The most obvious characteristic of a synchronous motor is its strict synchronism with the power line frequency. However, for most of its applications, this is not very important since a few rpm, more or less, would not significantly affect its usefulness. The reason the industrial user is likely to prefer a synchronous motor is its higher efficiency and the opportunity for the user to adjust the motor's power factor. However, these advantages come with the added complications of motor starting. Another important benefit of synchronous motors for very large horsepower or very slow rpm applications is that the overall installation is likely to be less expensive. Some people use the guideline that a synchronous motor should be considered whenever the motor horsepower is a larger number than the speed in rpm. While the induction motor might be less costly, the other selection criteria in this range of size often favors the synchronous motor.
A synchronous motor is almost identical to a generator of the same rating. The provisions for starting the motor are the major difference, although there may also be some difference in the length to diameter proportions as well. The induction motor has a large number of uninsulated conductors short-circuited upon themselves while the synchronous motor has a number of electromagnets built into its rotor. These electromagnets are energized by direct current which can be adjusted by external controls. Changing this excitation can change the power factor, either lowering the line current to the motor, and/or supplying magnetizing VAR's to the rest of the system and thereby raising the overall power factor for the plant.
The synchronous motor cannot be started as such. Another means of starting has to be built into the motor. This is why synchronous motors 
are only selected for applications with relatively infrequent starts. Most synchronous motors are started as induction motors, using a set of squirrel-cage rotor bars built into the faces of the iron cores of the rotor electromagnets. These bars also provide a stabilizing influence during normal operation. The motor should be unloaded when started because the starting torque provided by the rotor bars is usually much less than full rated power.
Another reason for starting the synchronous motor this way is to protect the DC coils wound around the rotor electromagnets. Otherwise, these coils would experience very large voltages (perhaps tens of thousands of volts) during starting. Frequently, this protection consists of short-circuiting the field windings either directly or through a field discharge resistor. When short-circuited, the large inductive reactance of these coils dissipates the intensity of the voltage without excessive current. As the rotor speed approaches synchronism, the over-voltage hazard drops to acceptable levels, the field shorting can be eliminated, and direct current supplied to the field coils. When this happens, the motor locks into step with the supply frequency and the motor operates in synchronism.
A specially designed motor controller performs these operations in the proper sequence and at the proper times during the starting process.
