Synchronous motor


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 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 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. 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, 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 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 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 to about. 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. This material has a wide hysteresis loop, 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. 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 motors

Usually made in larger sizes these motors require direct current to excite the rotor. This is most straightforwardly supplied through slip rings.
A brushless AC induction and rectifier arrangement can also be used.

Control techniques

A permanent magnet synchronous motor and reluctance motor requires a control system for operating.
There is a large number of control methods for synchronous machines, selected depending on the construction of the electric motor and the scope.
Control methods can be divided into:
The PMSMs can also operate on open-loop control, which is sometimes used for start-up thus enabling the position sensing operation.

Synchronous speed

The synchronous speed of a synchronous motor is given:

in RPM, by:
and in rad·s−1, by:
where:
  • is the frequency of the AC supply current in Hz,
  • is the number of magnetic poles,
  • is the number of pole pairs,.

    Examples

A single-phase, 4-pole synchronous motor is operating at an AC supply frequency of 50 Hz. The number of pole-pairs is 2, so
the synchronous speed is:
A three-phase, 12-pole synchronous motor is operating at an AC supply frequency of 60 Hz. The number of pole-pairs is 6, so the synchronous speed is:
The number of magnetic poles,, 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.

Construction

The principal components of electric motors are the stator and the rotor. Synchronous motor and induction motor stators are similar in construction. The construction of synchronous motor is similar to that of an alternator. The stator frame contains wrapper plate. Circumferential ribs and keybars are attached to the wrapper plate. To carry the weight of the machine, frame mounts and footings are required. The synchronous stator winding consists of a 3 phase winding. It is provided with a 3 phase supply, and the rotor is provided with a DC supply.
DC excited motors require brushes and slip rings to connect to the excitation supply. The field winding can be excited by a brushless exciter. Cylindrical, round rotors, are used for up to six poles.
In some machines or when a large number of poles are needed, a salient pole rotor is used.
Most synchronous motor construction uses a stationary armature and rotating field winding. This type of construction has an advantage over DC motor type where the armature used is of rotating type.