Synchronous motor

A synchronous electric motor is an AC electric motor in which, at steady state, ^[1] 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. Doubly fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator. ^[2]

Contents

• Types
• Non-excited
• Externally excited motors
• Control techniques
• Synchronous speed
• Examples
• Construction
• Operation
• Amortisseur winding
• Starting methods
• Applications, special properties, and advantages
• Use as synchronous condenser
• Steady-state stability limit
• Other
• Subtypes
• See also
• References
• Sources
• External links

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 ^[3] to industrial sizes. ^[1] In the fractional horsepower range, most synchronous motors are used to provide precise constant speed. These machines are commonly used in analog electric clocks, timers and related devices.

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) ^[4] and it can operate at leading or unity power factor and thereby provide power-factor correction.^{[ citation needed ]}

Types

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". ^[1]

The two major types of synchronous motors are distinguished by how the rotor is magnetized: non-excited and direct-current excited. ^[5]

Non-excited

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]

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. ^[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]

Reluctance

Main article: Reluctance motor

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] [18]} 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. ^[19]

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

Hysteresis motors have a solid, smooth, cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt steel. ^[18] 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. ^[18] 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.^{[ citation needed ]}

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.^{[ citation needed ]}

Externally excited motors

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

A brushless AC induction and rectifier arrangement can also be used. ^[20]

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

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: ^{[21] [22]}

• Scalar control
  • V/f control
• Vector control
  • Field oriented control
  • Direct torque control, a variant of VC
  • Feedback linearization
  • Passivity based control

The PMSMs can also operate on open-loop control, ^[23] which is sometimes used for start-up thus enabling the position sensing operation. ^[24]

Synchronous speed

The synchronous speed of a synchronous motor is given: ^[25]
in RPM, by:

${\displaystyle N_{s}=60{\frac {f}{P}}=120{\frac {f}{p}}}$

and in rad·s⁻¹, by:

${\displaystyle \omega _{s}=2\pi {\frac {f}{P}}=4\pi {\frac {f}{p}}}$

where:

• ${\displaystyle f}$ is the frequency of the AC supply current in Hz,
• ${\displaystyle p}$ is the number of magnetic poles,
• ${\displaystyle P}$ is the number of pole pairs (rarely, planes of commutation), ${\displaystyle P=p/2}$.

Construction

The principal components of electric motors are the stator and the rotor. ^[26] Synchronous motor and induction motor stators are similar in construction. ^[27] The construction of synchronous motor is similar to that of a synchronous alternator. ^[28] 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. ^[29] 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. ^[30] The field winding can be excited by a brushless exciter. ^[31] 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. ^{[32] [33]}

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. ^[34]

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. ^[3]

Large motors operating on commercial power include a squirrel-cage induction winding that provides sufficient torque for acceleration and also serves to damp motor speed oscillations. ^[3] Once the rotor nears the synchronous speed, the field winding becomes excited and the motor pulls into synchronization. Very large motor systems may include a "pony" motor that accelerates the unloaded synchronous machine before load is applied. ^{[35] [36]} Electronically controlled motors can be accelerated from zero speed by changing the frequency of the stator current. ^[37]

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. ^[3] Single-phase synchronous motors such as in electric wall clocks can freely rotate in either direction, unlike a shaded-pole type.

Costs are an important parameter for starters. ^[38] Rotor excitation is a possible way to resolve the issue. ^[39] In addition, starting methods for large synchronous machines include repetitive polarity inversion of the rotor poles during startup. ^[40]

Applications, special properties, and advantages

Use as synchronous condenser

Main article: Synchronous condenser

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. ^[41] The magnitude of current at this excitation is minimum. ^[41] Excitation voltage more than normal excitation is called over excitation voltage, excitation voltage less than normal excitation is called under excitation. ^[41] 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. ^[42]

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. ^[43]

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.^{[ citation needed ]}

Subtypes

• AC motor § Polyphase synchronous motor
• Stepper motor (may be synchronous or not)
• Synchronous brushless wound-rotor doubly-fed electric machine

See also

• Clock drive
• Doubly fed electric machine
• Short circuit ratio (synchronous generator)

Related Research Articles

An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

In electricity generation, a generator is a device that converts motion-based power or fuel-based power into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all the power for electrical grids.

