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PowerPedia:Electric motor

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An electric motor converts There was an error working with the wiki: Code[6] 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, There was an error working with the wiki: Code[50]s used on There was an error working with the wiki: Code[51]s often perform both tasks if the locomotive is equipped with There was an error working with the wiki: Code[52]s.

Operation

Most electric motors work by Magnetism, but motors based on other electromechanical phenomena, such as There was an error working with the wiki: Code[7] and the There was an error working with the wiki: Code[8] on any current-carrying wire contained within a magnetic field. The force is described by the Magnetic field and is perpendicular to both the wire and the magnetic field.

Most magnetic motors are rotary, but There was an error working with the wiki: Code[9], that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input Voltage is supplied. Depending upon the design of the machine, either the rotor or the stator can serve as the armature.

DC motors

One of the first electromagnetic rotary motors was invented by There was an error working with the wiki: Code[10].

A permanent There was an error working with the wiki: Code[53] was placed in the middle of the pool of mercury.

When a Current (electricity) was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but There was an error working with the wiki: Code[54] (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called There was an error working with the wiki: Code[55]s. A later refinement is the There was an error working with the wiki: Code[56].

Another early electric motor design used a reciprocating plunger inside a switched There was an error working with the wiki: Code[57] conceptually it could be viewed as an electromagnetic version of a two stroke Internal combustion engine. The modern DC motor was invented by accident in 1873, when There was an error working with the wiki: Code[58] connected a spinning There was an error working with the wiki: Code[59] to a second similar unit, driving it as a motor.

The classic Direct current motor has a rotating armature in the form of an electromagnet. A rotary switch called a There was an error working with the wiki: Code[11] reverses the direction of the electric current twice every cycle, to flow through the There was an error working with the wiki: Code[12] so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, There was an error working with the wiki: Code[60] keeps the classical motor going in the proper direction. (See the diagrams below.)

Wound field DC motor

The permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current, it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance that gives steady speed over a range of loads. Separate excitation is also common, with a fixed field voltage, the speed being controlled by varying the armature voltage. Further reductions in field current are possible to gain even higher speed but correspondingly lower torque, called "weak field" operation.

Theory

If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an electric motive force (EMF). This voltage is also generated during normal motor operation. The spinning of the motor produces a voltage known as the back EMF because it opposes the applied voltage on the motor. Therefore the voltage drop across a motor consists of the voltage drop due to this back EMF and the parasitic voltage drop resulting from the internal resistance of the armature's windings. The current through a motor is given by the following equation:

I = (V_{applied}-V_{backemf})/R_{armature}

The mechanical power produced by the motor is given by:

P = I V_{backemf}

Since the back EMF is proportional to motor speed, when an electric motor is first started or is completely stalled, there is zero back EMF. Therefore the current through the armature is much higher. This high current will produce a strong electric field which will start the motor spinning. As the motor spins, the back EMF increases until it is equal to the applied voltage minus the parasitic voltage drop. At this point there will be a smaller current flowing through the motor. Basically the following three equations can be used to find the speed, current, and back EMF of a motor under a load:

Load = V_{backemf} I

V_{applied} = I R_{armature} + V_{backemf}

V_{backemf} = speed Flux_{armature}

Speed control

Generally, the rotational speed of a DC motor is proportional to the voltage applied to it, and the There was an error working with the wiki: Code[13] (direction contactors).

The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device made of There was an error working with the wiki: Code[14]s. In a circuit known as a There was an error working with the wiki: Code[15], the average voltage applied to the motor is varied by switching the supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage gives the average voltage applied to the motor. Therefore, with a 100&nbspV supply and a 25% "on" time, the average voltage at the motor will be 25&nbspV. During the "off" time, the armature's inductance causes the current to continue flowing through a diode called a "flywheel diode", in parallel with the supply. At this point in the cycle, the supply current will be zero, and therefore the average motor current will always be higher than the supply current unless the percentage "on" time is 100%. At 100% "on" time, the supply and motor current are equal. The rapid switching wastes less energy than series resistors. An output There was an error working with the wiki: Code[16] is sometimes installed to smooth the average voltage applied to the motor and reduce motor noise. This method is also called pulse width modulation, or PWM, and is often controlled by a microprocessor.

