AC motor
An AC motor is an electric motor that operates using alternating current (AC) to convert electrical energy into mechanical energy, typically through the generation of a rotating magnetic field in its stator that interacts with the rotor to produce torque.[1] These motors are characterized by their ability to run at constant speeds determined by the supply frequency and number of poles, making them suitable for a wide range of applications where reliable, efficient power conversion is essential.[2] The fundamental principle of operation for most AC motors relies on electromagnetic induction, where the alternating current in the stator windings creates a rotating magnetic field that induces currents in the rotor, causing it to turn.[1] AC motors are broadly classified into two main types: induction motors (also known as asynchronous motors), which operate at a speed slightly below the synchronous speed due to slip, and synchronous motors, which rotate at exactly the synchronous speed locked to the supply frequency.[3] Induction motors account for over 80% of installed motor capacity in industry and, particularly three-phase squirrel-cage designs, dominate industrial use owing to their simplicity, robustness, low maintenance, and cost-effectiveness; they consume about 70% of manufacturing electricity worldwide (IEA estimates as of 2023).[4] Synchronous motors offer advantages in efficiency and power factor correction for large-scale applications.[3] Single-phase AC motors are common in residential settings, such as fans and pumps, but require auxiliary starting mechanisms.[5] AC motors find extensive applications across industries, including manufacturing equipment, HVAC systems, electric vehicles, and household appliances. In industrial applications like manufacturing and HVAC, as well as some EVs (e.g., Tesla models) and household appliances, AC induction motors are prevalent due to reliability and compatibility with standard power grids; synchronous types, such as permanent magnet synchronous motors, are common in many EVs.[1][6] They are increasingly integrated with variable frequency drives (VFDs) for speed control in modern systems, enhancing energy efficiency and adaptability.[5] Electric motor systems account for around 50% of global electricity consumption, with advancements in materials and designs continuing to focus on improving efficiency standards, such as those outlined by regulatory bodies like the International Energy Agency for premium efficiency motors (IEA estimates as of 2023).[4]Fundamentals
Definition and Classification
An AC motor is an electric motor driven by alternating current (AC), converting electrical energy into mechanical energy through the interaction of magnetic fields generated by the stator and rotor.[7] The device typically consists of a stationary stator with windings that produce a rotating magnetic field when energized by AC, which induces torque in the rotor to produce rotational motion.[3] This process relies on electromagnetic principles, either through induction for torque generation or synchronous alignment of fields.[7] AC motors are broadly classified into two main types based on the relationship between the rotor speed and the speed of the rotating magnetic field created by the stator: induction motors (also known as asynchronous motors) and synchronous motors. In induction motors, the rotor operates at a speed slightly less than the synchronous speed of the magnetic field, with torque produced by induced currents in the rotor via electromagnetic induction.[3] Synchronous motors, in contrast, have the rotor rotating at exactly the synchronous speed, locked in step with the magnetic field, often requiring additional excitation for the rotor field.[7] Within these categories, AC motors are further divided into single-phase and polyphase (typically three-phase) designs, where single-phase motors are suited for smaller, low-power applications like household appliances, while polyphase motors provide higher efficiency and power for industrial uses.[3] Compared to DC motors, AC motors eliminate the need for brushes and a commutator, resulting in simpler construction, reduced maintenance, and greater suitability for high-power applications due to their robustness and lower wear.[3] However, AC motors require an alternating current supply, whereas DC motors can operate directly from batteries or rectified sources, making DC types more common in precision speed control scenarios.[7]Operating Principles
