how do you change the speed of a 3 phase synchronous motor?

Question

what is it governed by?

Answer 1

Change the frequency or add more poles to the motor.

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

RPM = 120F / p

where:

RPM = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding

Answer 2

You could not change the speed of a 3 phase synchronous motor if it were connected directly to line power since it is the line power's frequency which governs the motor's speed.

Changing the DC excitation to the rotor would not cause much change in productive speed.

The only practical way to effect change over the speed is to run the motor off an inverter which basically takes incoming power and converts it to an appropriate running voltage but at a different frequency. You would then apply this power to the stator windings, leaving the DC rotor excitation unchanged.

Toshiba, among other manufacturers, makes such inverters with different maximum power ratings.

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