Since the series-wound DC motor develops its highest torque at low speed, it is often used in There was an error working with the wiki: Code[17] applications such as electric There was an error working with the wiki: Code[61]s, and There was an error working with the wiki: Code[62]s. Another application is starter motors for petrol and small diesel engines. Series motors must never be used in applications where the drive can fail (such as belt drives). As the motor accelerates, the armature (and hence field) current reduces. The reduction in field causes the motor to speed up (see 'weak field' in the last section) until it destroys itself. This can also be a problem with railway motors in the event of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far higher than they would do under normal circumstances. This cannot only cause problems for the motors themselves and the gears, but due to the differential speed between the rails and the wheels it can also cause serious damage to the rails and wheel treads as they heat and cool rapidly. Field weakening is used in some electronic controls to increase the top speed of an electric vehicle. The simplest form uses a contactor and field weakening resistor, the electronic control monitors the motor current and switches the field weakening resistor into circuit when the motor current reduces below a preset value (this will be when the motor is at its full design speed). Once the resistor is in circuit, the motor will increase speed above its normal speed at its rated voltage. When motor current increases, the control will disconnect the resistor and low speed torque is made available.

One interesting method of speed control of a DC motor is the There was an error working with the wiki: Code[18]. It is a method of controlling a DC motor (usually a shunt or compound wound) and was developed as a method of providing a speed-controlled motor from an AC supply, though it is not without its advantages in DC schemes. The AC supply is used to drive an AC motor, usually an induction motor that drives a DC generator or There was an error working with the wiki: Code[63]. The DC output from the armature is directly connected to the armature of the DC motor (usually of identical construction). The shunt field windings of both DC machines are excited through a variable resistor from the generator's armature. This variable resistor provides extremely good speed control from standstill to full speed, and consistent torque. This method of control was the de facto method from its development until it was superseded by solid state Thyristor systems. It found service in almost any environment where good speed control was required, from passenger lifts through to large mine pit head winding gear and even industrial process machinery and electric cranes. Its principal disadvantage was that three machines were required to implement a scheme (five in very large installations, as the DC machines were often duplicated and controlled by a tandem variable resistor). In many applications, the motor-generator set was often left permanently running, to avoid the delays that would otherwise be caused by starting it up as required. There are numerous legacy Ward-Leonard installations still in service.

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. In practice, the motor must be specially designed to cope with the AC current (There was an error working with the wiki: Code[19].

The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the There was an error working with the wiki: Code[20]. As a result such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently. Continuous speed control of a universal motor running on AC is very easily accomplished using a There was an error working with the wiki: Code[21] voltage of the AC power line).

Unlike AC motors, universal motors can easily exceed one revolution per cycle of the mains current. This makes them useful for appliances such as There was an error working with the wiki: Code[22], There was an error working with the wiki: Code[23] motors will exceed 10,000 RPM, There was an error working with the wiki: Code[24] all act to prevent overspeed.

With the very low cost of There was an error working with the wiki: Code[64] There was an error working with the wiki: Code[65]s, some applications that would have previously used a universal motor now use a pure DC motor, usually with a permanent magnet field. This is especially true if the semiconductor circuit is also used for variable-speed control.

The advantages of the universal motor and alternating-current distribution made installation of a low-frequency There was an error working with the wiki: Code[66] distribution system economical for some railway installations. At low enough frequencies, the motor performance is approximately the same as if the motor were operating on DC. Frequencies as low as 162/3&nbsphertz were employed.

