Jan 11, 2021
Comparison of five starting methods of motors
The electric equipment most familiar to electrical workers is 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. the way. So what is the difference between them?
1. Full pressure direct start
In the case where full voltage direct start is allowed in both the grid capacity and load, full voltage direct start can be considered.
The advantages are convenient operation and control, simple maintenance, and relatively economical. It is mainly used to start small-power motors. From the perspective of energy saving, motors larger than 11kW should not be used in this method.
2. Auto decompression start
The multi-tap decompression using the autotransformer can not only meet the needs of different loads for starting, but also get a larger starting torque. It is a decompression starting method that is often used to start a larger capacity motor.
Its biggest advantage is that the starting torque is relatively 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 tapping. It is still widely used today.
3. Y-Δ start
For a squirrel-cage asynchronous motor with a delta-connected stator winding in normal operation, if the stator winding is connected in a star shape when starting, and then connected to a triangle after the start is completed, the starting current can be reduced and its impact on the grid can be reduced. The impact. This starting method is called star delta decompression start, or simply star delta start (Y-Δ start).
When using star-delta starting, the starting current is only 1/3 of the original direct starting according to the delta connection. If the starting current during direct starting is calculated as 6～7Ie, the starting current is only 2～2.3 times when the star-delta starts. This means that when star-delta starting is used, the starting torque is also reduced to 1/3 of the original direct starting according to the delta connection.
Suitable for no-load or light-load startup occasions. And compared with any other decompression starter, its structure is the simplest and the price is the cheapest. In addition, the star-delta starting method has another advantage, that is, when the load is lighter, the motor can be operated in star connection. At this time, the rated torque can be matched with the load, which can increase the efficiency of the motor and save power consumption.
4. Soft starter
This uses the phase-shifting and voltage-regulating principle of the SCR to realize the voltage-regulating start of the motor, which is mainly used for the start-up control of the motor. The start-up effect is good but the cost is higher. Due to the use of thyristor components, the harmonic interference of the thyristor is relatively large, which has a certain impact on the power grid.
In addition, the fluctuation of the power grid will also affect the conduction of the thyristor components, especially when there are multiple thyristor devices in the same power grid. Therefore, the failure rate of thyristor components is relatively high, because power electronic technology is involved, so the requirements for maintenance technicians are also relatively high.
5. Frequency converter
The frequency converter is a motor control device with the highest technical content, the most complete control function 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 electronic 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 require high speed control.
Comparison of advantages and disadvantages of decompression start, soft start, and frequency conversion start
The decompression start, the star-delta start is common, the disadvantage is that the start torque is small, and it is only suitable for no-load or light-load start. The advantage is that it is cheap.
Soft start, you can set the start time and start initial torque to achieve soft start and soft stop of the equipment, and can limit the start current, and the price is moderate.
Frequency conversion start, can start smoothly according to the set time, and let the equipment run at the set frequency, the price is higher.
Comparison of performance principles of decompression start, soft start, and frequency conversion start
1. The soft starter is a combination of thyristor AC voltage regulation technology and power factor control technology. It realizes the soft start and soft stop of the motor through thyristor voltage regulation. It does not have the function of speed regulation.
2. Inverter is a motor control (speed regulation) device that uses the on-off action of power semiconductor devices to transform industrial frequency power into another frequency. Controlling the operation of the motor through frequency conversion (voltage also changes with frequency, such as constant v/f) is a real high-efficiency speed regulation method with high efficiency. The frequency converter can realize true soft start, soft stop and efficient speed regulation.
3. The common methods of decompression start are auto-decompression start and Y-Δ start. The biggest advantage of auto-decompression start is that the starting torque is large. When the winding tap is at 80%, start The torque can reach 64% of the direct start. 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 start. And compared with any other decompression starter, its structure is the simplest and the price is the cheapest. In addition, the star-delta starting method has another advantage, that is, when the load is lighter, the motor can be operated in star connection. At this time, the rated torque can be matched with the load, which can increase the efficiency of the motor and save power consumption.
Comprehensive analysis of decompression start, soft start and frequency conversion start
1. Price issues
Naturally, the inverter is the most expensive, and Y-Δ, auto-decompression start is relatively cheap. For projects with small investment, economy will become the first choice.
2. Controllable problems
Y-Δ, auto-decompression start is simple, but only start. But in occasions with a high degree of automation, it is estimated that it will be used less, and even soft start. However, regulating the motor through the frequency converter, including speed, voltage, etc., is far beyond the comparison of decompression start and soft start. Therefore, frequency converters are the first choice for large-scale or highly automated production lines.
3. 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 also incomparable to decompression start and soft start.
Since Y-Δ and auto-decompression start is relatively simple, it is naturally the simplest to maintain. I am actually against the use of soft start. If you do not choose a frequency converter, you will definitely choose Y-Δ or auto decompression start.
The inverter can complete the soft start and stop of the motor, so in the case of relatively large load, Y-Δ, auto-coupling decompression start or soft start are inferior to the inverter.
Comparing supplementary knowledge
1. Soft starter and inverter
Inverter and soft start equipment belong to the category of step-down start. The inverter achieves the purpose of step-down start by changing the frequency.
The soft start is the starting process from voltage 0 to full voltage by changing the conduction angle of the thyristor.
The frequency converter is controlled throughout, and the motor speed can be controlled at any time by the instrument signal. The soft starter can only reduce the voltage when the motor starts and stops.
2. Big analogy of motor starting mode
Common methods of motor starting: full voltage direct start, auto-coupling decompression start, Y-Δ start, soft start, frequency conversion start, etc.
When both the power grid and the load are allowed, the motor should be started directly because it is convenient to control and economical.
Auto-decompression starter is often used to start larger capacity squirrel-cage asynchronous motors. Although auto-decompression starter is an old-fashioned starting device, the use of autotransformer for multi-tap decompression can adapt to multiple loads. The need of starting, it can get a larger starting torque, and it is also widely used because it is equipped with a thermal relay and a low-voltage release and has a perfect 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 occasions, but this method has the simplest structure and the cheapest price, which can save power consumption during light-load operation.
All of the above start methods are step-by-step decompression start, and have obvious disadvantages, that is, a secondary impulse current occurs during the start.
3. Comparison between soft start and traditional decompression start
No surge current
When the soft starter starts the motor, by gradually increasing the conduction angle of the thyristor, the motor starting current linearly rises 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 prolongs the service life of the machine.
Has soft parking function
That is, it decelerates smoothly and stops gradually. It can overcome the shortcomings of instantaneous power failure, reduce the impact on heavy-duty machinery, avoid the water hammer effect of the elevation water supply system, and reduce equipment damage.
Adjustable startup parameters
According to the load conditions and the selection of power grid protection characteristics, it can be freely and steplessly adjusted to the best starting current.
The soft starter and the inverter are two products with completely different purposes. The inverter is used where speed regulation 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 for motor starting When, the output only changes the voltage and does not change the frequency.
The frequency converter has all the soft starter functions, but its price is much more expensive than the soft starter, and the structure is much more complicated.