Synchronous motor

The two major types of synchronous motors are distinguished by how the rotor is magnetized: non-excited and direct-current excited. Non-excited synchronous motor for a Teletype machine, non-excited rotor type, manufactured from 1930 to 1955 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. 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 . 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 Hysteresis motors have a solid, smooth, cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt steel. == Control techniques ==

A permanent magnet synchronous motor and reluctance motor requires a control system for operating (VFD or servo drive). 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: • Scalar control • V/f control • Vector control • Field oriented control • Direct torque control, a variant of VC • Feedback linearization 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: : N_s = 60\frac f P = 120\frac f p and in rad·s−1, by: : \omega_s = 2\pi\frac f P = 4\pi\frac f p where: • f is the frequency of the AC supply current in Hz, • p is the number of magnetic poles, • P is the number of pole pairs (rarely, planes of commutation), P = p/2. Examples 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: :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: :N_s = 60\;\frac\text{rpm}\text{Hz} \times \frac{60\text{ Hz}}{6} = 600\,\,\text{rpm} The number of magnetic poles, 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. ==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 (also called synchronous generator). The stator frame contains wrapper plate (except for wound-rotor synchronous doubly fed electric machines). 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, (also known as non-salient pole rotor) 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. ==Operation==

Electric motors generate power due to the interaction of the magnetic fields of the stator and the rotor. In synchronous motors, the stator carries 3 phase currents and produces 3 phase rotating magnetic flux (and therefore a rotating magnetic field). The rotor eventually locks in with the rotating magnetic field and rotates along with it. Once the rotor field locks in with the rotating magnetic field, the motor is said to be synched. A single-phase (or two-phase derived from single phase) stator is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by startup arrangements. ==Starting methods==

Above a certain size, synchronous motors cannot self-start. This property is due to rotor inertia; it cannot instantly follow the rotation of the stator's magnetic field. Since a synchronous motor produces no inherent average torque at standstill, it cannot accelerate to synchronous speed without a supplemental mechanism. Electronically controlled motors can be accelerated from zero speed by changing the frequency of the stator current. Small synchronous motors are commonly used in line-powered electric mechanical clocks or timers that use the power line frequency to run the gear mechanism at the correct speed. Such small synchronous motors are able to start without assistance if the moment of inertia of the rotor and its mechanical load are sufficiently small. The motor accelerates from slip speed to synchronous speed during an accelerating half cycle of the reluctance torque. Rotor excitation is a possible way to resolve the issue. In addition, starting methods for large synchronous machines include repetitive polarity inversion of the rotor poles during startup. ==Applications, special properties, and advantages==

Use as synchronous condenser 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. The magnitude of current at this excitation is minimum. 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. 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. Other 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 ''''.) • Record player turntables • Increased efficiency in low-speed applications (e.g. ball mills). ==Subtypes==

• • Stepper motor (may be synchronous or not) • Synchronous brushless wound-rotor doubly-fed electric machine ==See also==