synchronous machine

synchronous machine is one where the rotor rotates at synchrnous speed ie there is no slip.Consequently syn.machine is not self starting.However in practice the damper windings enable the machine to start as induction ( asynchronous motor) and machine pulls in to synchronism when the DC field winding is enegised.The starting required in Traction is very high which cannot be provided by syn machine.Secondly a asyn.machine is simple and ruuged .It has no DC winding and complications of DC swicthing.Rotors can be made to give high starting torque as required.Further on gradients both up and down performance of syn machine is inferior compared to Asyn.