A stepper motor, also known as step motor or stepping motor, is a Brushless DC electric motor that rotates in a series of small and discrete angular steps. Stepper motors can be set to any given step position without needing a position sensor for feedback. The step position can be rapidly increased or decreased to create continuous rotation, or the motor can be ordered to actively hold its position at one given step. Motors vary in size, speed, step resolution, and torque.

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually, the term refers to small rotating machines driven by automotive and other internal combustion engines.

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore needs no electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the current pulses that control the speed and torque of the motor. It is an improvement on the mechanical commutator (brushes) used in many conventional electric motors.

A DC motor is an electrical motor that uses direct current (DC) to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminium or copper conductors embedded in its surface. In operation, the non-rotating stator winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a transformer except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields in the stator and the currents in the rotor produce a torque on the rotor.

The shaded-pole motor is the original type of AC single-phase electric motor, dating back to at least as early as 1890. A shaded-pole motor is a motor, in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole to produce a weakly rotating magnetic field. When single phase AC supply is applied to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated. This auxiliary single-turn winding is called a shading coil. Currents induced in this coil by the magnetic field create a second electrical phase by delaying the phase of magnetic flux change for that pole enough to provide a 2-phase rotating magnetic field. The direction of rotation is from the unshaded side to the shaded (ring) side of the pole. Since the phase angle between the shaded and unshaded sections is small, shaded-pole motors produce only a small starting torque relative to torque at full speed. Shaded-pole motors of the asymmetrical type shown are only reversible by disassembly and flipping over the stator, though some similar looking motors have small, switch-shortable auxiliary windings of thin wire instead of thick copper bars and can reverse electrically. Another method of electrical reversing involves four coils.

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

Doubly fed electric machines, also slip-ring generators, are electric motors or electric generators, where both the field magnet windings and armature windings are separately connected to equipment outside the machine.

An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Because they can recover energy with relatively simple controls, induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure.

The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.

A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact.

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. While transformers are occasionally called "static electric machines", since they do not have moving parts, generally they are not considered "machines", but as electrical devices "closely related" to the electrical machines.

In electromagnetism, excitation is the process of generating a magnetic field by means of an electric current.

The switched reluctance motor (SRM) is a type of reluctance motor. Unlike brushed DC motors, power is delivered to windings in the stator (case) rather than the rotor. This simplifies mechanical design because power does not have to be delivered to the moving rotor, which eliminates the need for a commutator. However it complicates the electrical design, because a switching system must deliver power to the different windings and limit torque ripple. Sources disagree on whether it is a type of stepper motor.

A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil. The term synchronous refers here to the fact that the rotor and magnetic field rotate with the same speed, because the magnetic field is generated through a shaft-mounted permanent magnet mechanism, and current is induced into the stationary armature.

Electromagnetically induced acoustic noise, electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.

References

Sources

• Jordan, H.E. (2013). Energy-Efficient Electric Motors and their Applications. Springer US. ISBN   978-1-4899-1465-1 . Retrieved 2023-09-01.
• Finch, John W.; Giaouris, Damian (2008). "Controlled AC Electrical Drives" (PDF). IEEE Transactions on Industrial Electronics. 55 (2). Institute of Electrical and Electronics Engineers (IEEE): 481–491. doi:10.1109/tie.2007.911209. ISSN   0278-0046.
• Acarnley, P.P.; Watson, J.F. (2006). "Review of position-sensorless operation of brushless permanent-magnet machines" (PDF). IEEE Transactions on Industrial Electronics. 53 (2). Institute of Electrical and Electronics Engineers (IEEE): 352–362. doi:10.1109/tie.2006.870868. ISSN   0278-0046.
• Buja, G.S.; Kazmierkowski, M.P. (2004). "Direct Torque Control of PWM Inverter-Fed AC Motors—A Survey". IEEE Transactions on Industrial Electronics. 51 (4). Institute of Electrical and Electronics Engineers (IEEE): 744–757. doi:10.1109/tie.2004.831717. ISSN   0278-0046.

External links

Wikimedia Commons has media related to Synchronous motors .

• Synchronous motor animation