This characteristic is an important ingredient of AC drive design, which will be covered in the next topic. If we got a v ,50Hz 4 pole? Motors having class A or B insulation systems and temperature rises according to NEMA can operate satisfactorily at altitudes above feet. If either of these conditions occurs, the motor may be fixed on site or be removed for extensive repairs. Each coil in a pole group is connected to one phase of the three-phase power source.
Shunt Motor Operation
The rotor is the rotating part of the motor. The rotor consists of copper or aluminum bars, connected together at the ends with end rings. Refer to Figure Induction motor rotor construction The inside of the rotor is filled with many individual disks of steel, called laminations. The revolving field set up by the stator currents, cut the squirrel cage conducting aluminum bars of the rotor.
This causes voltage in these bars, called induced voltage. This voltage causes current to flow in the aluminum bars. The current sets up a magnetic field around the bars with corresponding north and south poles in the rotor. Torque is produced from the attraction and repulsion between these poles and the poles of the revolving stator field. The rotating stator magnetic field and induced voltage in the rotor bars also causes voltage in the stator and rotor cores.
The voltage in these cores cause small circulating currents to flow. These currents, called eddy currents, serve no useful purpose and result only in wasted power. To reduce these currents, the stator and rotor cores are constructed with laminations. These laminations are coated with insulating varnish and then welded together to form the core.
This type of core substantially reduces eddy current losses, but it does not eliminate them entirely. Engineers can design motors for almost any application by changing the design of the squirrel cage rotor and stator coils.
Characteristics such as speed, torque, and voltage are just a few of the features controlled by the designer. To standardize certain motor features, the National Electrical Manufacturers Association NEMA has established standards for a number of motor features.
The following section contains many of the features that will be helpful in selecting the right motor for a specific application. The speed of a squirrel cage motor depends on the frequency and the number of poles for which the motor is wound. The higher the frequency, the faster the motor operates.
The more poles the motor has, the slower it operates. The smallest number of poles ever used in a squirrel cage motor is two. A two-pole Hz motor will run at approximately rpm. As soon will be seen, the motor will always operate at a speed less than rpm. To find the approximate speed of any squirrel cage motor, the formula for synchronous speed can be used, which is actually the speed of the rotating magnetic field: The actual speed of the motor shaft is somewhat less than synchronous speed.
This difference between the synchronous and actual speeds is defined as slip. If the squirrel cage rotor rotated as fast as the stator field, the rotor bars would be standing still with respect to the rotating magnetic field.
No voltage would be induced in the rotor bars, and no magnetic flux would be cut by the rotor bars. The result would be no current set up to produce torque. Since no torque is produced, the rotor will slow down until sufficient current is induced to develop torque. When torque is developed, the rotor will accelerate to a constant speed.
Figure is a graphical representation of slip. Slip in an induction motor To summarize: This allows the rotor bars to cut through the stator magnetic fields and create a magnetic field in the rotor. The interaction of the stator and rotor magnetic fields produce the attraction needed to develop torque. When the load on the motor increases, the rotor speed decreases.
Then the rotating field cuts the rotor bars at a faster rate than before. This has the effect of increasing the current in the rotor bars and increasing the magnetic pole strength of the rotor. Basically, as the load increases, so does the torque output. Slip is usually expressed as a percentage and can easily be calculated using the following formula: A normal slip motor is often referred to as a constant speed motor because the speed changes very little with variations in load.
In specifying the speed of the motor on the nameplate, most motor manufacturers use the actual speed of the motor at rated load.
The term used is base speed. Base speed is a speed somewhat lower than the synchronous speed. It is defined as the actual rotor speed at rated voltage, rated hertz, and rated load. Direction of Rotation The direction of rotation of a squirrel cage induction motor depends on the motor connection to the power lines. Rotation can easily be reversed by interchanging any two input leads. Control of Torque and Horsepower As discussed earlier, horsepower takes into account the speed at which the shaft rotates.
It takes more horsepower to rotate the shaft fast, compared with rotating it slowly. Horsepower is a rate of doing work. By definition, 1 HP equals 33, ft-lb per minute. In other words, lifting a 33,pound weight 1 foot, in 1 minute would take 1 HP.
