Electric motor - electric machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor.

          An electric motor converts electrical energy into mechanical motion. The reverse task, that of converting mechanical motion into electrical energy, is accomplished by a generator or dynamo. In many cases the two devices differ only in their application and minor construction details, and some applications use a single device to fill both roles. For example, traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes.

          Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and the piezoelectric effect, also exist. The fundamental principle upon which electromagnetic motors are based is that there is a mechanical force on any wire when it is conducting electricity while contained within a magnetic field. The force is described by the Lorentz force law and is perpendicular to both the wire and the magnetic field.

          Most magnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. The rotor rotates because the wires and magnetic field are arranged so that a torque is developed about the rotor's axis.

AC motors.

          A typical AC motor consists of two parts:
          1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;
          2. An inside rotor attached to the output shaft that is given a torque by the rotating field.

          There are two fundamental types of AC motor depending on the type of rotor used:
          1. The synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency, and;
          2. The induction motor, which turns slightly slower, and typically (though not necessarily always) takes the form of the squirrel cage motor.

          The rotating magnetic field principle, though commonly credited to Nikola Tesla in 1882 or thereabouts, was employed by scientists such as Michael Faraday and James Clerk Maxwell in the 1820s. Tesla, however, exploited the principle to design a unique two-phase induction motor in 1883. Michael von Dolivo-Dobrowlsky invented the first modern three-phase "cage-rotor" in 1890. Introduction of the motor from 1888 onwards initiated what is known as the Second Industrial Revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888). The first successful commercial three phase generation and long distance transmission system was designed by Almerian Decker at Mill Creek No. 1 in Redlands California.

Тhree-phase AC induction motors.

          Where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is used commonly used, especially for higher-powered motors. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor.

          Through electromagnetic induction, the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no counterbalancing field will be produced in the rotor.

          Induction motors are the workhorses of industry and motors up to about 500 kW in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are different). Very large synchronous motors are made up to tens of thousands of kilowatts output, for pipeline compressors and wind-tunnel drives.

          There are two types of rotors used in induction motors.

          Squirrel Cage rotors: 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 takes its name from its shape - a ring at either end of the armature, with bars connecting the rings running the length of the rotor. It is typically cast aluminum 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 rings; high efficiency motors will often use cast copper in order 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 into synchronization with the stator's field. An unloaded squirrel cage motor at synchronous speed will only consume electrical power 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.

          A common type of squirrel cage motor is a shaded pole motor, found in most inexpensive low-noise and low-torque applications like fans. Shaded pole motors are inherently inefficient, and most incorporate some form of impedance protection to limit stalled current. With the exception of shaded pole motors, most squirrel cage motors are extremely efficient.

          Virtually every washing machine, dishwasher, record player, etc. uses some variant of a squirrel cage motor.

          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 an external 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.

          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. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available.)

          Several methods of starting a polyphase motor are used. Where the 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 (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is star-delta starting, where the motor coils are initially connected in wye for acceleration of the load, then switched to delta 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.

          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:

          Actual RPM 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 (0 RPM/full stall).

          The slip of the AC motor is calculated by:

          As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800.

          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.