اے سی موٹر
اے سی موٹر ایک ایسی موٹر ہوتی ہے جو کہ اے سی برقی رو پر کام کرتی ہے۔ اس کے دو حصے ہوتے ہیں۔ باہر والا حصہ برقی کوائلوں پر مشتمل ہوتا ہے جن کی مدد سے ایک ایسا مقناطیسی میدان بنتا ہے جو کہ ہر وقت گھومتا رہتا ہے۔ اندر والا حصہ ایک روٹر یا بیلن ہوتا ہے جو کہ ایک شافٹ سے منسلک وزن کر گھوماتا ہے۔
اے سی موٹروں کو ان میں استعمال ہونے والے بیلن کی بنیاد پر دو اقسام میں تقسیم کیا جاتا ہے۔
سنکرونس موٹریں ان کی گھمنے کی رفتار برقی رو کے تعدد کے عین برابر یا پھر اس کا کوئی عددی حاصل تقسیم ہوتی ہے۔ بیلن میں یا تو مستقل مقناطیس استعمال ہوتا ہے یا پر یہ ایک کوائل پر مشتمل ہوتا ہے جسے ایک گھس انگوٹھی (Slip Ring) سے ڈی سی برقی رو دی جادی ہے۔
انڈکشن موٹریں یہ موٹریں ہمیشہ برقی سپلائی کے تعدد سے کم رفتار سے گھومتی ہیں۔ ان کے بیلن میں نہ تو بوئی مستقل مقناطیس ہوتا ہے اور نہ ہی اسے کو باہر سے سپلائی دے کر برقی مقناطیس بناتے ہیں بلکہ یہ انڈکسن کے اصول سے خود ہی مقناطیس بن جاتا ہے۔ اس کی وجہ وہ بیرونی مقناطیسی میدان ہوتا ہے جو کہ باہر والی کوائلیں بناتی ہیں۔
- 1 تاریخی پس منظر
- 2 تھری فیز اے سی انڈکشن میٹر
- 3 Squirrel-cage rotors
- 4 Wound rotor
- 4.1 Three-phase AC synchronous motors
- 4.2 Two-phase AC servo motors
- 4.3 Single-phase AC induction motors
- 4.4 Single-phase AC synchronous motors
- 4.5 Electronically commutated motors
- 5 See also
- 6 References
تاریخی پس منظر[ترمیم]
سن 1982 میں ایک سرب موجد نکولا ٹیسلا نے یہ اصول دریافت کیا کہ اے سی برقی رو کو استعمال کرتے ہوۓ ہم ایسا مقناطیسی میدان بنا سکتے ہیں جو کہ ہر وقت گھومتا رہے۔ اس مقناطیسی میدان کا عمل بالکل ایسا ہی ہو گا جس طرح کہ ایک گھومتے ہوۓ مقناطیس کا ہوتا ہے۔ اس کی یہی دریافت اے سی موٹر کی ایجاد میں کار فرما ہے۔ اس سے قبل ڈی سی موٹریں ہی استعمال ہوتی تھیں۔ لیکن ڈی سی موٹروں میں قباحت یہ تھی کہ ان میں کاربن برش استعمال کرنا پڑتے تھے۔ کاربن برش اس لیے استعمال کرنا ضروی تھا کہ موٹر میں مقناطیسی میدان بنانے وانی کوائلوں میں کرنٹ کی سمت اس وقت تبدیل کر دی جاۓ جب یہ مقناطیسی میدان میں اس مقام پر ہو جہاں مقناطیسی میدان کی دھکیلنے کی قوت کم ہو جاتی ہے۔ یہ کاربن برش وقت کے ساتھ گھس جاتے ہیں اور ان کو تبدیل کرنا ضروری ہو جاتا ہے۔ اس کے علاوہ جب کرنٹ کی سمت تبدیل ہوتی ہے اس وقت شعلے بھی نکلتے ہیں جو کہ کوائل میں کرنٹ میں اچانک تبدیلی کی وجہ سے ہوتے ہیں (گرائنڈر مشین کی موٹر ڈی سی ہوتی ہے اس لیے اس میں اس مسائل کا سامنا اکثر کرنا پڑتا ہے
اس لیے ٹیسلا نے اس تحقیق کا بیڑا اٹھایا کا کوائل میں مقناطیسی میدان کی سمت کو بغیر اس کے کنکشن تبدیل کیے' بدلا جا سکے۔ اس طرح کئی مسائل سے بچا جا سکتا ہے۔ چنانچہ اے سی مشین کی ایجاد نے ایک انقلاب بپا کر دیا اور اب ہر طرف اے سی موٹروں کی اجارہ داری ہے
تھری فیز اے سی انڈکشن میٹر[ترمیم]
Where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is commonly used, especially for higher-powered motors. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor.
