This is the most important thing. Nobody knows about what you did as much as you do know. You are just explaining your work to few people who might understand it but probably will not!
This is due to the fact that the seminar is an "informal" meeting where the majority of the audiance are your friends doing masters in other areas within the engineering discipline but not necessarily the same as you are. The professors that you invite may not show up. Don't be offended, this is normal.
The best thing about the seminar is that it allows you to produce the slides and view graphs that you need for the defense quite early. You try them in front of an audience and can measure your performance. Try to speak clearly, concisely and to answer the questions in an technicaly sound manner even if you don't know the right answer. You are doing research and it is possible that you don't know the answers to several points, however it is important that you attack the problem in a good way and suggest ways to solve it.
Once it is done, ask your professors as well as your friends about the presentation speed, clarity and about your voice tone. Ask also about the quality of the graphs, their visibilty from a long distance and any enhancements that can be made. Then use this information in order to perfom better on the defense day.