The operating principles of AC motors revolve around the generation of a rotating magnetic field in the stator and its interaction with the rotor to produce torque. In polyphase AC motors, typically three-phase systems, the stator windings are spatially displaced by 120 electrical degrees and energized by balanced polyphase currents of equal magnitude but phase-shifted by 120 degrees. This configuration results in a magnetic field that rotates at a constant magnitude and speed, rather than pulsating as in single-phase setups. The rotating magnetic field arises from the vector sum of the individual phase fields, where each phase produces a pulsating flux along its axis, but their superposition yields a smooth rotation.[8][9] The speed of this rotating magnetic field, known as the synchronous speed N_s, determines the motor's fundamental operating pace and is derived from the relationship between the supply frequency and the number of magnetic poles. For a motor with p poles and supply frequency f in hertz, the synchronous speed in revolutions per minute is given by N_s = \frac{120 f}{p}, which follows from the fact that the field completes f / (p/2) cycles per second, or $60 f / (p/2) = 120 f / p revolutions per minute. This formula establishes the baseline speed for all AC motors, with the field rotating in the direction determined by the phase sequence of the currents.[10][11] Torque production occurs through the electromagnetic interaction between the stator's rotating field and currents induced in the rotor conductors, governed by Faraday's law of electromagnetic induction. As the rotating field sweeps past the rotor, it induces an electromotive force (EMF) in the rotor windings or conductors according to Faraday's law, where the induced EMF is proportional to the rate of change of magnetic flux linkage. For a concentrated winding, the root-mean-square value of the induced EMF per phase in the stator or rotor is approximated by E = 4.44 f N \Phi, where f is the frequency, N is the number of turns per phase, and \Phi is the flux per pole; this equation originates from the integration of the sinusoidal flux variation over one cycle, yielding a form factor of 1.11 times 4 times the average EMF.[12][13] The induced rotor currents create a secondary magnetic field that interacts with the stator field, producing a force on the rotor conductors per Lorentz's force law, resulting in net torque that drives rotation.[12][14] AC motors operate in either synchronous or asynchronous modes based on the rotor's response to the stator field. In synchronous motors, the rotor locks into step with the rotating magnetic field, achieving exactly the synchronous speed N_s with zero slip, as the rotor's magnetic poles align directly with the stator field's poles for continuous torque. In contrast, asynchronous motors, such as induction types, exhibit slip where the rotor speed N_r lags behind N_s by a fractional amount s = (N_s - N_r)/N_s, typically 1-5% at full load, allowing relative motion that sustains induced currents and torque. This slip is essential for induction motors but absent in synchronous operation.[15] The power flow in AC motors converts electrical input to mechanical output through sequential stages, with losses occurring primarily in the stator, rotor, and core. Alternating current enters the stator, where a portion is dissipated as stator copper losses (I²R in windings) and stator core losses (hysteresis and eddy currents due to flux alternation). The remaining air-gap power transfers across to the rotor, inducing rotor currents that incur rotor copper losses (also I²R) and rotor core losses (proportional to slip frequency). The converted mechanical power emerges as output torque times speed, further reduced by friction and windage losses, yielding overall efficiency typically 85-95% for industrial motors. Core losses, concentrated in laminated iron to minimize eddy currents, represent 15-25% of total losses and depend on flux density and frequency.[16][17][18]Historical Development
Early Concepts and Experiments
The foundational concept for alternating current (AC) motors emerged from Michael Faraday's discovery of electromagnetic induction in 1831, when he demonstrated that a changing magnetic field could induce an electric current in a nearby conductor using his "induction ring" apparatus.[19] This principle established the reciprocal relationship between electricity and magnetism, enabling the conversion of mechanical energy into electrical energy and vice versa, which later underpinned motor operation.[20] Building on Faraday's work, Hippolyte Pixii constructed the first practical AC generator in 1832, a hand-cranked magneto-electric machine that produced alternating current through a rotating permanent magnet near stationary coils.