Principle of three - phase asynchronous motor

When the three-phase stator winding of the motor (the difference between a 120 - degree Angle), after the ventilation with balanced three-phase alternating current (ac), will produce a rotating magnetic field, the rotating magnetic field cutting rotor winding, thus produce induced current in the rotor winding is closed (rotor winding path), the rotor of current-carrying conductor under the effect of the stator rotating magnetic field will produce electromagnetic force, as to form the electromagnetic torque on the motor shaft, motor rotation, and the motor rotation direction is the same as the rotating magnetic field direction.

The name "induction motor" derives from the induced current that is generated in a conductor when it cuts a magnetic field line.

The combined action of induced current and magnetic field exerts a driving force on the motor rotor.

We have the closed coil ABCD rotating around the axis xy in the magnetic field B.If a magnetic field is rotated clockwise and the closed coil is subjected to variable magnetic flux, an induced electromotive force is generated, which generates an induced current (Faraday's Law).According to Lenz's law, the direction of the current is: the effect of the induced current always impedes the cause of the induced current.Therefore, each conductor is subjected to a Lorentz force F that moves in the opposite direction relative to the induced magnetic field.

A simple way to determine the direction of F is to use the right-handed three-handed rule (the effect of the magnetic field on the current is to place the thumb in the direction of the inductive magnetic field and the index finger in the direction of the action.

Place your middle finger in the direction of the induced current.In this way, the closed coil is subjected to a certain torque and thus rotates in the same direction as the induction field, which is called the rotating field.The load torque is balanced by the electric torque produced by the closed coil rotation.

Generation of rotating magnetic field:

The three sets of windings differ from each other by 120 degrees. Each set of windings is supplied by one phase of a three-phase AC power supply.

The windings intersect the ac currents with the same phase displacement, and each group generates an AC sinusoidal magnetic field.The magnetic field is always along the same axis, and when the current in the winding is at its peak, so is the magnetic field.The magnetic field generated by each set of windings is the result of two magnetic fields rotating in opposite directions, both of which are constant and equal to half of the peak magnetic field.This magnetic field is rotated during the power supply period.The speed depends on the power frequency (F) and the pole log (P).This is called the "synchronous speed" slip and the driving torque exists only if the closing coil has induced current.The torque is determined by the current in the closed coil and exists only when the magnetic flux in the loop changes.Therefore, there must be a velocity difference between the closed coil and the rotating magnetic field.Therefore, the motor that follows this principle is called "asynchronous motor".

The difference between the synchronous speed (NS) and the closed coil speed (n) is called the "slip" and is expressed as a percentage of the synchronous speed.

S =[(NS-n)/ ns] x 100% (s is subscript)

During operation, the rotor current frequency is the power supply frequency times the slip.When the motor is started, the rotor current frequency is at the maximum value, equal to the stator current frequency.The rotor current frequency decreases with the increase of motor speed.The slip at constant steady state is related to the motor load.It is affected by the supply voltage, if the load is lower, the slip is smaller, if the motor supply voltage is lower than the rated value, the slip increases.

Synchronous speed the synchronous speed of three-phase asynchronous motor is directly proportional to the power supply frequency and inversely proportional to the logarithm of the stator.

For example, in ns=60 F/P formula, NS -- synchronous speed, unit is R /lmin F-frequency, unit is Hz. The logarithm of p magnetic poles gives the rotating magnetic field speed or synchronous speed corresponding to different number of magnetic poles at industrial frequencies of 50Hz, 60Hz and 100Hz.

In fact, even if the voltages are correct, if the power supply frequency is higher than the rated frequency of the asynchronous motor, one may not be able to increase the motor speed.Its mechanical and electrical capacity must first be determined.Due to slip, the speed of the loaded asynchronous motor is slightly lower than the synchronous speed shown in the tab

1 2