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Type of AC 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; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator. 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

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:

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 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.

Hysteresis motors have a solid, smooth, cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt steel. The power may be supplied from a separate source or from a generator directly connected to the motor shaft.

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 a synchronous alternator. 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.

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.

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.

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. 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.