How many kilowatts can a three-phase asynchronous motor generally start directly?

The three-phase asynchronous motor is a common type of motor, widely used in industrial production and daily life. In practical applications, the starting method of the three-phase asynchronous motor has an important impact on its performance and life. This article will introduce the starting method of the three-phase asynchronous motor in detail, especially the scope and limitations of direct starting.

1. The basic principle of three-phase asynchronous motor

The three-phase asynchronous motor is an electric motor that works on the principle of electromagnetic induction. It consists of two parts: the stator and the rotor. The stator is the fixed part of the motor, usually composed of three-phase windings. The rotor is the rotating part of the motor, usually composed of a squirrel cage or wound structure. When three-phase alternating current passes through the stator winding, a rotating magnetic field is generated in the stator. This rotating magnetic field cuts the rotor conductor, thereby generating an induced current in the rotor. The induced current interacts with the rotating magnetic field to generate an electromagnetic force, which causes the rotor to rotate.

1. Starting method of three-phase asynchronous motor

There are mainly the following starting methods for three-phase asynchronous motors:

(1) Direct starting: Direct starting means connecting the motor directly to the power supply so that it is subjected to the rated voltage during starting. Direct starting has the advantages of simple starting and low cost, but the starting current is large, which has a certain impact on the power grid and the motor itself.

(2) Star-delta starting: Star-delta starting is a commonly used starting method suitable for motors with higher power. During starting, the three-phase windings of the motor are connected in a star shape to reduce the starting current. After starting, the motor is switched to a delta connection to provide the rated voltage and rated power.

(3) Autotransformer starting: Autotransformer starting uses an autotransformer to reduce the starting voltage of the motor, thereby reducing the starting current. After starting, the autotransformer is removed to allow the motor to run at rated voltage.

(4) Soft starter start: A soft starter is an electronic control device that can gradually increase the voltage during the motor start-up process to achieve smooth start-up. Soft starters have the advantages of smooth start-up and little impact on the power grid, but they are relatively expensive.

1. Application scope of direct starting of three-phase asynchronous motor

Direct starting is suitable for small three-phase asynchronous motors, generally with a power of less than 7.5 kilowatts. This is because when starting directly, the starting current of the motor is large, which has a certain impact on the power grid and the motor itself. For large motors, direct starting may cause problems such as grid voltage drop and motor overheating.

1. Limitations of direct starting of three-phase asynchronous motors

(1) Large starting current: When starting directly, the starting current of the motor is about 5-7 times the rated current. This will cause the grid voltage to drop and affect the normal operation of other equipment.

(2) Small starting torque: When starting directly, the starting torque of the motor is small, about 1.5-2 times the rated torque. This may cause the motor to fail to start or start with difficulty.

(3) Impact on the motor itself: During direct starting, the motor is subjected to a large starting current and torque, which may cause problems such as motor overheating and insulation damage, thus affecting the life of the motor.

1. Selection of starting methods for three-phase asynchronous motors

When choosing the starting method for a three-phase asynchronous motor, it is necessary to comprehensively consider factors such as the motor's power, starting torque, and grid conditions. For small motors, direct starting is possible; for large motors, star-delta starting, autotransformer starting, or soft starter starting is recommended.

1. in conclusion

The starting method of a three-phase asynchronous motor has an important impact on its performance and life. Direct starting is suitable for motors with lower power, but there are problems such as large starting current and small starting torque. When choosing a starting method, it is necessary to make comprehensive considerations based on the actual situation of the motor. By reasonably choosing a starting method, the operating efficiency and life of a three-phase asynchronous motor can be improved.