Think of your audience.
Your summary should serve as a brief introduction into and overview of your research. The purpose of that summary is to allow your audience to understand your research, discuss it with you, and give you helpful feedback.
Just condensing your dissertation may not help them with their task of helping you.
What I would do is create an interesting narrative. Look at some TED talks to give you an idea what that might entail. Generally, it begins with a very brief (one to a few sentences) summary of what you research, goes on to explain why you research this (i.e. why is this important and to whom), then you explain how you research it, but keep your explanations general and avoid jargon, to that an intelligent non-expert can understand it (imagine that you write for the New York Times), and finally you explain the problems that you currently face and ask your audience for ideas in how to solve them.
That is, on every level of your narrative you draw your audience in. First you make your research relevant to them. Then you explain it in a non-boring way, then you ask them to get thinking about how to do it better.