By using the familiar formula below, we can determine the horsepower developed by an AC induction motor. By inserting the known information into the formula, we calculate that the motor develops approximately 0. As the formula shows, horsepower is directly related to the speed of motor shaft.
If the shaft turns twice as fast 10 rpm , the motor will develop almost. We can see the general rules of thumb for torque developed versus speed by reviewing Table Torque developed will vary slightly on lower HP and rpm motors or non-standard motors.
As seen in Table , at higher synchronous speeds, the induction motor develops less torque compared with lower speeds. We can also see that the higher the number of poles, the larger the amount of torque developed. Basically, more poles mean stronger magnetic fields that will be produced.
With more magnetic flux interacting with rotor flux, a stronger twisting motion will result, thereby developing more torque. An induction motor is built to supply this extra torque needed to start the load. The speed torque curve for a typical induction motor is seen in Figure Occasionally a sudden overload will be placed on a motor. To keep the motor from stalling every time an overload occurs, motors have what is called a breakdown torque. The breakdown torque point is much higher than the rated load torque point.
For this reason, it takes quite an overload to stall the motor. Operating a motor overloaded for an extended period of time will cause an excessive heat buildup in the motor and may eventually burn up the motor windings. The locked rotor torque of a motor is the minimum torque, which it will develop at rest for all angular positions of the rotor. This capability is true with rated voltage and frequency applied. The pull-up torque of a motor is the minimum torque developed by the motor when accelerating from rest to the breakdown torque point.
For motors that do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed. The breakdown torque of a motor is the maximum torque that it will develop. This capability is true with rated voltage and frequency applied, without an abrupt drop in speed.
Rated load speed is normally considered base speed. Base speed means actual rotor speed when rated voltage, frequency, and load are applied to the motor. The above torque designations are all very important to the motor designer. Essentially, motors can be designed with emphasis on one or more of the above torque characteristics to produce motors for various applications.
An improvement in one of these torque characteristics may adversely affect some other motor characteristic. Motors are often exposed to damaging atmospheres such as excessive moisture, steam, salt air, abrasive or conducting dust, lint, chemical fumes, and combustible or explosive dust or gases. To protect motors, a certain enclosure or encapsulated windings and special bearing protection may be required.
Motors exposed to the following conditions may require special mountings or protection: Many types of enclosures are available. A few of the most common types are listed here, many of which are the same designations as for DC motors. The open motor enclosure type has ventilation openings that permit passage of external cooling air over and around the windings.
The open drip-proof ODP enclosure type is constructed so that drops of liquid or solids falling on the machine from a vertical direction cannot enter the machine. This could be abbreviated DPFG—drip-proof fully guarded. The guarded enclosure type has all ventilation openings limited to specified sizes and shapes.
This enclosure prevents insertion of fingers or rods and limits accidental contact with rotating or electrical parts. The splash-proof enclosure type is so constructed that drops of liquid or solid particles falling on the motor cannot enter. The totally enclosed enclosure type prevents the free exchange of air between the inside and outside of the case, but is not airtight. It is a popular motor for use in dusty, dirty, and corrosive atmospheres.
The totally enclosed blower-cooled enclosure type is totally enclosed and is equipped with an independently powered fan to blow cooling air across the external frame. A TEBC motor is commonly used in constant torque, variable-speed applications. The encapsulated enclosure has windings that are covered with a heavy coating of material to protect them from moisture, dirt, abrasion, etc. Some encapsulated motors have only the coil noses coated. In motors with pressure-embedded windings, the encapsulation material impregnates the windings, even in the coil slots.
With this complete protection, the motors can often be used in applications that demand totally enclosed motors. The explosion-proof enclosure is totally enclosed and built to withstand an explosion of gas or vapor within it. It also prevents ignition of gas or vapor surrounding the machine by sparks, flashes, or explosions that may occur within the machine casing. The typical method of starting a three-phase induction motor is by connecting the motor directly across the power line. Line starting a motor is done with a three-phase contactor.
To adequately protect the motor from prolonged overload conditions, motor overloads are installed, typically in the same enclosure as the three-phase contactor.
These overloads OLs operate as heater elements—heating to the point of opening the circuit, and mechanically disconnecting the circuit Figure Line starting a motor Overloads can be purchased with a specific time designed into the element.