[21] Pixii's device, though rudimentary and primarily a generator, illustrated the feasibility of generating AC and influenced early thinkers by showing how rotational motion could interact with magnetic fields to produce oscillatory currents, hinting at potential reversal for motoring applications.[21] In the mid-1880s, Elihu Thomson conducted pioneering demonstrations of alternating current phenomena, including the repulsion effects between AC-carrying conductors, which he used to exhibit basic AC motor-like behaviors in laboratory settings.[22] These experiments highlighted AC's potential for dynamic magnetic interactions, such as oscillating fields that could drive motion, and helped build empirical understanding of AC's advantages over direct current for certain mechanical conversions.[23] A pivotal theoretical breakthrough occurred in 1882 when Nikola Tesla, then 25, conceived the idea of a rotating magnetic field while walking in a Budapest park, visualizing how two phase-displaced AC currents could produce a continuously rotating field to drive a rotor without direct electrical connection.[24] This insight, inspired by his prior work on AC arc lighting systems in Paris, shifted focus from static or pulsating fields to smooth rotation, addressing key limitations in prior AC experiments.[25] Early AC concepts faced significant challenges with single-phase systems, which generated only pulsating magnetic fields incapable of producing steady torque and self-starting rotation in motors.[26] Polyphase arrangements, involving multiple offset currents, became essential to create the true rotating fields needed for efficient, reliable operation, contrasting sharply with the inefficiencies and starting difficulties of single-phase approaches.Key Inventions and Commercialization
In 1885, Italian engineer Galileo Ferraris independently developed the concept of a rotating magnetic field using two-phase alternating currents offset by 90 degrees, demonstrating a prototype induction motor that operated without commutators.[27] This foundational work laid the theoretical groundwork for polyphase AC motors, though Ferraris did not pursue patents or commercialization.[27] Nikola Tesla advanced these ideas through practical inventions, filing U.S. Patent No. 381,968 for an electromagnetic motor and related polyphase systems in October 1887, with grants issued in May 1888.[28] These patents described a complete polyphase AC induction motor system, including generators and transformers, enabling efficient power transmission and motor operation.[29] Tesla's designs were publicly demonstrated at the 1893 World's Columbian Exposition in Chicago, where Westinghouse Electric showcased over 200,000 AC-powered lights and rotating motors, proving the viability of polyphase systems.[30] Parallel to these efforts, Charles Proteus Steinmetz contributed essential mathematical tools for AC motor design while working at General Electric in the 1890s. Steinmetz formulated the law of hysteresis using complex numbers and phasor analysis, allowing engineers to predict motor efficiency and performance without physical prototypes.[31] His 1893 publication on alternating-current phenomena provided the analytical framework that standardized AC circuit calculations, facilitating scalable motor production.[32] Commercialization accelerated when George Westinghouse licensed Tesla's polyphase patents in July 1888, integrating them into AC power distribution systems to compete with Edison's DC networks.[33] This partnership culminated in the 1895 Adams Power Plant at Niagara Falls, the first large-scale hydroelectric facility using Westinghouse's AC generators and Tesla's motor designs to transmit 5,000 horsepower over 20 miles.[34] The plant's success validated AC motors for industrial applications, powering Buffalo's streetcars and factories by 1896.[35] By the early 20th century, adaptations for residential use led to single-phase AC motor developments, such as the shaded-pole design, which simplified starting mechanisms for household appliances like fans and refrigerators.[36] These motors, building on polyphase principles but requiring only one power line, enabled widespread adoption in homes as electrification expanded.[37]Induction Motors
Construction and Basic Operation
