The most familiar electrical equipment to electrical workers is probably the electric motor. There are many ways to start an electric motor, including direct starting, autotransformer starting, Y-Δ reduced voltage starting, soft starter starting, and frequency converter starting. So what are the differences between them?
1. Full pressure direct start
If both the grid capacity and the load allow for full-voltage direct starting, it can be considered. Its advantages include convenient operation and control, simple maintenance, and relatively low cost. It is mainly used for starting small-power motors; from an energy-saving perspective, this method is not suitable for motors larger than 11kW.
2. Autotransformer reduced voltage starting
Using a multi-tap autotransformer to reduce voltage can meet the starting needs of different loads and obtain a larger starting torque. It is a reduced voltage starting method that is often used to start large-capacity motors.
Its greatest advantage is its large starting torque; when the winding tap is at 80%, the starting torque can reach 64% of that of direct starting. Furthermore, the starting torque can be adjusted by tapping. It is still widely used today. 3. Y-Δ Starting: For squirrel-cage induction motors with a delta-connected stator winding during normal operation, if the stator winding is connected in a star configuration during startup and then switched to a delta configuration after startup, the starting current can be reduced, lessening its impact on the power grid. This starting method is called star-delta reduced-voltage starting, or simply star-delta starting (Y-Δ starting).
When using star-delta starting, the starting current is only 1/3 of that when starting directly with a delta connection. If the starting current during direct starting is 6-7 Ie, then the starting current during star-delta starting is only 2-2.3 times that. This means that when using star-delta starting, the starting torque is also reduced to 1/3 of that when starting directly with a delta connection.
Suitable for no-load or light-load starting applications. Compared to any other reduced-voltage starter, it has the simplest structure and is the cheapest. In addition, the star-delta starting method has the advantage that the motor can run in a star connection when the load is light. In this case, the rated torque can match the load, thus improving the motor's efficiency and saving power consumption. 4. Soft starter: This uses the phase-shifting voltage regulation principle of thyristors to achieve voltage regulation starting of the motor. It is mainly used for motor starting control, with good starting effect but higher cost. Because it uses thyristors, the harmonic interference is relatively large when the thyristors are working, which has a certain impact on the power grid.
Furthermore, fluctuations in the power grid can also affect the conduction of thyristor components, especially when there are multiple thyristor devices on the same power grid. Therefore, thyristor components have a relatively high failure rate. Because they involve power electronics technology, the requirements for maintenance technicians are also high. 5. Frequency Converters: Frequency converters are the most technologically advanced motor control devices in the modern motor control field, offering the most comprehensive control functions and the best control effect. They regulate the speed and torque of the motor by changing the frequency of the power grid. Because they involve power electronics and microcomputer technology, they are expensive and require highly skilled maintenance technicians. Therefore, they are mainly used in fields requiring speed regulation and high speed control requirements.
Comparison of the advantages and disadvantages of pressure-reduced starting, soft starting, and variable frequency starting.
Reduced-voltage starting, commonly star-delta starting, has the disadvantage of low starting torque and is only suitable for no-load or light-load starting. Its advantage is its low price. Soft starting allows for setting the starting time and initial starting torque to achieve soft start and soft stop of the equipment, and can limit the starting current; it is moderately priced. Variable frequency starting (VFD) can smoothly start according to a set time and allow the equipment to operate at a set frequency; it is more expensive. A comparison of the performance principles of soft starters, VFDs, and reduced-voltage starting is provided.
1. A soft starter combines thyristor AC voltage regulation technology with power factor control technology. It achieves soft starting and soft stopping of the motor through thyristor voltage regulation, but does not have speed regulation function. 2. A frequency converter is a motor control (speed regulation) device that uses the switching action of power semiconductor devices to convert the mains frequency power supply to another frequency. By controlling the motor operation through frequency conversion (the voltage also changes with the frequency, such as a constant V/F), it is a truly efficient speed regulation method with very high efficiency. Frequency converters can achieve true soft starting, soft stopping, and efficient speed regulation. 3. Reduced voltage starting commonly uses two methods: autotransformer starting and Y-Δ starting. The biggest advantage of autotransformer starting is its large starting torque; when its winding tap is at 80%, the starting torque can reach 64% of that of direct starting. Furthermore, the starting torque can be adjusted by tapping. It is still widely used today. Y-Δ is suitable for no-load or light-load starting applications. Compared with any other reduced voltage starter, it has the simplest structure and the lowest price. In addition, star-delta starting has another advantage: when the load is light, the motor can run in a star connection. In this case, the rated torque can match the load, which improves the motor's efficiency and saves power. A comprehensive analysis of soft starters, frequency converters, and reduced-voltage starters: 1. Price: Frequency converters are naturally the most expensive, while Y-Δ and autotransformer reduced-voltage starters are relatively cheaper. For projects with smaller investments, economy becomes the primary consideration.
