Motivating students to the extent they can is arguably a partial responsibility of undergraduate lecturer. At the graduate level, I think its more a nice to have.
I think the lecture’s responsibility with regard to motivation is primarily the following. First, to encourage and cultivate passion of the students (mostly by living it out & by pointing out heroes). Second, to encourage and cultivate their curiosity. And third is to help students make connections of relevance (help the students with a practical and perhaps emotional ). And they can also help student setting their goals and connect students with the campus resources.
If a lecturer doesn’t motivate, then it will be difficult for the students to take an interest in the course that the lecturer teaches. A lot of being a lecturer is leadership, that is, to inspire other people to do things that they might not do otherwise. By leading students, inspiring students, the subject materials become accessible to the students because they are willing to work a bit hard to understand them.
The students also have a part in motivation. It is important that the students take an interest in learning the materials required. Students need to provide feedback to the lecturer to let the lecturer knows if they are learning and more importantly, what is not be learned. So motivation is two way communication.
And then to conclude that, I think the term mentoring would be better than motivating. A good college lecturer mentors his or her students. Note that I use the word lecturer who is more than a teacher. Students need to be motivated if they expect to succeed, and frankly, any college student especially a graduate student is already motivated. It is when these students become discouraged that a good mentor can help. But in my experience, only the student can sort this out.