Oct 31, 2020
Soft starter control and comparison of various motor starting methods
The soft starter is especially suitable for various pump loads or fan loads, where soft start and soft stop are required.
Similarly, for variable load conditions and long-term light-load operation of the motor, and only short-term or momentary heavy-load occasions, the application of a soft starter (without bypass contactor) has the effect of light-load energy saving.
Motor soft starters generally use high-power bidirectional thyristors to form a three-phase AC voltage regulating circuit, and a microprocessor and signal acquisition and protection links to constitute a controller. By controlling the trigger angle of the thyristor, the output voltage of the thyristor voltage regulating circuit is adjusted to realize the motor Energy-saving and protection functions of non-contact step-down soft start, soft stop, no-load, light-load. To this end, by comparing the soft start mode with the traditional start mode, the characteristics of the soft starter are analyzed.
The difference between soft start and general buck start
When the motor is started, reducing the voltage applied to the stator winding of the motor can reduce the starting current of the motor. Generally, step-down start means that the voltage change applied to the stator winding of the motor during the start-up process is sudden and sudden, mainly including "Y-△" step-down start and self-coupling transformer step-down start, etc.; while soft start uses voltage regulation The device automatically increases the starting voltage continuously and smoothly within the specified starting time until it reaches the rated voltage.
If the general step-down start is used, the start process will be jumpy and not smooth, so it is also called a hard start. It is not suitable for occasions where stable start is required for the production process. The soft start voltage increases continuously and steadily from the initial voltage. During the starting process, the motor torque is smooth instead of jumping. The starting process is smooth, so it is called soft start.
The working principle of the soft starter is that when the motor is started, the conduction angle of the thyristor is controlled by the electronic circuit to gradually increase the terminal voltage of the motor at a set speed until it reaches the full voltage, so that the motor can start without impact to control the motor soft The process of starting. When the motor is started and reaches the rated voltage, the three-phase bypass contactor is closed, and the motor is directly put into grid operation.
If it is a light load, during normal operation, the required lower end voltage is also maintained, which increases the power factor of the motor and the efficiency. When the motor is stopped, the conduction angle of the thyristor is also controlled to slowly reduce the voltage at the motor terminal to 0, so as to achieve soft shutdown.
Soft start characteristics
(1) The starting current rises to the set value with a certain slope, without impact on the grid.
(2) The current negative feedback is introduced during the starting process, and the starting current rises to the set value to make the motor start smoothly.
(3) Not affected by the fluctuation of the grid voltage. Since the soft start takes the current as the set value, when the grid voltage fluctuates up and down, by increasing or decreasing the conduction angle of the thyristor and adjusting the terminal voltage of the motor, the constant value of the starting current can still be maintained to ensure the normal start of the motor.
(4) According to the requirements of different loads on the motor, the starting current setting value can be steplessly adjusted, and the motor starting time can be changed to achieve the best starting time control.
Comparison of various starting methods of motors
Because asynchronous motors have the advantages of simple structure, small size, low price, reliable operation, convenient maintenance, high operating efficiency and good working characteristics, they are widely used on electric drive platforms. However, it has the disadvantage of large starting current (generally, the starting current is 4-7 times the rated current, and the starting current of some domestic motors is as high as 8-12 times the rated current after actual measurement). Excessive starting current will adversely affect the normal operation of the motor itself, the power grid and other electrical equipment, and will produce instantaneous excessive torque on the motor shaft (up to 1.6 to 2.0 times the full load torque of the motor) and twist the motor Shafts, damage to the keyway, damage and other equipment connected to the shaft will cause the motor to heat up and affect its life, and the voltage loss of the power supply line will increase, which may damage the normal operation of other electrical equipment connected in parallel on the same power supply line.
The relevant state departments have clearly stipulated that the voltage drop of the power grid cannot exceed 15% when the motor starts. For motors with larger capacity, measures should be taken to reduce the starting current of the motor. Usually asynchronous motors always run at full voltage. The magnetic field of the motor is almost unchanged from no load to full load. Therefore, the magnetizing current is approximately the same under all loads.
By comparing various starting methods of asynchronous motors:
When the motor is started at full voltage, the impact on the power grid is the greatest, and the impact time is also the longest; and the commonly used step-down start is a hard start. Although the impact on the power grid is relatively small, it involves a coil voltage switching process, so Unfavorable links with secondary shocks;
Since the soft start is set to a starting current that does not affect the grid before starting, the current is slowly increased to the set current, so there is no inrush current, the impact on the grid is minimal, and the impact of starting torque can be eliminated.
Soft starter control and comparison of various motor starting methods
Use a soft starter connected in series between the power supply and the controlled motor to control the conduction angle of its internal thyristor, so that the motor input voltage gradually rises from zero with a preset function relationship, until the start is completed, and the motor is given full voltage, namely It is a soft start. According to the different needs of different industries, there are also different requirements for the starting method of the soft starter. Generally, there are the following methods:
1. Slope boost soft start
This kind of starting method is the simplest. It does not have current closed-loop control and only adjusts the conduction angle of the thyristor to increase it as a function of time. The disadvantage is that due to the unlimited current, in the motor starting process, sometimes a large inrush current will be generated to damage the thyristor, which has a greater impact on the power grid, and is rarely used in practice.
2. Step start
Turn on, that is, make the starting current quickly reach the set value in the shortest time, which is a step start. By adjusting the starting current setting value, a quick start effect can be achieved.
3. Slope constant current soft start
This kind of starting method is to increase the starting current gradually in the initial stage of motor starting, and keep constant when the current reaches the preset value until the starting is completed. This starting method is the most widely used starting method, especially suitable for starting fans and pumps.
4. Pulse shock start
In the starting stage, let the thyristor turn on with a relatively large current in a very short period of time and then fall back, and then increase linearly according to the original set value, and connect to constant current start. This starting method is rarely used in general loads, and is suitable for starting occasions with heavy loads and large static friction needs to be overcome.
5. Voltage double ramp start
During the starting process, the output torque of the motor increases with the voltage. When starting, an initial starting voltage Us is provided. Us can be adjusted according to the load. Adjust Us to be greater than the static friction torque of the load. When the output voltage reaches the speed voltage Ur, The motor basically reaches the rated speed. The soft starter automatically detects the up-speed voltage during the starting process, and when the motor reaches the rated speed, the output voltage reaches the rated voltage.
6. Current limit start
Current limiting starting is a soft starting method that limits the starting current of the motor to not exceed a certain set value during the starting process of the motor. The output voltage increases rapidly from zero until the output current reaches the preset current limit Im, and then the output current I is maintained. The advantage of this starting method is that the starting current is small and can be adjusted as needed.
Soft starter control and comparison of various motor start methods
Starter (soft starter) is a motor control device that integrates motor soft start, soft stop, light load energy saving and multiple protection functions. The soft starter uses a three-phase parallel thyristor as a voltage regulator, which is connected between the power supply and the motor stator.
Such a circuit is a three-phase fully controlled bridge rectifier circuit. When using the soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on. The motor works on the mechanical characteristics of the rated voltage to achieve smooth starting, reduce the starting current, and avoid starting over-current tripping.
When the motor reaches the rated number of revolutions, the start-up process ends, and the soft starter automatically replaces the completed thyristor with a bypass contactor to provide the rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and extend the service life of the soft starter , Improve its work efficiency, and make the grid avoid harmonic pollution