Through electromagnetic induction, the time changing and reversing (alternating in direction polyphase currents) rotating magnetic field induces a time changing and reversing (alternating in direction)current in the conductors in the rotor; this sets up a time changing and counterbalancing moving electromagnetic field that causes the rotor to turn in the direction the field is rotating. The rotor always moves (rotates) slightly behind the phase peak of the primary magnetic field of the stator and is thus always moving slower than the rotating magnetic field produced by the polyphase electrical supply.
Induction motors are the workhorses of industry and motors up to about 500 kW (670 horsepower) in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are different). Very large induction motors are capable of tens of thousands of kW in output, for pipeline compressors, wind-tunnel drives and overland conveyor systems.
There are two types of rotors used in induction motors: squirrel cage rotors and wound rotors.
Most common AC motors use the squirrel cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage takes its name from its shape - a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper in order to reduce the resistance in the rotor.
In operation, the squirrel cage motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor into synchronization with the stator's field. An unloaded squirrel cage motor at synchronous speed will consume electrical power only to maintain rotor speed against friction and resistance losses; as the mechanical load increases, so will the electrical load - the electrical load is inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.
This is why, for example, a squirrel cage blower motor may cause the lights in a home to dim as it starts, but doesn't dim the lights when its fanbelt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.
In order to prevent the currents induced in the squirrel cage from superimposing itself back onto the supply, the squirrel cage is generally constructed with a prime number of bars, or at least a small multiple of a prime number (rarely more than 2). There is an optimum number of bars in any design, and increasing the number of bars beyond that point merely serves to increase the losses of the motor particularly when starting.
An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter.
Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available, but this is never used in household appliances, because it can cause electrical interference and because of high power requirements.)
Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (Direct-on-line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is (Star-Delta, YΔ) starting, where the motor coils are initially connected in star for acceleration of the load, then switched to delta when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.
This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor.
The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
- Ns = Synchronous speed, in revolutions per minute
- F = AC power frequency
- p = Number of poles per phase winding
Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).
The slip of the AC motor is calculated by:
- Nr = Rotational speed, in revolutions per minute.
- S = Normalised Slip, 0 to 1.
As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.
The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.
Three-phase AC synchronous motors[ترمیم]
If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate synchronously with the rotating magnetic field produced by the polyphase electrical supply.
The synchronous motor can also be used as an alternator.
Nowadays, synchronous motors are frequently driven by transistorized variable-frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.
One use for this type of motor is its use in a power factor correction scheme. They are referred to as synchronous condensers. This exploits a feature of the machine where it consumes power at a leading power factor when its rotor is over excited. It thus appears to the supply to be a capacitor, and could thus be used to correct the lagging power factor that is usually presented to the electric supply by inductive loads. The excitation is adjusted until a near unity power factor is obtained (often automatically). Machines used for this purpose are easily identified as they have no shaft extensions. Synchronous motors are valued in any case because their power factor is much better than that of induction motors, making them preferred for very high power applications.
Some of the largest AC motors are pumped-storage hydroelectricity generators that are operated as synchronous motors to pump water to a reservoir at a higher elevation for later use to generate electricity using the same machinery. Six 350-megawatt generators are installed in the Bath County Pumped Storage Station in Virginia, USA. When pumping, each unit can produce 563,400 horsepower (420 megawatts).
Two-phase AC servo motors[ترمیم]
A typical two-phase AC servo motor has a squirrel-cage rotor and a field consisting of two windings: 1) a constant-voltage (AC) main winding, and 2) a control-voltage (AC) winding in quadrature with the main winding as to produce a rotating magnetic field. The electrical resistance of the rotor is made high intentionally so that the speed-torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load.