Classes 10, 20, and 30 are the usual ratings for industrial use. The current draw from a typical induction motor, as well as the torque produced can be seen in Figure Line starting an induction motor, as shown in Figure , would allow the motor to develop rated torque, as soon as the motor starter button is pressed. This is because across the line, the motor has the benefit of full voltage, current, and frequency Hz.
As long as the input power is of rated value, the motor would develop the torque as seen in Figure , from zero to base speed. Motor torque and inrush current line starting If the ratio of voltage to hertz is maintained, then the motor will develop the rated torque that it was designed to produce. This characteristic is an important ingredient of AC drive design, which will be covered in the next topic. There may be applications where full torque is not desirable when the motor is started: If the feed conveyor has uncapped full bottles on the conveyor, full torque when the conveyor is started would be a not-so-good situation.
The bottles would spill all of their contents. In cases like that, a reduced torque type of start would be required. High-horsepower motors connected to compressors would be an example. In these cases, a reduced voltage start would be required. Reducing the starting current may be accomplished in any one of the following ways.
The primary resistor or reactance method uses series reactance or resistance to reduce the current during the first seconds. After a preset time interval, the motor is connected directly across the line.
This method can be used with any standard induction motor. The auto transformer method uses an auto transformer to directly reduce voltage and the current for the first few seconds. This method can also be used with any standard induction motor. The wye-delta method applies the voltage across the Y connection to reduce the current during the first few seconds.
After a preset time interval, the motor is connected in delta mode permitting full current. This type of induction motor must be constructed with wye-delta winding connections.
The part-winding method uses a motor design that has two separate winding circuits. Upon starting, only one winding circuit is engaged and current is reduced. After a preset time interval, the full winding of the motor is connected directly across the line. This type of motor must have two separate winding circuits. To avoid winding overheating and damage, the time between first and second winding connections is limited to 4 seconds maximum.
When reviewing ratings, it is also necessary to review several design features of the induction motor. Induction motor design classifications, characteristics, and ratings will now be reviewed in detail. These designs take into consideration starting current and slip, as well as torque. These motor design classes should not be confused with the various classes of wire insulation, which are also designated by letter.
Figure indicates the relative differences in torque, given a specific motor NEMA design class. The motors indicated are all line started. As seen in Figure , the major differences are in the starting torque and peak or breakdown torque capabilities.
Figure illustrates the general relationship between current, slip, efficiency, and power factor. Kilowatts kW are measured with a wattmeter, and kilovolt-amperes kVA are measured with a voltmeter and ammeter. A power factor of one 1. Power factor is highest near rated load, as seen in Figure Speed, slip, efficiency and power factor relationships.
Current draw in amperes is proportional to the actual load on the motor in the area of rated load. At other loads, current draw tends to be more nonlinear Figure Another rating specified on motor nameplates is locked rotor kVA per horsepower. Some manufacturers use the designation locked rotor amps. The nameplate codes are a good indicator of the starting current in amperes. A lower code letter indicates a low starting current and a high code letter indicates a high starting current for a specific motor horsepower rating.
Calculating the starting current can be accomplished using the following formula: Taking a number approximately halfway in-between and substituting in the formula, we get: Therefore, the starting current is approximately amperes. The starting current is important because the purchaser of the motor must know what kind of protection overload to provide. The installation must also include power lines of sufficient size to carry the required currents and properly sized fuses.
An insulation system is a group of insulating materials in association with conductors and the supporting structure of a motor. Insulation systems are divided into classes according to the thermal rating of the system. Four classes of insulation systems are used in motors: Do not confuse these insulation classes with motor designs previously discussed. Those design classes are also designated by letter. Another confusion factor is the voltage insulation system classes of the sta-tor windings.
Those classes are also designated by class B, F, and H, for example. More review of motor voltage insulation characteristics will be done in topic 4. At this point, we will review the temperature insulation classes, common in standard industrial induction motors operated across the line.
Typical materials used include cotton, paper, cellulous acetate films, enamel-coated wire, and similar organic materials impregnated with suitable substances. Typical materials include mica, glass fiber, asbestos, and other materials, not necessarily inorganic, with compatible bonding substances having suitable thermal stability. Typical materials used include mica, glass fiber, asbestos, silicone elastomer, and other materials, not necessarily inorganic, with compatible bonding substances, such as silicone resins, having suitable thermal stability.