2. Controllability: Y-Δ and autotransformer reduced-voltage starting are simple, but they only provide starting capabilities. In highly automated applications, they are likely to be used less frequently, and even soft starting is less common. However, controlling the motor via a frequency converter, including speed and voltage, is far superior to reduced-voltage or soft starting. Therefore, frequency converters are the preferred choice for large or highly automated production lines. 3. Network Communication: Frequency converters can achieve network monitoring through their integrated or extended communication ports. Soft starting can also perform some monitoring, but real-time motor monitoring is unmatched by reduced-voltage or soft starting. 4. Maintenance: Because Y-Δ and autotransformer reduced-voltage starting are relatively simple, maintenance is also the simplest. I am actually against using soft starting; if not a frequency converter, I would definitely choose Y-Δ or autotransformer reduced-voltage starting directly. Frequency converters can achieve soft starting and soft stopping of the motor, so in applications with relatively large loads, Y-Δ, autotransformer reduced-voltage starting, or soft starting are all inferior to frequency converters.
Supplementary Knowledge Comparison 1: Soft Starters and Frequency Converters Both frequency converters and soft starters fall under the category of reduced-voltage starting. Frequency converters achieve reduced-voltage starting by changing the frequency. Soft starters achieve the starting process from 0 voltage to full voltage by changing the conduction angle of the thyristor.
A frequency converter provides full-range control and can control the motor speed at any time using instrument signals. A soft starter only reduces voltage during motor startup and shutdown. 2: General Comparison of Motor Starting Methods Common motor starting methods include: direct full-voltage starting, autotransformer starting, Y-Δ starting, soft starting, and frequency converter starting. When both the power grid and load permit, direct starting is preferable because it is convenient to operate and control, and relatively economical. Autotransformer starting is often used to start larger capacity squirrel-cage induction motors. Although it is an older type of starting device, the multi-tap reduction of the autotransformer allows it to adapt to various load starting needs and provides greater starting torque. Furthermore, it is widely used because it is equipped with thermal relays and undervoltage release devices, providing comprehensive overload and undervoltage protection. Star-delta starting has excellent current characteristics but poor torque characteristics, so it is only suitable for no-load or light-load starting. However, this method has the simplest structure and is the cheapest, saving power consumption during light-load operation. All of the above starting methods belong to stepped reduced-voltage starting, which has a significant drawback: a secondary inrush current occurs during the starting process. 3: Comparison between soft starting and traditional reduced-voltage starting methods
① No inrush current. When starting a motor, the soft starter gradually increases the thyristor conduction angle, causing the motor starting current to rise linearly from zero to the set value. This causes no impact on the motor, improves power supply reliability, ensures smooth starting, reduces impact torque on the load machinery, and extends machine life. ② It has a soft stop function, i.e., smooth deceleration and gradual stopping. It can overcome the drawbacks of instantaneous power failure stopping, reduce the impact on heavy-duty machinery, avoid the water hammer effect in high-altitude water supply systems, and reduce equipment damage. ③ The starting parameters are adjustable. They can be freely and steplessly adjusted to the optimal starting current according to the load conditions and the characteristics of the power grid relay protection.
Soft starters and frequency converters are two completely different products. Frequency converters are used where speed regulation is required, and their output changes both voltage and frequency. Soft starters are actually voltage regulators used when starting a motor, and their output only changes the voltage without changing the frequency.
Variable frequency drives (VFDs) have all the functions of soft starters, but they are much more expensive and have a much more complex structure.
