circuits for speed control of squirrel cage induction motors using thyristors?

Thyristors are no longer used for this application. IGBT are now used. The problem with thyristors is that they require a commutating current to turn them off and so they were only used in early generation drives when transistors could not be produced to handle the power.

The basic power circuit can be found in most manufacturers manuals. I have put the link to the Toshiba site that you can download a manual from or go to ABB, Allen Bradley, Siemens, Square D etc etc etc.

http://www.toshiba.com/ind/downloads_main.jsp

The control circuits are much more complex and I would see little point in trying to make one from any practical point of view bearing in mind the low cost of drives.

The only way to control the speed of a squirrel cage motor is by using a variable frequency drive (VFD). The VFD contains a rectifier to change incoming AC to DC. An inverter then changes the DC back to AC at a controlled frequency. VFDs can be constructed using thyristors, but transistors have been used for many years.

If just the motor voltage is reduced, the motor torque capability will be reduced. If the load is a fan or centrifugal pump, the motor speed will be reduced to a speed at which the reduced load torque matches the motor torque capability. If the load does not have a torque requirement that is lower at reduced speeds, the motor will stall when the voltage is reduced.

Look at:

http://en.wikipedia.org/wiki/Adjustable-speed_driv...

http://en.wikipedia.org/wiki/Inverter_%28electrica...

http://en.wikipedia.org/wiki/Variable-frequency_dr...

it particularly is an exceedingly technical question, isn't it? i'm going to project a guess: an inductions motor is created from electrical energy, a wound motor is a hand crank. And a synchronous motor has a hamster interior.