Comparison of the starting mode of the motor

03/08/2019

The advantages and disadvantages of decompression start, soft start, and frequency conversion start.

Decompression start, the common is the star-delta start, the disadvantage is that the starting torque is small, only suitable for no-load or light-load start. The advantage is that the price is cheap.

Soft start, can set the start time and starting initial torque to achieve soft start and soft stop for the device, and can limit the starting current, the price is moderate.

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

Soft start, frequency converter, decompression start performance principle comparison.

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

2. The frequency converter is a motor control (speed regulation) device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device. The motor is controlled by frequency conversion (the voltage also changes with frequency, such as v/f constant), which is a true high-efficiency speed regulation method with high efficiency. The frequency converter enables true soft start, soft stop and efficient speed regulation.

3, the common way to start the decompression is auto-decompression start-up and Y-Δ start. The biggest advantage of auto-coupling and decompression 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 use in unloaded or light-loaded start-ups and is the simplest and cheapest in comparison to any other pressure-reducing starter. In addition, the star-delta starting method has the advantage that when the load is light, the motor can be operated in 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.

Soft start, frequency converter, decompression start comprehensive analysis

1, the price problem.
Naturally, the inverter is the most expensive, and Y-Δ and auto-decompression start-up are relatively inexpensive. For projects with smaller investments, economics will be the first choice;

2. Controllable problems.
Y-Δ, auto-coupling decompression is simple to start, but it is just a start. However, in the case of high degree of automation, it is estimated that it will be used less, or even softer. The regulation of the motor through the frequency converter, including the speed and voltage, is far from being comparable to the decompression start and soft start. Therefore, the inverter is the first choice for large or highly automated production lines.

3. Network communication.
The frequency converter itself can be monitored by its own integrated or extended communication port. Soft start can also do some monitoring, but real-time monitoring of the motor is also unmatched by decompression start and soft start.

4. Maintenance.
Since the Y-Δ, auto-coupling decompression startup itself is relatively simple, it is also the easiest to maintain naturally. I am actually against the use of soft start, if you do not choose the inverter, you will definitely choose Y-Δ or auto-decompression start.

5. The inverter can complete the soft start and soft stop of the motor, so in the case of relatively large load, Y-Δ, auto-decompression start or soft start can not compare with the inverter.

Supplementary knowledge comparison

1: soft starter and inverter
Both the inverter and the soft start device belong to the buck start range. The inverter achieves the purpose of step-down startup by changing the frequency.

Soft start is achieved by changing the conduction angle of the thyristor to start from voltage 0 to full voltage.
The frequency converter is controlled in full range, and the motor speed can be controlled by the instrument signal at any time. The soft starter can only be used for voltage reduction when the motor starts and stops.

2: Motor analog mode is a large analogy
Common methods for starting the motor: full-voltage direct start, auto-decompression start-up, Y-Δ start, soft start, variable frequency start, etc.

In the case where both the grid and the load are allowed, the motor is preferably started directly because the steering control is convenient and economical.

Auto-coupling decompression is often used to start larger-capacity squirrel-cage asynchronous motors. Although auto-coupling decompression is an old-fashioned starting device, multi-tap decompression using autotransformers can accommodate multiple loads. The need for starting, but also to get a larger starting torque, coupled with the installation of thermal relays and low voltage trips with perfect overload and loss of pressure protection is widely used.
The star-delta starting mode has good current characteristics and poor torque characteristics, so it is only suitable for occasions with no load or light load starting, but this method is the simplest and the cheapest, and can save power consumption in light load operation.

All of the above starting methods belong to the staged decompression starting, and there are obvious disadvantages, that is, secondary inrush current occurs during starting.

3: Comparison between soft start and traditional decompression start mode

The difference between soft start and traditional decompression start mode is:

1 no inrush current. When the soft starter starts the motor, the motor starting current is linearly increased from zero to a set value by gradually increasing the conduction angle of the thyristor. No impact on the motor, improve the reliability of power supply, start smoothly, reduce the impact torque on the load machine, and extend the service life of the machine.

2 There is a soft parking function, that is, smooth deceleration, and gradually stop, 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.

3 The starting parameters are adjustable, and can be adjusted steplessly to the optimal starting current according to the load condition and the grid relay protection characteristics.

Soft starters and frequency converters are two completely different applications.

The frequency converter is used for speed regulation. The output not only changes the voltage but also changes the frequency. The soft starter is actually a voltage regulator. When the motor starts, the output only changes the voltage and does not change the frequency.

The frequency converter has all the soft starter functions, but it is much more expensive than the soft starter and the structure is much more complicated.