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Christian Tabios
08-14-2014 03:18 PM

How can I get the transient reactance of an induction machine?

Dear all, how can I get the transient reactance of an induction machine? Is this equal to the subtransient reactance?
08-14-2014 05:53 PM
Top #2
Ing. Ole Knudsen
08-14-2014 05:53 PM
First: No, transient and subtransient reactance is not the same.
Sub-transient refers to the first few cycles after the load of a generator has changed, while transient refers to the period of normally a few seconds until the system has stabilised.
You can find several references on the net, such as this one:
08-14-2014 08:41 PM
Top #3
Spir Georges GHALI
08-14-2014 08:41 PM
Dear Christian

In adding to what Mr. Ole said, we can calculate these Reactances by using the following formulas :

- Sub-Transient Reactance : X″d = Un^2 . x″d / 100 . S
- Transient Reactance : X′d = Un^2 . x′d / 100 . S
- Synchronous Reactance : Xo = Un^2 . xo / 100 . S

Where :
- S : Apparent Power
- Un : Voltage between 2 phases
- x″d : Percentage " % " of Sub-Transient Reactance ( typically 20% )
- x′d : Percentage " % " of Transient Reactance ( typically 30% )
- xo : Percentage " % " of Synchronous Reactance ( typically 6% )
Noting that these percentages should be defined by the manufactures.
08-14-2014 11:22 PM
Top #4
Manoj Badave
08-14-2014 11:22 PM
Dear Mr. Ole,
A very good paper on the subject!
Dear Cristian,
As mentioned by other, these are based on the design parameters of the machine and hence generally specified by the manufacturer in data sheet.
For academic interest could you explain where do you require it?
08-15-2014 02:12 AM
Top #5
Alan Maltz
08-15-2014 02:12 AM
Ole, Nice paper are synchronous generator reactances, but Cristian is looking for info on induction machines. Typically X"d and X'd are so small for induction machines that they are ignored in any type of study except where the induction motors are very large and constitute a large proportion of the total load. Since induction machines aren't separately excited there's usually no amortisseur winding (which gives us X"d), large field winding (which gives us X'd), or field current to decay and add much to the short circuit current beyond the first cycle or two, though it can add to the residual voltage (but that's a whole other topic).

08-15-2014 04:23 AM
Top #6
Tan KuiLi
08-15-2014 04:23 AM
read the datasheet
08-15-2014 06:29 AM
Top #7
Ing. Ole Knudsen
08-15-2014 06:29 AM
Hi Alan,
I did not fully read the question, and of course, have always associated X"d and X'd with generators only.
One minor correction to your post, though, I presume you mean to say that the reactances for an IM are so HIGH, that they can largely be ignored?
08-15-2014 08:42 AM
Top #8
Alan Maltz
08-15-2014 08:42 AM
Ole, I suspected that you'd pick that up, but in the simplified equivalent model of an induction motor we're talking about the series branch (X) before we get to the parallel branches containing the magnetizing and slip portion. In a synchronous machine X"<X'<X, how else would the current be higher during the transient portion. Nice try though! Alan
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