Single-phase AC induction motors[ترمیم]
Three-phase motors inherently produce a rotating magnetic field. However, when only single-phase power is available, the rotating magnetic field must be produced using other means. Several methods are commonly used:
A common single-phase motor is the shaded-pole motor, which is used in devices requiring low starting torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle, thus producing a low level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor accelerates the torque builds up to its full level as the principal (rotationally stationary) magnetic field is rotating relative to the rotating rotor. Such motors are difficult to reverse without significant internal alterations.
Split-phase induction motor[ترمیم]
Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.
In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding..
The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.
Capacitor start motor[ترمیم]
A capacitor start motor is a split-phase induction motor with a starting capacitor inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.
Resistance start motor[ترمیم]
A resistance start motor is a split-phase induction motor with a starter inserted in series with the startup winding, creating capacitance. This added starter provides assistance in the starting and initial direction of rotation.
Permanent-split capacitor motor[ترمیم]
Another variation is the permanent-split capacitor (PSC) motor (also known as a capacitor start and run motor). This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch, and the start windings (second windings) are permanently connected to the power source (through a capacitor), along with the run windings. PSC motors are frequently used in air handlers, blowers, and fans (including ceiling fans) and other cases where a variable speed is desired.
A capacitor ranging from 3 to 25 microfarads is connected in series with the start windings and remains in the circuit during the run cycle. The start windings and run windings are identical in this motor, and reverse motion can be achieved by reversing the wiring of the 2 windings, with the capacitor connected to the other windings as start windings. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds. Also, provided all 6 winding connections are available separately, a 3 phase motor can be converted to a capacitor start and run motor by commoning two of the windings and connecting the third via a capacitor to act as a start winding.
Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2005.
Single-phase AC synchronous motors[ترمیم]
Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio turntables, and tape drives; formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The shaded-pole synchronous motor is one version.
Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).
Electronically commutated motors[ترمیم]
Such motors have an external rotor with a cup-shaped housing and a radially magnetized permanent magnet connected in the cup-shaped housing. An interior stator is positioned in the cup-shaped housing. The interior stator has a laminated core having grooves. Windings are provided within the grooves. The windings have first end turns proximal to a bottom of the cup-shaped housing and second end turns positioned distal to the bottom. The first and second end turns electrically connect the windings to one another. The permanent magnet has an end face remote from the bottom of the cup-shaped housing. At least one galvano-magnetic rotor position sensor is arranged opposite the end face of the permanent magnet so as to be located within a magnetic leakage of the permanent magnet and within a magnetic leakage of the interior stator. The at least one rotor position sensor is designed to control current within at least a portion of the windings. A magnetic leakage flux concentrator is arranged at the interior stator at the second end turns at a side of the second end turns facing away from the laminated core and positioned at least within an angular area of the interior stator in which the at least one rotor position sensor is located.
ECM motors are increasingly being found in forced-air furnaces and HVAC systems to save on electricity costs as modern HVAC systems are running their fans for longer periods of time (duty cycle). The cost effectiveness of using ECM motors in HVAC systems is questionable, given that the repair (replacement) costs are likely to equal or exceed the savings realized by using such a motor.[حوالہ درکار]
- ^ [|Dominion Resources, Inc.] (2007), Bath County Pumped Storage Station, http://www.dom.com/about/stations/hydro/bath.jsp, retrieved 2007-03-30
- ^ Split Phase Induction Motor section in Neets module 5: Introduction to Generators and Motors
- ^ 3.0 3.1 3.2 3.3 3.4 3.5 George Shultz, George Patrick Shultz (1997). "Transformers and Motors". Newnes. page 159 of 336. http://books.google.com/books?id=kfIC04vdXYcC&pg=PA159&lpg=PA159&dq=%22Permanent+split-capacitor+motor%22&source=web&ots=4Nutz_NKeT&sig=Q_dTJYxKNCYlVMdLCBT0xVwPNRw&hl=en&sa=X&oi=book_result&resnum=10&ct=result. Retrieved 2008-09-26.
- ^ http://www.ferret.com.au/c/Maxon-Motor-Australia/EC-max-16-2-wire-electronically-commutated-motors-available-from-Maxon-Motor-Australia-n817712