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An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip rings as in other types of motor. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Single-phase versions are used in small appliances. Their speed is determined by the frequency of the supply current, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is sometimes used for induction motors generally.
Three-phase induction motors.
Three Phase AC Motor Theory
Motors have been described as a transformer with a rotating secondary. Motors, generators, and transformers are similar in that their basic principle of operation involves induction. The premise for motor operation is that if you can create a rotating magnetic field in the stator of the motor, it will induce a voltage in the armature that will have magnetic properties causing it to ‘chase’ the field in the stator. This premise applies to AC motors that employ a squirrel cage rotor, and it is probably the most simple and basic of all motor designs. The three phase motor is widely used in industry because of it’s low maintenance characteristics. Due to the nature of three phase power, creating a rotating magnetic field in the stator of this motor is simple and straight forward.
The strength of the magnetic field changes, as the current flow in the coils of wire around thestator poles change . Just as the current in the field windings rise and fall 120 electrical degrees apart, so does the resulting magnetic field on the pole face. In other words, when the magnetic polarity on pole ‘A’ reaches it’s peak, 120 electrical degrees later pole ‘B’ will reach it’s peak, and 120 electrical degrees after that, pole ‘C’ will reach it’s peak. Then the cycle repeats itself, and a rotating magnetic field is developed in the stator of the motor. Swap any two of the three pole face polarities by changing any two leads of the supply voltage,
and you will see that the, ‘ABC’ order reverses. Phase rotation of the supply voltage is not always the same from one electrical service to the next, and the electrician has a fifty-fifty chance of getting the correct direction of rotation when connecting a three phase motor. The phase rotation of the electrical service can be determined with the use of a phase rotation meter, but the way the individual motor was manufactured also determines it’s direction of rotation. Every three phase motor should be energized for a very brief moment, in order to check rotation before being placed into service.
The synchronous speed of an AC motor is the rotation rate of the rotating magnetic field created by the stator. It is always an integer fraction of the supply frequency. The synchronous speed ns in revolutions per minute (RPM) is given by:
where f is the frequency of the AC supply current in Hz and p is the number of magnetic pole pairs per phase. For example, a small 3-phase motor typically has six magnetic poles organized as three opposing pairs 120° apart, each powered by one phase of the supply current. So there is one pair of poles per phase, which means p = 1, and for a line frequency of 50 Hz the synchronous speed is 3000 RPM.
Typical torque curve as a function of slip (slip is represented by g here, equivalent to s in the formula at left).
Slip s is the rotation rate of t the rotor rotation speed in rpm. It is zero at synchronous speed and 1 (100%) when the rotor is stationary. The slip determines the motor’s torque. Since the short-circuited rotor windings have small resistance, a small slip induces a large current in the rotor and produces large torque. At full rated load, typical values of slip are 4-6% for small motors and 1.5-2% for large motors, so induction motors have good speed regulation and are considered constant-speed motors.
The torque exerted by the motor as a function of slip is given by a torque curve. Over a motor’s normal load range, the torque line is close to a straight line, so the torque is proportional to slip. As the load increases above the rated load, increases in slip provide less additional torque, so the torque line begins to curve over. Finally at a slip of around 20% the motor reaches its maximum torque, called the "breakdown torque". If the load torque reaches this value, the motor will stall. At values of slip above this, the torque decreases. In 3-phase motors the torque drops but still remains high at a slip of 100% (stationary rotor), so these motors are self-starting. The starting torque of an induction motor is less than other types of motor, but still around 300% of rated torque. In 2-pole single-phase motors, the torque goes to zero at 100% slip (zero speed), so these require alterations to the stator such as shaded poles to provide starting torque.
Typical winding pattern for a 3 phase, 4 pole motor (phases here are labelled U, V, W). Note the interleaving of the pole windings and the resulting quadrupole field.
The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90 degrees from the single-phase supply and feeds it to the second motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field. There are three types of rotor: squirrel cage rotors made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ring rotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel. For information on die-cast copper rotors in energy-efficient induction motors, see: Copper die-cast rotors.
As mentioned earlier the induction motor is essentially a constant-speed motor. Its speed of rotation is determined by the synchronous speed. Many motor applications, however, require wide variation in motor speed. This can be achieved by varying the stator frequency of the motor thereby varying the synchronous speed. Let us now see the relationship between supply voltage and frequency As mentioned earlier the induction motor is essentially a constant-speed motor. Its speed of rotation is determined by the synchronous speed. Many motor applications, however, require wide variation in motor speed. This can be achieved by varying the stator frequency of the motor thereby varying the synchronous speed. Let us now see the relationship between supply voltage and frequency.
Typical torque curves for different line frequencies. By varying the line frequency with an inverter, induction motors can be kept on the stable part of the torque curve above the peak over a wide range of rotation speeds. However, the inverters can be expensive, and fixed line frequencies and other start up schemes are often employed instead.
The theoretical unloaded speed (with slip approaching zero) of the induction motor is controlled by the number of pole pairs and the frequency of the supply voltage.
When driven from a fixed line frequency, loading the motor reduces the rotation speed. When used in this way, induction motors are usually run so that in operation the shaft rotation speed is kept above the peak torque point; then the motor will tend to run at reasonably constant speed. Below this point, the speed tends to be unstable and the motor may stall or run at reduced shaft speed, depending on the nature of the mechanical load.
Before the development of semiconductor power electronics, it was difficult to vary the frequency, and induction motors were mainly used in fixed speed applications. However, many older DC motors have now been replaced with induction motors and accompanying inverters in industrial applications.