There are many ways to start a motor. Including direct start, autotransformer decompression start, Y-Δ voltage reduction start, soft starter start, inverter start and so on.
So what are the differences between them?

1. Full-voltage direct start

When both the grid capacity and load allow full-voltage direct start, full-voltage direct start can be considered.
The advantages are convenient operation and control, simple maintenance, and relatively economical. It is mainly used for starting small-power motors. From the perspective of saving electricity, motors greater than 11kW are not suitable for this method.

2. Autotransformer decompression start

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 start method that is often used to start larger capacity motors.
Its biggest advantage is that 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.

3. Y-Δ starting

For squirrel cage asynchronous motors with normally operating stator windings in delta connection, if the stator windings are connected in star at the time of starting and then in delta after starting, the starting current can be reduced and its impact on the power grid can be reduced. This starting method is called star-delta pressure reduction 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 in 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 using star-delta starting, the starting torque is also reduced to 1/3 of the original direct starting in delta connection.
It is suitable for no-load or light-load starting occasions. And compared with any other pressure reduction 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 light, the motor can be operated 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-shift voltage regulation principle of thyristor to realize the voltage regulation start of the motor. It is mainly used for the start-up control of the motor. The start-up effect is good but the cost is high. Because thyristor components are used, the harmonic interference of thyristor is large when it works, 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.

5. Frequency converter

The frequency converter is the 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 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 high speed control requirements.

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

For 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 starting can set the starting time and the initial starting torque to achieve soft starting and soft stopping of the equipment, and can limit the starting current. The price is moderate.

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

B. Comparison of performance principles of reduced pressure starting, soft starting and variable frequency starting

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

2. 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 efficient speed regulation method with high efficiency. The frequency converter can achieve true soft start, soft stop and efficient speed regulation.

3. The common methods of decompression starting are auto-coupling decompression starting and Y-Δ starting. The biggest 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 direct starting. And the starting torque can be adjusted by tapping. It is still widely used.

Y-Δ is suitable for no-load or light-load starting 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 light, the motor can be operated 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.

C. Comprehensive analysis of decompression starting, soft starting and variable frequency starting

1. Price issue
Naturally, the frequency converter is the most expensive, and Y-Δ and auto-coupling decompression starting are relatively cheap. For projects with small investment, economy will become the first choice.

2. Controllability issues

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

3. Network communication

The frequency converter itself can realize network monitoring through its own 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 decompression start and soft start.

4. Maintenance

Since Y-Δ and auto-coupling decompression start are relatively simple in themselves, they are naturally the simplest to maintain. I am actually very opposed to using soft start. If I don’t choose a frequency converter, I will definitely choose Y-Δ or auto-coupling decompression start directly.

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 decompression start or soft start are not as good as the frequency converter.

D. Comparison of supplementary knowledge
1. Soft starter and frequency converter

Frequency converter and soft starter equipment both belong to the category of reduced voltage starting. Although the frequency converter will reduce the voltage after reducing the frequency, the constant torque is full voltage.
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 fully controlled, and the motor speed can be controlled by the instrument signal at any time. The soft starter can only play the purpose of reducing the voltage when the motor starts and stops.

2. 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 better to start the motor directly because it is easy to operate and control and more economical.
Auto-coupling decompression starter is often used to start larger capacity squirrel cage asynchronous motors. Although auto-coupling decompression starter is an old-fashioned starting device, it can adapt to the needs of starting a variety of loads and obtain a larger starting torque by using the multi-tap decompression of the autotransformer. In addition, it is widely used because it is equipped with thermal relays and low-voltage releases and has perfect overload and undervoltage 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, and can save power consumption during light-load operation.
All of the above starting methods belong to step-down starting, which has obvious disadvantages, that is, secondary impact current occurs during the starting process.

3. Comparison between soft starter and traditional decompression starting method

3.1 No impact current
When starting the motor, the soft starter gradually increases the conduction angle of the thyristor 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 prolongs the service life of the machine.

3.2 Soft stop function
That is, smooth deceleration and gradual shutdown. It can overcome the disadvantages of instant 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.

3.3 Adjustable starting parameters
According to the load conditions and the characteristics of the power grid relay protection, the optimal starting current can be freely and steplessly adjusted.

Soft starters and frequency converters are two completely different products. Frequency converters are used in places where speed regulation is required. Their output not only changes the voltage but also changes the frequency at the same time. Soft starters are actually voltage regulators. When used for motor starting, the output only changes the voltage and does not change the frequency.

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