Motor and Generator Terminologies

Dynamo Development
The first generators and motors were called dynamos or dynamoelertric machines. Dynamo is from the Greek word dynamis which means power.

Webster defines dynamoelectric as "relating to the conversion of mechanical energy into electrical energy or vice versa". The word motor is from the Latin word motus which means one that imparts motion or prime mover. The dynamo was the result of the efforts of several people, in different countries, in the mid-nineteenth century, to make electricity work for them.

Definitions

Dynamo:
From the Greek word dynamis, which means power
Dynamoelectric:
Relating to the conversion by induction of mechanical energy into electrical energy or vice versa

Dynamoelectric machine:
A dynamo or generator
Motor:
From the Latin word motus, one that imparts motion, prime mover. A device that changes electrical energy into mechanical energy.
Generator:
A device that changes mechanical energy into electrical energy. Although the terms AC and DC generator are in common usage, a generator is normally considered to be a device that provides DC current.
Alternator:
A device that changes mechanical energy into an alternating current electrical energy, an AC generator.

Landmarks Of Electric Motor Development

The core function of electric motors is to convert electrical current into mechanical force. The history of motors can be related to times when fundamentals of electromagnetic induction were introduced. In early 1800, three popular scientists Oersted, Faraday and Gauss came up with the basic principals of electromagnetic induction.
In 1820, Andre Ampere and Hans Oersted made the most fascinating invention. They discovered that electric current produces magnetic field leading to the invention of the basic DC motor some ten years later. No body in particular is acclaimed with the sole invention of the DC motor, as it was a gradual process with involvement of many people
Michael Faraday from England set to prove the theory proposed by Ampere and Oersted. In 1921, he successfully demonstrated in his experiment by converting electrical energy into motion. His motor was made of a free-hanging wire that was plunged into a puddle of mercury. A permanent magnet was placed in the centre of the mercury pool. On passing through the wire, it rotated around the magnet. It proved that the current resulted in a circular magnetic field around the wire. This is the simplest form of electric motor.
Ten years later, it was Joseph Henry who built an improved motor based upon Faraday’s experimental motor. He constructed a device whose rotating part was an electromagnet with a horizontal axis. The motion resulted in two vertical permanent magnets, alternately attracted and repelled at end of the electromagnet. This made the magnet sway back and forth at 75 cycles per minute.
Till this stage, use of electromagnetic field in motors was restricted to lab experiments. A major development took place with William Sturgeon’s invention of commutator. He is credited with the discovery of first rotary electric motor. Sturgeon made use of horseshoe electromagnets to build rotating and stationary magnetic fields. His shunt wound DC motor was the first to produce a continuous rotary motion using all essentials of modern-day DC motors.
Another early electric motor design used a reciprocating plunger inside a switched solenoid; an electromagnetic version of a two-stroke internal combustion engine. A remarkable fact points that the modern day version of motor was actually an accident. In the year 1873, Zénobe Gramme accidentally linked a spinning dynamo to a similar unit, driving it as a motor.
The accident proved to be successful. And the journey of development started from then on. There are many amusing facts and chapters in the development of the present day motor. Motor is the core of many hi-tech electronic, electro-mechanical and electrical gadgets all over the world.
1820 The discovery of electromagnetism Hans Christian Oersted, Danish
1827 The statement of the law of electric conduction, Ohm's law George S. Ohm, German

1830 The discovery of electromagnetic induction Joseph Henry, American
1831 The discovery of electromagnetic induction Michael Faraday, English

The first practical dynamo, about 1867

Types of Motors

Electric motors are practically used everywhere round us. Starting with your kitchen fan, microwave oven, refrigerator, vacuum cleaners, hair dryers to car heaters and radiators, everything uses a motor. Following are the main types of motors.

1. AC Motor
There are two main types of AC motors. The most popular and simple motor is the three-phase AC induction motor. It is also known as the squirrel cage motor. The synchronous motor rotates at exactly the supply frequency or submultiples of the supply frequency. A typical AC motor consists of two parts:
o An exterior stationary stator with coils that uses AC current to produce a revolving magnetic field
o An interior rotor linked to the output shaft that employs torque using the rotating field
2. Stepper Motors

Stepper motor is an electro-mechanical device that converts electrical current into torque output. It popularly finds application as a positioning device for precision control. The motor works by converting electrical impulses into distinct mechanical rotatory motion. Normally this motion of a stepper motor is measured in degrees, or in steps.
3. DC Motor

While an electric motor converts electrical energy into mechanical force, the reverse if true for DC motor. DC motors convert mechanical force into electrical energy with the use of a generator or dynamo. DC motors can be primarily divided into Brushed DC motors and Brushless DC motors

4. Brushed DC motors

The brushed DC motor has an ancient history. In this type of motor, a permanent magnetic field is produced in the stator with the help of permanent magnets or electro-magnetic windings. If the field is created by permanent magnets, the motor is called a permanent magnet DC motor. If it is generated using electromagnetic windings, the motor is named as Shunt wound DC motor.


