# The order of Transfer function

Discussion in 'Homework Help' started by mo2015mo, Oct 19, 2013.

1. ### mo2015mo Thread Starter Member

May 9, 2013
157
1
hello guys

I have confused about determine the order(degree) of the transfer function T(s) (System)

As the attached photo,,,, Why my doctor consider the Field-Controlled DC motor T.F is 2nd-order sys. although i see those just are three poles ????(& the highest power of S is three)??
While the Armature-Controlled DC motor T.F. is 3rd-order sys. & the highest power of S is 3rd ??
Which one of them is Correct??

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Last edited: Oct 19, 2013

May 9, 2013
157
1
3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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706
As far as I can see they are both 3rd order.

4. ### WBahn Moderator

Mar 31, 2012
23,552
7,207
There isn't enough information to go from the pictorial representatin to the transfer function, but the presence of an additional coil in the second one implies it would be a higher order.

I'm not sure of the nomenclature for s-domain transfer functions. In general you pick up an additional 's' in the denominator as a result of the transform itself. So a second order system would have a third-degree polynomial in the denominator. It might make sense (or be convention) to talk about the order of the transfer function as one less than the degree of the characteristic polynomial so that a second order system has a second order transfer function. But that is pure speculation on my part.

5. ### primeq New Member

May 15, 2012
3
0
diagrams aside (agree with WBahn that the diagrams alone are not sufficient to determine order of the system) it's a fact that the Laplace-domain's 3rd-order polynomial makes this a third-order transfer-function. (if you multiply the denominator expression you will see that the highest exponent of s is 3 - hence "3rd order"

6. ### LvW Well-Known Member

Jun 13, 2013
769
104
Yes - that`s also my opinion. The order of a transfer function is identical to the order of the denominator if the transfer function is written in normalized form (denominator is a polynominal in s).
Looking to the given functions only (ignoring the pictures) I can see no fundamental difference between both functions.