company news

Three phase asynchronous motor has many advantages, but it also has two fatal weaknesses: one is poor starting performance; the other is poor starting performance; Second, the speed regulation performance is poor. With the development of power semiconductor technology and the application of high power transistor in the field of industrial control, two typical products have greatly improved the two weaknesses of asynchronous motor, one is inverter, the other is soft starter. Frequency converter improves the speed regulation performance of asynchronous motor; The soft starter improves the starting performance of asynchronous motor. There is an article in front of a question and answer in the form of a brief introduction of the inverter related knowledge, this article and you share the basic knowledge of soft starter.

Soft starter has many functions, such as soft start, soft stop, pump control function, timing and low-speed operation, so it has incomparable advantages in power equipment control of industrial and civil buildings. The features and precautions of soft starter are as follows:

1. Features

(1) During the soft start time Ts, the terminal voltage of the motor increases gradually from the set initial value to full voltage, and the motor accelerates smoothly and steplessly. At the same time, ts can be adjusted (usually between 0.5 and 60s), which is conducive to the cooperation with low-voltage circuit breaker and prevents the instantaneous release of low-voltage circuit breaker from tripping when the motor starts.

(2) Pump control function when the pump starts and stops, the water flow impacts the pipeline, resulting in serious water hammer effect. Although waterway specialty has taken some measures to eliminate water hammer effect, if the soft starter with pump control function is adopted, the water hammer effect can be completely eliminated, the workload of mechanical maintenance can be reduced, the cost of system maintenance can be saved, and the reliability of water supply can be ensured. It is worth mentioning that this function is optional and can be indicated on the supply drawing.

(3) Generally, the overload protection function of motor is realized by thermal element. It is difficult for thermal element to avoid the starting current of motor in the process of motor starting. In order to avoid maloperation, users often increase the action current of the thermal element, which makes the overload protection not sensitive enough and makes the overload protection useless. The built-in overload protection function of soft starter is to select different tripping characteristic curves according to the weight of starting load (i.e. the length of starting time), so as to avoid false tripping during starting. Because its overload protection is programmable, it provides more flexibility for users. The soft starter with built-in overload protection function can also save the thermal relay and make the wiring in the control cabinet easier.

(4) The parameter test function can select and display the operation parameters of the motor in the display window by pressing the key, such as three-phase voltage value, three-phase current value, power factor, operation time, etc., without adding any instrument. This function enables users to conveniently query various parameters of the motor in the operation process.

(5) Because the fire pump is a fire emergency facility, it is not in use for a long time, so it is easy to get stuck. Once the fire occurs, if it can not work normally, it will affect the smooth progress of fire fighting and cause serious consequences《 Code for electrical design of civil buildings stipulates that regular automatic test and automatic detection measures should be taken for important fire-fighting electrical equipment such as fire pump (including spray pump), smoke exhaust fan and positive pressure forced draft fan. The soft starter has the function of timing and low-speed operation, which can start and stop automatically according to the time set by the user. It plays the role of regular automatic test and automatic detection for the fire pump, and improves the reliability and emergency of the fire facilities.

(6) Principle and wiring soft starter adopts three-phase anti parallel thyristor as voltage regulator, and connects it between power supply and motor stator (see Figure 1 for specific wiring diagram). When the soft starter is used to start the motor, the output voltage of the thyristor increases gradually, and the motor accelerates gradually until the thyristor is fully on. The motor works on the mechanical characteristics of rated voltage to realize smooth starting, reduce starting current and avoid starting over-current trip. When the motor reaches the rated speed, the starting process ends.

2. Problems needing attention

(1) Some soft starters have a variety of built-in protection functions, such as stall and locked rotor test, phase to phase balance, under load protection, under voltage protection, over-voltage protection, etc., which play a further role in motor protection. In design, protection functions should be selected or disabled by programming according to specific conditions《 The code for design of low voltage power distribution stipulates that the overload protection should act on the signal rather than the cut-off circuit for the lines with more losses caused by sudden power failure than overload. For the fire pump control system, the protection of the fire pump motor is not an important problem. Some protection functions can be disabled by programming, only acting on the signal. The overload protection function mentioned above should also only act on the signal.

(2) Because the soft starter itself has no short circuit protection, in order to protect the thyristor, the fast fuse should be used (the breaking time of low-voltage circuit breaker is long, 0.1s, which can not effectively protect the thyristor). In design, the corresponding fast fuse can be selected according to the product samples provided by the manufacturer and the rated current of the soft starter.

(3) When the soft starter makes the motor stop, only the thyristor does not conduct, and there is no electrical isolation between the motor and the power supply. If the line and motor after the soft starter are overhauled at this time, it is not safe, so the circuit breaker should be added before the soft starter in the motor primary control circuit.

(4) Because the soft starter uses thyristor and other nonlinear devices, when the power of soft starter is large or the number of soft starters is large, the generated high-order harmonics will cause adverse effects on the power grid and interfere with the electronic equipment in the building. At this time, a bypass contactor can be installed (see Fig. 2). After the soft starter makes the motor start smoothly to the normal speed, the contactor K is closed and the soft starter is short circuited. That is, after the start-up, the high-power thyristor is in a non-conductive state to reduce the interference of high-order harmonics on the power grid and electronic equipment.

(5) When the soft starter passes through the current, it will produce heat dissipation. When installing, it should pay attention to leave a certain space above and below it, so that the air can flow through its power module. When the rated current of the soft starter is large, the fan should be used for cooling, and the power supply of the fan can be taken from the secondary circuit of the motor control system.

Above all, soft starter is a new type of starting device with excellent performance, which is the preferred product of AC asynchronous motor starter.