The stator of an induction motor consists of a laminated iron core with distributed polyphase windings, typically three-phase, arranged in slots to produce a rotating magnetic field when energized by alternating current.[38] The core is made from high-grade silicon steel laminations to minimize eddy current losses, and the windings are connected in either star or delta configuration depending on the voltage requirements. This rotating magnetic field revolves at the synchronous speed N_s = \frac{120 f}{p}, where f is the supply frequency in hertz and p is the number of poles.[39] The rotor is the rotating component, mounted on a shaft and separated from the stator by a small air gap. Induction motors feature two main rotor types: squirrel-cage and wound-rotor (slip-ring). The squirrel-cage rotor is a cylindrical laminated core with parallel slots containing conductive bars, usually aluminum or copper, short-circuited at both ends by end rings, forming a cage-like structure. This design is simple, rugged, and requires no external connections. The wound-rotor consists of a laminated core with polyphase windings similar to the stator, connected to slip rings on the shaft for external access, allowing insertion of resistance for control. Unlike synchronous motors, the induction motor rotor has no direct electrical connection for excitation; instead, it relies on induced currents.[38][39] In operation, the stator's rotating magnetic field induces voltages and currents in the rotor conductors via electromagnetic induction, following Faraday's and Lenz's laws. These rotor currents create a secondary magnetic field that interacts with the stator field, producing torque that drives the rotor in the direction of the field. The rotor speed N_r is slightly less than the synchronous speed N_s, with the difference known as slip s = \frac{N_s - N_r}{N_s}, typically 1-5% at full load for efficient operation. This slip is essential for continuous torque production, as zero slip would eliminate relative motion and induction. Induction motors are self-starting under balanced polyphase supply due to the inherent asymmetry in the rotor field interaction.[38][39]Polyphase Squirrel-Cage Motors
Polyphase squirrel-cage motors represent the most prevalent type of induction motor, distinguished by their rotor construction that eliminates the need for external electrical connections. The rotor consists of a cylindrical core made from laminated steel sheets with slots containing conductive bars, typically of die-cast aluminum, which are short-circuited at both ends by continuous end rings to form a cage-like structure.[40] This design induces currents in the rotor bars via the rotating magnetic field from the polyphase stator windings, enabling torque production without brushes or slip rings.[41] The symmetrical bar arrangement ensures uniform impedance regardless of rotor position, contributing to smooth operation.[42] To enhance starting performance, particularly for loads requiring high initial torque, variants such as double-cage and deep-bar rotors are employed. In a double-cage rotor, an outer cage of high-resistance bars is positioned near the rotor surface alongside an inner cage of low-resistance bars; at startup, the skin effect confines induced currents primarily to the outer cage, increasing effective resistance and thus starting torque.[43] As the motor accelerates, currents distribute more evenly, reducing resistance for efficient running. Deep-bar rotors achieve a similar effect through elongated slots with bars of varying cross-section, where the skin effect elevates rotor resistance during high-slip conditions at startup, providing torque boosts up to 200-250% of full-load value before transitioning to normal operation.[44] These modifications leverage the skin effect—the tendency of alternating currents to concentrate near conductor surfaces—to optimize torque without altering the basic cage structure. These motors exhibit robust characteristics suited to demanding environments, including simplicity in construction with no moving contacts, inherent ruggedness against mechanical stress, and self-starting capability under balanced polyphase supply. The torque-speed curve features a gradual rise from locked-rotor torque (typically 150-200% of full-load torque) to a peak breakdown torque of 175-300% occurring at slips of 20-30%, beyond which torque declines sharply toward synchronous speed.[45] Full-load operation occurs at low slips (1-5%), ensuring high efficiency. To meet diverse application needs, the National Electrical Manufacturers Association (NEMA) classifies polyphase squirrel-cage motors into designs A through D, each tailored to specific torque and current profiles:| NEMA Design | Locked-Rotor Torque (% of Full-Load) | Breakdown Torque (% of Full-Load) | Full-Load Slip (%) | Typical Applications |
|---|---|---|---|---|
| A | 70-275 | 175-300 | ≤5 | Machine tools, fans with light loads |
| B | 75-190 | 200-250 | ≤5 | General-purpose: pumps, compressors, fans |
| C | 200-250 | 190-225 | 1-5 | High-inertia loads like crushers |
| D | 275+ | 175-200 | 5-13 | Very high starting torque needs, e.g., conveyors |
Polyphase Wound-Rotor Motors
