# root locus question - root locus 'begins' at finite open loop pole locations

This is a general question about root locus plots. I notice that there is a source that says:

"The root locus begins at the finite and infinite poles of G(s)H(s)".

Note the part that says 'finite and INFINITE poles'.

Should the part that says 'infinite' be omitted? The root locus only begins at the FINITE open loop pole locations, right?

If the statement is indeed correct - about cases where a root locus may start at 'infinite' pole locations, then are there any examples of such cases?

Thanks all!

## 1 Answer

You can do a root locus plot of a system with more zeros than poles, even though such a thing is physically impossible. If you do, you find poles coming in towards your zeros, just as a system with an excess of poles has them running off to infinity.

• That is really excellent help Tim. That makes proper sense now! Absolutely appreciated. The examples we had been dealing with were mainly finite open loop poles outnumbering the finite open loops. I totally overlooked cases of the reverse situation - where the number of finite open zeroes outnumber the finite open loop poles. Thanks once again! – Kenny Sep 27 '19 at 4:08