AC motors

In 1882, There was an error working with the wiki: Code[25] principle, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, There was an error working with the wiki: Code[67] independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Introduction of Tesla's motor from 1888 onwards initiated what is known as the There was an error working with the wiki: Code[68], making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888).http://www.tfcbooks.com/tesla/system.htm Before the invention of the rotating magnetic field, motors operated by continually passing a conductor through a stationary magnetic field (as in There was an error working with the wiki: Code[69]s).

Tesla had suggested that the There was an error working with the wiki: Code[26]s from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a There was an error working with the wiki: Code[70]. "Tesla's Early Years". There was an error working with the wiki: Code[71]. Tesla would later attain There was an error working with the wiki: Code[1], Electric Motor (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.

{| width=44% border="0" class="wikitable" style="float:right"

|-

! Stator energy || Rotor energy|| Total energy supplied || Power developed

|- align="right"

| 10 || 90 || 100 || 900

|- align="right"

| 50 || 50 || 100 || 2500

|}

In the induction motor, the field and armature were ideally of equal field strengths and the field and armature cores were of equal sizes. The total energy supplied to operate the device equaled the sum of the energy expended in the armature and field coils. U.S. Patent 0416194, "Electric Motor", December 1889. The power developed in operation of the device equaled the product of the energy expended in the armature and field coils. U.S. Patent 0416194, "Electric Motor", December 1889.

There was an error working with the wiki: Code[72] later invented a three-phase "cage-rotor" in 1890. A successful commercial There was an error working with the wiki: Code[73] of generation and long-distance transmission was designed by Almerian Decker at Mill Creek No. 1 http://www.electrichistory.com in Redlands California. http://www.redlandsweb.com

Components and types

A typical AC motor consists of two parts:

# An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and

# 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:

The synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency, and

The induction motor, which turns slightly slower, and typically (though not necessarily always) takes the form of the There was an error working with the wiki: Code[27] motor.

Three-phase AC induction motors

Where a polyphase electrical supply is available, the There was an error working with the wiki: Code[28]) AC induction motor is 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 There was an error working with the wiki: Code[74], 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 (670 There was an error working with the wiki: Code[75]) 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 capable of tens of thousands of kW in 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 There was an error working with the wiki: Code[76], 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.

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 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, as an example, a squirrel cage blower motor may cause the lights in a home to dim as it starts, but doesn't dim the lights when its fanbelt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.

Virtually every There was an error working with the wiki: Code[29], There was an error working with the wiki: Code[77], 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, but this is never used in household appliances, because it can cause electrical interference and because of high power requirements.)

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 (Direct-on-line, DOL). 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 Transformer, 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 There was an error working with the wiki: Code[78].

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:

: N_{s} = 120F/p

where

:Ns = Synchronous speed, in revolutions per minute

:F = AC power frequency

:p = Number of poles per phase winding

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:

:S = (N_{s} - N{r})/N_{s}

where

:Nr = Rotational speed, in revolutions per minute.

:S = Normalised Slip, 0 to 1.

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 There was an error working with the wiki: Code[79] mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.

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 There was an error working with the wiki: Code[80].

Nowadays, synchronous motors are frequently driven by transistorized There was an error working with the wiki: Code[30]. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.

Synchronous motors are occasionally used as traction motors the There was an error working with the wiki: Code[81] may be the best-known example of such use.

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.

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 There was an error working with the wiki: Code[82], which is used in devices requiring low Torque, such as There was an error working with the wiki: Code[83]s 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 (There was an error working with the wiki: Code[84]), 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 There was an error working with the wiki: Code[85]s such as There was an error working with the wiki: Code[86]s and There was an error working with the wiki: Code[87]s. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special There was an error working with the wiki: Code[88] in conjunction with a There was an error working with the wiki: Code[89].

In the split-phase motor, the startup winding is designed with a higher Electrical resistance than the running winding. This creates an There was an error working with the wiki: Code[31] 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 There was an error working with the wiki: Code[90] 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.