Polyphase wound-rotor motors, also known as wound-rotor induction motors, feature a rotor constructed with polyphase windings, typically three-phase, that are similar in design to the stator windings and wound around the rotor core to match the number of stator poles.[50] These rotor windings are connected to slip rings mounted on the rotor shaft, which allow access to the rotor circuit from the exterior without direct electrical connection to the rotating parts.[51] The slip rings facilitate the attachment of external resistors or a rheostat, enabling manual or automatic adjustment of the rotor resistance.[50] In operation, the stator's rotating magnetic field induces currents in the rotor windings, producing torque that drives the rotor at a speed slightly less than the synchronous speed, with the difference defined as slip.[50] Inserting external resistance into the rotor circuit increases the effective rotor resistance, which reduces the starting current while simultaneously boosting the starting torque by steepening the torque-slip curve near zero speed.[51] By varying this resistance during startup or operation, the motor achieves smooth acceleration and speed control below synchronous speed, with higher resistance yielding lower speeds under constant load.[50] This adjustment modifies the torque-slip relationship to optimize performance for specific load conditions.[51] The primary advantages of polyphase wound-rotor motors include their ability to deliver high starting torque with limited inrush current, making them suitable for demanding applications such as cranes, hoists, and elevators where abrupt loads are common.[52] They also provide effective speed regulation and a wide adjustable speed range through simple resistance variation, offering strong running torque once operational.[50] However, these motors incur higher initial costs due to the complex rotor windings and slip ring assembly, and they require more frequent maintenance to service the brushes and slip rings, which are prone to wear and potential arcing.[50] Additionally, the external resistance method dissipates energy as heat, reducing overall efficiency unless a recovery system is employed.[51] In modern applications, polyphase wound-rotor motors have largely been supplanted by variable frequency drives (VFDs) paired with standard induction motors, which offer more efficient and precise speed control without mechanical components like slip rings.[52] VFDs enable stepless speed adjustment by varying the supply frequency, improving energy efficiency and eliminating maintenance issues associated with rotor access.[52] Nevertheless, wound-rotor designs persist in niche scenarios involving high-inertia loads, such as slabbing mills or hammer mills, where their inherent high starting torque and compatibility with VFDs for enhanced control remain advantageous.[52]Single-Phase Induction Motors
Single-phase induction motors adapt the basic induction motor principle to operate on single-phase AC power supplies, which are common in residential and light commercial settings. Unlike polyphase motors, a single-phase stator winding produces only a pulsating magnetic field that alternates in magnitude but does not rotate, resulting in zero net starting torque as the forward and backward rotating field components cancel each other at standstill.[53] To overcome this challenge and initiate rotation, these motors incorporate auxiliary starting mechanisms, such as additional windings or shading coils, to create a temporary phase difference between the main and auxiliary magnetic fields, effectively simulating a rotating field during startup.[54] The primary types of single-phase induction motors include split-phase, capacitor-start, and shaded-pole designs, each employing distinct methods to achieve the necessary phase shift for starting. In split-phase motors, an auxiliary winding with higher resistance and fewer turns is placed perpendicular to the main winding, producing a 30-degree phase lag in the auxiliary current to generate starting torque, typically 150-200% of full-load torque.[55] Capacitor-start motors enhance this by inserting a capacitor in series with the auxiliary winding, achieving a closer to 90-degree phase shift for higher starting torque, up to 300-400% of full-load, after which a centrifugal switch disconnects the auxiliary circuit once the motor reaches about 75% of synchronous speed.[56] Shaded-pole motors, the simplest and cheapest variant, use short-circuited copper rings (shading coils) on a portion of each pole to induce eddy currents that create a small time delay in the magnetic flux, providing low starting torque (25-75% of full-load) suitable for very small motors under 1/20 horsepower.