Synchronous motor

Single-phase 60 Hz 1800 RPM synchronous motor for Teletype machine, non-excited rotor type, manufactured from 1930 to 1955

In non-excited motors, the rotor is made of steel. It rotates in step with the stator's rotating magnetic field, so it has an almost-constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high-retentivity steel such as cobalt steel. These are manufactured in permanent magnet, reluctance and hysteresis designs:[6]

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.[7] 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.[8] Machines that use ferrite magnets have lower power density and torque density when compared with neodymium machines.[8]

PMSMs have been used as gearless elevator motors since 2000.[9]

Most PMSMs require a variable-frequency drive to start them.[10][11][12][13][14] However, some incorporate a squirrel cage in the rotor for starting—these are known as line-start or self-starting.[15] 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[16] and field oriented control.[17] However, these methods suffer from relatively high torque and stator flux ripples.[18] Predictive control and neural network controllers cope better with these issues.[18][19]

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.[3][20] 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.[21]

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.

Hysteresis motors have a solid, smooth_, cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt steel.[20] This material has a wide hysteresis loop (high coercivity), meaning once it is magnetized in a given direction, it requires a high magnetic field to reverse the magnetization. The rotating stator field causes each small volume of the rotor to experience a reversing magnetic field. Because of hysteresis the phase of the magnetization lags behind the phase of the applied field. Thus the axis of the magnetic field induced in the rotor lags behind the axis of the stator field by a constant angle δ, producing torque as the rotor tries to "catch up" with the stator field. As long as the rotor is below synchronous speed, each particle of the rotor experiences a reversing magnetic field at the "slip" frequency that drives it around its hysteresis loop, causing the rotor field to lag and create torque. The rotor has a 2-pole low reluctance bar structure.[20] As the rotor approaches synchronous speed and slip goes to zero, this magnetizes and aligns with the stator field, causing the rotor to "lock" to the rotating stator field.

A major advantage of the hysteresis motor is that since the lag angle δ is independent of speed, it develops constant torque from startup to synchronous speed. Therefore, it is self-starting and doesn't need an induction winding to start it, although many designs embed a squirrel-cage conductive winding structure in the rotor to provide extra torque at start-up.

Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.

Externally excited motor, 1917. The exciter is on the left.

Usually made in larger sizes (larger than about 1 horsepower or 1 kilowatt) these motors require direct current (DC) to turn (excite) the rotor. This is most straightforwardly supplied through slip rings.

A brushless AC induction and rectifier arrangement can also be used.[22]

The power may be supplied from a separate source or from a generator directly connected to the motor shaft.

A single-phase, 4-pole (2-pole-pair) synchronous motor is operating at an AC supply frequency of 50 Hz. The number of pole-pairs is 2, so the synchronous speed is:

${\displaystyle N_{s}=60\;{\frac {\text{rpm}}{\text{Hz}}}\times {\frac {50{\text{ Hz}}}{2}}=1500\,\,{\text{rpm}}}$

A three-phase, 12-pole (6-pole-pair) synchronous motor is operating at an AC supply frequency of 60 Hz. The number of pole-pairs is 6, so the synchronous speed is:

${\displaystyle N_{s}=60\;{\frac {\text{rpm}}{\text{Hz}}}\times {\frac {60{\text{ Hz}}}{6}}=600\,\,{\text{rpm}}}$

The number of magnetic poles, ${\displaystyle p}$, is equal to the number of coil groups per phase. To determine the number of coil groups per phase in a 3-phase motor, count the number of coils, divide by the number of phases, which is 3. The coils may span several slots in the stator core, making it tedious to count them. For a 3-phase motor, if you count a total of 12 coil groups, it has 4 magnetic poles. For a 12-pole 3-phase machine, there will be 36 coils. The number of magnetic poles in the rotor is equal to the number of magnetic poles in the stator.

Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization and the rotor no longer follows the rotating magnetic field.

Since the motor cannot produce torque if it falls out of synchronization, practical synchronous motors have a partial or complete squirrel-cage damper called an amortisseur winding to stabilize operation and facilitate starting.

Because this winding is smaller than that of an equivalent induction motor and can overheat on long operation, and because large slip-frequency voltages are induced in the rotor excitation winding, synchronous motor protection devices sense this condition and interrupt the power supply (out of step protection).[33]

V-curve of a synchronous machine

By varying the excitation of a synchronous motor, it can be made to operate at lagging, leading and unity power factor. Excitation at which the power factor is unity is termed normal excitation voltage.[40] The magnitude of current at this excitation is minimum.[40] Excitation voltage more than normal excitation is called over excitation voltage, excitation voltage less than normal excitation is called under excitation.[40] When the motor is over excited, the back emf will be greater than the motor terminal voltage. This causes a demagnetizing effect due to armature reaction.[41]

The V curve of a synchronous machine shows armature current as a function of field current. With increasing field current armature current at first decreases, then reaches a minimum, then increases. The minimum point is also the point at which power factor is unity.[42]

This ability to selectively control power factor can be exploited for power factor correction of the power system to which the motor is connected. Since most power systems of any significant size have a net lagging power factor, the presence of overexcited synchronous motors moves the system's net power factor closer to unity, improving efficiency. Such power-factor correction is usually a side effect of motors already present in the system to provide mechanical work, although motors can be run without mechanical load simply to provide power-factor correction. In large industrial plants such as factories the interaction between synchronous motors and other, lagging, loads may be an explicit consideration in the plant's electrical design.

${\displaystyle \mathbf {T} =\mathbf {T} _{\text{max}}\sin(\delta )}$

where,

${\displaystyle \mathbf {T} }$ is the torque

${\displaystyle \delta }$ is the torque angle

${\displaystyle \mathbf {T} _{\text{max}}}$ is the maximum torque

here,

${\displaystyle \mathbf {T} _{\text{max}}={\frac {{\mathbf {3} }{\mathbf {V} }{\mathbf {E} }}{{\mathbf {X_{s}} }{\omega _{s}}}}}$

When load is applied, torque angle ${\displaystyle \delta }$ increases. When ${\displaystyle \delta }$ = 90° the torque will be maximum. If load is applied further then the motor will lose its synchronism, since motor torque will be less than load torque.[43][44] The maximum load torque that can be applied to a motor without losing its synchronism is called steady state stability limit of a synchronous motor.[43]

Synchronous motors are especially useful in applications requiring precise speed or position control:

• Speed is independent of the load over the operating range of the motor.
• Speed and position may be accurately controlled using open loop controls (e.g. stepper motors).
• Low-power applications include positioning machines, where high precision is required, and robot actuators.
• They will hold their position when a DC current is applied to both the stator and the rotor windings.
• A clock driven by a synchronous motor is in principle as accurate as the line frequency of its power source. (Although small frequency drifts will occur over any given several hours, grid operators actively adjust line frequency in later periods to compensate, thereby keeping motor-driven clocks accurate; see Utility frequency § Stability.)
• Record player turntables
• Increased efficiency in low-speed applications (e.g. ball mills).