report on modern AC motors design and uses.

assignment 1.

Ac machine design.
AC Motor - Basics of AC Motor Design Engineering
A synchronous and synchronous electric motors are the two main categories of ac motors. The induction ac motor is a common form of asynchronous motor and is basically an ac transformer with a rotating secondary. The primary winding (stator) is connected to the power source and the shorted secondary (rotor) carries the induced secondary current. Torque is produced by the action of the rotor (secondary) currents on the air-gap flux. The synchronous motor differs greatly in design and operational characteristics, and is considered a separate class of ac motor.

Induction AC Motors: Induction ac motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction ac motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.
AC motors typically feature rotors, which consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This ac motor "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. The speed at which the ac motor magnetic field rotates is the synchronous speed of the ac motor and is determined by the number of poles in the stator and the frequency of the power supply: ns = 120f/p, where ns = synchronous speed, f = frequency, and p = the number of poles.
Synchronous speed is the absolute upper limit of ac motor speed. If the ac motor's rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. When ac motors are running, the rotor always rotates slower than the magnetic field. The ac motor's rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the ac motor's rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed, and na = actual speed.

TYPES OF AC MOTOR DESIGN
Polyphase AC Motors: Polyphase squirrel-cage ac motors are basically constant-speed machines, but some degree of flexibility in operating characteristics results from modifying the rotor slot design. These variations in ac motors produce changes in torque, current, and full-load speed. Evolution and standardization have resulted in four fundamental types of ac motors.
AC Motors - Designs A and B: General-purpose ac motors with normal starting torques and currents, and low slip. Fractional-horsepower polyphase ac motors are generally design B. Because of the drooping characteristics of design B, a polyphase ac motor that produces the same breakdown (maximum) torque as a single-phase ac motor cannot attain the same speed-torque point for full-load speed as single-phase ac motors. Therefore, breakdown torque must be higher (a minimum of 140% of the breakdown torque of single-phase, general-purpose ac motors) so that full-load speeds are comparable.
AC Motors - Design C: High starting torque with normal starting current and low slip. AC motors are normally used where breakaway loads are high at starting, but which normally run at rated full load and are not subject to high overload demands after running speed has been reached.
AC Motors - Design D: High slip, ac motor starting torque, low starting current, and low full-load speed. Because of the high slip, speed can drop when fluctuating loads are encountered. This ac motor design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve.
AC Motors - Design F: Low starting torque, low starting current, and low slip. These AC motors are built to obtain low locked-rotor current. Both locked-rotor and breakdown torque are low. Normally these ac motors are used where starting torque is low and where high overloads are not imposed after running speed is reached.

Wound-rotor AC Motors: Squirrel-cage ac motors are relatively inflexible with regard to speed and torque characteristics, but a special wound-rotor ac motor has controllable speed and torque. Application of wound-rotor ac motors is markedly different from squirrel-cage ac motors because of the accessibility of the rotor circuit. AC motor performance characteristics are obtained by inserting different values of resistance in the rotor circuit.
Wound-rotor ac motors are generally started with secondary resistance in the rotor circuit. The ac motor resistance is sequentially reduced to permit the motor to come up to speed. Thus, ac motors can develop substantial torque while limiting locked-rotor current. This secondary ac motor resistance can be designed for continuous service to dissipate heat produced by continuous operation at reduced speed, frequent acceleration, or acceleration with a large inertia load. External resistance gives ac motors a characteristic that results in a large drop in rpm for a fairly small change in load. Reduced ac motor speed is provided down to about 50% rated speed, but efficiency is low.
Multispeed AC Motors: Consequent-pole ac motors are designed for one speed. By physically reconnecting the leads, a 2:1 speed ratio can be obtained. Typical synchronous speeds for 60-Hz ac motors are: 3,600/1,800 rpm (2/4 pole), 1,800/900 rpm (4/8 pole), and 1,200/600 rpm (6/12 pole).
Two-winding ac motors have two separate windings that can be wound for any number of poles so that other speed ratios can be obtained. However, ratios greater than 4:1 are impractical because of ac motor size and weight. Single-phase multispeed ac motors are usually variable-torque design, but constant-torque and constant-horsepower ac motors are available.
Power output of multispeed ac motors can be proportioned to each different speed. These ac motors are designed with output horsepower capacity in accordance with one of the following load characteristics.
AC Motors - Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.
AC Motors - Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.
AC Motors - Constant horsepower: These ac motors develop the same horsepower at each speed and the torque is inversely proportional to the speed. Typical applications for ac motors include machine tools such as drills, lathes, and milling machines.
AC Motors - Single-phase AC Motors: Single-phase induction ac electric motors are commonly fractional-horsepower types, although single-phase integral-horsepower are available in the lower horsepower range. The most common fractional-horsepower single-phase ac motors are split-phase, capacitor-start, permanent split-capacitor, and shaded pole.
The ac motors come in multispeed types, but there is a practical limit to the number of speeds obtained. Two, three, and four-speed motors are available, and speed selection may be accomplished by consequent-pole or two-winding methods.
Single-phase ac electric motors run in the direction in which they are started; and they are started in a predetermined direction according to the electrical connections or mechanical setting of the starting means. General-purpose ac motors may be operated in either direction, but the standard ac motor rotation is counterclockwise when facing the end opposite the drive shaft. AC motors can be reconnected to reverse the direction of rotation.


