The working principle of three-phase asynchronous motor
The three-phase asynchronous motor is a kind of induction motor, stator currents pass into the after, part of the magnetic flux through the circuit loop, and in which the induced current. Changes in short circuit loop current impede flux, resulting in a short circuit loop flux part and no short circuit ring part is produced with the phase difference, thus forming a rotating magnetic field. After power up, the rotor winding isdue to the relative motion exists between magnetic fields and the induced electromotive force and current, namely the rotating magnetic field and the rotor has relative speed, and generates electromagnetic torque and magnetic field interaction,the rotor turn up, to achieve energy conversion.
When the three-phase stator winding of the motor (the difference between the 120electrical degrees), pass into the three-phase AC, will produce a rotating magnetic field, the rotating magnetic field cutting rotor windings, thus induced current in the rotor winding (rotor winding is a closed path), load flow rotor conductor will generate electromagnetic force in the stator rotating magnetic field, so as to form theelectromagnetic torque on a rotating shaft of the motor, the driving motor, and themotor rotating direction and rotating magnetic field direction are the same.
When the conductor cutting magnetic line of force in a magnetic field, induced current in the conductor, "the name of the resulting induction motor". The combined effect ofinduced current and the magnetic field applied to the rotor of the motor driving force.
We let the closed coil ABCD in the magnetic field around the axis of rotation of the Bxy. If in a clockwise direction magnetic field, closed coil subjected to variable flux,induced electromotive force, the electromotive force induced current (Faraday's law).According to Lenz's law, the direction of the current as: induced current effect alwayshinder the cause of induction current. Therefore, the Lorentz force each conductorunder direction of motion relative to the induction magnetic field instead of F.
A simple method to determine each conductor F direction is the use of three to the right hand is assigned (the effect of magnetic field on current the thumb in theinduction in the direction of the magnetic field, the index finger force direction. Thefingers in the direction of the induced current. In this way, the closed coil bear certaintorque, which along with the induction magnetic field the same direction rotating, the magnetic field is called rotating magnetic field. Electric torque closed coil produced by the rotation of the load torque balance.
Start up and operation
(1) according to the law of electromagnetic force, in the role of the induced EMF, the rotor conductor will produce consistent with the direction of induction electromotive force induced current. By the electromagnetic force of rotor conductor ampacity ofmagnetic field generated in the stator (in the direction of the force with the left hand dingze judge), the formation of electromagnetic force electromagnetic torque motor rotor shaft, the driving motor rotating magnetic field along the direction of rotation of the rotor, when with mechanical load on the shaft of the motor, then outputmechanical energy. Since no short-circuit ring part of the flux ratio flux short-circuit ring part of the lead, the motor rotation direction and rotation direction of the samemagnetic field.
(2) when the three-phase asynchronous motor access three-phase AC power supply(the difference between the 120 electrical degrees), three-phase magnetic motive force of three-phase stator winding through the three-phase symmetrical current generation (stator rotating MMF) and produce a rotating magnetic field, the magnetic field N0 synchronous speed along the stator and rotor space for inner circle clockwiserotation.
(3) the rotating magnetic field and the rotor conductors have relative cutting motion,according to the principle of electromagnetic induction, rotor conductor (rotor windingis a closed path) induced electromotive force and the induced current (Induction EMF direction with the right hand dingze judge).
Slip
Only when the closing coil induced current, exist the driving torque. Torque is determined by the current closing coil, and only when the magnetic flux ring changesexist. Therefore, must have the speed difference between the closing coil and rotating magnetic field. Therefore, in accordance with the principle of the motor work is called "asynchronous motor". Synchronous speed (NS) and closing coil velocity (n)difference between the called "slip", expressed as a percentage of the synchronousspeed. S=[(ns-n) / ns] x 100% (s index) in the process of operation, the rotor currentfrequency for power frequency multiplied by slip. When the starting of motor, the rotor current frequency is maximum is equal to the frequency of stator current.
The rotor current frequency decreased gradually with the increase of motor speed. In the steady state slip and motor load relationship. It is influenced by the voltage of the power supply, if the load is low, then the slip is small, if the motor power supply voltage is lower than the rated value, then the slip increases.
Frequency synchronization speed and power of synchronous speed three phase asynchronous motor is proportional to, and inversely proportional to the logarithm of stator.
For example: ns=60 f/p in the NS - synchronous speed, the unit is r/lmin f- frequency,the unit is Hz,
P number of pole pairs are given in 50Hz, 60Hz and 100Hz industrial frequency,corresponding to different pole numbers of a rotating magnetic field rotation orsynchronous speed.
In fact, even if the voltage. Correct, if the power supply frequency is higher than the rated frequency asynchronous motor, one may not be able to improve the speed of motor. Must first determine its mechanical and electrical capacity. Due to the presence of slip, asynchronous motor with load speed slightly lower than the synchronous speed provided in the table. The rotation direction change motor, realizes the change of phase sequence of power supply, namely the exchange into the three-phasevoltage motor received any two-phase motor terminals on the line.
Because the induction current in three-phase asynchronous motor rotor coil is caused by the rotor conductor and the magnetic field relative motion. Three phase asynchronous motor rotor speed is not synchronized with the rotating magnetic field,more will not exceed the speed of a rotating magnetic field. If the rotor three-phase asynchronous motor speed and rotation speed of the rotating magnetic field into equal size, so there is no relative motion between the magnetic field and the rotor, the conductor shall not cut magnetic lines, so the rotor winding will not generate induced electromotive force and current, rotor three-phase asynchronous motor conductor in a magnetic field is not affected by electromagnetic force and the the rotation of the rotor. Thus the rotor rotation speed of three-phase asynchronous motor can not be the same with the rotating magnetic field, the synchronous speed is always less thanthe rotating magnetic field. But in the special operating mode (such as power generation brake), three-phase rotor asynchronous motor speed can be higher than the synchronous speed.
Produce a rotating magnetic field
The three winding groups differ by 120 degrees, each set of windings are powered by a three-phase AC power supply in a.
Alternating current winding and has the same electrical phase shifts of the cross, eachproduces a sine wave AC magnetic field. The magnetic field is along the same axis,when the winding current in the peak, the magnetic field is located in the peak. Each winding generates a magnetic field which is the results of the two magnetic fieldrotating in opposite direction, the two magnetic field values are constant, equivalent tohalf the peak magnetic field. The magnetic field rotation is done in the power supplyperiod. The speed depends on the frequency of the power supply (f) and number of pole pairs (P). This is called "synchronous speed"
