Soft Starting and Optimizing Technology of AC Asynchronous Motor
1. Harm of Direct Start
Direct starting is a relatively simple way of starting. When starting, the motor is directly connected to the power grid through the gate or contactor. The advantage of direct starting is that the starting equipment is simple and the starting speed is fast, but the harm of direct starting is great.
(1) Power grid impact: excessive starting current (no-load starting current can reach 4-7 times of rated current, 8-10 times or more when starting with load) will cause voltage drop of power grid, affect the normal operation of other electrical equipment, and may cause undervoltage protection action, resulting in harmful tripping of equipment. At the same time, excessive starting current will heat the motor windings, thus accelerating insulation aging and affecting the life of the motor.
(2) Mechanical impact: Too much impact torque often causes the breakage of cage bar, end ring and insulation wear of stator end winding of motor rotor, leading to breakdown burner, twist of rotating shaft, damage of coupling, transmission gear and belt tear, etc.
(3) Impacts on production machinery: sudden change of pressure during start-up often causes damage to pump system pipes and valves, shortens service life, affects transmission accuracy, and even affects normal process control.
All of these bring a threat to the safe and reliable operation of the equipment, but also cause excessive loss of starting energy, especially when starting and stopping frequently. Therefore, there are the following restrictions on direct starting of motors:
(1) Whether the production machinery permits the direct starting of the driving motor is a prerequisite;
(2) The capacity of the motor shall be no more than 10%-15% of the capacity of the power supply transformer;
(3) The voltage drop (U) during starting should be no more than 15% of the rated voltage. For medium and high power motors, it is generally not allowed to start directly, but a certain starting equipment is required to complete the normal starting work.
2. New Electronic Soft Starter
With the development of power electronics technology and computer control technology, a series of electronic starting control devices have been developed at home and abroad, which are used for starting control of asynchronous motors to replace the traditional step-down starting devices. The main circuit of the new electronic soft starter generally adopts the thyristor voltage regulating circuit, which consists of six thyristors in two reverse parallel connection and is connected in series to the three-phase power supply circuit of the motor. When the micro-computer control system of the starter receives the starting instruction, it carries out relevant calculation, outputs the trigger signal of the thyristor, and adjusts the output voltage according to the designed mode by controlling the conduction angle of the thyristor, so as to control the starting process of the motor. When the starting process is completed, the general starter will suck up the bypass contactor, short-circuit all the main thyristor circuits, so that the motor can be directly put into power grid operation, in order to avoid unnecessary power loss.
Direct starting is a relatively simple way of starting. When starting, the motor is directly connected to the power grid through the gate or contactor. The advantage of direct starting is that the starting equipment is simple and the starting speed is fast, but the harm of direct starting is great.
(1) Power grid impact: excessive starting current (no-load starting current can reach 4-7 times of rated current, 8-10 times or more when starting with load) will cause voltage drop of power grid, affect the normal operation of other electrical equipment, and may cause undervoltage protection action, resulting in harmful tripping of equipment. At the same time, excessive starting current will heat the motor windings, thus accelerating insulation aging and affecting the life of the motor.
(2) Mechanical impact: Too much impact torque often causes the breakage of cage bar, end ring and insulation wear of stator end winding of motor rotor, leading to breakdown burner, twist of rotating shaft, damage of coupling, transmission gear and belt tear, etc.
(3) Impacts on production machinery: sudden change of pressure during start-up often causes damage to pump system pipes and valves, shortens service life, affects transmission accuracy, and even affects normal process control.
All of these bring a threat to the safe and reliable operation of the equipment, but also cause excessive loss of starting energy, especially when starting and stopping frequently. Therefore, there are the following restrictions on direct starting of motors:
(1) Whether the production machinery permits the direct starting of the driving motor is a prerequisite;
(2) The capacity of the motor shall be no more than 10%-15% of the capacity of the power supply transformer;
(3) The voltage drop (U) during starting should be no more than 15% of the rated voltage. For medium and high power motors, it is generally not allowed to start directly, but a certain starting equipment is required to complete the normal starting work.
2. New Electronic Soft Starter
With the development of power electronics technology and computer control technology, a series of electronic starting control devices have been developed at home and abroad, which are used for starting control of asynchronous motors to replace the traditional step-down starting devices. The main circuit of the new electronic soft starter generally adopts the thyristor voltage regulating circuit, which consists of six thyristors in two reverse parallel connection and is connected in series to the three-phase power supply circuit of the motor. When the micro-computer control system of the starter receives the starting instruction, it carries out relevant calculation, outputs the trigger signal of the thyristor, and adjusts the output voltage according to the designed mode by controlling the conduction angle of the thyristor, so as to control the starting process of the motor. When the starting process is completed, the general starter will suck up the bypass contactor, short-circuit all the main thyristor circuits, so that the motor can be directly put into power grid operation, in order to avoid unnecessary power loss.