326 Copyright 1991 IEEE All Rights Reserved

IEEE Std 241-1990 IEEE RECOMMENDED PRACTICE FOR

9.5.11 Rotating Machines

Machine reactances are usually expressed in terms of percent reactance % Xm, or per unit reactance Xpu, on the normalrated kVA of the machine. Either the subtransient reactance X or the transient reactance X should be selected,depending on the type of short-circuit current calculation required. Motor-rated kVA can be estimated, given the motorhp, as follows:

9.5.12 Motors Rated 600 V or Less

In systems of 600 V or less, the large motors (that is, motors over 50 hp) are usually few in number and represent onlya small portion of the total connected hp. These large motors can be represented individually, or they can be combinedwith the smaller motors, representing the complete group as one equivalent motor in the impedance diagram. Smallmotors are turned off and on frequently, so it is practically impossible to predict which ones will be on line when ashort circuit occurs. Therefore, all small motors are generally assumed to be running, and all are considered as onelarge motor.

Where more accurate data are not available, the following procedures may be used in representing the combinedreactance of a group of miscellaneous motors:

1) In all 208 V systems and 480 V commercial building systems, a substantial portion of the load consists oflighting; so assume that the running motors are grouped at the transformer secondary bus and have areactance of 25% on a kVA base equal to 50% of the transformer kVA rating.

2) Groups of small induction motors that are served by a motor control center can be represented by consideringthe group to have a reactance of 25% on a kVA rating equal to the connected motor hp.

Example

1) Conversion to per unit on a 10 000 kVA base (kVAb). A 500 hp, 0.8 pf synchronous motor has a subtransientreactance Xd of 15%

NOTE Refrigeration chillers (such as those made by Carrier, Trane, and York) have motors rated in kW. IEC-type motors havekW ratings. To obtain an equivalent kVA, divide kW by an assumed power factor of about 0.85.

Type of Machine Rated kVA

All

(exact)

Induction motors and 0.8 power factor synchronous motors rated hp (approximate)

1.0 power factor synchronous motors 0.8 rated hp (approximate)

V ratedI rated 3

1000---------------------------------

Xpu

%Xd

100------------

kVAbmotor kVA---------------------------

15100---------

10000500

--------------- 3.0= = =