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Application of soft starter in the control of civil building power equipment

Sep 24, 2020

Application of soft starter in the control of civil building power equipment

I. Overview 

   AC squirrel cage asynchronous motor is widely used in the driving of mechanical equipment due to its simple structure, convenient control and high efficiency. Most of the mechanical equipment in civil buildings, such as fire pumps, sprinkler pumps, living pumps, refrigeration units, etc., are powered by AC squirrel cage asynchronous motors. When the building has a high number of floors or a large scale, the rated power of the motors of these mechanical equipment is usually large. For example, the rated power of a fire pump is usually between 55kW and 150kW. These devices will generate a larger starting current during the starting process, resulting in a larger voltage drop. Therefore, proper selection of the starting mode has important meanings such as reducing the power supply capacity and ensuring the reliability of building power supply. Because of this, the soft starter will have broad prospects in the application of civil construction.

   2. Comparison of starting methods of power equipment such as water pumps in civil buildings

   As we all know, the squirrel-cage asynchronous motor has a large starting current and a long starting time when it is started at full voltage. When the power of the motor is large, the starting current is very large (the starting current is 5-8 times the rated current). A large starting current will cause a voltage drop in the power distribution system, affect the normal operation of other electrical equipment connected to the same transformer or the same power supply line, and even cause the diesel generator set to stall and shut down. At the same time, due to the large starting torque, it will have an impact on the load, increase the wear of the transmission components and additional maintenance. Therefore, when the capacity of the motor is large (generally more than 20%-30% of the power supply capacity), the reduced voltage starting is adopted.

   The traditionally used methods of step-down starting include Y/△ changeover starting and autotransformer step-down starting. Although these two starting methods can reduce the starting current, the peak current (secondary impulse current) that is several times the rated current will still be generated at the moment of switching and applying full voltage after the step-down starting process is completed. The current will have an impact on the power distribution system, and the destructive dynamic torque generated at the same time will cause mechanical vibration of the pump motor, which is very harmful to the rotor and intermediate gear of the motor, and increases the power consumption of the power supply line.

  Soft starter is also a kind of step-down starter. It uses a microprocessor with advanced performance to control the conduction of high-power thyristor components in a reasonable and orderly manner, so that a gradually increasing smooth AC voltage is applied to the AC motor, so that the motor can gradually progress according to preset methods and parameters Accelerate to achieve soft start. It can be seen that the use of a soft starter can achieve a smooth, uniform and stable start of the large motor, avoid the impact on the power grid when the large motor starts, reduce mechanical vibration and noise, and reduce the power consumption of the power supply line.

   Three, the application of soft starter in civil building power equipment control

  The soft starter has multiple functions such as soft start, soft stop, pump control, and timing low speed operation. Therefore, it has incomparable advantages over traditional step-down starters in the control of power equipment in the field of civil construction. Take the fire pump in a high-rise building as an example to analyze the outstanding advantages of the soft starter in the starting and operation control of the fire pump. The maximum capacity of the fire pump in the high-rise building is 110KW, one for one and one for use. Use a certain type of soft starter from a certain company.

  1, soft start” As mentioned before, during the starting time ts, the terminal voltage of the motor gradually increases from the set initial value to the full voltage, and the motor accelerates smoothly and steplessly. At the same time, the starting time ts can be adjusted (usually between 0.5-60 seconds), which is conducive to the cooperation with the air switch and prevents the instantaneous release of the air switch from accidentally tripping when the motor starts.

  2, pump control function  When the water pump starts and stops, the water flow hits the pipeline, causing serious "water hammer effect". Although the waterway profession has taken some measures to eliminate the water hammer effect, if a soft starter with pump control function is used, the water hammer effect can be completely eliminated, the workload of mechanical maintenance is reduced, the system maintenance cost is saved, and the water supply is reliable. . It is worth mentioning that this function is optional and should be marked on the order drawing.

  3, overload protection function   general motor overload protection is realized by thermal elements. It is difficult for the thermal element to avoid the starting current of the motor and cause malfunction during the motor starting. In order to avoid misoperation, users often increase the operating current of the thermal element, making the overload protection useless. The built-in overload protection function of the soft starter is implemented by electronic devices based on the I2t algorithm. It can select different tripping characteristic curves according to the starting load, that is, the length of the starting time to avoid false trips during the starting process. Because its overload protection is programmable, it provides users with greater flexibility. In addition, the use of a soft starter with built-in overload protection can eliminate thermal relays, making wiring in the control cabinet simpler and faster.

