Soft start
2023-09-15 12:12:11
The voltage is slowly increased from zero to the rated voltage, so that the starting current of the motor during the starting process is controlled by the uncontrollable overload current in the past.
Brief introduction of soft start and working principle The voltage is gradually increased from zero to the rated voltage, so that the starting current of the motor during the starting process is controlled by the uncontrollable overload current in the past. And the starting current can be adjusted as needed. There is no impact torque in the whole process of starting the motor, but a smooth start-up operation. This is the soft start.
Soft starter (soft starter) is a novel motor control device that integrates motor soft start, soft stop, light load energy saving and various protection functions. It is called Soft Starter in foreign countries. Its main component is a three-phase parallel thyristor connected in series between the power source and the controlled motor and its electronic control circuit. Different methods are used to control the conduction angle of the three opposite parallel thyristors, so that the input voltage of the controlled motor changes according to different requirements, and different functions can be realized. Soft starters and frequency converters are two completely different applications. The inverter is used for speed regulation. The output not only changes the voltage but also changes the frequency. The soft starter is actually a voltage regulator. When the motor starts, the output only changes the voltage and does not change the frequency. The frequency converter has all the soft starter functions, but it is much more expensive than the soft starter and the structure is much more complicated. The motor soft starter uses a soft starter connected in series between the power supply and the controlled motor to control the conduction angle of the internal thyristor, so that the input voltage of the motor gradually rises from zero with a preset function relationship until the end of the start, and the motor is given to the motor. The full voltage, that is, the soft start, during the soft start process, the motor starting torque is gradually increased, and the rotational speed is gradually increased.
The soft starter uses three opposite parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. Such a circuit is a three-phase fully controlled bridge rectifier circuit. When the motor is started by the soft starter, the output voltage of the thyristor is gradually increased, and the motor is gradually accelerated until the thyristor is fully turned on. The motor operates on the mechanical characteristic of the rated voltage to achieve a smooth start, reduce the starting current, and avoid starting the overcurrent trip. When the motor reaches the rated number of revolutions, the start-up process ends. The soft starter automatically replaces the completed thyristor with a bypass contactor to provide a rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and prolong the service life of the soft starter. To improve the efficiency of its work, and to avoid harmonic pollution in the power grid. The soft starter also provides a soft stop function. The soft stop is opposite to the soft start process. The voltage gradually decreases and the number of revolutions gradually decreases to zero, avoiding the torque shock caused by free parking. The difference between soft start and traditional decompression start mode is:
(1) No inrush current. When the soft starter starts the motor, the motor starting current is linearly increased from zero to a set value by gradually increasing the conduction angle of the thyristor. No impact on the motor, improve the reliability of power supply, start smoothly, reduce the impact torque on the load machine, and extend the service life of the machine.
(2) There is a soft parking function, that is, smooth deceleration, and gradually stop, it can overcome the shortcomings of instantaneous power failure, reduce the impact on heavy-duty machinery, avoid the water hammer effect of the elevation water supply system, and reduce equipment damage.
(3) The starting parameters are adjustable. According to the load condition and the grid relay protection characteristics, it can be freely and steplessly adjusted to the optimal starting current.
Comparison of several large-scale motor starting methods At present, China's economy has entered a new stage of development, with more and more large-scale enterprises and large-scale equipment, more and more applications of large-scale motors (5000kW to 60,000kW), and large-scale motor starting methods. More and more attention has been paid.
Social development is phased. At different stages, people's production methods, production tools and daily necessities are very different. In the 1980s and 1990s, China’s economic strength was still weak. At that time, small cement and small steel developed rapidly. The application of 1000kW～4000kW motor grew rapidly, and the liquid starter adapted to the economic foundation at that time appeared. Economical and practical, it solves some problems in motor starting. It played a certain role in the economic development at that time. By the turn of the century, China's economic strength has been greatly developed, production methods and production tools have also been greatly developed, and the motor capacity has also increased greatly. People began to fail to meet the low performance of liquid starters, so the thyristors are connected in series. The application of the type (solid state) soft starter began to increase, and then the switch transformer type soft starter and the magnetic saturation reactor type (magnetron) starter appeared. The frequency converter was used for the soft start of the motor. Currently, these four products are the mainstream products in the large motor starting market, and the liquid starting devices are used in small (5000 kW or less) motors. In addition, two old-fashioned starting methods (autotransformers and transformer-motor units) are also often found on start-ups of motors below 20000 kW.