Universal AC Motors: Universal ac motors operate with nearly equivalent performance on direct current or alternating current up to 60 Hz. AC motors differ from a dc motors due to the winding ratios and thinner iron laminations. DC motors runs on ac, but with poor efficiency. Universal ac motors can operate on dc with essentially equivalent ac motor performance, but with poorer commutation and brush life than for an equivalent dc motor.
An important characteristic of universal ac motors is that it has the highest horsepower-per-pound ratio of any ac motor because it can operate at speeds many times higher than that of any other 60-Hz electric motor.
When operated without load, universal ac motors tend to run away, speed being limited only by windage, friction, and commutation. Therefore, large universal ac motors are nearly always connected directly to a load to limit speed. On portable tools such as electric saws, the load imposed by the gears, bearings, and cooling fan is sufficient to hold the no-load speed down to a safe value.
With a universal ac motor, speed control is simple, since electric motor speed is sensitive to both voltage and flux changes. With a rheostat or adjustable autotransformer, ac motor speed can be readily varied from top speed to zero.
Synchronous AC Motors: Synchronous ac motors are inherently constant-speed electric motors and they operate in absolute synchronism with line frequency. As with squirrel-cage induction ac motors, speed is determined by the number of pairs of poles and is always a ratio of the line frequency.
Synchronous ac motors are made in sizes ranging from subfractional self-excited units to large-horsepower, direct-current-excited ac motors for industrial drives. In the fractional-horsepower range, synchronous ac motors are used primarily where precise constant speed is required.
In large horsepower sizes applied to industrial loads, synchronous ac motors serve two important functions. First, ac motors provide highly efficient means of converting ac energy to mechanical power. Second, ac motors can operate at leading or unity power factor, thereby providing power-factor correction.
There are two major types of synchronous ac motors: nonexcited and direct-current excited electric motors.
Nonexcited Electric Motors are made in reluctance and hysteresis designs. These electric motors employ a self-starting circuit and require no external excitation supply.
Dc-excited Electric Motors come in sizes larger than 1 hp, and require direct current supplied through slip rings for excitation. Direct current may be supplied from a separate source or from a dc generator directly connected to the ac motor shaft.
Single-phase or polyphase synchronous electric motors can't start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the electric motor field is rotating at a synchronous speed, the electric motor must be accelerated before it can pull into synchronism. Accelerating from zero speed requires slip until synchronism is reached. Therefore, separate starting means must be employed.
In self-starting electric motor designs, fhp sizes use starting methods common to induction electric motors (split-phase, capacitor-start, repulsion-start, and shaded-pole). The electrical characteristics of these electric motors cause them to automatically switch to synchronous operation.
Although the dc-excited electric motor has a squirrel cage for starting, called an amortisseur or damper winding, the inherent low starting torque and the need for a dc power source requires a starting system that provides full electric motor protection while starting, applies dc field excitation at the proper time, removes field excitation at rotor pull out (maximum torque), and protects the electric motor's squirrel-cage winding against thermal damage under out-of-step conditions.
The electric motor's pull-up torque is the minimum torque developed from standstill to the pull-in point. This torque must exceed load torque by a sufficient margin so that a satisfactory rate of acceleration is maintained under normal voltage conditions.


