The ac synchronous motor
In a synchronous motor, torque can be produced at synchronous speed. This is achieved by a field winding, generally wound on the rotor, and dc excited so that it produces a rotor flux which is stationary relative to the rotor. Torque is produced when the rotating three-phase field produced by currents in the stator winding and the rotor field are stationary relative to each other, hence there must be physical rotation of the rotor at synchronous speed ns in order that its field travels in step with the stator field axis. At any other speed a rotor pole would approach alternately a stator ‘north’ pole field, then a ‘south’ pole field, changing the resulting torque from a positive to a negative value at a frequency related to the speed difference, the mean torque being zero.
A typical inverter for variable speed control automatically regulates the main stator voltage to be in proportion to motor frequency. It is possible to arrange an excitation control loop which monitors the main stator voltage and increases the excitation field voltage proportionately.
The ac synchronous motor has attractive features for inverter variable speed drive applications, particularly at ratings of 40 kW and above. Not least is overall cost when compared with a cage induction motor and inverter, or a dc shunt wound motor with converter alternatives. In applications requiring a synchronous speed relationship between multiple drives, or precise speed control of single large drives the ac synchronous motor with inverter control system appears attractive, freedom from brushgear maintenance, good working efficiency and power factor being the main considerations.