The Working Principle of Three-Phase Motors

YouTube：https://youtube.com/shorts/ep0e7YGo4tA

Based on the law of electromagnetic induction and the effect of magnetic fields on electric currents, the following is a detailed introduction for you:

### Generation of Rotating Magnetic Field

- **Three-Phase AC Power Supply**: A three-phase motor is connected to a three-phase AC power supply, and its three-phase windings are spaced 120° electrical angles apart in space. Since the phases of the three-phase AC power supply differ from each other by 120°, the magnitudes and directions of the currents they generate at different times are also different.

- **Composite Magnetic Field**: When three-phase currents flow into the three-phase windings, three alternating magnetic fields that are 120° apart from each other in space will be generated inside the motor. These three magnetic fields superimpose on each other and combine into a rotating magnetic field in space, and its rotation direction is related to the phase sequence of the three-phase currents.

### Rotation of the Rotor - **Generation of Induced Current**: The rotating magnetic field cuts the rotor windings. According to the law of electromagnetic induction, induced electromotive forces and induced currents will be generated in the closed rotor windings.

- **Formation of Electromagnetic Force**: The induced currents in the rotor windings will be affected by electromagnetic forces in the rotating magnetic field. According to the left-hand rule, these electromagnetic forces will generate a torque on the rotor, causing the rotor to rotate in the direction of the rotating magnetic field.

### Energy Conversion

- **Conversion of Electrical Energy into Mechanical Energy**: During the operation of a three-phase motor, the stator windings absorb electrical energy from the power supply. Through electromagnetic induction, induced currents are generated in the rotor windings, which then generate electromagnetic torques to make the rotor rotate, thus realizing the conversion of electrical energy into mechanical energy.

- **Rotational Speed and Slip**: The rotational speed of the rotor is always slightly lower than that of the rotating magnetic field. There is a speed difference between them, which is called slip. The ratio of the slip to the rotational speed of the rotating magnetic field is called the slip ratio, which reflects the operating state of the motor. What are the starting methods of three-phase motors? There are several starting methods for three-phase motors. The following are some common starting methods:

### Direct Starting - **Principle**: Directly connect the three-phase motor to the power supply with the rated voltage, and the motor starts directly under the full voltage.

- **Characteristics**: The starting current is large, generally 4 to 7 times the rated current. The starting torque is large. The starting equipment is simple, has a low cost, and is easy to operate. However, it has a great impact on the power grid and may cause fluctuations in the grid voltage, affecting the normal operation of other equipment. It is suitable for low-power motors or situations where the power grid has a large capacity.

### Reduced-Voltage Starting

- **Reduced-Voltage Starting by Connecting Resistors in Series with the Stator**:

- **Principle**: When starting the motor, connect resistors in series with the stator windings. The voltage across the stator windings is reduced through the voltage division of the resistors, thereby reducing the starting current.

- **Characteristics**: The starting current is reduced, but the starting torque is also reduced accordingly. Moreover, a certain amount of electrical energy will be consumed by the resistors, and the efficiency is relatively low. It is suitable for occasions of light-load or no-load starting.

- **Reduced-Voltage Starting with an Autotransformer**: - **Principle**: Use an autotransformer to reduce the voltage across the stator windings of the motor. When starting, connect the taps of the autotransformer to the stator windings of the motor, and then directly connect the motor to the power supply after starting.

- **Characteristics**: Different voltage reduction ratios can be selected as needed. The starting current and starting torque can be reduced or increased in a certain proportion, and the impact on the power grid is relatively small. However, the autotransformer is large in size, has a high cost, and is relatively complicated to maintain.

- **Star-Delta Reduced-Voltage Starting**: - **Principle**: When starting the motor, connect the stator windings in a star configuration. At this time, the voltage across the stator windings is 1/√3 of the rated voltage, and the starting current and starting torque are also reduced accordingly. When the rotational speed of the motor approaches the rated rotational speed, then switch the stator windings to a delta connection, and the motor operates normally under the rated voltage.

- **Characteristics**: The starting current and starting torque are reduced to 1/3 of those in direct starting. The impact on the power grid is relatively small, and the cost is relatively low. However, the starting torque is small, and it is only suitable for light-load or no-load starting occasions. Moreover, the stator windings of the motor must be connected in a delta configuration.

### Soft Starting - **Principle**: Control the starting voltage and current of the motor through a soft starter, so that the voltage gradually increases and the current gradually increases during the starting process, realizing a smooth start.

- **Characteristics**: The starting current is small, the impact on the power grid is small, the starting process is smooth, and it can effectively protect the motor and mechanical equipment. Meanwhile, the soft starter also has various protection functions, such as over-current protection, over-heat protection, and phase-loss protection. However, the price of the soft starter is relatively high, and its maintenance is relatively complicated.

### Variable-Frequency Starting

- **Principle**: Utilize a frequency converter to change the frequency and voltage of the power supply, thereby realizing the stepless speed regulation starting of the motor. During the starting process, the frequency converter can automatically adjust the output frequency and voltage according to the load condition of the motor, so that the motor starts in the best state.

- **Characteristics**: The starting current is small, the impact on the power grid is small, the starting process is smooth, the stepless speed regulation of the motor can be realized, the operating parameters can be automatically adjusted according to the load condition, and the energy-saving effect is obvious. However, the frequency converter is expensive, has relatively high requirements for the operating environment, and is complicated to maintain.