Dc-excited AC Electric Motors (cont.) The electric motor's reluctance torque results from the saliency (preferred direction of magnetization) of the rotor pole pieces and pulsates at speeds below synchronous. It also has an influence on electric motor pull-in and pull-out torques because the unexcited salient-pole rotor tends to align itself with the stator electric motor magnetic field to maintain minimum magnetic reluctance. The electric motor's reluctance torque may be sufficient to pull into synchronism a lightly loaded, low-inertia system and to develop approximately a 30% pull-out torque.
The electric motor's synchronous torque is torque developed after excitation is applied, and represents the total steady-state torque available to drive the load. It reaches maximum at approximately 70° lag of the rotor behind the rotating stator magnetic field. This maximum value is actually the pull-out torque.
Pull-out torque is the maximum sustained torque the electric motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor electric motors, and 175 to 200% for 0.8-leading-power-factor electric motors.
Pull-in torque of a synchronous electric motor is the torque that it develops when pulling its connected inertia load into synchronism upon application of excitation. Pull-in torque is developed during transition from slip speed to synchronous speed, as electric motors change from induction to synchronous operation. It is usually the most critical period in starting a synchronous electric motor. Torques developed by the amortisseur and field windings become zero at synchronous speed. At the pull-in point, therefore, only the reluctance torque and the synchronizing torque provided by exciting the field windings are effective.
Timing Electric Motors: Timing electric motors are rated under 1/10 hp and are used as prime movers for timing devices. Since the electric motor is being used as a timer, it must run at a constant speed.
Timing Electric Motors (cont.) Ac and dc electric motors can be used as timing motors. Dc electric timing motors are used for portable applications, or where high acceleration and low speed variations are required. These electric motors offer advantages, which include starting torque as high as ten times running torque, efficiency from 50 to 70%, and relatively easy speed control. But some form of speed governor, either mechanical or electronic, is required.
Ac motors use readily available power, are lower in cost, have improved life, and do not generate RFI. However, ac motors cannot be readily adapted to portable applications, have relatively low starting torques, and are much less efficient than dc motors.
AC Servo Motors: Ac servo motors are used in ac servomechanisms and computers which require rapid and accurate response characteristics. To obtain these characteristics, servo motors have small-diameter high-resistance rotors. The small diameter provides low inertia for fast starts, stops, and reversals, while the high resistance provides a nearly linear speed-torque relationship for accurate control.
Servo motors are wound with two phases physically at right angles or in space quadrature. Servo motors feature a fixed or reference winding is excited from a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servoamplifier. The servo motor windings are usually designed with the same voltage-turns ratio, so that power inputs at maximum fixed-phase excitation and at maximum control-phase signal are in balance.
In an ideal servo motor, torque at any speed is directly proportional to the servo motor's control-winding voltage. In practice, however, this relationship exists only at zero speed because of the inherent inability of an induction servo motor to respond to voltage input changes under conditions of light load.
The inherent damping of servo motors decreases as ratings increase, and the servo motors have a reasonable efficiency at the sacrifice of speed-torque linearity. Most larger servo motors have integral auxiliary blowers to maintain temperatures within safe operating ranges. Servo motors are available in power ratings from less than 1 to 750 W, in sizes ranging from 0.5 to 7-in. OD. Most servo motors are available with modular or built-in gearheads.

reference

http://www.motor-design.com

report on 3 phase motors and its uses.

ASSINGMENT 2.

3 phase motors and its uses

INTRODUCTION.
An AC motor is an electric motor that is driven by an alternating current. It consists of two basic 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 types of AC motors, depending on the type of rotor used. The first is the synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet.
The second type is the induction motor, which turns slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.

3 Phase Motors
3 Phase Electric Motors Are More Efficient
The most common type of 3 phase electrical load is the 3 phase electric motor. A 3 phase motor is more compact and less costly than a 1-phase motor of the same voltage class and rating; also 1-phase AC motors above 10 HP (7.5 kW) are not as efficient and thus not usually manufactured. A 3 phase induction motor has a simple design, inherently high starting torque, and high efficiency. Such motors are applied in industry for 3 phase pumps, fans, blowers, compressors, conveyor drives, and many other types of 3 phase motor-driven equipment. There are a lot of benefits to using a 3 phase electric motor over a single phase electric motor. Large air conditioning equipment (for example, most air conditioning units above 2.5 tons (8.8 kW) cooling capacity) use 3 phase motors for reasons of economy and efficiency. Read more about other 3 phase power loads. Read more about 3 phase motors below.
Most electric power is distributed in the form of 3-phase AC. Therefore, before proceeding any further you should understand what is meant by 3 phase power. Basically, the power company generators produce electricity by rotating (3) coils or windings through a magnetic field within the generator . These coils or windings are spaced 120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three (3) lines as in three-phase power. 3 phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage we need and maintain the proper phasing or polarity.

