Comparison of five starting methods for motors

The electric equipment that electrical workers are most familiar with should be the motor. There are many ways to start the motor, including direct start, auto-decompression start, Y-Δ step-down start, soft starter start, inverter start, etc. Way. So what's the difference between them?

 

1 Direct start with full pressure

 

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

Advantages: easy operation and control, simple maintenance, and relatively economical. It is mainly used for starting small power motors. From the perspective of saving electric energy, this method is not suitable for motors larger than 11kw.

 

2Auto-decompression starting

 

Using the multi-tap pressure reduction of the autotransformer can not only adapt to the starting needs of different loads, but also obtain greater starting torque. It is a pressure reduction starting method that is 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 that at direct starting. And the starting torque can be adjusted by tapping. It is still widely used today.

 

3Y-Δ start

 

For a squirrel-cage asynchronous motor whose stator winding is connected in delta during normal operation, if the stator winding is connected in star shape when starting, and then connected in delta after starting, the starting current can be reduced and its impact on the power grid can be reduced. impact. This starting method is called star-delta decompression starting, or simply star-delta starting (Y-Δ starting).

 

When using star-delta starting, the starting current is only 1/3 of the original direct starting according to delta connection. If the starting current during direct starting is calculated as 6~7Ie, then 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 based on delta connection.

 

Suitable for no-load or light-load starting situations. Compared with any other decompression starter, it has the simplest structure and the cheapest price. In addition, the star-delta starting method has another advantage, that is, when the load is light, the motor can be run in star connection. At this time, the rated torque can match the load, which can improve the efficiency of the motor and save power consumption.

 

4 soft starter

 

This uses the phase-shifting voltage regulation principle of the thyristor to realize the voltage-regulated starting of the motor. It is mainly used for the starting control of the motor. The starting effect is good but the cost is high. Due to the use of thyristor components, harmonic interference is greater when the thyristor is working, which has a certain impact on the power grid.

In addition, fluctuations in 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, and because it involves power electronics technology, the requirements for maintenance technicians are also high.

 

5 frequency converter

 

Frequency converter is a motor control device with the highest technical content, the most complete control functions and the best control effect 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 speed control requirements.​ 

 

Comparison of the advantages and disadvantages of decompression start, soft start and variable frequency start

 

• Decompression start: Star-delta start is common. The disadvantage is that the starting torque is small and it is only suitable for no-load or light-load start. The advantage is that it is cheap.

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

• Variable frequency starting: It can start smoothly according to the set time and allow the equipment to run at the set frequency. The price is higher.

 

Comparison of soft start, frequency converter, and decompression starting performance principles

 

• The soft starter is a combination of thyristor AC voltage regulation technology and power factor control technology. It realizes motor soft start and soft stop through thyristor voltage regulation and does not have speed regulation function.

   

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

   

• The most common methods of decompression starting are auto-decompression starting and Y-Δ starting. The biggest advantage of auto-decompression starting is that the starting torque is large. When the winding tap is at 80%, the starting torque It can reach 64% of the direct starting value. And 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 situations. Compared with any other decompression starter, it has the simplest structure and the cheapest price.

  In addition, the star-delta starting method has another advantage, that is, when the load is light, the motor can be run in star connection. At this time, the rated torque can match the load, which can improve the efficiency of the motor and save power consumption.

 

Comprehensive analysis of soft start, frequency converter and decompression start

 

• The price issue is naturally that frequency converters are the most expensive, while Y-Δ and auto-decompression starting are relatively cheap. For projects with smaller investment, economy will become the first choice;

• Controllable issues
  Y-Δ, auto-coupling decompression is easy to start, but it is just a start. But in situations with a high degree of automation, it is estimated that it will be used less, and even less soft. The control of motors through frequency converters, including speed, voltage, etc., is far beyond what decompression start and soft start can match. Therefore, frequency converters are the first choice in large or highly automated production lines.

• Network communication
  The inverter itself can realize network monitoring through its own integrated or expanded communication port. Soft start can also do some monitoring, but to achieve real-time monitoring of the motor, it is incomparable to decompression start and soft start.

• In terms of maintenance
  Since Y-Δ and auto-decompression startup are relatively simple, maintenance is naturally the easiest. I am actually very opposed to using soft starter. If I don't choose a frequency converter, I will definitely choose Y-Δ or auto-decompression starter directly.

   

The frequency converter can realize the soft start and soft stop of the motor, so in situations where the relative load is relatively large, Y-Δ, auto-coupling decompression start or soft start are not as good as the frequency converter.

 

Supplementary knowledge comparison

 

1 Soft starter and frequency converter

Both frequency converters and soft start equipment 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 a starting process from voltage 0 to full voltage by changing the conduction angle of the thyristor.

 

The frequency converter is fully controlled and can control the motor speed at any time by instrument signals. The soft starter can only reduce the voltage when the motor starts and stops.
 

2 Big analogy of motor starting methods

Common methods for starting motors: full voltage direct start, auto-decompression start, Y-Δ start, soft start, variable frequency start, etc.
When both the grid and the load allow it, it is better to start the motor directly, because it is convenient to control and more economical.

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

The star-delta starting method has good current characteristics but poor torque characteristics, so it is only suitable for no-load or light-load starting situations. However, this method has the simplest structure and the cheapest price. Power consumption can be saved during light load operation.

The above starting methods are all staged pressure reduction starting, which have obvious shortcomings, that is, a secondary surge current occurs during the starting process.
 

3 Comparison between soft start and traditional decompression starting methods

 

①. No inrush current.
When starting the motor, the soft starter gradually increases the thyristor conduction angle to linearly increase the motor starting current from zero to the set value. There is no impact on the motor, which improves the reliability of power supply, starts smoothly, reduces the impact torque on the load machinery, and extends the service life of the machine.

②. It has a soft parking function, that is, smooth deceleration and gradual shutdown. It can overcome the shortcomings of instant power outage and shutdown, reduce the impact on heavy-load machinery, and avoid the water hammer effect of high-altitude water supply systems. Reduce equipment damage.

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

 

Soft starters and frequency converters are two products with completely different uses. Frequency converters are used where speed control is required. Its output not only changes the voltage but also changes the frequency at the same time. The soft starter is actually a voltage regulator, used for starting motors. When , the output only changes the voltage and does not change the frequency.

 

The frequency converter has all the functions of a soft starter, but its price is much more expensive than a soft starter and its structure is much more complex.