It is called "NYM"-command.
a certain interval?
1) Default interval is 10 minutes. You would change it in your torrc ( Configuration file ) read the f.. manual.
2) Also, you can always change circuit by telnet, You need to connect to Tor's control port, thereafter send there right command. Again, read the manual. You must login with you password.
3) In Unix-like systems, you would always change the circuit by one SIGNAL, often it is SIGHUP ( Number 1 ).
kill -1 `pidof tor`
works for me.
kill -HUP `pidof tor`
Daemon programs sometimes use SIGHUP as a signal to restart themselves, the most common reason for this being to re-read a configuration file that has been changed.
Here, in Tor, "re-read a configuration file" - includes a building of new circuit for you.