① Full-voltage direct starting

When both the grid capacity and the load allow full-voltage direct starting, full-voltage direct starting can be considered.

Advantages: convenient operation and control, simple maintenance, and relatively economical. Mainly used for starting small-power motors. From the perspective of saving electricity, this method is not suitable for motors greater than 11kw. ② Autotransformer decompression starting

The multi-tap decompression of the autotransformer can not only meet the needs of starting different loads, but also obtain a larger starting torque. It is a decompression starting method often used to start larger capacity motors. Advantages: The starting torque is large. When the winding tap is at 80%, the starting torque can reach 64% of the direct start. And the starting torque can be adjusted by the tap. It is still widely used.

③Y-Δ starting

For squirrel-cage asynchronous motors with a normally operating stator winding connected in a triangle, if the stator winding is connected in a star shape at the time of starting and then in a triangle shape after the starting is completed, the starting current can be reduced and its impact on the grid can be reduced. This starting method is called star-delta decompression starting, or simply star-delta starting (Y-Δ starting).

 

When star-delta starting is used, the starting current is only 1/3 of the original direct starting with delta connection. If the starting current during direct starting is 6-7Ie, the starting current during star-delta starting is only 2-2.3 times. This means that when star-delta starting is used, the starting torque is also reduced to 1/3 of the original direct starting with delta connection.

It is suitable for no-load or light-load starting occasions. And compared with other decompression starters, its structure is simple and the price is cheap. In addition, the star-delta starting method has another advantage, that is, when the load is light, the motor can be operated under star connection. At this time, the rated torque can match the load, which can improve the efficiency of the motor and save power consumption.

④Soft starter

This uses the phase-shift voltage regulation principle of thyristors to achieve voltage regulation starting of motors. It is mainly used for starting control of motors. The starting effect is good but the cost is high. Because thyristor components are used, the harmonic interference of thyristors is large when they work, which has a certain impact on the power grid.

In addition, the fluctuation of the power grid will also affect the conduction of thyristor components, especially when there are multiple thyristor devices in the same power grid. Therefore, the failure rate of thyristor components is high. Because it involves power electronics technology, the requirements for maintenance technicians are also high. ⑤Inverter

The inverter is a motor control device with high technical content, full control functions and good control effects in the field of modern motor control. It adjusts the speed and torque of the motor by changing the frequency of the power grid. Because it involves power electronics technology and microcomputer technology, the cost is high and the requirements for maintenance technicians are also high. Therefore, it is mainly used in fields that require speed regulation and have high requirements for speed control.

Comparison of advantages and disadvantages of reduced pressure starting, soft starting and variable frequency starting

Reduced pressure starting: The most common one is star-delta starting. The disadvantage is that the starting torque is small and it is only suitable for no-load or light-load starting. The advantage is that it is cheap.

Soft start: You can set the start time and the initial start torque to achieve soft start and soft stop of the equipment, and can limit the starting current. The price is moderate.

Frequency conversion start: It can start smoothly according to the set time and let the equipment run at the set frequency. The price is relatively high.Comparison of performance principles of soft start, frequency converter, and pressure reduction start

The soft starter is a combination of thyristor AC voltage regulation technology and power factor control technology. It uses thyristor voltage regulation to achieve motor soft start and soft stop, and does not have speed regulation function.

The frequency converter is a motor control (speed regulation) device that uses the on-off function of power semiconductor devices to convert the industrial frequency power supply to another frequency. Controlling the motor operation through frequency conversion (the voltage also changes with the frequency, such as constant v/f) is a truly effective speed regulation method with high efficiency. The frequency converter can achieve true soft start, soft stop and effective speed regulation.

The common methods of decompression starting are auto-coupling decompression starting and Y-Δ starting. The advantage of auto-coupling decompression starting is that the starting torque is large. When the winding tap is at 80%, the starting torque can reach 64% of the direct starting. And the starting torque can be adjusted by the tap. It is still widely used.