Large-scale motor-driven equipment is generally the core equipment of the enterprise, which directly affects the production status of the enterprise. Therefore, people should pay special attention to its start-up, and reasonable selection of the starting device will bring great economic benefits to the enterprise. However, motor starting technology is not a core technology of an enterprise. Many industrial electrical workers have little time to study the differences between various starting methods, often resulting in inappropriate choices, and sometimes even have to make a second Choose to cause undue losses to the company. Therefore, it is very important to faithfully describe the performance and differences of various starting methods.
1. Cause grid voltage fluctuation, affecting the operation of other equipment in the same grid. When the motor is directly started at full voltage, the starting current will reach 4~7 times of the rated current. When the capacity of the motor is relatively large, the starting current will cause the grid voltage. The sharp decline has affected the normal operation of other equipment in the grid. When starting soft start, the starting current is generally 2~3 times of the rated current, and the grid voltage fluctuation rate is generally within 10%, which has little impact on other equipment.
2. Impact on the grid
The impact of a large current directly driven by a super large motor on the power grid is almost similar to the impact of a three-phase short circuit on the power grid, which often causes power oscillations and makes the power grid unstable.
2 The starting current contains a large number of high-order harmonics, which will cause high-frequency resonance with the grid circuit parameters, causing malfunctions such as relay protection malfunction and automatic control failure. The starting current is greatly reduced during soft start, and the above effects can be completely eliminated.
3. Damage motor insulation and reduce motor life
The Joule heat generated by a large current repeatedly acts on the outer insulation of the wire to accelerate the aging of the insulation and reduce the life.
The mechanical force generated by the 2 large currents causes the wires to rub against each other, reducing the insulation life.
3 When the high-voltage switch is closed, the jitter of the contact will generate an operating over-voltage on the stator winding of the motor, sometimes reaching more than 5 times the applied voltage. Such a high over-voltage will cause great damage to the motor insulation. During soft start, the maximum current is reduced by about half, and the instantaneous heat generation is only about 1/4 of the straight-up, and the insulation life will be greatly extended. When the soft start, the motor terminal voltage can be adjusted from zero, and the over-voltage damage can be completely eliminated.
4. Electric power damage to the motor
The large current generates a large impact force on the stator coil of the motor and the squirrel cage of the rotor, which may cause failures such as loose clamping, deformation of the coil, and breakage of the squirrel cage. At the time of soft start, since the maximum current is small, the impact force is greatly reduced.
5. For the mechanical equipment " target=_blank> damage to the mechanical equipment, the starting torque at the direct start is about twice the rated torque. Such a large torque is suddenly added to the stationary mechanical equipment "target=_blank> mechanical equipment It will accelerate gear wear and even toothing, accelerate belt wear and even break the belt, accelerate the fatigue of the blade and even break the blade.
The torque of the soft start does not exceed the rated torque, and the above drawbacks can be completely overcome.
When the decompression start is adopted, the above hazard is only reduced to a certain extent; when the soft start is adopted, the above hazard almost completely disappears; the direct start of the independent transformer power supply mode can only alleviate the fluctuation of the grid voltage, and other hazards are all Still exist.
