One method utilizes a series of short-circuited copper bars inserted through the outer extremities of the salient poles. It is evident from equation (2.81) that synchronous motors have no starting torque and the rotor must be run up to synchronous speed by some alternative means. Where T max is the maximum rated torque and δ is the load angle. In this case, a high starting torque can be developed by using the information of the rotor initial position. If the speed of the motor is increased or decreased from the synchronous speed, then a current will be induced in the damper winding and will develop a torque to oppose the change in speed.Īs for the starting method of PMSMs used in variable speed drives, we can start the motor slowly at a reduced frequency by using a PWM inverter. In addition, the damper winding has another function to help keeping the synchronous motor in synchronism. When the synchronous motor is starting, the torque will be produced by the induced current in the damping winding, so it can start as an induction motor. In this case, the rotor has a special damping winding for the purpose of starting, which is similar to the squirrel-cage arrangements of an induction motor. A second method is to start the synchronous motor as an induction motor. As a simple method, a separate motor is used to drive the rotor to run close to the synchronous speed. There are some techniques employed to start a synchronous motor. Once the rotor reaches a speed close to the synchronous speed (>95%) through some starting means, the field winding will be excited, so the motor locks into synchronization.
Therefore we need some means for starting synchronous motors.