3 Phase Power Is More Efficient Than Single Phase
Three phase electricity powers large industrial loads more efficiently than single-phase electricity. When single-phase electricity is needed, It is available between any two phases of a three-phase system, or in some systems , between one of the phases and ground. By the use of three conductors a 3 phase system can provide 173% more power than the two conductors of a single-phase system. Three-phase power allows heavy duty industrial equipment to operate more smoothly and efficiently. 3 phase power can be transmitted over long distances with smaller conductor size.

reference

http://www.engineersedge.com/motors/generators_types.htm

report on DC machines design and its application.

ASSINGMENT 3

DC MOTORS DESIGN


DC Motors

INTROCUCTION.
Industrial applications use dc motors because the speed-torque relationship can be varied to almost any useful form -- for both dc motor and regeneration applications in either direction of rotation. Continuous operation of dc motors is commonly available over a speed range of 8:1. Infinite range (smooth control down to zero speed) for short durations or reduced load is also common.
Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting).
Dynamic braking (dc motor-generated energy is fed to a resistor grid) or regenerative braking (dc motor-generated energy is fed back into the dc motor supply) can be obtained with dc motors on applications requiring quick stops, thus eliminating the need for, or reducing the size of, a mechanical brake.
Dc motors feature a speed, which can be controlled smoothly down to zero, immediately followed by acceleration in the opposite direction -- without power circuit switching. And dc motors respond quickly to changes in control signals due to the dc motor's high ratio of torque to inertia.
DC Motor types: Wound-field dc motors are usually classified by shunt-wound, series-wound, and compound-wound. In addition to these, permanent-magnet and brushless dc motors are also available, normally as fractional-horsepower dc motors. Dc motors may be further classified for intermittent or continuous duty. Continuous-duty dc motors can run without an off period.
DC Motors - Speed control: There are two ways to adjust the speed of a wound-field dc motor. Combinations of the two are sometimes used to adjust the speed of a dc motor.

DC Motor - Shunt-field control: Reel drives require this kind of control. The dc motor's material is wound on a reel at constant linear speed and constant strip tension, regardless of diameter.Control is obtained by weakening the shunt-field current of the dc motor to increase speed and to reduce output torque for a given armature current. Since the rating of a dc motor is determined by heating, the maximum permissible armature current is approximately constant over the speed range. This means that at rated current, the dc motor's output torque varies inversely with speed, and the dc motor has constant-horsepower capability over its speed range.
Dc motors offer a solution, which is good for only obtaining speeds greater than the base speed. A momentary speed reduction below the dc motor's base speed can be obtained by overexciting the field, but prolonged overexcitation overheats the dc motor. Also, magnetic saturation in the dc motor permits only a small reduction in speed for a substantial increase in field voltage.
Dc motors have a maximum standard speed range by field control is 3:1, and this occurs only at low base speeds. Special dc motors have greater speed ranges, but if the dc motor's speed range is much greater than 3:1, some other control method is used for at least part of the range.
Armature-voltage DC Motor Control: In this method, shunt-field current is maintained constant from a separate source while the voltage applied to the armature is varied. Dc motors feature a speed, which is proportional to the counter emf. This is equal to the applied voltage minus the armature circuit IR drop. At rated current, the torque remains constant regardless of the dc motor speed (since the magnetic flux is constant) and, therefore, the dc motor has constant torque capability over its speed range.

Armature-voltage DC Motor Control (cont.): Horsepower varies directly with speed. Actually, as the speed of a self-ventilated motor is lowered, it loses ventilation and cannot be loaded with quite as much armature current without exceeding the rated temperature rise.
DC Motors - Selection: Choosing a dc motor and associated equipment for a given application requires consideration of several factors.
DC Motors - Speed range: If field control is to be used, and a large speed range is required, the base speed must be proportionately lower and the motor size must be larger. If speed range is much over 3:1, armature voltage control should be considered for at least part of the range. Very wide dynamic speed range can be obtained with armature voltage control. However, below about 60% of base speed, the motor should be derated or used for only short periods.
DC Motors - Speed variation with torque: Applications requiring constant speed at all torque demands should use a shunt-wound dc motor. If speed change with load must be minimized, a dc motor regulator, such as one employing feedback from a tachometer, must be used.
When the dc motor speed must decrease as the load increases, compound or series-wound dc motors may be used. Or, a dc motor power supply with a drooping volt-ampere curve could be used with a shunt-wound dc motor.
DC Motors - Reversing: This operation affects power supply and control, and may affect the dc motor's brush adjustment, if the dc motor cannot be stopped for switching before reverse operation. In this case, compound and stabilizing dc motor windings should not be used, and a suitable armature-voltage control system should supply power to the dc motor.