5. Brushless DC motor

A brushless DC motor is an electric motor with similar operations as the DC motor. The only difference is that the role of rotor and stator are inverted in the brushless DC motor. The motor’s rotor has a set of permanent magnets while the stator here consists of electromagnets. As the name suggests, the motor does not use brushes, but the function of commutator takes place by an electronic circuit. It switches the current to various stator coils as and when required.
6. Linear Motor
A linear motor is essentially an electric motor with an unrolled stator. It results in the Linear motor producing linear force along its length instead of conventional torque as in other motors.
7. Servo motor
Servo is a tiny motor with specific function. This motor can select between vertical or horizontal polarization. Popularly used for motion controls in electronic gadgets and robots and computer hard disc drives, the motor works more like an alternator. It uses special circuit to make them rotate electrically. Some servomotors are used in reverse to generate AC current
8. Universal motors
A variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) at the same time, and hence the mechanical force generated is always in the same direction.
9. 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 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 in order to reduce the resistance in the rotor.
10. 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.
11. Three-phase AC synchronous motors
If connections to the rotor coils of a three-phase motor are taken out on Slip-rings and fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate in synchronism with the rotating magnetic field produced by the polyphase electrical supply.
The synchronous motor can also be used as an alternator.
12. Two-phase AC servo motors
A typical two-phase AC servo motor has a squirrel-cage rotor and a field consisting of two windings: 1) a constant-voltage (AC) main winding, and 2) a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. The electrical resistance of the rotor is made high intentionally so that the speed-torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load.
13. Single-phase AC induction motors
Three-phase motors inherently produce a rotating magnetic field. However, when only single-phase power is available, the rotating magnetic field must be produced using other means. Several methods are commonly used.
A common single-phase motor is the shaded-pole motor, which is used in devices requiring low torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle, thus producing the required rotating magnetic field.
Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.
In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.
The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.
In a capacitor start motor, a starting capacitor is inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.
Another variation is the Permanent Split-Capacitor (PSC) motor (also known as a capacitor start and run motor). This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch and the second winding is permanently connected to the power source. PSC motors are frequently used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds. Also provided all 6 winding connections are available separately, a 3 phase motor can be converted to a capacitor start and run motor by commoning two of the windings and connecting the third via a capacitor to act as a start winding.
Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2006.
14. Single-phase AC synchronous motors
Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio turntables, and tape drives; formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The shaded-pole synchronous motor is one version.
Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).
15.Torque motors
A torque motor. In this mode, the motor will apply a steady torque to the load (hence the name). A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In this application, driven from a low voltage, the characteristics of these motors allow a relatively-constant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, (and so delivering a higher torque), the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches. In the computer world, torque motors are used with force feedback steering wheels.
15. Stepper motors
Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a large iron core with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the motor may not rotate continuously; instead, it "steps" from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards.
Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position "between" the "cog" points and thereby rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.
Stepper motors can be rotated to a specific angle with ease, and hence stepper motors are used in computer disk drives, where the high precision they offer is necessary for the correct functioning of, for example, a hard disk drive or CD drive. Only very old harddrives (from the pre-gigabyte era) use stepper motors, newer drives use voice coil based systems.
Stepper motors were upscaled to be used in electric vehicles under the term SRM switched reluctance machine.
16. Permanent magnet motor
A permanent magnet motor is the same as the conventional dc machine except the fact that the field winding is replaced by permanent magnets. By doing this, the machine would act like a constant excitation dc machine (separately excited dc machine).
These motors usually have a small rating, ranging up to a few horsepower. They are used in small appliances, battery operated vehicles, for medical purposes, in other medical equipment such as x-ray machines. These motors are also used in toys, and in automobiles as auxiliary motors for the purposes of seat adjustment, power windows, sunroof, mirror adjustment, blower motors, engine cooling fans and the like.
The latest developments are PSM motors for electric vehicles. - High efficiency - Minimal locking moment and torque surface undulation - Small space requirements, compact dimensions - Low weight source [1]
17.Brushless DC motors
Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. This limits the maximum speed of the machine. The current density per unit area of the brushes limits the output of the motor. The imperfect electric contact also causes electrical noise. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance. The commutator assembly on a large machine is a costly element, requiring precision assembly of many parts.