Very large motors are of high value and play a central role in production. A little fault will cause great economic losses, and it is necessary to adopt perfect protection. For example, for a motor we can't expect its insulation to be completely consistent. There may be a weak link at a certain point. It can pass through the factory test, but under the long-term impact, this weak link will gradually become the first. It is revealed to shorten its life. If we take a soft start, we can greatly extend the life of the motor. Which of the two schemes is worthwhile? This is obvious.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
Brief introduction of soft start and working principle The voltage is gradually increased from zero to the rated voltage, so that the starting current of the motor during the starting process is controlled by the uncontrollable overload current in the past. And the starting current can be adjusted as needed. There is no impact torque in the whole process of starting the motor, but a smooth start-up operation. This is the soft start.
Soft starter (soft starter) is a novel motor control device that integrates motor soft start, soft stop, light load energy saving and various protection functions. It is called Soft Starter in foreign countries. Its main component is a three-phase parallel thyristor connected in series between the power source and the controlled motor and its electronic control circuit. Different methods are used to control the conduction angle of the three opposite parallel thyristors, so that the input voltage of the controlled motor changes according to different requirements, and different functions can be realized. Soft starters and frequency converters are two completely different applications. The inverter is used for speed regulation. The output not only changes the voltage but also changes the frequency. The soft starter is actually a voltage regulator. When the motor starts, the output only changes the voltage and does not change the frequency. The frequency converter has all the soft starter functions, but it is much more expensive than the soft starter and the structure is much more complicated. The motor soft starter uses a soft starter connected in series between the power supply and the controlled motor to control the conduction angle of the internal thyristor, so that the input voltage of the motor gradually rises from zero with a preset function relationship until the end of the start, and the motor is given to the motor. The full voltage, that is, the soft start, during the soft start process, the motor starting torque is gradually increased, and the rotational speed is gradually increased.
The soft starter uses three opposite parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. Such a circuit is a three-phase fully controlled bridge rectifier circuit. When the motor is started by the soft starter, the output voltage of the thyristor is gradually increased, and the motor is gradually accelerated until the thyristor is fully turned on. The motor operates on the mechanical characteristic of the rated voltage to achieve a smooth start, reduce the starting current, and avoid starting the overcurrent trip. When the motor reaches the rated number of revolutions, the start-up process ends. The soft starter automatically replaces the completed thyristor with a bypass contactor to provide a rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and prolong the service life of the soft starter. To improve the efficiency of its work, and to avoid harmonic pollution in the power grid. The soft starter also provides a soft stop function. The soft stop is opposite to the soft start process. The voltage gradually decreases and the number of revolutions gradually decreases to zero, avoiding the torque shock caused by free parking. The difference between soft start and traditional decompression start mode is:
(1) No inrush current. When the soft starter starts the motor, the motor starting current is linearly increased from zero to a set value by gradually increasing the conduction angle of the thyristor. No impact on the motor, improve the reliability of power supply, start smoothly, reduce the impact torque on the load machine, and extend the service life of the machine.
(2) There is a soft parking function, that is, smooth deceleration, and gradually stop, it can overcome the shortcomings of instantaneous power failure, reduce the impact on heavy-duty machinery, avoid the water hammer effect of the elevation water supply system, and reduce equipment damage.
(3) The starting parameters are adjustable. According to the load condition and the grid relay protection characteristics, it can be freely and steplessly adjusted to the optimal starting current.
Comparison of several large-scale motor starting methods At present, China's economy has entered a new stage of development, with more and more large-scale enterprises and large-scale equipment, more and more applications of large-scale motors (5000kW to 60,000kW), and large-scale motor starting methods. More and more attention has been paid.
Social development is phased. At different stages, people's production methods, production tools and daily necessities are very different. In the 1980s and 1990s, China’s economic strength was still weak. At that time, small cement and small steel developed rapidly. The application of 1000kW～4000kW motor grew rapidly, and the liquid starter adapted to the economic foundation at that time appeared. Economical and practical, it solves some problems in motor starting. It played a certain role in the economic development at that time. By the turn of the century, China's economic strength has been greatly developed, production methods and production tools have also been greatly developed, and the motor capacity has also increased greatly. People began to fail to meet the low performance of liquid starters, so the thyristors are connected in series. The application of the type (solid state) soft starter began to increase, and then the switch transformer type soft starter and the magnetic saturation reactor type (magnetron) starter appeared. The frequency converter was used for the soft start of the motor. Currently, these four products are the mainstream products in the large motor starting market, and the liquid starting devices are used in small (5000 kW or less) motors. In addition, two old-fashioned starting methods (autotransformers and transformer-motor units) are also often found on start-ups of motors below 20000 kW.
