Why do single-phase motors require additional mechanisms to start compared to three-phase motors?

In the intricate world of electric motors, the distinction between single-phase and three-phase motors lies not just in their power supply but also in the nuances of their operation. One of the most striking disparities between these motor types is the necessity for single-phase motors to employ additional starting mechanisms, a requirement conspicuously absent in their three-phase counterparts. Delving into the heart of this discrepancy reveals a fundamental difference in the initiation of motion, rooted deeply in the physics of electromagnetic induction.
At the core of the matter lies the concept of the rotating magnetic field. In single-phase asynchronous motor, this ethereal force, generated by the harmonious interplay of three alternating currents, serves as the catalyst for motion. As the currents cycle through their phases, they generate magnetic fields that collectively weave around the motor's stator, creating a mesmerizing dance of magnetic flux. This dynamic field, akin to a cosmic whirlwind within the motor, induces currents in the rotor windings, propelling it into motion with grace and efficiency.
However, the landscape changes drastically when we shift our gaze to single-phase motors. Here, the story takes on a more intricate hue. With only a solitary phase coursing through the motor's veins, the creation of a rotating magnetic field becomes an elusive dream. In the absence of synchronized currents, the stator lies dormant, bereft of the swirling energy that propels its three-phase brethren into action.
Thus, the single-phase motor faces a conundrum: how to bridge the chasm between stasis and motion without the luxury of a rotating magnetic field. The solution lies in the ingenuity of additional starting mechanisms, each a testament to human creativity in harnessing the forces of nature.
One such mechanism is the starting winding, an auxiliary coil nestled within the motor's stator. Energized during startup, this winding creates a magnetic field slightly out of phase with the main winding. Though humble in its origin, this phase disparity sparks the ember of motion, imparting an initial push to the rotor and setting it on its journey.

Complementing the starting winding is the stalwart companion, the capacitor. Acting as a reservoir of energy, the capacitor provides the necessary phase shift to kickstart the motor into action. By introducing a lag between voltage and current, the capacitor bestows upon the motor the semblance of a rotating magnetic field, however faint. This modest addition to the motor's arsenal proves instrumental in surmounting the inertia that shackles the rotor at rest.
Yet another ally in the quest for motion is the centrifugal switch, a sentinel guarding the gateway to operation. Positioned strategically within the motor, this device stands vigilant, poised to engage and disengage the starting mechanisms at the appropriate juncture. As the motor gathers speed, the centrifugal force exerted by the spinning rotor triggers the switch, signaling the denouement of the startup saga and the commencement of steady-state operation.
In essence, the need for additional starting mechanisms in single-phase motors arises from the absence of a natural rotating magnetic field, the hallmark of three-phase power systems. In its absence, human ingenuity rises to the occasion, devising an array of mechanisms to bridge the chasm between stasis and motion. Through the symbiotic interplay of starting windings, capacitors, and centrifugal switches, the single-phase asynchronous motor emerges from its slumber, poised to fulfill its purpose with vigor and determination.