Y-Δ is suitable for no-load or light-load starting occasions. And compared with other decompression starters, its structure is simple and the price is cheap. In addition, the star-delta starting method has another advantage, that is, when the load is light, the motor can be operated under the star connection. At this time, the rated torque can be matched with the load, which can improve the efficiency of the motor and save power consumption.

Comprehensive analysis of soft start, frequency converter, and decompression starting

The price problem is naturally that the frequency converter is expensive, and Y-Δ and auto-coupling decompression starting are relatively cheap. For projects with smaller investments, economy will become their choice; controllable issues

Y-Δ and auto-coupling decompression starting are simple, but they are just starting. However, in situations with a high degree of automation, it is estimated that it will be used less, and even soft start will be used less. The control of motors by frequency converters, including speed and voltage, is far from comparable to reduced pressure start and soft start. Therefore, frequency converters are ideal for large or highly automated production lines.

Network communication

The frequency converter itself can realize network monitoring through its integrated or extended communication port. Soft start can also do some monitoring, but to achieve real-time monitoring of the motor, it is also incomparable to reduced pressure start and soft start.

Maintenance

Since Y-Δ and auto-coupling reduced pressure start are relatively simple in themselves, they are naturally easy to maintain. The frequency converter can complete the soft start and soft stop of the motor, so in situations with relatively large loads, Y-Δ, auto-coupling reduced pressure start or soft start are not as good as frequency converters.Supplementary knowledge comparison

1: Soft starter and frequency converter

Frequency converters and soft start devices both belong to the category of reduced voltage start. The frequency converter achieves the purpose of reduced voltage start by changing the frequency.

Soft start is to achieve the starting process from voltage 0 to full voltage by changing the conduction angle of the thyristor.

The frequency converter is a full-range control, and the motor speed can be controlled by the instrument signal in various time periods. The soft starter can only reduce the voltage when the motor starts and stops.

 

2: A general analogy of motor starting methods

Common methods for motor starting: full-voltage direct starting, auto-coupling pressure reduction starting, Y-Δ starting, soft starting, variable frequency starting, etc.

When both the power grid and the load allow, it is advisable to start the motor directly because it is easy to operate and control, and it is relatively economical.

 

Auto-coupling pressure reduction starting is often used to start large-capacity squirrel cage asynchronous motors. Although auto-coupling pressure reduction starting is an old-fashioned starting device, the multi-tap pressure reduction of the autotransformer can not only meet the needs of starting a variety of loads, but also obtain a larger starting torque. In addition, it is widely used because it is equipped with a thermal relay and a low-voltage release and has perfect overload and undervoltage protection.

 

The star-delta starting method has a good current characteristic, but a poor torque characteristic, so it is only suitable for no-load or light-load starting occasions, but this method has a simple structure and a low price, and can save power consumption in light-load operation.

 

All of the above starting methods belong to step-down starting, which has disadvantages, that is, secondary impact current occurs during the starting process.

3: Comparison between soft starting and traditional decompression starting methods ①. No impact current.

When starting the motor, the soft starter gradually increases the thyristor conduction angle to make the motor starting current rise linearly from zero to the set value. There is no impact on the motor, which improves the power supply reliability, starts smoothly, reduces the impact torque on the load machinery, and extends the service life of the machine.

 

②. There is a soft stop function, that is, smooth deceleration and gradual shutdown. It can overcome the disadvantages of instantaneous power failure and shutdown, reduce the impact on heavy-load machinery, avoid the water hammer effect of the high-altitude water supply system, and reduce equipment damage.

 

③. The starting parameters are adjustable. According to the load conditions and the characteristics of the power grid relay protection, it can be freely and steplessly adjusted to the best starting current.

Soft starters and frequency converters are two products with different uses. Frequency converters are used where speed regulation is required, and their output not only changes the voltage but also the frequency; soft starters are actually voltage regulators, and when used for motor starting, their output only changes the voltage without changing the frequency.

Frequency converters have all the functions of soft starters, but they are much more expensive than soft starters and their structure is much more complicated.