Just saw this on the Globalspec web site.
http://www.globalspec.com/MSKennedy/...eed_020802.pdf
It details the other speed controller Joe mentioned. These are used allot for brushless dc servo motors. The pulse width period can also have an effect on brush - commuter wear. Too long a period can cause wear.This link was posted in another thread this year. It may be of interest to review
Speed controller