For service conditions other than usual, the precautions listed below must be considered. The larger motor will be loaded below full capacity so the temperature rise will be less and overheating reduced. Service factor refers to rated motor power and indicates permissible power loading that may carried by the motor. For example, a 1. Altitude does not exceed feet meters. Motors having class A or B insulation systems and temperature rises according to NEMA can operate satisfactorily at altitudes above feet.
However, in locations above feet, a decrease in ambient temperature must compensate for the increase in temperature rise, as seen in Table Motors having a service factor of 1. Operation outside these limits or unbalanced voltage conditions can result in overheating or loss of torque and may require using a larger-horsepower motor.
Operation outside of these limits results in substantial speed variation and causes overheating and reduced torque.
Mounting Surface and Location. The mounting surface must be rigid and in accordance with NEMA specifications. Location of supplementary enclosures must not seriously interfere with the ventilation of the motor.
AC motors can be divided into two major categories—asynchronous and synchronous. The induction motor is probably the most common type of asynchronous motor meaning speed is dependent on slip. Another type of asynchronous motor is the wound rotor motor.
This type of motor has controllable speed and torque because of the addition of a secondary resistance in the rotor circuit. A third type of popular asynchronous motor is the single-phase motor. The single-phase AC motor will not be covered because of their limited use in industrial applications when connected with variable-frequency drives.
The synchronous motor is inherently a constant-speed motor, when operated directly across the line. This type of motor operates in synchronism with the line frequency. Two types of synchronous motors are non-excited and DC-excited. The basic principles of AC induction motors have been previously covered. In this section, attention will be given to motor designations, ratings, and designs.
NEMA frame motors are in widespread use throughout U. This motor design was developed before the s and has well served many types of fixed-speed applications. In and , NEMA evaluated standard frame sizes and re-rated the frame standards.
The result was smaller diameter motor frames e. As the re-rating took place, the frame sizes numbers were reduced, as was the amount of iron in the stator. However, with smaller-diameter frames comes more efficiency and faster response to changes in magnetic flux. Figure indicates the construction of a standard AC induction motor. All the major motor components are identified. It should be noted that all standard motors include a small rectangular slot, cut lengthwise in the shaft, called a keyway or keyseat.
This slot includes a tapered-cut rectangular piece of steel, call a key. The key is inserted into the keyway and pressure-fit snugly to mechanically connect the shaft and coupler or connection device, such as a pulley or gear.
As seen in Figure , the induction motor is a fairly simple device. However, precision engineering is required to create small tolerances and air gaps that will allow maximum efficiency and torque generation. A brief description of each classification will be presented here, followed by a comparison to IEC frame motors. This type of motor is designed for general-purpose use and accounts for the largest share of induction motors sold.
This type of motor has a high starting torque, with a relatively normal starting current and low slip. The type of load applied to a design C is one where breakaway loads are high upon start. The loads, however, would be normally run at the rated point, with very little demand for overload. This type of motor has a high starting torque, high slip, but also low full load speed. This type of motor is known for high efficiency and is used mainly where the starting torque requirements are low.
Fans and centrifugal pumps make up the bulk of applications using this type of motor. Figure indicates the NEMA designs and compares design with rated starting current and speed.
This fact must be reviewed when sizing the proper overload heater elements. For this reason, the air gap should be minimal.
Very small gaps may pose mechanical problems in addition to noise and losses. Windings are wires that are laid in coils , usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current.
Electric machines come in two basic magnet field pole configurations: In the salient-pole machine the pole's magnetic field is produced by a winding wound around the pole below the pole face. In the nonsalient-pole , or distributed field, or round-rotor, machine, the winding is distributed in pole face slots.
Some motors have conductors that consist of thicker metal, such as bars or sheets of metal, usually copper , alternatively aluminum. These are usually powered by electromagnetic induction. A commutator is a mechanism used to switch the input of most DC machines and certain AC machines. It consists of slip-ring segments insulated from each other and from the shaft.