DC Motors - Duty cycle: Direct current motors are seldom used on drives that run continuously at one speed and load. Motor size needed may be determined by either the peak torque requirement or heating.
DC Motors - Peak torque: The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Dc motor brush and commutator damage depends on sparking severity and duration. Therefore, the dc motor's peak torque depends on the duration and frequency of occurrence of the overload. Dc motor peak torque is often limited by the maximum current that the power supply can deliver.
Dc motors can commutate greater loads at low speed without damage. NEMA standards specify that machines powered by dc motors must deliver at least 150% rated current for 1 min at any speed within rated range, but most dc motors do much better.
DC Motors - Heating: Dc motor temperature is a function of ventilation and electrical/mechanical losses in the machine. Some dc motors feature losses, such as core, shunt-field, and brush-friction losses, which are independent of load, but vary with speed and excitation.
The best method to predict a given dc motor's operating temperature is to use thermal capability curves available from the dc motor manufacturer. If curves are not available, dc motor temperature can be estimated by the power-loss method. This method requires a total losses versus load curve or an efficiency curve.
For each portion of the duty cycle, power loss is obtained and multiplied by the duration of that portion of the cycle. The summation of these products divided by the total cycle time gives the dc motor's average power loss. The ratio of this value to the power loss at the motor rating is multiplied by the dc motor's rated temperature rise to give the approximate temperature rise of the dc motor when operated on that duty cycle.


DIAGRAM REPRESENTATION.



motor design.

SCHEMATIC FOR SOME OF THE DC MOTORS.

REFERENCE
http://www.motor-design.com

report on modern specialty motors design.

Assingnment 4.

modern specialty motors design




Definite and special-purpose motors .

INTRODUCTION.
Definite-purpose motors handle specific applications and have well-established NEMA standards. They are produced in high volume, and are low in cost when compared to general-purpose motors with the same ratings.
To apply a definite-purpose motor for a duty other than that for which it was intended must be carefully considered. Modifications can be made easily and inexpensively. Other modifications may require special tooling, increasing cost. The electrical characteristics of the motor must be checked very closely against the load. Testing the motor with the application is recommended.
Instrument motors: Instrument motor definitions vary with users and manufacturers of motors. Generally, an instrument motor is a precision motor with fractional or subfractional horsepower ratings. Sizes range from 1/2 to 3/4hp units.
Gearmotors: A gearmotor consists of a gear-reduction unit with an integral or flange-mounted rotor. The main advantage of a gearmotor is that the driving shaft may be coupled directly to the driven shaft. Belts, pulleys, chains, or additional gearing to step down motor speed are not needed. Also, coupling or belting of a motor to a separate speed-reducer unit is eliminated.
Torque requirements: Starting and running torques are considered separately because starting characteristics of the motor and gearing differ. Applications needing high breakaway torques require careful selection of the motor because split-phase, polyphase, capacitor-start, and brush-type motors have large starting torques.



APPLICATION
Applications with high inertias should be analyzed by the gearmotor manufacturer. This problem is critical with self-locking right-angle gearmotors. Since rotor and load are rigidly connected by the gear train, both must stop in the same time. In severe cases, momentary power failure may be all that is necessary for a high inertial load to destroy the gear train.
Overhung loads are applied to the output shaft of the gearmotor whenever the gearmotor is connected to the applications requiring cams, belts, or gearing. Applications requiring cams, hoisting drums, or switches at the output shaft can also cause very high overhung loads on gearhead bearings.
It is inherent with gearmotors that overhung load capacity decreases as the delivered torque increases. This reduction is caused by internal gear-reduction forces. In small fhp gearheads, reducer bearings may be fully loaded by the reactions resulting from rated torque. Under these conditions, no additional overhung load may be placed on the output shaft.
When the overhung-load capacity of a gearmotor is too small, it may be possible to obtain oversize shafts and bearings. A third or outboard bearing is sometimes used to support the end of the driveshaft. However, it is extremely important that this bearing is properly aligned to prevent excessive shaft and bearing loads. Thrust loads are most severe with vertical-shaft units and gearmotors driving a lead screw or axial actuating device. It is often less expensive to specify a heavy-duty unit rather than extensively modify a normal-duty unit when loads are excessive.
Toothless motors: Some motors can be made with toothless armatures, thanks to powerful modern magnets. Coils are wound and assembled outside the motor, then inserted and secured as a unit in the armature.
Toothless construction provides more space for armature coils and provides higher current ratings. Iron losses are cut by 50%, and armature inductance approaches that for cup motors. Toothless motors do not cog at low speed. And because coil insertion is easier, motor diameters can be smaller.
In toothless motors that use ceramic magnets, flux density in the air gap is much lower than in conventional motors. The low density results from the large air gap, an inherent characteristic of toothless construction.
Toothless motors equipped with rare-earth magnets, however, operate at high flux densities. These motors contain magnets that are about the same length as the air gap. With this relationship, rare-earth magnets operate at flux levels that are close to the magnet's maximum energy product.

