I. Water resistance and thermal resistance
1. Water resistance soft starter is a starting method in which a liquid resistor is connected in series in the stator or rotor circuit of the motor. During motor starting, the device adjusts the motor voltage by changing the distance between the plates, thereby changing the liquid resistance between the plates.
The biggest advantage of water resistance starting is:
A. Low price. Because it does not use modern power electronic devices, but instead uses mechanical adjustment of the electrode distance, the technology is low-tech, so the price is relatively low.
B. It can be used in wound-rotor motors. Because the water resistance can be connected in series with the rotor circuit of the wound-rotor motor, it can start under heavy load.
However, the disadvantages of water resistance are also obvious:
A. Large size. Because the switching control characteristics of power electronic devices cannot be utilized, the voltage during motor startup can only be reduced in the water resistance solution. If a 1000KW motor is started, the average power consumed by the water resistance during startup can reach several hundredKW. Such a large power is ultimately converted into heat, requiring a very large solution tank to withstand this power, thus occupying a lot of space.
B. Poor safety. Because water resistance relies on liquid to reduce the voltage of the three phases of the motor stator, insulation is difficult, and improper handling may cause accidents such as fire and high-voltage grounding.
C. Water resistance thermometers also have disadvantages such as short lifespan and poor environmental adaptability. The presence of a moving electrode requiring mechanical adjustment reduces the reliability of the device. Furthermore, ambient temperature significantly affects its starting performance; for example, temperatures below the freezing point of the solution can cause it to freeze, rendering the device unusable.
D. Inability to start frequently and unsuitable for multi-device operation. After the initial start, restarting requires waiting for the solution temperature to drop, which takes a long time. Otherwise, the solution may vaporize excessively due to overheating. Similarly, if the system is designed for multi-device operation, and the water resistance power is designed for a single unit, the motors cannot start sequentially and must wait. If the water resistance power is designed for multiple units cumulatively, it greatly increases the system cost and size, making it less competitive in terms of price.
E. High maintenance costs. Water resistance systems have problems such as water evaporation and pollution. If the water change time is not properly controlled, even bigger problems may occur.
2. Both thermal resistors and water resistors belong to the category of liquid resistors. The difference is that the thermal resistor eliminates the moving plates, relying on the temperature resistance characteristics of the solution to limit the motor current. It has all the disadvantages of liquid resistors, except that it does not have moving plates, which improves mechanical reliability. However, the change in solution resistance is limited and greatly affected by temperature, significantly reducing the success rate of starting. Moreover, this characteristic makes it completely uncontrollable, leaving it entirely at the mercy of the elements.
II. Soft starting using a switching transformer
By connecting the primary winding of a transformer in series with the stator circuit of the motor, and controlling the short-circuiting of the secondary winding of the transformer by a thyristor, the primary winding current is changed, thereby controlling the motor's starting process. This soft-start method utilizes the voltage reduction principle of the transformer, avoiding the voltage equalization problem in direct series connection of thyristors, thus circumventing the technical difficulties.
However, this method also has some drawbacks: although the voltage on the thyristor is reduced by N times through the transformer, according to the principle of transformer, the current carried by the thyristor must be increased by N times. In motors with slightly larger capacity, thyristors need to be connected in parallel. It can be seen that this method does not reduce the capacity of the thyristor (carrying voltage * current) and adds an extra switching transformer.
III. Magnetic Soft Start
This method utilizes the principle of magnetic amplification. A small control current is added to the control coil of the saturated reactor, altering the reactor's saturation level and thus changing the starting current of the motor connected in series with the reactor. This method employs the principle of magnetic amplification to achieve the goal of controlling a larger power with a smaller power.
The advantage of this method is that the size and cost of inductors are lower than those of transformers.
shortcoming:
1. Slow response: Due to the lag of current in the inductor connected in series in the stator circuit, it takes about 1 second or more from the time the control system issues a control command to the time the inductor reaches the set requirement of the command.
2. Low power factor: Since the power factor of the motor is already low when it starts, the inductance in the stator further reduces the power factor, resulting in low utilization of the starting current.
3. It is noisy, especially during startup.
IV. Soft start using thyristor serial insertion
This is a starting method widely used in medium and high voltage applications abroad, and it is the mainstream starting method internationally. It utilizes thyristor series technology, transmitting control signals through optical fiber to simultaneously turn the thyristor series on and off, thereby controlling the motor's starting process.
Thyristor-based soft starting has many advantages, such as:
1. Small size and light weight.
2. Rapid control and quick response.
3. Can be started frequently.
4. High reliability, with a guaranteed start-up success rate.
Given the above advantages, thyristor-based soft starting is the best performing starting method among the various soft starting methods besides frequency converter starting.
Regarding the price, due to the high technical threshold, such products were previously all imported, giving people the impression of being expensive. However, with the increasing maturity of semiconductor power device technology, the price of semiconductor components, and the involvement of strong domestic companies, the price of thyristor-based soft starters has reached an acceptable level. Therefore, from a developmental perspective, thyristor-based soft starters will inevitably replace other soft starter methods and become the mainstream.
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