The motor's armature current is supplied through stationary brushes in contact with the revolving commutator, which causes required current reversal, and applies power to the machine in an optimal manner as the rotor rotates from pole to pole. In light of improved technologies in the electronic-controller, sensorless-control, induction-motor, and permanent-magnet-motor fields, externally-commutated induction and permanent-magnet motors are displacing electromechanically-commutated motors.
A DC motor is usually supplied through slip ring commutator as described above. AC motors' commutation can be either slip ring commutator or externally commutated type, can be fixed-speed or variable-speed control type, and can be synchronous or asynchronous type. Universal motors can run on either AC or DC. Variable-speed controlled AC motors are provided with a range of different power inverter , variable-frequency drive or electronic commutator technologies. The term electronic commutator is usually associated with self-commutated brushless DC motor and switched reluctance motor applications.
Electric motors operate on three different physical principles: By far, the most common is magnetism. In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft. One, or both, of these fields must be made to change with the rotation of the motor. This is done by switching the poles on and off at the right time, or varying the strength of the pole. The main types are DC motors and AC motors,  the former increasingly being displaced by the latter.
AC electric motors are either asynchronous or synchronous. Once started, a synchronous motor requires synchronism with the moving magnetic field's synchronous speed for all normal torque conditions. In synchronous machines, the magnetic field must be provided by means other than induction such as from separately excited windings or permanent magnets. A fractional-horsepower FHP motor either has a rating below about 1 horsepower 0.
Many household and industrial motors are in the fractional-horsepower class. By definition, all self-commutated DC motors run on DC electric power. Most DC motors are small permanent magnet PM types. They contain a brushed internal mechanical commutation to reverse motor windings' current in synchronism with rotation.
A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator, a long-lasting rotary electrical switch that periodically reverses the flow of current in the rotor windings as the shaft rotates. Thus, every brushed DC motor has AC flowing through its rotating windings. Current flows through one or more pairs of brushes that bear on the commutator; the brushes connect an external source of electric power to the rotating armature.
The rotating armature consists of one or more coils of wire wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet an electromagnet. The magnetic field produced by the armature interacts with a stationary magnetic field produced by either PMs or another winding a field coil , as part of the motor frame.
The force between the two magnetic fields tends to rotate the motor shaft. The commutator switches power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops like a compass needle does , but rather keeps rotating as long as power is applied.
Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. Sparks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections — and hence coil ends — momentarily while crossing the gaps.
Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity , limits the output of the motor.
The making and breaking of electric contact also generates electrical noise ; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance on larger motors or replacement on small motors.
The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor. While most commutators are cylindrical, some are flat discs consisting of several segments typically, at least three mounted on an insulator.
Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking.
Small brushes are also desirable for lower cost. Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses lower efficiency and accelerated brush and commutator wear. DC machines are defined as follows: A PM permanent magnet motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque.
Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control. PM fields stators are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings.
Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.
To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy.
With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly-fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles to ensure starting, regardless of rotor position and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.
In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. Efficiency for a BLDC motor of up to The BLDC motor's characteristic trapezoidal counter-electromotive force CEMF waveform is derived partly from the stator windings being evenly distributed, and partly from the placement of the rotor's permanent magnets.
Also known as electronically commutated DC or inside out DC motors, the stator windings of trapezoidal BLDC motors can be with single-phase, two-phase or three-phase and use Hall effect sensors mounted on their windings for rotor position sensing and low cost closed-loop control of the electronic commutator. They have several advantages over conventional motors:. Modern BLDC motors range in power from a fraction of a watt to many kilowatts. They also find significant use in high-performance electric model aircraft.
The SRM has no brushes or permanent magnets, and the rotor has no electric currents. Instead, torque comes from a slight misalignment of poles on the rotor with poles on the stator. The rotor aligns itself with the magnetic field of the stator, while the stator field windings are sequentially energized to rotate the stator field. The magnetic flux created by the field windings follows the path of least magnetic reluctance, meaning the flux will flow through poles of the rotor that are closest to the energized poles of the stator, thereby magnetizing those poles of the rotor and creating torque.
As the rotor turns, different windings will be energized, keeping the rotor turning. SRMs are used in some appliances  and vehicles. A commutated electrically excited series or parallel wound motor is referred to as a universal motor because it can be designed to operate on AC or DC power. A universal motor can operate well on AC because the current in both the field and the armature coils and hence the resultant magnetic fields will alternate reverse polarity in synchronism, and hence the resulting mechanical force will occur in a constant direction of rotation.
