motor loaded circuits

[powerjunx]

Everytime i look up some fault calculation a low voltage system having a motor loaded circuits, it appears the fault current contribution of SMALL MOTOR and LARGE MOTOR were approximated to a value of 20% to 25% reactance, respectively. Why and how this value obtained?

As i refer to IEEE 115-19xx, "guide and test procedure for synchronous machine transient parameters" a table of typical machine reactance and time constants were tabulated.. i found out that 20%-25% reactance falls right under the motor with specific rpm.. Does this tabulated typical reactance were used as the basis for small and large motor? How this value affected by RPM, HP/KW, ROTOR CONSTRUCTION, etc?

all comments are welcome!

thank you..

 

[motorspert]

The fault current is very approximately the same as the starting current. Most motors are designed with a starrting current in the range 450% - 600% which gives a starting reactamce band of 16% - 22%, so i am not surprised the fault reactances are in a narrow band.

I think it is just a fact of life for all motors

 

[dpc]

ANSI C37 contains the best guidance (or most commonly used, anyway) for determine motor reactances to be used for fault calculations (in the US). IEC has their own standards for this.

For small motors, the subtransient reactance is almost never known, so some approximation is required. You also have to take into account the impedance of the cable feeding the motor. This adds considerable resistance for small motors.