special-purpose motors design

EXAMPLE

This example as meet all the above standards(ISOS ,NEMA)

With this in mind, UL did not write the new standards to be more stringent, but instead to accomplish three tasks:
1. To address new and emerging motor and motor-control technologies that were either not addressed or envisioned when the requirements were first written.
2. To clarify and to remove identified ambiguities from the existing standards so that manufacturers more clearly understand all of the requirements and their intent.
3. To provide alternatives. There is not just one way to build a motor, and, similarly, there should not be just a single way to meet the intent of a safety requirement.
Where the legacy of standards consisted simply of UL 1004 – Electric Motors and UL 2111 – Overheating Protection for Motors, the new standards are written as a more functional family that better categorize and organize requirements for specific types of rotating machines. This family scheme should be very familiar to those who are accustomed to IEC Standards. The new motor series of standards now has the following:
UL 1004-1 – Rotating Machinery: This contains requirements common to all rotating machines.
UL 1004-2 – Impedance Protected Motors: This contains requirements specific to this design.
UL 1004-3 – Thermally Protected Motors: This contains requirements specific to motors protected by any one of five different technologies of thermal motor protector devices.
UL 1004-4 – Electric Generators: This contains requirements for component electric generators, sometimes called generator heads.
UL 1004-5 – Fire Pump Motors: This contains requirements unique to that specific application.
In addition, there are three pending standards to further define requirements and further expand the family of motor standards. These include:
Electronically Protected Motors: This will address both BLDC (electronically commutated) motors as well as conventional motors protected by electronic circuitry.
Servo and Stepper Motors: This contains requirements specific to these very specialized motors.
Inverter Duty Motors: This contains requirements specific to the evaluation of motors intended for variable-speed drives or other non-sinusoidal AC supplies.
This family of standards architecture enables UL to build on a continuum of more focused requirements for the various types of rotating machinery. The segregation of the various standards also avoids the result of an enormous, unintelligible document. All of that should shed some light on how UL is addressing the first of the three reasons for introducing a family of standards. Regarding the second goal, it is a given that everyone tries to write standards that are crystal clear and intuitive. Unfortunately, as standards age and numerous revisions are appended, rearranged and tacked on, the original intended clarity inevitably suffers. Eventually, a complete rewrite is required to restore the intended precision, transparency, and user friendliness. As for the third goal, it remains important to understand that the intent of any requirement should be to provide more than one solution to a problem. Inflexibility in the consideration of alternatives simply stifles innovation and creativity in both motor design and appliance design. As noted, UL does not envision its role as the “Safety Police” throwing up barriers, but rather as colleagues of manufacturers desiring to bring safer products to market faster.

reports on different Generator design and application.

ASSINGMENT 5

Different Generator design and application


Generators types and design

dynamo
The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.
Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.
A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution and solid state electronic AC to DC power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a curiosity.
Other rotating electromagnetic generators
Without a commutator, the dynamo is an example of an alternator, which is a synchronous singly-fed generator. With an electromechanical commutator, the dynamo is a classical direct current (DC) generator. The alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid for non-destructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.
Other types of generators, such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator, do not incorporate permanent magnets or field windings (i.e, electromagnets) that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.
The full output performance of any generator can be optimized with electronic control but only the doubly-fed generators or the brushless wound-rotor doubly-fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits

MHD generator.
A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25Mw demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980's, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. [1] MHD generators operated as a topping cycle are currently (2007) less efficient than combined-cycle gas turbines.

power considaration

references