Operating at normal power line frequencies , universal motors are often found in a range less than watts. Universal motors also formed the basis of the traditional railway traction motor in electric railways. In this application, the use of AC to power a motor originally designed to run on DC would lead to efficiency losses due to eddy current heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid un-laminated iron and they are now rarely used.
An advantage of the universal motor is that AC supplies may be used on motors that have some characteristics more common in 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 commutator. Such motors are used in devices, such as food mixers and power tools, that are used only intermittently, and often have high starting-torque demands. Multiple taps on the field coil provide imprecise stepped speed control.
Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor causing the motor to run on half-wave rectified AC. Universal motors also lend themselves to electronic speed control and, as such, are an ideal choice for devices like domestic washing machines.
The motor can be used to agitate the drum both forwards and in reverse by switching the field winding with respect to the armature. Whereas SCIMs cannot turn a shaft faster than allowed by the power line frequency, universal motors can run at much higher speeds. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable.
They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed 10, rpm , while many similar miniature grinders exceed 30, rpm. The design of AC induction and synchronous motors is optimized for operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power such as supplied for fixed-speed application from the AC power grid or for variable-speed application from VFD controllers.
An AC motor has two parts: An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action. An induction motor resembles a rotating transformer, because the stator stationary part is essentially the primary side of the transformer and the rotor rotating part is the secondary side. Polyphase induction motors are widely used in industry.
SCIMs have a heavy winding made up of solid bars, usually aluminum or copper, joined by rings at the ends of the rotor. When one considers only the bars and rings as a whole, they are much like an animal's rotating exercise cage, hence the name.
Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars determines the speed-torque characteristics. At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to be in the outer parts of the rotor cage. As the motor accelerates, the slip frequency becomes lower, and more current is in the interior of the winding. By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit.
However, the majority of such motors have uniform bars. In a WRIM, the rotor winding is made of many turns of insulated wire and is connected to slip rings on the motor shaft. An external resistor or other control devices can be connected in the rotor circuit. Resistors allow control of the motor speed, although significant power is dissipated in the external resistance. A converter can be fed from the rotor circuit and return the slip-frequency power that would otherwise be wasted back into the power system through an inverter or separate motor-generator.
The WRIM is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range.
By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero speed to full speed. This type of motor also offers controllable speed.
Motor speed can be changed because the torque curve of the motor is effectively modified by the amount of resistance connected to the rotor circuit. Increasing the value of resistance will move the speed of maximum torque down.
If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced. When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal.
Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant.
The speed regulation and net efficiency is also very poor. A torque motor is a specialized form of electric motor that can operate indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, 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 gaming world, torque motors are used in force feedback steering wheels. Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor.
In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small adjustments to the amount of current applied to the motor.
If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being applied to the motor, causing the return spring to pull back and close the throttle.
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the AC and resulting in a magnetic field that drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip to produce torque. One type of synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to the rotor.
The rotor poles connect to each other and move at the same speed hence the name synchronous motor. Another type, for low load torque, has flats ground onto a conventional squirrel-cage rotor to create discrete poles. Yet another, such as made by Hammond for its pre-World War II clocks, and in the older Hammond organs, has no rotor windings and discrete poles. It is not self-starting. The clock requires manual starting by a small knob on the back, while the older Hammond organs had an auxiliary starting motor connected by a spring-loaded manually operated switch.
Finally, hysteresis synchronous motors typically are essentially two-phase motors with a phase-shifting capacitor for one phase. They start like induction motors, but when slip rate decreases sufficiently, the rotor a smooth cylinder becomes temporarily magnetized. Its distributed poles make it act like a permanent magnet synchronous motor PMSM. The rotor material, like that of a common nail, will stay magnetized, but can also be demagnetized with little difficulty.
Once running, the rotor poles stay in place; they do not drift. Low-power synchronous timing motors such as those for traditional electric clocks may have multi-pole permanent magnet external cup rotors, and use shading coils to provide starting torque. Telechron clock motors have shaded poles for starting torque, and a two-spoke ring rotor that performs like a discrete two-pole rotor.
