranging from A - V. The per unit impedance will be the reciprocal of the associated lock rotor number. Make sure that you are not looking at the NEMA enclosure code! These are not the same.

For Chillers and some other types of motors, look for the locked rotor or starting amperes. Per Unit Impedance = Full Load Amps / Lock Rotor or Starting Amperes.

Robert is correct about using the motor code to determine the subtransient impedance Z. In order to get to the subtransient reactance Xd", you need to also adjust the starting PF. IEEE 399 indicates that is around 0.20 for large motors and references "Industrial Power Systems Handbook", 1955, by Donald Beeman. I got a copy of that reference which by the way is a good reference and is referenced multiple times in the Color Book series. It has a graphical figure for starting PF at around 0.62 for 5 HP and on a (starting PF Linear Y-Axis x HP log scale X-Axis) is near linear to about 300 HP at 0.22, and then curves to 0.16 pf at 1000 HP.

For new motors under design, you can use motor manufacturers LRA or motor code and if the subtransient reactance is not given, use the Beeman figure for starting pf.