Trumpy, Scott35 did a good job, but if you need more on AC motors, see
http://www.lmphotonics.com/m_control.htm and
http://www.lmphotonics.com/slipring.htm Mark Empson, the website owner in Christchurch, NZ, is well respected for internet contributions on electrical subjects. He is also very knowledgeable about starting methods and often-questionable energy-saving motor controllers.
