1. Hazards of direct starting
Direct starting is a relatively simple starting method. When starting, the motor is directly connected to the power grid through a knife or a contactor. The advantages of direct starting are that the starting equipment is simple and the starting speed is fast, but the harm of direct starting is great:
(1) Grid impact: Excessive starting current (no-load starting current can reach 4 to 7 times the rated current, and 8 to 10 times or more when starting with load) will cause the grid voltage to drop and affect other applications. The normal operation of the electrical equipment may also cause the undervoltage protection to act, resulting in harmful tripping of the equipment. At the same time, excessive starting current will make the motor windings heat up, thereby accelerating insulation aging and affecting the life of the motor;
(2) Mechanical impact: Excessive impact torque often causes the motor rotor cage bars, end rings to break and the stator end winding insulation to wear, resulting in breakdown of the machine, twisting of the rotating shaft, damage to the coupling, transmission gear and belt tearing Wait;
(3) Impact on production machinery: The sudden change of pressure during the starting process often causes damage to the pipelines and valves of the pump system, shortens the service life, affects the transmission accuracy, and even affects the normal process control.
All of these pose a threat to the safe and reliable operation of the equipment, and also cause excessive starting energy consumption, especially when starting and stopping frequently. Therefore, the following restrictions apply to the direct starting of the motor:
(1) Whether the production machinery allows the direct start of the drag motor, which is a prerequisite;
(2) The capacity of the motor should not be greater than 10% to 15% of the capacity of the power supply transformer;
(3) The voltage drop △U during the starting process should not be greater than 15% of the rated voltage. For medium and high power motors, direct starting is generally not allowed, and certain starting equipment is required to complete the normal starting work.
2. New electronic soft starter
With the development of power electronic technology and microcomputer control technology, a series of electronic starting control devices have been developed at home and abroad, which are used for the starting control of asynchronous motors to replace the traditional step-down starting equipment. The main circuit of the new electronic soft starter generally adopts the thyristor voltage regulator circuit. The voltage regulator is composed of six thyristors in reverse parallel connection, which are connected in series to the three-phase power supply line of the motor. When the starter's microcomputer control system receives the start command, it will perform relevant calculations, output the trigger signal of the thyristor, and control the conduction angle α of the thyristor to make the starter adjust the output voltage according to the designed mode to control the motor's output voltage. start process. When the starting process is completed, the general starter will pull in the bypass contactor, short-circuit all the thyristor main circuits, and make the motor directly put into the power grid to avoid unnecessary power loss.