When the stator winding carries a sinusoidal current which is being fed from a single-phase supply, a sinusoidal space distributed mmf, whose peak or maximum value pulsates (alternates) with time, is produced in the air gap.As the motor approaches synchronous speed, F R becomes almost equal to F S and a nearly perfect revolving field is produces. The flux rotates counterclockwise in the same direction as the rotor and it rotates at synchronous speed irrespective of the actual speed of the rotor. The value of F R increases with increasing speed, becoming almost equal to F S at synchronous speed.Equally important is the fact that FR does not reach its maximum value at the same time as F S does, in effect, F R lags almost 90o behind F S, owing to the inductance of the rotor The combined action of F S and F R produces a revolving magnetic field, similar to that in a three-phase motor. These currents produce an ac flux F R which act at right angle to the stator flux F s.It causes current I r to flow in the rotor bars facing the stator poles. This voltage increases as the rotor speed increases.As soon as the rotor begins to turn, a speed emf E is induced in the rotor conductors, as they cut the stator flux F S. The principle of operation of a single-phase induction motor can be explained from the cross-field theory.The single-phase induction motor operation can be described by two methods This method of starting is generally not convenient for large motors.
Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. However, if the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor. The field polarity reverses after each half cycle but the field does not rotate. Principle of Operation of Single Phase Induction MotorĪ single-phase induction motor is not self starting but requires some starting means.The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner.
0 kommentar(er)