  4. Parameter test function  can select and display the operating parameters of the motor, such as three-phase voltage value, three-phase current value, power factor, running time, etc., in the display window by pressing the button, without adding any instruments. This function allows users to conveniently query various parameters of the motor during operation.

  5. Regular automatic test of fire-fighting water pumps. Because fire-fighting water pumps are fire-fighting emergency facilities, they are usually not used for a long time, and the phenomenon of pump jams is prone to occur. Once a fire occurs, if it fails to operate normally, it will affect the smooth progress of fire fighting and cause serious consequences. Article 24.6.6 of "Code for Electrical Design of Civil Buildings" JGJ/T16-92 stipulates that "fire pumps (including spray pumps), exhaust fans, positive pressure blowers and other important fire fighting electrical equipment should be tested regularly and automatically tested. Measures." The soft starter has a timing low-speed operation function, which can automatically start and stop according to the time set by the user. It plays a role of regular automatic test and automatic detection for the fire pump, and improves the performance of the fire pump as a fire protection facility. Reliability, contingency. However, it will be difficult to automatically test the fire water pump regularly with the traditional reduced pressure starting method.

   Fourth, the problems that should be paid attention to when using soft starters in civil building power equipment control

   1. Some soft starters have a variety of built-in protection functions, such as stall and locked-rotor test, phase balance, underload protection, undervoltage protection, overvoltage protection, etc., which provide further protection for the motor. When designing, the protection function should be selected by programming or make some protection functions invalid according to the specific situation. Article 4.3.5 of "Code for Design of Low-Voltage Power Distribution" GB50054-95 stipulates: "For lines where sudden power failure causes more losses than overload, the overload protection should act on the signal and not on the circuit." In the aforementioned fire pump control system, since the fire pump is a fire emergency load, it works at the critical moment of fire fighting. When a fire occurs, the most important problem is that the fire pump can run to draw water into the fire pipe network, but the protection of the fire pump motor is not an important issue. Therefore, some of the protection functions mentioned above can be disabled by programming, or the protection can act on the signal instead of stopping the fire pump motor. The aforementioned overload protection function should only act on signals.

  2, branch circuit protection” Since the soft starter itself has no short-circuit protection, in order to protect the thyristor in it, a fast fuse should be used (the opening time of the automatic air switch is longer, 0.1 second, which cannot effectively protect the thyristor). When designing, select the corresponding fast fuse according to the product sample provided by the manufacturer and the rated current of the soft starter.

   3. When the soft starter brakes the motor to stop, only the thyristor does not conduct, and there is no electrical isolation between the motor and the power supply. It is unsafe if the circuit and motor behind the soft starter are inspected at this time. Therefore, a circuit breaker should be added before the soft starter in the primary control circuit of the motor.

  4. Due to the rapid development of information and network technology, modern buildings, especially smart buildings, have more and more electronic devices, which have higher requirements for the quality of the power grid. Since the soft starter uses non-linear devices such as thyristor, when the power of the soft starter is large or the number of units is large, the high-order harmonics generated will have a negative impact on the power grid and produce electronic equipment in the building. Timidly. At this time, a bypass contactor can be installed. After the soft starter makes the motor start smoothly to the normal speed, the contactor KM is closed and the soft starter is short-circuited. That is, after the start is completed, the high-power thyristor is in a non-conducting state, reducing the interference of high-order harmonics to the power grid and electronic equipment.

   5. The soft starter will dissipate heat when passing current. When installing, pay attention to leaving a certain space above and below it so that air can flow through its power module. When the rated current of the soft starter is large, the fan should be used to cool down. The power supply of the fan can be taken from the secondary circuit of the motor control system.

   5. Conclusion

   In short, the soft starter is a new type of starting device with excellent performance, and it is the first choice for AC asynchronous motor starters. However, since the current soft starters are mainly imported products, the price is relatively high. As mentioned earlier, the price of the 150-B240NBD soft starter of A-B company is more than 20,000 yuan, which is difficult for most owners to accept. Now domestic manufacturers such as Hunan Carrier Company have also launched their own products. It is hoped that more and more inexpensive and good-quality products will appear on the market, so that soft starters can replace traditional starters in the field of civil construction and quickly promote their applications and bring benefits.