Sep 19, 2020
Application of Soft Starter in Water Pump Motor of Water Supply System
AC squirrel cage asynchronous motor is the most widely used electrical equipment in oil fields. But it is a load dominated by back EMF, that is, he uses back EMF to balance the applied voltage. The back electromotive force of the motor gradually increases with the increase of the rotor speed. The back electromotive force of the motor is o at the beginning of the start, so the inrush current is very large when starting. When the motor capacity is large, it will cause interference or even harm to the power grid and other loads. Safe operation of the power grid. In order to solve this problem, a variety of step-down starting methods are used: early series reactance or resistance starting methods, autotransformer step-down starting methods, star-delta conversion starters, etc.
The soft starter made with thyristor AC voltage regulation technology was promoted in the 1970s. Later, NASA engineer Nova combined power factor control technology and used microcomputer instead of analog control circuit to develop into a modern one. Intelligent soft starter. At present, many electrical companies in the world produce soft starters. GE's soft starters have a maximum power of 850 kW, a rated voltage of 500V, and a maximum starting current of 5 900A. American BS company also produces medium voltage 6-13kV, 8kV synchronous or asynchronous motors with a maximum power of 100,000 kW.
1 The working principle of soft starter
The main circuit of the soft starter is actually a thyristor voltage and speed regulation circuit, which changes the trigger conduction angle. , You can adjust the output voltage of the thyristor voltage regulator circuit. Its main feature is that the torque of the motor is approximately proportional to the square of the stator voltage.
When starting the motor with a soft starter, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage.
2 Application on pumps of water supply system
Application in Tuha Oilfield: Tuha Oilfield has successively used a 300kW smart soft starter from XXX Company in the first water source water transfer system, and realized the soft start of three motors by using its one-to-three function. A 160kW XXX soft starter was used to drive three 132kW motors in the Shanshan Zhuan Water Pumping Station, and AITISTART starters plus reactive power compensation technology were widely used in the power distribution devices of various wells in the oil field, realizing the water supply of the Tuha Oilfield System soft start and power saving operation. The pump control method is selected to provide smooth stepless motor acceleration, and reduce the damage to the motor and pump by reducing torque fluctuations. From the operating situation. To achieve satisfactory results.
2.1 Realization of pump control function
The pump control function is one of the motor control methods selected according to the characteristic curve of the pump. This function smoothly accelerates and decelerates the motor to reduce water hammer during the start and stop of the pump. The microprocessor analyzes the change of the motor speed, and issues instructions to control the motor and reduce the possibility of surge in the system. The start-up time can be set in the range of o~30s, and the stop time can be set in the range of 0~120s.
2.2 Operating status
The soft starter can have 4 operating states:
(1) Cross-over operation mode The thyristor is in full conduction state, the motor works in full voltage mode, and the voltage harmonic components can be completely ignored. This mode is often used for short-term repetitive motors, as shown in Figure 2.
(2) Contactor bypass mode When the motor reaches full speed, the bypass contactor is used to replace the soft starter that has completed the starting task, which can reduce the heat loss of the thyristor and improve the efficiency of the system. In this working mode, it is possible to use one soft starter to start multiple motors.
(3) Energy-saving operation mode When the motor load is light. The soft starter automatically reduces the voltage applied to the motor stator, reducing the excitation component of the motor current, thereby improving the power factor of the motor.
(4) Since the soft starter in the voltage regulation and speed regulation mode is realized by the principle of thyristor voltage regulation, it can be used for voltage regulation and speed regulation operation. Because the internal resistance of the motor rotor is very small, its speed regulation range is very small. The slip rate is adjusted between o~Sm. To obtain a wide range of speed regulation, it is necessary to put an appropriate resistance in series in the motor rotor.
3 Features after applying the soft starter
(1) With the pump control function, the relevant parameters of the motor can be detected according to the pump load characteristics and the output torque can be automatically adjusted to minimize mechanical shock.
(2) Phase balance. Using the function of phase balance, the SMC controller can continuously monitor the three-phase line voltage and automatically adjust the output voltage to balance the three-phase current of the motor.
(3) Energy saver, because the water injection pump is running at power frequency, the motor is under light load or under load. The SMC controller with energy-saving function uses its built-in feedback circuit to continuously detect the motor load. Because the thyristor can control the output voltage, the power loss of the motor is reduced by reducing the motor terminal voltage.
(4) The built-in overload protection function can save the thermal relay, making the control cabinet wiring simpler and faster.
(5) You can read the operating parameters of the motor, such as voltage, current, power, kWh, and operating time without adding any sensors or meters.
(6) The controller can support multiple communication protocols. Such as RS232/422/485-DFl. DH-485, etc., provide a communication interface for computer data acquisition and centralized control.
The soft start device is used on the pump motor, compared with the autotransformer. Improved starting characteristics, reduced mechanical damage, reduced volume, coupled with flexible use and complete functions, is an ideal motor drive device.