[55] Direction reversal in these motors is accomplished by interchanging the connections of the auxiliary winding leads with respect to the main winding, which swaps the direction of the rotating field.[54] Compared to polyphase induction motors, single-phase versions exhibit lower efficiency (typically 50-80% versus 85-95%) and reduced starting torque due to the auxiliary mechanisms' limitations and higher power losses from phase imbalance, making them less suitable for heavy-duty applications.[55] Despite these drawbacks, they are widely used in household appliances such as ceiling fans, refrigerators, washing machines, and air conditioners, where single-phase power availability and fractional horsepower ratings (up to 5 HP) suffice.[56] Capacitor motors represent an advanced category within single-phase induction designs, optimizing performance through strategic capacitor use for phase correction. Permanent-split capacitor (PSC) motors connect a run capacitor continuously in series with the auxiliary winding, providing a moderate phase shift (around 45 degrees) for smooth operation at constant speed and improved efficiency (up to 75%), ideal for applications like blowers and pumps requiring quiet, vibration-free performance without a starting switch.[55] In contrast, capacitor-start capacitor-run motors employ two capacitors: a larger electrolytic start capacitor for high initial torque (200-400% of full-load) and a smaller oil-filled run capacitor that remains connected for better running efficiency and power factor, commonly found in compressor drives.[54] The equivalent circuit for these capacitor motors models the auxiliary branch as an impedance with the capacitor's reactance (-jX_c) in series, which supplies leading current to the auxiliary winding, balancing the inductive lag of the main winding and approximating a two-phase system for enhanced torque production.[53]Synchronous Motors
Construction and Basic Operation
The stator of a synchronous motor features a laminated iron core with distributed polyphase windings, typically three-phase, designed to produce a rotating magnetic field when energized by alternating current, similar to the stator in an induction motor.[57] This rotating field revolves at synchronous speed N_s = \frac{120 f}{p}, where f is the supply frequency in hertz and p is the number of poles.[58] The rotor is the distinguishing component, constructed either as a salient-pole type with projecting poles or a cylindrical (non-salient) type for higher speeds, and excited by direct current supplied through slip rings and brushes to create a constant magnetic field.[57] Alternatively, permanent magnets can be embedded in or mounted on the rotor surface for excitation without slip rings, offering maintenance-free operation in certain designs.[57] Unlike the rotor in an induction motor, which relies on induced currents and operates with slip, the synchronous rotor's field locks precisely with the stator's rotating field, ensuring zero slip and constant speed synchronized to the supply frequency.[59] In operation, the interaction between the stator's rotating field and the rotor's fixed field produces torque that maintains synchronization; the rotor aligns such that its north pole follows the stator's south pole, resulting in steady-state rotation at N_s.[60] Load application causes a shift in the spatial angle (load angle \delta) between the rotor and stator fields, generating synchronizing torque T_s to balance the mechanical load.[61] This torque is given byT_s = \frac{3 V E_f \sin \delta}{2 \pi f X_s}
where V is the terminal voltage per phase, E_f is the field-induced voltage per phase, \delta is the load angle, f is the frequency, and X_s is the synchronous reactance per phase.[61] Pull-out torque occurs at \delta = 90^\circ, representing the maximum load the motor can handle before losing synchronism and stalling.[61] Synchronous motors lack inherent starting torque due to the absence of relative motion between fields at standstill, so they cannot self-start under synchronous conditions.[62] A common starting method uses amortisseur (damper) windings—short-circuited copper or brass bars embedded in the rotor pole faces—enabling induction motor action during startup with the field unexcited, accelerating the rotor to near N_s.[63] Once near synchronous speed, DC excitation is applied to the rotor field, pulling it into lockstep with the stator field.[57] Sudden load changes can cause the rotor to oscillate around its equilibrium position, known as hunting, potentially leading to instability if undamped.[63] Damper windings also serve to damp these oscillations by inducing currents that produce opposing torques, stabilizing the rotor and preventing prolonged hunting.[63]