Common methods of starting an electric motor
1. Full pressure direct start
If both the grid capacity and the load allow for full-voltage direct starting, then full-voltage direct starting can be considered.
The advantages are convenient operation and control, simple maintenance, and relatively economical operation. It is mainly used for starting small-power motors. From the perspective of saving energy, 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 start. Furthermore, the starting torque can be adjusted via the tap. It is still widely used today.
3. Y-Δ start
For a squirrel-cage induction motor with a delta-connected stator winding during normal operation, connecting the stator winding in a star configuration during startup and then reconnecting it in a delta configuration after startup can reduce the starting current and lessen 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 of allowing the motor to operate in a star connection when the load is light. In this case, the rated torque can be matched to the load, thus improving the motor's efficiency and saving power consumption.
4. Soft starter
This method utilizes the phase-shifting voltage regulation principle of thyristors to achieve voltage regulation starting of motors. It is mainly used for motor starting control, offering good starting performance but at a higher cost. Because of the use of thyristors, harmonic interference is significant during operation, impacting the power grid. Furthermore, power grid fluctuations can affect the conduction of the thyristors, especially when multiple thyristor devices are connected to the same grid. Therefore, the failure rate of thyristors is relatively high, and due to the involvement of power electronics technology, the requirements for maintenance technicians are also high.
5. Frequency converter
Variable frequency drives (VFDs) are the most technologically advanced, feature-rich, and effective motor control devices in modern motor control. They regulate the speed and torque of a 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 that require speed regulation and have high speed control requirements.
Comparison of the advantages and disadvantages of pressure-reduced starting, soft starting, and variable frequency starting.
Reduced-pressure starting, commonly using 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 cost.
Soft start allows you to set the start time and initial starting torque to achieve soft start and soft stop of the equipment, and can limit the starting current. It is also reasonably priced.
Variable frequency start-up can smoothly start the equipment according to the set time and keep it running at the set frequency, but it is more expensive.
Comparison of the performance principles of soft starters, frequency converters, and reduced voltage starters
1. A soft starter is a combination of thyristor AC voltage regulation technology and 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 mains frequency power to another frequency. By controlling motor operation through frequency conversion (where voltage also changes with frequency, e.g., V/F remains constant), it is a truly efficient speed regulation method with very high efficiency. Frequency converters can achieve true soft start, soft stop, and efficient speed regulation.
3. Common methods of reduced-voltage starting include autotransformer starting and Y-Δ starting. The biggest advantage of autotransformer starting is its larger 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.
Y-Δ starting is 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 operate in a star connection when the load is light. In this case, the rated torque can be matched with the load, which improves the motor's efficiency and thus saves power consumption.
Comprehensive Analysis of Soft Starter, Frequency Converter, and Reduced Voltage Starter
1. Price issue
Variable frequency drives (VFDs) are naturally the most expensive, while Y-Δ and autotransformer reduced voltage starters are relatively cheaper. For projects with smaller investments, cost-effectiveness becomes the primary consideration.
2. Controllability issues
Y-Δ and autotransformer reduced-voltage starting are simple, but they only provide starting functionality. 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 starting and soft starting. Therefore, frequency converters are the preferred choice for large-scale or highly automated production lines.
3. Network communication
Inverters can achieve network monitoring through their integrated or expanded communication ports. Soft starters can also perform some monitoring, but for real-time monitoring of motors, they are unmatched by reduced-voltage starting and soft starters.
4. Maintenance
Since Y-Δ and autotransformer reduced-voltage starting methods are inherently simple, they are also the simplest to maintain. Therefore, if a frequency converter is not selected, choosing Y-Δ or autotransformer reduced-voltage starting is more suitable.
5. Functional load
5. Variable frequency drives (VFDs) can achieve soft start and soft stop of motors. Therefore, in situations with relatively large loads, Y-Δ, autotransformer reduced voltage starting, or soft start are not as good as VFDs.
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 start is a process of starting from 0 to full voltage by changing the conduction angle of the thyristor.
A frequency converter provides full-process control and can control the motor speed at any time using instrument signals, while a soft starter can only reduce voltage when the motor is starting and stopping.
2. Major Comparison of Motor Starting Methods
Common methods for starting electric motors include: direct full-voltage starting, autotransformer starting, Y-Δ starting, soft starting, and frequency conversion starting.
When both the power grid and the load permit, direct starting of the motor is preferable because it is convenient to operate and control, and is also more economical.
Autotransformer reduced voltage starters are often used to start large-capacity squirrel-cage induction motors. Although autotransformer reduced voltage starters are an old-fashioned starting device, they can adapt to the starting needs of various loads and obtain a larger starting torque by utilizing the multi-tap reduction of the autotransformer. In addition, they are widely used because they are equipped with thermal relays and undervoltage release devices, which provide complete 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 applications. However, this method has the simplest structure and the lowest price, and can save power consumption during light-load operation.
All of the above starting methods belong to stepped reduced voltage starting, which has obvious drawbacks, namely, the occurrence of secondary inrush current during the starting process.
3. Comparison between soft start and traditional reduced pressure start method
The difference between soft start and traditional reduced pressure start is:
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 the machine's service life.
② It has a soft stop function, that is, smooth deceleration and gradual stopping. It can overcome the drawbacks of instantaneous power failure and stop, 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. They can be selected according to the load conditions and the characteristics of the power grid relay protection, and can be freely and steplessly adjusted to the optimal starting current.
Soft starters and frequency converters are two completely different products.
A frequency converter is used where speed regulation is required; its output changes both voltage and frequency. A soft starter is actually a voltage regulator used when starting a motor; its output only changes the voltage and does not change 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.
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