These problems are eliminated in the brushless motor. In this motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switch synchronised to the motor's position. Brushless motors are typically 85-90% efficient, whereas DC motors with brushgear are typically 75-80% efficient.
Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors. In effect, they act as three-phase synchronous motors containing their own variable-frequency drive electronics. A specialized class of brushless DC motor controllers utilize EMF feedback through the main phase connections instead of Hall effect sensors to determine position and velocity. These motors are used extensively in electric radio-controlled vehicles.
Brushless DC motors are commonly used where precise speed control is necessary, computer disk drives or in video cassette recorders the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products such as fans, laser printers and photocopiers. They have several advantages over conventional motors:
• Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to much-improved life of the fan's bearings.
• Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers.
• The same Hall effect devices that provide the commutation can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan okay" signal.
• The motor can be easily synchronized to an internal or external clock, leading to precise speed control.
• Brushed motors cannot be used in the vacuum of space because they will weld themselves into an immovable position.
• Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels.
Modern DC brushless motors range in power from a fraction of a watt to many kilowatts. Larger brushless motors up to about 100 kW rating are used in electric vehicles. They also find significant use in high-performance electric model aircraft.
18. Coreless DC motors
Nothing in the design of any of the motors described above requires that the iron (steel) portions of the rotor actually rotate; torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless DC motor, a specialized form of a brush DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a winding-filled cylinder inside the stator magnets, a basket surrounding the stator magnets, or a flat pancake (possibly formed on a printed wiring board) running between upper and lower stator magnets. The windings are typically stabilized by being impregnated with epoxy resins.
Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air.
These motors were commonly used to drive the capstan(s) of magnetic tape drives and are still widely used in high-performance servo-controlled systems.
19. Linear motors
A linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing a torque (rotation), it produces a linear force along its length by setting up a traveling electromagnetic field.
Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground.
20. Doubly-fed electric motor
Doubly-fed electric motors or Doubly-Fed Electric Machines incorporate two independently powered multiphase winding sets that actively participate in the energy conversion process (i.e., doubly-fed) with at least one of the winding sets electronically controlled for synchronous operation from sub-synchronous to super synchronous speeds. As a result, doubly-fed electric motors are synchronous machines with an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as Singly-Fed Electric Machines, which incorporate a single active winding set. In theory, this attribute has attractive cost, size, and efficiency ramifications compared to Singly-Fed Electric Machines but Doubly-fed motors are difficult to realize in practice.
The Wound-Rotor Doubly-Fed Electric Machines, the Brushless Wound-Rotor Doubly-Fed Electric Machine, and the so-called Brushless Doubly-Fed Electric Machines are the only examples of synchronous doubly-fed electric machines.
21. Singly-fed electric motor
Singly-fed electric motors or Singly-Fed Electric Machines incorporate a single multiphase winding set that actively participate in the energy conversion process (i.e., singly-fed). Singly-fed electric machines operate under either Induction (i.e., Asynchronous) or Synchronous principles. The active winding set can be electronically controlled for optimum performance. Induction machines exhibit startup torque and can operate as standalone machines but Synchronous machines must have auxiliary means for startup and practical operation, such as an electronic controller. Singly-fed electric machines have an effective constant torque speed range up to synchronous speed (i.e., 3600 rpm @ 60 Hz and 2 Poles) for a given excitation frequency.
The Induction (Asynchronous) motors (i.e., squirrel cage rotor or wound rotor), Synchronous motors (i.e., field-excited, Permanent Magnet or brushless DC motors, Reluctance motors, etc.), which are discussed on this page, are examples of Singly-fed motors. By far, Singly-fed motors are the predominantly installed type of motors.
22. Dual mechanical port motor
The Dual Mechanical Port Electric Motors (or DMP electric motor) is considered a new electric motor concept. More accurately, DMP electric motors are actually two electric motors (or generators) occupying the same package. Each motor operates under traditional electric motor principles. The electrical ports, which may include electronic support, of the electric motors are tied to a single electrical port while two mechanical ports (shafts) are available externally. Theoretically, the physical integration of the two motors into one is expected to increase power density by efficiently utilizing otherwise wasted magnetic core real-estate. The mechanics of the integration, such as for the two mechanical shafts, may be quite exotic.

Nanomotor constructed at UC Berkeley. The motor is about 500nm across: 300 times smaller than the diameter of a human hair
Researchers at University of California, Berkeley, have developed rotational bearings based upon multiwall carbon nanotubes. By attaching a gold plate (with dimensions of order 100nm) to the outer shell of a suspended multiwall carbon nanotube (like nested carbon cylinders), they are able to electrostatically rotate the outer shell relative to the inner core. These bearings are very robust; Devices have been oscillated thousands of times with no indication of wear. The work was done in situ in an SEM. These nanoelectromechanical systems (NEMS) are the next step in miniaturization that may find their way into commercial aspects in the future.
Notice: The thin vertical string seen in the middle, is the nanotube to which the rotor is attached. When the outer tube is sheared, the rotor is able to spin freely on the nanotube bearing.