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 There was an error working with the wiki: Code[32]s, and There was an error working with the wiki: Code[91]s formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The There was an error working with the wiki: Code[92] is one version.

Because There was an error working with the wiki: Code[93] 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).

Torque motors

A torque motor is a specialized form of induction motor which is capable of operating indefinitely at stall (with the rotor blocked from turning) without damage. 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 There was an error working with the wiki: Code[94]s or There was an error working with the wiki: Code[95]es.

Stepper motors

Closely related in design to three-phase AC synchronous motors are There was an error working with the wiki: Code[96]s, 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 There was an error working with the wiki: Code[97]. 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 "There was an error working with the wiki: Code[33] 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.

Description

A stepper motor is a brushless, synchronous Electric motor that can divide a full rotation into a large number of steps, for example, 200 steps.

This is achieved by increasing the numbers of poles (on both rotor and stator), taking care that they have no common denominator. Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Like an Electric motor, it is ideally driven by sinusoidal current, allowing a stepless operation, but this puts some burden on the controller. When using an 8-bit digital There was an error working with the wiki: Code[98], 256 microsteps per step are possible. As a There was an error working with the wiki: Code[99] produces unwanted ohmic heat in the controller, There was an error working with the wiki: Code[100] is used instead to regulate the mean current. Simpler models switch voltage only for doing a step, thus needing an extra current limiter: for every step, they switch a single cable to the motor. Bipolar controllers can switch between supply voltage, ground, and unconnected. Unipolar controllers can only connect or disconnect a cable, because the voltage is already hard wired. Unipolar controllers need center-tapped windings.

Typically a bipolar stepper motor has 4 wires coming out of it. Each pair of wires is connected to the ends of two coils. The wires must be toggled between ground and voltage, which is usually accomplished via a set of There was an error working with the wiki: Code[101]. Unipolar stepper motor has 5 or 6 wires. The extra wires are referred to as "common." They are located in the middle of the two coils and consistently supply voltage to the coils. The 4 wires that are located at the ends of the coils now switch between unconnected and ground. This helps reduce number of transistors required in the circuit. One way to distinguish common wire from a coil-end wire is by measuring the resistance. Resistance between common wire and coil-end wire is always half of what it is between coil-end and coil-end wires. This is due to the fact that there is actually twice the length of coil between the ends and only half from center (common wire) to the end.

It is possible to drive unipolar stepper motors with bipolar drivers. The idea is to connect the output pins of the driver to 4 transistors. The transistor must be grounded at the emitter and the driver pin must be connected to the base. Collector is connected to the coil wire of the motor.

Stepper motors are rated by the There was an error working with the wiki: Code[34] during each step. The Voltage rating (if there is one) is almost meaningless. The motors also suffer from EMF, which means that once the coil is turned off it starts to generate current because the motor is still rotating. There needs to be an explicit way to handle this extra current in a circuit otherwise it can cause damage and affect performance of the motor.

Applications

Computer-controlled stepper motors are one of the most versatile forms of There was an error working with the wiki: Code[35]. Stepper motors are used in floppy disk drives, flatbed scanners, printers, plotters and many more devices. Note that There was an error working with the wiki: Code[102]s no longer use stepper motors to position the read/write heads, instead utilising a There was an error working with the wiki: Code[103] and servo feedback for head positioning.

Stepper motors can also be used for positioning of valve pilot stages, for fluid control systems.

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.

Brushless DC motors

Many of the limitations of the classic There was an error working with the wiki: Code[104] DC motor are due to the need for brushes to press against the commutator. This creates There was an error working with the wiki: Code[105]. 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 There was an error working with the wiki: Code[106]. 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 Direct current motors and stepper motors lies the realm of the There was an error working with the wiki: Code[36]. 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 There was an error working with the wiki: Code[37] 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 There was an error working with the wiki: Code[38] vehicles.

