Synchronous motor - Which feedback systems can be used in servo control?

Question:
Why is it not possible to operate synchronous motors in servo control with incremental encoders or simple Sin/Cos encoders?

Answer:
During operation of synchronous machines in servo control (e. g. at the 9300 servo inverter and ECS servo system or 9400 Servo Drives and 8400 Inverter Drives TopLine), the stator winding is exposed to a current flow depending on the position of the salient poles. The information on the rotor position is supplied by the feedback system:

• Resolver => Mounting position of the encoder at the motor shaft and adjusted pole position in the drive controller, e. g. C0058 (9300, ECS), C00058/1 (9400) or C0926/1 (8400 TopLine), 0x2C03/2 (i950) 
  
• Absolute value encoder => Pole position information in the encoder and adjusted pole position in the drive controller, e. g. C0058 (9300, ECS), C00058/2 (9400) or C0926/2 (8400 TopLine), 0x2C03/4 (i950)

In contrast to these absolute feedback systems (at least absolute at one revolution), the incremental encoder and the simple Sin/Cos encoder are not able to make an absolute rotor position available. As a result these encoders can only be used for asynchronous machines as a feedback.

Under the restriction that a pole position identification is carried out every time after mains switching on, an incremental encoder or a sin-cos encoder or a resolver with the number of pole pairs unequal to one and unequal to the number of pole pairs of the rotor can restrictedly be used. It has to be checked in the individual case how far this seems sensible and practicable under practical points of view (e. g. in case of linear motors). The pole position identification has to be started by corresponding device commands at motor standing still (see to this under the keywords 'pole position identification' or 'pole angle adjustments' in the corresponding documentation from the drive controller).

Further information about the functional principle of synchronous motors see also under Wikipedia.org: Synchronous motor