A single-phase induction motor is a rotating electric machine powered by a single-phase AC supply, featuring a single-phase stator winding. When connected to a single-phase AC source, it generates an alternating, pulsating magnetic field within the air gap between the stator and the rotor; consequently, a single-phase induction motor is not self-starting. In AC machines, the flow of AC current through the stator winding establishes an armature magnetomotive force (MMF), which significantly influences the motor's energy conversion process and operational performance. Thus, when a single-phase AC current flows through a single-phase winding, it produces a pulsating MMF; this MMF can be resolved into two rotating MMFs of equal magnitude but opposite direction. This resolution establishes a forward-rotating magnetic field and a backward-rotating magnetic field within the air gap. These two rotating magnetic fields cut across the rotor conductors, inducing electromotive forces (EMFs) and currents within them.
The interaction between these induced currents and the magnetic fields generates both a forward electromagnetic torque and a backward electromagnetic torque. The forward torque attempts to drive the rotor in the forward direction, while the backward torque attempts to drive it in the reverse direction. The superposition of these two torques constitutes the resultant torque that drives the motor's rotation.
For both the forward and backward rotating magnetic fields, the relationship between their magnitude and the slip ratio is identical to that observed in three-phase induction motors. If the motor's rotational speed is denoted as *n*:
For the forward-rotating magnetic field, the slip ratio is: s+ = (n1 - n) / n1 = s
For the backward-rotating magnetic field, the slip ratio is: s- = (-n1 - n) / -n1 = s
Based on the torque-slip (T-s) characteristic curve of a single-phase induction motor, its primary characteristics can be summarized as follows:
(1) When n = 0 (at standstill) and s = 1, the net torque T = T+ + T- = 0. This indicates that a single-phase induction motor possesses no starting torque; therefore, unless additional measures are implemented, the motor cannot initiate rotation on its own.
(2) When s ≠ 1, the net torque T ≠ 0; however, the direction of T is not fixed but depends on whether the slip *s* is positive or negative.
(3) Due to the presence of the backward torque, the resultant torque is reduced; consequently, single-phase induction motors exhibit a relatively low overload capacity. Working Principle of Capacitor-Start Motors
During startup, switch K closes, creating a phase difference of approximately 90° between the currents in the two windings (I1 and I2). This generates a rotating magnetic field, causing the motor to begin rotating. Once normal operating speed is attained, the centrifugal switch opens, thereby disconnecting the starting winding.
Working Principle of Shaded-Pole Single-Phase Motors
When current flows through the stator, a portion of the magnetic flux passes through the short-circuiting ring, inducing a current within it. The current flowing in the short-circuiting ring opposes changes in the magnetic flux; consequently, a phase difference arises between the magnetic flux generated in the shaded (ring-equipped) section of the pole and that generated in the unshaded section. This phase difference creates a rotating magnetic field, causing the rotor to rotate.
Regarding the T-s (Torque-Slip) characteristic curve of a single-phase induction motor, the motor's direction of rotation is clockwise. This is because the magnetic flux in the unshaded section of the pole leads-i.e., precedes in phase-the magnetic flux in the shaded section.