Brushless DC motors are commonly used where precise speed control is necessary, computer There was an error working with the wiki: Code[39]s, There was an error working with the wiki: Code[107]s and There was an error working with the wiki: Code[108]s. 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 There was an error working with the wiki: Code[109]s.

Without a There was an error working with the wiki: Code[40] 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 There was an error working with the wiki: Code[110] 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.

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.

Description

A brushless DC motor (BLDC) is an AC synchronous Electric motor that from a modeling perspective looks very similar to a DC motor. Sometimes the difference is explained as an electronically-controlled commutation system, instead of a mechanical commutation system, although this is misleading, as physically the two motors are completely different. (The rest of this article assumes the reader is familiar with the principles of electrical motors.)

Three subtypes exist:

The Electric motor type has three electrical connections

The There was an error working with the wiki: Code[111] type may have more poles on the stator.

The There was an error working with the wiki: Code[112] has all its poles on the stator, and a magnetic core on the rotor.

In a conventional (brushed) DC-motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush-system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature-coil closest to the stationary stator (permament magnet.)

In a BLDC motor, the brush-system/commutator assembly is replaced by an intelligent electronic controller. The controller performs the same power-distribution found in a brushed DC-motor, only without using a commutator/brush system. The controller contains a bank of There was an error working with the wiki: Code[41] in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called "sensorless" controllers. (The BLDC motor has a trapezoidal backemf, while a brushless AC motor has a sinousoidal backemf.)

BLDC motors can be constructed in two different physical configurations: In the 'conventional' configuration, the permanent magnets are mounted on the spinning armature (rotor.) The stator coils surround the rotor. In the 'outrunner' configuration, the radial-relationship between the coils and magnets are reversed the stator coils form the center (core) of the motor, while the permanent magnets spin on an overhanging rotor which surrounds the core. In all BLDC motors, the stator-coils are stationary.

Comparison with brushed-DC motors

BLDC motors offer several advantages over brushed DC-motors, including higher reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of There was an error working with the wiki: Code[113] (EMI.) BLDC's main disadvantage is higher cost, which arises from two issues: First, BLDC motors require high-power MOSFET devices in the fabrication of the electronic speed controller. Brushed DC-motors can be regulated by a comparatively trivial variable-resistor (potentiometer or rheostat), which is inefficient but also satisfactory for cost-sensitive applications. BLDC motors need a more expensive There was an error working with the wiki: Code[114], called an electronic speed controller, to offer the same type of variable-control. Second, when comparing manufacturing techniques between BLDC and brushed motors, many BLDC designs require manual-labor, to hand-wind the stator coils. On the other hand, brushed motors use armature coils which can be inexpensively machine-wound.

BLDC motors are considered more efficient than brushed DC-motors. This means for the same input power, a BLDC motor will convert more electrical power into mechanical power than a brushed motor. The enhanced efficiency is greatest in the no-load and low-load region of the motor's performance curve. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency.

Applications

BLDC motors can potentially be deployed in any field-application currently fulfilled by brushed DC motors. Cost prevents BLDC motors from replacing brushed motors in most common areas of use. Nevertheless, BLDC motors have come to dominate many applications: Consumer devices such as computer There was an error working with the wiki: Code[42]s, CD/DVD players, and PC cooling fans use BLDC motors almost exclusively. Low speed, low power brushless DC motors are used in There was an error working with the wiki: Code[115]s. High power BLDC motors are found in Electric vehicles and some industrial machinery. These motors are essentially AC synchronous motors with permanent magnet rotors.....

Model aircraft scene

Recently, an increase in the popularity of electric-powered There was an error working with the wiki: Code[43]s. BLDC motors sold as parts kits allow the buyer to save money through additional assembly work.

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 There was an error working with the wiki: Code[116], 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 There was an error working with the wiki: Code[117]) running between upper and lower stator magnets. The windings are typically stabilized by being impregnated with There was an error working with the wiki: Code[118] resins.