Large-scale motor-driven equipment is generally the core equipment of the enterprise, which directly affects the production status of the enterprise. Therefore, people should pay special attention to its start-up, and reasonable selection of the starting device will bring great economic benefits to the enterprise. However, motor starting technology is not a core technology of an enterprise. Many industrial electrical workers have little time to study the differences between various starting methods, often resulting in inappropriate choices, and sometimes even have to make a second Choose to cause undue losses to the company. Therefore, it is very important to faithfully describe the performance and differences of various starting methods.
1. Cause grid voltage fluctuation, affecting the operation of other equipment in the same grid. When the motor is directly started at full voltage, the starting current will reach 4~7 times of the rated current. When the capacity of the motor is relatively large, the starting current will cause the grid voltage. The sharp decline has affected the normal operation of other equipment in the grid. When starting soft start, the starting current is generally 2~3 times of the rated current, and the grid voltage fluctuation rate is generally within 10%, which has little impact on other equipment.
2. Impact on the grid
The impact of a large current directly driven by a super large motor on the power grid is almost similar to the impact of a three-phase short circuit on the power grid, which often causes power oscillations and makes the power grid unstable.
2 The starting current contains a large number of high-order harmonics, which will cause high-frequency resonance with the grid circuit parameters, causing malfunctions such as relay protection malfunction and automatic control failure. The starting current is greatly reduced during soft start, and the above effects can be completely eliminated.
3. Damage motor insulation and reduce motor life
The Joule heat generated by a large current repeatedly acts on the outer insulation of the wire to accelerate the aging of the insulation and reduce the life.
The mechanical force generated by the 2 large currents causes the wires to rub against each other, reducing the insulation life.
3 When the high-voltage switch is closed, the jitter of the contact will generate an operating over-voltage on the stator winding of the motor, sometimes reaching more than 5 times the applied voltage. Such a high over-voltage will cause great damage to the motor insulation. During soft start, the maximum current is reduced by about half, and the instantaneous heat generation is only about 1/4 of the straight-up, and the insulation life will be greatly extended. When the soft start, the motor terminal voltage can be adjusted from zero, and the over-voltage damage can be completely eliminated.
4. Electric power damage to the motor
The large current generates a large impact force on the stator coil of the motor and the squirrel cage of the rotor, which may cause failures such as loose clamping, deformation of the coil, and breakage of the squirrel cage. At the time of soft start, since the maximum current is small, the impact force is greatly reduced.
5. For the mechanical equipment " target=_blank> damage to the mechanical equipment, the starting torque at the direct start is about twice the rated torque. Such a large torque is suddenly added to the stationary mechanical equipment "target=_blank> mechanical equipment It will accelerate gear wear and even toothing, accelerate belt wear and even break the belt, accelerate the fatigue of the blade and even break the blade.
The torque of the soft start does not exceed the rated torque, and the above drawbacks can be completely overcome.
When the decompression start is adopted, the above hazard is only reduced to a certain extent; when the soft start is adopted, the above hazard almost completely disappears; the direct start of the independent transformer power supply mode can only alleviate the fluctuation of the grid voltage, and other hazards are all Still exist.
Very large motors are of high value and play a central role in production. A little fault will cause great economic losses, and it is necessary to adopt perfect protection. For example, for a motor we can't expect its insulation to be completely consistent. There may be a weak link at a certain point. It can pass through the factory test, but under the long-term impact, this weak link will gradually become the first. It is revealed to shorten its life. If we take a soft start, we can greatly extend the life of the motor. Which of the two schemes is worthwhile? This is obvious.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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