Doubly fed electric motors have two independent multiphase winding sets, which contribute active i. Two independent multiphase winding sets i. Doubly-fed electric motors are 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 have only one active winding set. A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings may offset the saving in the power electronics components.
Difficulties with controlling speed near synchronous speed limit applications. Nothing in the principle of any of the motors described above requires that the iron steel portions of the rotor actually rotate. If the soft magnetic material of the rotor is made in the form of a cylinder, then except for the effect of hysteresis torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless or ironless DC motor , a specialized form of a permanent magnet DC motor.
The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only the magnet wire and the bonding material. The rotor can fit inside the stator magnets; a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux.
A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the housing for the motor, and likewise provides a return path for the flux.
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 one 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.
Overheating might be an issue for coreless DC motor designs. Modern software, such as Motor-CAD , can help to increase the thermal efficiency of motors while still in the design stage.
The vibrating alert of cellular phones is sometimes generated by tiny cylindrical permanent-magnet field types, but there are also disc-shaped types that have a thin multipolar disc field magnet, and an intentionally unbalanced molded-plastic rotor structure with two bonded coreless coils.
Metal brushes and a flat commutator switch power to the rotor coils. Related limited-travel actuators have no core and a bonded coil placed between the poles of high-flux thin permanent magnets. These are the fast head positioners for rigid-disk "hard disk" drives. Although the contemporary design differs considerably from that of loudspeakers, it is still loosely and incorrectly referred to as a "voice coil" structure, because some earlier rigid-disk-drive heads moved in straight lines, and had a drive structure much like that of a loudspeaker.
The printed armature or pancake motor has the windings shaped as a disc running between arrays of high-flux magnets. The magnets are arranged in a circle facing the rotor with space in between to form an axial air gap.
The technology has had many brand names since its inception, such as ServoDisc. The printed armature originally formed on a printed circuit board in a printed armature motor is made from punched copper sheets that are laminated together using advanced composites to form a thin rigid disc. The printed armature has a unique construction in the brushed motor world in that it does not have a separate ring commutator.
The brushes run directly on the armature surface making the whole design very compact. An alternative manufacturing method is to use wound copper wire laid flat with a central conventional commutator, in a flower and petal shape.
The windings are typically stabilized with electrical epoxy potting systems. These are filled epoxies that have moderate, mixed viscosity and a long gel time. The unique advantage of ironless DC motors is the absence of cogging torque variations caused by changing attraction between the iron and the magnets.
Parasitic eddy currents cannot form in the rotor as it is totally ironless, although iron rotors are laminated. These motors were originally invented to drive the capstan s of magnetic tape drives, where minimal time to reach operating speed and minimal stopping distance were critical.
Pancake motors are widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices. Due to the variety of constructions now available, the technology is used in applications from high temperature military to low cost pump and basic servos.
Another approach Magnax is to use a single stator sandwiched between two rotors. This yokeless axial flux motor offers a shorter flux path, keeping the magnets further from the axis. The design allows has zero winding overhang; percent of the windings are active. This is enhanced with the use of rectangular-section copper wire. The motors can be stacked to work in parallel. Instabilities are minimized by ensuring that the two rotor discs put equal and opposing forces onto the stator disc.
The rotors are connected directly to one another via a shaft ring, cancelling out the magnetic forces. Magnax motors range in size from.
A servomotor is a motor, very often sold as a complete module, which is used within a position-control or speed-control feedback control system. Servomotors are used in applications such as machine tools, pen plotters, and other process systems. Motors intended for use in a servomechanism must have well-documented characteristics for speed, torque, and power. Dynamic response characteristics such as winding inductance and rotor inertia are also important; these factors limit the overall performance of the servomechanism loop.
Large, powerful, but slow-responding servo loops may use conventional AC or DC motors and drive systems with position or speed feedback on the motor. As dynamic response requirements increase, more specialized motor designs such as coreless motors are used. AC motors' superior power density and acceleration characteristics compared to that of DC motors tends to favor permanent magnet synchronous, BLDC, induction, and SRM drive applications. A servo system differs from some stepper motor applications in that the position feedback is continuous while the motor is running.