Because the rotor is much lighter in weight (There was an error working with the wiki: Code[44]. This is especially true if the windings use There was an error working with the wiki: Code[45] 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 There was an error working with the wiki: Code[119](s) of There was an error working with the wiki: Code[120] drives and are still widely used in high-performance servo-controlled systems.

Linear motors

A There was an error working with the wiki: Code[121] 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 There was an error working with the wiki: Code[46] (There was an error working with the wiki: Code[122]) train, where the train "flies" over the ground.

Nano motor

Researchers at There was an error working with the wiki: Code[47]. 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 There was an error working with the wiki: Code[123] to which the rotor is attached. When the outer tube is sheared, the There was an error working with the wiki: Code[124] is able to spin freely on the There was an error working with the wiki: Code[125].

The process and technology can be seen in this render.

Physicists build world's smallest motor using nanotubes and etched silicon

Research Project

There was an error working with the wiki: Code[126]

Westminster motor kits

Westminster motor kits are prefabricated kits for making educational There was an error working with the wiki: Code[48]s in school. They are available to buy from educational science suppliers and allow students to build their own motors from a couple of magnets and a meter or so of plastic coated wire.

The instructions that come with the kits are aimed at an adult readership so child friendly notes should be given to schoolchildren.

The most common causes for them not to work are:

One of the magnets being turned the wrong way round.

Insulation issues: the spindles are made of metal and have to be insulated with sticky tape.

Poor Contact with the 'brushes'

Related articles

General:

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Electric motor

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Components:

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Scientists and engineers:

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Nikola Tesla

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Applications:

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Electric vehicle

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Other:

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Electrical generator

List of electronics topics

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External articles and references

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General references

Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, New York, 1978, ISBN 0-07-020974-X.

Edwin J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric Machinery, copyright American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902

Kuphaldt, Tony R., Lessons In Electric Circuits &mdash Volume II, Chapter 13 AC MOTORS

A.O.Smith: The AC's and DC's of Electric Motors

http://www.tfcbooks.com/tesla/system.htm

"Tesla's Early Years". PBS.

U.S. Patent 0416194, "Electric Motor", December 1889.

U.S. Patent 0416194, "Electric Motor", December 1889.

http://www.electrichistory.com

http://www.redlandsweb.com

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Shanefield D. J., Industrial Electronics for Engineers, Chemists, and Technicians, William Andrew Publishing, Norwich, NY, 2001. A self-teaching textbook that briefly covers electric motors, transformers, speed controllers, wiring codes and grounding, There was an error working with the wiki: Code[149]s, digital, etc. Easy to read and understand, up to an elementary level on each subject, not a suitable reference book for technologists already working in any of those fields.

How to Properly Select a Step Motor

EDUCYPEDIA Stepper Motor Links

Stepper Motor Basics

Introduction to Stepping Motors

Jones on Stepping Motors

Control unipolar motor with bipolar driver

Selecting the Proper Size Stepper Motor

Fitzgerald/Kingsley/Kusko (Fitzgerald/Kingsley/Umans in later years), Electric Machinery, classic text for junior and senior electrical engineering students. Originally published in 1952, 6th edition published in 2002. Authors still listed as Fitzgerald/Kingsley/Umans although Fitzgerald and Kingsley are now deceased.

Bedford, B. D., Hoft, R. G., et al., "Principles of Inverter Circuits". John Wiley & Sons, Inc. New York 1964 ISBN 0471061344 (Inverter circuits are used for There was an error working with the wiki: Code[49])

B. R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters: Operation, Control, and Performance" (New York: John Wiley, 1971).

Electric Motors and Generators, explanations with animations from the University of New South Wales.

The Numbers Game: A Primer on Single-Phase A.C. Electric Motor Horsepower Ratings, Kevin S. Brady.

See also

Directory:Gemini Electric Motor

- PowerPedia

- Main Page

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