The value of 100% slip (standstill condition) is the same for both the fields.Įxplain motor action from standstill to running condition?Īt standstill, s = 1 so that impedances of the two circuits are equal. Thus the point of zero slip for one field corresponds to 200% slip for the other. Note that each rotating field tends to drive the rotor in the direction in which the field rotates.
Therefore, single-phase induction motor is not self-starting. The alternating flux produced by the stator winding can be presented as the sum of two rotating fluxes 1 and 2, each equal to one half of the maximum value of alternating flux and each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.Īt standstill, these two torques are equal and opposite and the net torque developed is zero. What is the position of rotor at standstill?Ĭonsider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. Note that the resultant vector of two revolving flux vectors is a stationary vector that oscillates in length with time along X-axis. Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. This theory is based on the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions. The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. Nor can it be employed fur a motor located at some inaccessible spot.Įxplain why single phase induction motor does not start in the light of Double-Field Revolving Theory? 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.
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. What is basic method to start single phase induction motor and why it is not preferred? 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. The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. Why single phase induction motor does not self starts? Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means. What is major difference between a 3-phase induction motor and a single-phase induction motor?
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