Three-phase asynchronous motors are a type of induction motor, with the following structure and technical characteristics:
I. Structure
Three-phase asynchronous motors primarily consist of two fundamental components: the stator (fixed part) and the rotor (rotating part).
1. Stator: The stator is the stationary part of the motor, primarily composed of the core and windings. The core is typically constructed from laminated silicon steel sheets to minimize eddy current and hysteresis losses. The windings are wound around the core in a specific pattern, forming three-phase windings AX, BY, and CZ. These windings are spatially offset by 120 electrical degrees to generate a rotating magnetic field.
2. Rotor: The rotor is the rotating part of the motor, also composed of an iron core and windings. Based on winding structure, rotors are classified into squirrel-cage and wound types. Squirrel-cage rotors feature a simple structure, reliable operation, and low cost, making them widely adopted. Wound rotors incorporate three-phase windings connected via slip rings and brushes to external resistors, enabling adjustment of the motor's starting performance and speed.
Additionally, three-phase asynchronous motors include auxiliary components such as end covers and fans to ensure proper operation and heat dissipation.
II. Technical Performance
1. Rotating Magnetic Field: When symmetrical three-phase alternating current is applied to the stator windings, it generates a rotating magnetic field that moves clockwise at synchronous speed n1 within the space between the stator and rotor. This rotating magnetic field is the key factor driving the motor's rotation.
2. Slip Rate: Due to the rotor rotating in the same direction but at a different speed than the stator's rotating magnetic field, a slip rate exists. The slip rate is a crucial physical quantity for induction motors, indicating the difference between the rotor speed n and the magnetic field speed n0. Under rated operating conditions, the slip rate is typically very small, ranging from approximately 0.015 to 0.06.
3. Key Performance Parameters:
● Efficiency (η): Efficiency is the ratio of output power to input power. High efficiency indicates the motor can convert more input power into output power.
● Power Factor (COSΦ): Power factor is the ratio of active power to apparent power. It measures the efficiency of electrical equipment. A low power factor indicates significant reactive power consumption in the circuit for alternating magnetic field conversion, increasing line power supply losses.
● Locked-rotor Torque (Starting Torque) TK: The torque generated when voltage is applied to a stationary motor (n=0, s=1) is called starting torque. The ratio of starting torque to rated torque is the starting torque multiple, characterizing the motor's starting capability.
● Locked-rotor current (starting current) IA: This refers to the inrush current when electrical equipment (inductive load) first starts. The ratio of starting current to rated current is called the starting current multiple. The starting current of an asynchronous motor does not significantly affect the motor itself, as starting occurs for an extremely short duration (instantaneously). However, it does exert a certain impact on the power grid.
● Maximum Torque TMAX: The highest rotational torque a motor can generate under rated conditions when load increases without causing sudden stoppage. The ratio of maximum torque to rated torque is called the maximum torque multiplier (also known as the overload factor), indicating the motor's overload capability. Typically, the maximum torque multiplier for general-purpose motors ranges from 2.0 to 2.3.
● Noise: Motor noise can be categorized into electromagnetic noise and mechanical noise. Electromagnetic noise arises from time- and space-varying magnetic forces acting between motor components. Mechanical noise includes ventilation noise, noise generated by housing resonance, and noise caused by bearing quality issues.
4. Rated Values:
● Rated Voltage: The voltage at which the motor operates normally, representing the optimal voltage for sustained operation.
● Rated Frequency: China's industrial power supply frequency is specified as 50Hz.
● Rated Power: The effective power output at the motor shaft under rated operating conditions.
● Rated Current: The line current in the stator windings when rated voltage is applied and rated power is output at the shaft.
● Rated Speed: The rotational speed achieved at rated power.
In summary, three-phase asynchronous motors offer advantages such as simple structure, reliable operation, high efficiency, and high power factor, making them widely adopted in industrial production.