1, start directly

The advantages of direct start-up are less required equipment, simple start-up mode and low cost. Theoretically, all motors can be started directly. For motors that start frequently, the capacity of the transformer that needs to provide power is more than 5 times that of the motor; For motors that don't start frequently, the capacity of the transformer that needs to provide power is more than 3 times that of the motor; This requirement is easy to realize for small-capacity motors, so most small-capacity motors are started directly without step-down starting. For large-capacity motors, on the one hand, the capacity of power supply lines and transformers is difficult to meet the requirements of direct starting of motors; On the other hand, the strong starting current impacts the power grid and the motor, which affects the service life of the motor and is unfavorable to the stable operation of the power grid. Therefore, whether it is a large-capacity motor or a motor that cannot be started directly, step-down starting should be adopted.

Direct start-up can realize short-distance running, inching control, speed control and positive and negative rotation control of the motor with bakelite switch, iron shell switch and air switch (circuit breaker), and can also realize long-distance running, inching control, speed control, positive and negative rotation control and automatic control of the motor with limit switch, AC contactor and time relay.

2. Use autotransformer start

When the autotransformer is used for starting, the starting current and starting torque of the motor decrease in direct proportion to the square of its terminal voltage, and a larger starting speed can be obtained under the same starting current. If the starting voltage drops to 65% of the rated voltage, the starting current is 42% of the full-pressure starting current and the starting torque is 42% of the full-pressure starting torque.

The advantage of autotransformer startup is that it can be directly controlled by manual operation or automatically controlled by AC contactor. It is durable and has low maintenance cost. It is suitable for no-load and light-load starting of all asynchronous motors and has been widely used in production practice. The disadvantage is that manual operation needs to be equipped with expensive autotransformer box (autotransformer box), and automatic control needs to be equipped with autotransformer, AC contactor and other starting equipment and components.

3, y-delta step-down start

The stator winding is a triangle-connected motor, which is connected with Y when starting, and turns to triangle operation when the speed is close to the rated speed. When starting in this way, the stator winding of each phase is reduced to 58% of the power supply voltage, the starting current is 33% of that of direct starting, and the starting torque is 33% of that of direct starting. Small starting current, small starting torque.

The advantage of Y-△ step-down starting is that it can be realized without adding starting equipment, but it can only be used for triangle-connected motors, and large asynchronous motors cannot be started under heavy load.

4, rotor series resistance start

Wound-wound three-phase asynchronous motor, the rotor winding is connected with the resistor through the slip ring. External series resistance is equivalent to the increase of internal resistance of rotor winding, which reduces the induced current of rotor winding. From a certain point of view, the motor is like a transformer, and the secondary current is very small, which is equivalent to the motor excitation winding current of the primary winding of the transformer. According to the characteristics of the motor, the series resistance of the rotor will reduce the speed of the motor, improve the rotating torque and have better starting performance.

In this startup mode, because the resistance is constant, the startup resistance is divided into several levels, which can be cut off step by step during the startup process, so that a smoother startup process can be obtained.

According to the above analysis, in order to obtain more stable starting characteristics, it is necessary to increase the number of starting stages, which will complicate the equipment. The starting mode of series connection of frequency sensitive rheostat on the rotor is adopted to make the starting more stable.

The starting principle of the frequency-sensitive rheostat is as follows: when the stator winding of the motor is connected to the power supply, when the motor is started, because the frequency-sensitive rheostat is connected in series, the rotor speed of the motor is very low and the starting current is very small, so the rotor frequency is high, f2≈f 1, and the iron loss of the frequency-sensitive rheostat is very large. With the increase of rotating speed, the frequency of rotor current decreases gradually, and the impedance of inductor decreases accordingly. This is equivalent to stepless elimination of resistance during start-up. When the rotating speed rises to a stable value, the frequency sensitive resistor is short-circuited and the starting process ends.

Although the rotor series resistance or frequency sensitive rheostat has good starting performance, it can start under heavy load. Because it is only suitable for wound three-phase asynchronous motors with expensive price and complicated structure, it is only used in various industries with high requirements for starting control and speed control, such as elevators, conveyors and traveling cranes.

5, soft starter

Soft starter is a new motor control device which integrates motor soft start, soft stop, light load energy saving and various protection functions. It is called soft starter abroad. It is mainly composed of three anti-parallel brake pipe AC voltage regulators connected in series between the power supply and the controlled motor. By adopting different methods and changing the trigger angle of thyristor, the output voltage of thyristor voltage regulating circuit can be adjusted. During the whole starting process, the output of the soft starter is a smooth step-up process until the thyristor is completely turned on and the motor works at the rated voltage.

The soft starter has the advantages of low voltage starting and low starting current, and is suitable for all no-load and light-load asynchronous motors. The disadvantage is that the starting torque is small, which is not suitable for heavy-load starting of large motors.

6, frequency converter

Frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency, which can realize soft start, variable frequency speed regulation, improve operation accuracy, change power factor, over-current/over-voltage/overload protection and other functions.

Compared with other control devices, the subtlety of the frequency converter lies in the proportional change of frequency and voltage, that is, the output voltage of the frequency converter is controlled while changing the frequency, so that the magnetic flux of the motor remains constant, similar to the constant power speed regulation mode, and the phenomena of weak magnetism and magnetic saturation are avoided.

Although the frequency converter is expensive, good in performance and complex in structure, it is simple to use. It is the best equipment for modern motor control.