By Dan Levine

(Reuters) - A U.S. judge narrowed a patent lawsuit brought by Google's Motorola Mobility unit against Microsoft Corp, finding that parts of three Motorola patents are invalid.

The ruling on Thursday came from U.S. District Judge James Robart in Seattle, who is overseeing a tandem lawsuit in which Microsoft claims Motorola deserves only a small royalty on many of its telecommunications patents. Google bought Motorola for $12.5 billion, partly for its intellectual property stockpile.

Apple Inc and Microsoft have been litigating in courts around the world against Google and partners such as Samsung Electronics Co Ltd, which use the Android operating system on their mobile devices.

Apple contends that Android is basically a copy of its iOS smartphone software and Microsoft holds patents it contends cover a number of Android features.

Motorola had sought up to $4 billion a year for its wireless and video patents, while Microsoft argues its rival deserves about $1 million a year.

If U.S. District Judge James Robart decides Google deserves only a small royalty, then its Motorola patents would be a weaker bargaining chip for Google to negotiate licensing deals with rivals. A ruling on that issue is expected sometime in the next several weeks.

After Microsoft filed its case over the royalty amounts, Motorola sued Microsoft on three patents that cover video technology. In an order on Thursday, Robart found that parts of those patents are "indefinite," which means Motorola's patent language did not distinctly articulate the invention. Other parts of those patents are still active in the lawsuit.

A Google spokesperson declined to comment and a Microsoft representative could not immediately comment when reached.

The case in U.S. District Court, Western District of Washington is Microsoft Corp. vs. Motorola Inc., 10-cv-1823.

(Reporting By Dan Levine in San Francisco. Editing by Andre Grenon)