In principle, squirrel-cage induction motors are suitable for all applications where speed regulation is not required. Currently, the application range is AC 380V (660V/1140V is also possible), with motor power ranging from several kilowatts to 800kW. Soft starters are particularly suitable for various pump or fan loads requiring soft starting and stopping. Similarly, for variable load conditions, where the motor operates under light load for extended periods and only experiences short periods or momentary heavy loads, using a soft starter (without a bypass contactor) provides energy savings under light load conditions.
Motor soft starters typically use high-power bidirectional thyristors to construct a three-phase AC voltage regulating circuit, and a microprocessor and signal acquisition and protection circuits to form a controller. By controlling the firing angle of the thyristors, the output voltage of the thyristor voltage regulating circuit is adjusted to achieve contactless reduced-voltage soft starting, soft stopping, and energy-saving and protection functions for the motor under no-load and light-load conditions. Therefore, this paper compares the soft starting method with traditional starting methods to analyze the characteristics of soft starters.
The difference between soft start and regular reduced voltage start
When starting a motor, reducing the voltage applied to the stator windings can decrease the starting current. Generally, reduced-voltage starting refers to a sudden, instantaneous change in the voltage applied to the stator windings during the starting process. This mainly includes "Y-Δ" reduced-voltage starting and autotransformer reduced-voltage starting, etc. Soft starting, on the other hand, uses a voltage regulating device to automatically and smoothly increase the starting voltage continuously and smoothly within a specified starting time until it reaches the rated voltage.
If a conventional reduced-voltage starter is used, the starting process is abrupt and uneven, hence the name "hard start." It is not suitable for applications where a stable start is required by the production process. A soft starter, on the other hand, increases the voltage continuously and smoothly from the initial voltage. During the starting process, the motor torque is smooth rather than abrupt, resulting in a stable starting process, hence the name "soft start."
The working principle of a soft starter is that when the motor starts, the electronic circuit controls the conduction angle of the thyristors to gradually increase the motor's terminal voltage at a set rate until it reaches full voltage, thus enabling the motor to achieve a shock-free start and control the soft start process of the motor. When the motor has started and reached the rated voltage, the three-phase bypass contactor closes, and the motor is directly connected to the power grid for operation.
Under light load, the required lower terminal voltage is maintained during normal operation to increase the motor's power factor and efficiency. When the motor stops, the motor terminal voltage is gradually reduced to 0 by controlling the conduction angle of the thyristors, thus achieving a soft stop.
Features of soft start
(1) The starting current rises to the set value at a certain slope, without impacting the power grid.
(2) During the startup process, current negative feedback is introduced. After the startup current rises to the set value, the motor starts smoothly.
(3) Unaffected by grid voltage fluctuations. Since soft start is based on current as the set value, when the grid voltage fluctuates, the motor terminal voltage can be adjusted by increasing or decreasing the conduction angle of the thyristor, thus maintaining a constant starting current and ensuring normal motor start-up.
(4) The starting current setting can be steplessly adjusted to change the motor starting time and achieve optimal starting time control in response to different load requirements.
Comparison of various motor starting methods
Asynchronous motors are widely used in electric drive platforms due to their advantages such as simple structure, small size, low price, reliable operation, convenient maintenance, high operating efficiency, and good working characteristics. However, they have the disadvantage of high starting current (generally 4 to 7 times the rated current, and the starting current of some domestic motors has been measured to be as high as 8 to 12 times the rated current). Excessive starting current can adversely affect the normal operation of the motor itself, the power grid, and other electrical equipment. It can generate an instantaneous excessive torque on the motor shaft (up to 1.6 to 2.0 times the full-load torque of the motor), twisting the motor shaft, damaging the keyway, damaging other equipment connected to the shaft, causing the motor to overheat and affecting its lifespan, increasing voltage drop in the power supply line, and potentially disrupting the normal operation of other electrical equipment connected in parallel on the same power supply line.
Relevant national departments have long stipulated that the voltage drop across the power grid during motor startup should not exceed 15%. For larger capacity motors, measures should be taken to reduce the starting current. Asynchronous motors typically operate at full voltage. The magnetic field of a motor remains 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 starts at full voltage, the impact on the power grid is the greatest and the impact time is the longest; while the commonly used reduced voltage starting, also known as hard starting, has a smaller impact on the power grid, but because it involves a coil voltage switching process, there is a disadvantageous secondary impact; soft starting, because a starting current that does not affect the power grid is set before starting, and the current increases slowly to the set current, there is no inrush current, the impact on the power grid is minimal, and it can eliminate the impact of starting torque.
A soft starter is a device that uses a soft starter connected in series between the power supply and the controlled motor. By controlling the conduction angle of its internal thyristors, the motor input voltage gradually increases from zero according to a preset function until the start-up is complete, supplying the motor with full voltage. Depending on the needs of different industries, the starting method of the soft starter varies. Generally, the following methods are available:
ramp-up soft start
This starting method is the simplest, but it lacks closed-loop current control. It only adjusts the thyristor conduction angle to increase it in a certain function of time. Its disadvantage is that, because it doesn't limit current, a large inrush current can sometimes be generated during motor startup, damaging the thyristor and significantly impacting the power grid. Therefore, it is rarely used in practice.
Step start
A step start is when the starting current reaches the set value as quickly as possible upon power-on. This rapid start-up can be achieved by adjusting the starting current setting.
ramp constant current soft start
This starting method involves gradually increasing the starting current during the initial stage of motor startup. Once the current reaches a preset value, it remains constant until startup is complete. This is the most widely used starting method, especially suitable for starting loads such as fans and pumps.
Pulse shock start
During the initial startup phase, the thyristor is allowed to conduct with a large current for a very short period before dropping back down, and then linearly rising again according to the original set value to connect to constant current starting. This starting method is rarely used in general loads and is suitable for heavy-load starting applications where significant static friction needs to be overcome.
Voltage dual-slope start
During startup, the motor's output torque increases with voltage, providing an initial starting voltage Us. Us is adjustable according to the load; by adjusting Us to be greater than the static friction torque of the load, the motor reaches its rated speed when the output voltage reaches the speed-up voltage Ur. The soft starter automatically detects the speed-up voltage during startup, ensuring the output voltage reaches the rated voltage when the motor reaches its rated speed.
Rate limiting start
Current-limiting start is a soft-start method that limits the starting current of a motor to a certain set value during the starting process. The output voltage increases rapidly from zero until the output current reaches the preset current limit Im, and then the output current is maintained. The advantage of this starting method is that the starting current is small and can be adjusted as needed.
A soft starter is a motor control device that integrates soft starting, soft stopping, light-load energy saving, and multiple protection functions. The soft starter uses three anti-parallel thyristors as voltage regulators, connected between the power supply and the motor stator. This circuit resembles a three-phase fully controlled bridge rectifier circuit. When starting the motor with a soft starter, the thyristor output voltage gradually increases, and the motor gradually accelerates until the thyristors are fully conducting. The motor operates at its rated voltage mechanical characteristics, achieving smooth starting, reducing starting current, and avoiding overcurrent tripping. Once the motor reaches its rated speed, the starting process ends, and the soft starter automatically replaces the thyristors with a bypass contactor, providing the rated voltage for normal motor operation. This reduces thyristor heat loss, extends the soft starter's lifespan, improves its efficiency, and prevents harmonic pollution of the power grid. The soft starter also provides a soft stopping function, which is the reverse of the soft starting process. The voltage gradually decreases, and the speed gradually drops to zero, avoiding torque surges caused by free stopping.
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