A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current. Synchronous motors use permanent magnets or electromagnets for rotors, and electromagnets for stators. The stator creates a magnetic field that rotates in time with the oscillations of the current. The rotor turns in step with the stator field at the same rate and as a result, provides a second synchronized rotating magnet field. Synchronous and induction motors are the most widely used AC motors. Synchronous motors rotate at a rate locked to the line frequency since they do not rely on induction to produce the rotor's magnetic field. Induction motors require slip: the rotor must rotate at a frequency slightly slower than the AC alternations in order to induce current in the rotor. Small synchronous motors are used in timing applications such as in synchronous clocks, timers in appliances, tape recorders and precision servomechanisms in which the motor must operate at a precise speed; accuracy depends on the power line frequency, which is carefully controlled in large interconnected grid systems. Synchronous motors are available in self-excited, fractional to industrial sizes. In the fractional power range, most synchronous motors are used to provide precise constant speed. These machines are commonly used in analog electric clocks, timers and related devices. Doubly fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator. In typical industrial sizes, the synchronous motor provides an efficient means of converting AC energy to work (electrical efficiency above 95% is normal for larger sizes) and it can operate at leading or unity power factor and thereby provide power-factor correction. Synchronous motors fall under the category of synchronous machines that also includes synchronous generators. Generator action occurs if the field poles are "driven ahead of the resultant air-gap flux by the forward motion of the prime mover". Motor action occurs if the field poles are "dragged behind the resultant air-gap flux by the retarding torque of a shaft load".
Types The two major types of synchronous motors are distinguished by how the rotor is magnetized: non-excited and direct-current excited.
Non-excited
In non-excited motors, the external stator field magnetizes the rotor, inducing the magnetic poles needed to turn the rotor. The rotor rotates in step with the stator's rotating magnetic field, so it has an almost-constant magnetic field through it. The rotor is made of a high-retentivity steel such as cobalt steel. These are manufactured in permanent magnet, reluctance and hysteresis designs:
Permanent-magnet
A permanent-magnet synchronous motor (PMSM) uses permanent magnets embedded in the rotor to create a constant magnetic field. The stator carries windings connected to an AC electricity supply to produce a rotating magnetic field (as in an asynchronous motor). At synchronous speed the rotor poles lock to the rotating magnetic field. PMSMs are similar to brushless DC motors. Neodymium magnets are the most common, although rapid fluctuation of neodymium magnet prices triggered research in ferrite magnets. Due to inherent characteristics of ferrite magnets, the magnetic circuit of these machines needs to be able to concentrate the magnetic flux, typically leading to the use of spoke type rotors. Machines that use ferrite magnets have lower power density and torque density when compared with neodymium machines. PMSMs have been used as gearless elevator motors since 2000. Most PMSMs require a variable-frequency drive to start them. However, some incorporate a squirrel cage in the rotor for starting—these are known as line-start or self-starting. These are typically used as higher-efficiency replacements for induction motors (owing to the lack of slip), but must ensure that synchronous speed is reached and that the system can withstand torque ripple during starting. PMSMs are typically controlled using direct torque control and field oriented control.
Reluctance
Reluctance motors have a solid steel cast rotor with projecting (salient) toothed poles. Typically there are fewer rotor than stator poles to minimize torque ripple and to prevent the poles from all aligning simultaneously—a position that cannot generate torque. The size of the air gap in the magnetic circuit and thus the reluctance is minimum when the poles align with the stator's (rotating) magnetic field, and increases with the angle between them. This creates torque that pulls the rotor into alignment with the nearest pole of the stator field. At synchronous speed the rotor is thus "locked" to the rotating stator field. This cannot start the motor, so the rotor poles usually have squirrel-cage windings embedded in them, to provide torque below synchronous speed. The machine thus starts as an induction motor until it approaches synchronous speed, when the rotor "pulls in" and locks to the stator field. Reluctance motor designs have ratings that range from fractional horsepower (a few watts) to about 22 kW. Small reluctance motors have low torque, and are generally used for instrumentation applications. Moderate torque, multi-horsepower motors use squirrel cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in a drive system can operate at exactly the same speed. The power supply frequency determines motor operating speed.
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