Motor Impedance Xd''
Motor Impedance Xd''
(OP)
Hi, I am currently working on a short circuit study and a load flow study. Can anyone tell me what typical Xd'' values would be for a 3ph 11100kW 13.2kV motor? So far I have seen that the value of 0.17 would be an ok assumption? Please provide some insight on this issue.
Thanks
Thanks





RE: Motor Impedance Xd''
If this is an induction motor, the motor contribution will decay rapidly, within a few cycle. If it is a synchronous motor, then not quite so rapidly.
The X"d value is at best an approximation of a complex electromechanical system's transient response characterisitics.
For a motor this large, you may be able to obtain more accurate data from the motor manufacturer, if they are still in business.
RE: Motor Impedance Xd''
RE: Motor Impedance Xd''
jbartos - You said that the value could be lower or higher depending on number of poles. The motors I am dealing with have 4 poles. Some motor drive pumps and others are just fans. Given this info will my values would be lower or higher than what is shown in IEEE Std 141-1993?
Once again guys, thanx for the help.
RE: Motor Impedance Xd''
1. IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power System Analysis, (Brown Book),
Table 7-3 - Typical values of motor impedances and kVA ratings to use when exact values are not known.
on page 180.
"Individual large induction motors, usually medium voltage = Xd"=0.67"
I do not see that 0.67 realistic. This could be inquired about at IEEE:
http://www.ieee.org
There is also the following Note:
"Motor impedances are in per unit on motor voltage and kVA rating. Xd" for induction motors is approximately equal to the locked-rotor reactance." (Agreed.)
"For induction motors, the locked-rotor reactance is the reciprocal value of the locked-rotor current." (Agreed.)
Now, if you check a manufacturer literature, e.g. Baldor Motors and Drives AC Motor Data Catalog 502 April 1992, you will see that higher pole (lower RPM) induction motors have smaller Lock-Rotor amps. This implies that the Xd" is higher for the higher-pole squirrel cage induction motor. If you consider often used LRA/FLA=6.0 p.u., you will get Xd"=1/6=0.167~0.17p.u. as you indicated in your posting. If the large machine has LRA the same you can use 0.17pu=Xd". Some motors have on their nameplates Nema Code Letter, e.g. G that has kVA / HP range from 5.6 to 6.29; therefore, LRA/FLA = 6.0 is approximately right. If your large motor has a different LRA or NEMA Code Letter, then you need to adjust Xd".