A stepper system inherently operates open-loop - relying on the motor not to "miss steps" for short term accuracy - with any feedback such as a "home" switch or position encoder being external to the motor system. As long as power is on, a bidirectional counter in the printer's microprocessor keeps track of print-head position. Stepper motors are a type of motor frequently used when precise rotations are required. In a stepper motor an internal rotor containing permanent magnets or a magnetically soft rotor 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 rotary 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 stepper motor may not rotate continuously; instead, it "steps"—starts and then quickly stops again—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, and it may change direction, stop, speed up or slow down arbitrarily at any time.
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.
This mode of operation is often called microstepping. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system. As drive density increased, the precision and speed limitations of stepper motors made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems.
The term "voice coil" in this connection is historic; it refers to the structure in a typical cone type loudspeaker. This structure was used for a while to position the heads. Modern drives have a pivoted coil mount; the coil swings back and forth, something like a blade of a rotating fan. Nevertheless, like a voice coil, modern actuator coil conductors the magnet wire move perpendicular to the magnetic lines of force. Stepper motors were and still are often used in computer printers, optical scanners, and digital photocopiers to move the optical scanning element, the print head carriage of dot matrix and inkjet printers , and the platen or feed rollers.
Likewise, many computer plotters which since the early s have been replaced with large-format inkjet and laser printers used rotary stepper motors for pen and platen movement; the typical alternatives here were either linear stepper motors or servomotors with closed-loop analog control systems.
So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one coil, draw very little power, and have a permanent magnet rotor. The same kind of motor drives battery-powered quartz clocks. Some of these watches, such as chronographs, contain more than one stepping motor. Closely related in design to three-phase AC synchronous motors, stepper motors and SRMs are classified as variable reluctance motor type.
A linear motor is essentially any electric motor that has been "unrolled" so that, instead of producing a torque rotation , it produces a straight-line force along its length. Linear motors are most commonly induction motors or stepper motors.
Linear motors are commonly found in many roller-coasters where the rapid motion of the motorless railcar is controlled by the rail. They are also used in maglev trains , where the train "flies" over the ground. On a smaller scale, the era HP A pen plotter used two linear stepper motors to move the pen along the X and Y axes.
The fundamental purpose of the vast majority of the world's electric motors is to electromagnetically induce relative movement in an air gap between a stator and rotor to produce useful torque or linear force.
According to Lorentz force law the force of a winding conductor can be given simply by:. The most general approaches to calculating the forces in motors use tensors. Where rpm is shaft speed and T is torque , a motor's mechanical power output P em is given by, . For a linear motor, with force F expressed in newtons and velocity v expressed in meters per second,.
In an asynchronous or induction motor, the relationship between motor speed and air gap power is, neglecting skin effect , given by the following:. Since the armature windings of a direct-current or universal motor are moving through a magnetic field, they have a voltage induced in them. This voltage tends to oppose the motor supply voltage and so is called " back electromotive force emf ". The voltage is proportional to the running speed of the motor. The back emf of the motor, plus the voltage drop across the winding internal resistance and brushes, must equal the voltage at the brushes.
This provides the fundamental mechanism of speed regulation in a DC motor. If the mechanical load increases, the motor slows down; a lower back emf results, and more current is drawn from the supply.
This increased current provides the additional torque to balance the new load. In AC machines, it is sometimes useful to consider a back emf source within the machine; as an example, this is of particular concern for close speed regulation of induction motors on VFDs. Motor losses are mainly due to resistive losses in windings, core losses and mechanical losses in bearings, and aerodynamic losses, particularly where cooling fans are present, also occur. Losses also occur in commutation, mechanical commutators spark, and electronic commutators and also dissipate heat.
To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:. It is possible to derive analytically the point of maximum efficiency. Various regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher-efficiency electric motors.
Eric Laithwaite  proposed a metric to determine the 'goodness' of an electric motor: From this, he showed that the most efficient motors are likely to have relatively large magnetic poles. However, the equation only directly relates to non PM motors. All the electromagnetic motors, and that includes the types mentioned here derive the torque from the vector product of the interacting fields.
For calculating the torque it is necessary to know the fields in the air gap. Once these have been established by mathematical analysis using FEA or other tools the torque may be calculated as the integral of all the vectors of force multiplied by the radius of each vector.