AC motor
An AC motor is an electric motor driven by an alternating current. 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.
Less common, AC linear motors operate on similar principles as rotating motors but have their stationary and moving parts arranged in a straight line configuration, producing linear motion instead of rotation.
Operating principles
The two main types of AC motors are induction motors and synchronous motors. The induction motor always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor AC winding. As a result, the induction motor cannot produce torque near synchronous speed where induction is irrelevant or ceases to exist. In contrast, the synchronous motor does not rely on slip-induction for operation and uses either permanent magnets, salient poles, or an independently excited rotor winding. The synchronous motor produces its rated torque at exactly synchronous speed. The brushless wound-rotor doubly fed synchronous motor system has an independently excited rotor winding that does not rely on the principles of slip-induction of current. The brushless wound-rotor doubly fed motor is a synchronous motor that can function exactly at the supply frequency or sub to super multiple of the supply frequency.Other types of motors include eddy current motors, and AC and DC mechanically commutated machines in which speed is dependent on voltage and winding connection.
History
technology was rooted in Michael Faraday's and Joseph Henry's 1830–31 discovery that a changing magnetic field can induce an electric current in a circuit. Faraday is usually given credit for this discovery since he published his findings first.In 1832, French instrument maker Hippolyte Pixii generated a crude form of alternating current when he designed and built the first alternator. It consisted of a revolving horseshoe magnet passing over two wound-wire coils.
Because of AC's advantages in long-distance high voltage transmission, there were many inventors in the United States and Europe during the late 19th century trying to develop workable AC motors. The first person to conceive of a rotating magnetic field was Walter Baily, who gave a workable demonstration of his battery-operated polyphase motor aided by a commutator on 28 June 1879, to the Physical Society of London. Describing an apparatus nearly identical to Baily's, French electrical engineer Marcel Deprez published a paper in 1880 that identified the rotating magnetic field principle and that of a two-phase AC system of currents to produce it. Never practically demonstrated, the design was flawed, as one of the two currents was “furnished by the machine itself.” In 1886, English-born American engineer Elihu Thomson built an AC motor by expanding upon the induction-repulsion principle and his wattmeter. In 1887, American inventor Charles Schenk Bradley was the first to patent a two-phase AC power transmission with four wires.
"Commutatorless" alternating current induction motors seem to have been independently invented by Galileo Ferraris and Nikola Tesla. Ferraris demonstrated a working model of his single-phase induction motor in 1885, and Tesla built his working two-phase induction motor in 1887 and demonstrated it at the American Institute of Electrical Engineers in 1888. In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation; Tesla, in the same year, was granted a United States patent for his own motor. Working from Ferraris's experiments, Mikhail Dolivo-Dobrovolsky introduced the first three-phase induction motor in 1890, a much more capable design that became the prototype used in Europe and the U.S. He also invented the first three-phase generator and transformer and combined them into the first complete AC three-phase system in 1891. The three-phase motor design was also worked on by the Swiss engineer Charles Eugene Lancelot Brown, and other three-phase AC systems were developed by German technician Friedrich August Haselwander and Swedish engineer Jonas Wenström.
Induction motor
Slip
If the rotor of a squirrel cage motor were to run at the true synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of revolutions per minute slower than synchronous speed. Because the rotating field effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly. Even with no load, internal mechanical losses prevent the slip from being zero.The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
where
- is the synchronous speed, in revolutions per minute ;
- is the AC power frequency, in cycles per second ; and
- is the number of poles per phase winding.
Actual for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2–3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip.
The slip of the AC motor is calculated by:
where
- is the rotational speed, in revolutions per minute; and
- is the normalised slip ratio, ranging from 0 to 1.
The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.
This kind of rotor is the basic hardware for induction regulators, which is an exception of the use of rotating magnetic field as pure electrical application.
Polyphase cage rotor
Most common AC motors use the squirrel-cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel-cage refers to the rotating exercise cage for pet animals. The motor takes its name from the shape of its rotor "windings"- a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper to reduce the resistance in the rotor.In operation, the squirrel-cage motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor almost into synchronization with the stator's field. An unloaded squirrel-cage motor at rated no-load speed will consume electrical power only to maintain rotor speed against friction and resistance losses. As the mechanical load increases, so will the electrical load – the electrical load is inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.
This is why a squirrel-cage blower motor may cause household lights to dim upon starting, but does not dim the lights on startup when its fan belt is removed. Furthermore, a stalled squirrel-cage motor will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current overheating and destruction of the winding insulation is the likely outcome.
Many washing machines, dishwashers, larger standalone fans, compressors, etc. use some variant of a squirrel-cage motor.
Polyphase wound rotor
An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to a controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable-speed wound rotor drives, the slip-frequency energy is captured, rectified, and returned to the power supply through an inverter. With bidirectionally controlled power, the wound rotor becomes an active participant in the energy conversion process, with the wound rotor doubly fed configuration showing twice the power density.Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common.
Several methods of starting a polyphase motor are used. Where a large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals. Where it is necessary to limit the starting inrush current, the motor is started at reduced voltage using either series inductors, an autotransformer, thyristors, or other devices. A technique sometimes used is star-delta starting, where the motor coils are initially connected in star configuration for acceleration of the load, then switched to delta configuration when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.
This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor.