Analysis and research on soft start technology of AC asynchronous motor Paper Keywords: asynchronous motor; Soft start; Speed control b

Abstract: In this paper, the soft start of AC asynchronous motor is analyzed and studied, and a comprehensive control scheme for starting and running of asynchronous motor is put forward. 1 preface

At present, industrial and mining enterprises use a large number of AC asynchronous motors (including 380V/660V low-voltage motors and 3KV/6KV medium-voltage motors), and a considerable number of asynchronous motors and their transmission systems are still in uneconomical operation, wasting a lot of electricity. Investigate its reason, is roughly caused by the following circumstances:

(1) Because most motors adopt direct starting mode, starting current of 8~ 10 times not only causes shocks and accidents to power grid and traction system, but also causes huge energy loss.

② When selecting the motor capacity, it is often one-sided to pursue a large safety margin, and layers are overweight. The motor capacity is too large, resulting in the phenomenon of "big horse-drawn car", which leads to the motor deviating from the optimal operating point and reducing the operating efficiency and power factor.

(3) Considering the operation economy of electric production machinery itself, the electric drive system is often required to have the ability of variable pressure and variable speed regulation. If it is driven at constant speed and constant voltage, it will inevitably cause a lot of extra power loss.

2 Soft start of asynchronous motor

Due to the continuous renewal and development of industrial production machinery, higher and higher requirements are put forward for the starting performance of motors, which can be summarized as follows:

① Find the starting torque of the motor that is large enough and can be steadily improved and the mechanical characteristic curve that meets the requirements;

(2) The minimum starting current;

③ The starting equipment is as simple, economical and reliable as possible, and the starting operation is convenient;

④ The power consumption at startup should be as small as possible.

According to the above contradictory requirements and the actual situation of the power grid, there are usually two starting modes: one is the direct starting mode under rated voltage, and the other is the step-down starting mode.

2. 1 Danger of direct starting

① Power grid influence: Excessive starting current (4~7 times of rated current under no load and 8~ 10 times or more under load) will cause the voltage of power grid to drop, affect the normal operation of other electrical equipment, and may also cause undervoltage protection to act, resulting in harmful tripping of equipment. At the same time, excessive starting current will make the motor winding heat up, thus accelerating insulation aging and affecting the life of the motor.

② Mechanical impact: Excessive impact torque often causes the rotor cage bars and end rings of the motor to break, and the insulation of the stator end windings is worn, leading to breakdown and burning; Torsion of rotating shaft, damage of coupling and transmission gear, tearing of belt, etc.

(3) Impact on production machinery: sudden change of pressure at start-up often causes damage to pipes and valves of pump system and shortens service life; It affects the transmission accuracy and even the normal process control.

2.2 Application occasions and performance comparison of the old step-down starting mode:

The purpose of step-down starting is to reduce the starting current, but it also reduces the starting torque. For heavy load starting, production machinery with large peak load cannot be started in this way. Traditional step-down starting has the following methods:

(1) star/delta converter: This method is suitable for the motor with delta connection of stator windings in normal operation. The stator has six connectors leading out and connected to the transfer switch, which adopts star connection when starting and switches to triangle connection after starting. The starting voltage is 220 volts and the working voltage is 380 volts. The advantages of this starting equipment are simple starting equipment and low energy consumption in the starting process. Disadvantages are secondary current shock, high equipment failure rate and frequent maintenance, which are not suitable for frequently started equipment.

(2) step-down starting of autotransformer: the high voltage side of three-phase autotransformer (also called compensator) is connected to the power grid, and the low voltage side is connected to the motor. Generally, there are several taps, and different voltage ratios can be selected for different starting torques. Cut off after the motor is started. Its advantage is that the starting voltage can be selected, such as 0.65.0.8 or 0.9UN, to meet the requirements of different loads. Disadvantages are large volume, heavy weight, high consumption of non-ferrous metals, high failure rate and high maintenance cost.

(3) For wound asynchronous motor, frequency-sensitive rheostat or water resistance can be connected in series in the rotor winding to realize starting, and then cut off after starting. However, frequency-sensitive rheostat has high cost and large water resistance loss.

It is worth pointing out that all kinds of old-fashioned step-down starting methods have their own advantages and disadvantages, but they all have one common advantage: no harmonic pollution. 2.3 New electronic soft starter

The so-called "soft start" is actually a step-down start-up process according to the preset control mode. At present, soft starters generally have the following starting modes: (1) current-lImiting soft start: current-limiting soft start, as the name implies, is a soft start mode that limits the starting current of the motor to not exceed a certain set value (im) during the starting process. It is mainly used for step-down starting of load under light load. Its output voltage rises rapidly from zero until its output current reaches the preset current limit Im. Then, the advantage of this starting mode is that the starting current is small and can be adjusted as needed. (The limit value Im of starting current must be set according to the starting torque of the motor. If the Im setting is too small, the startup will fail or the motor will burn out. ) has little effect on the grid voltage. Its disadvantages are that it is difficult to know the starting pressure drop, the pressure drop space can not be fully utilized, the starting torque is lost and the starting time is long.

(2) Torque control starting: it is mainly used for heavy-load starting, and the output voltage is controlled according to the law of linear increase of motor starting torque. Its advantages are stable starting and good flexibility, which is beneficial to the traction system and reduces the impact on the power grid. This is the best way to start with heavy load. Its disadvantage is long startup time.

(3) Torque-jump control starting is also used for heavy-load starting, just like torque control starting. The difference is that jumping torque is used to overcome the static torque of traction system at the moment of starting, and then the torque rises steadily, which can shorten the starting time. However, the sudden jump will send a sharp pulse to the power grid and interfere with other loads, so special attention should be paid when using it. (4) Voltage-controlled starting is adopted for light-load starting, which can make the motor get the maximum starting torque and shorten the starting time as much as possible under the premise of ensuring the starting pressure drop, and is the best light-load soft starting method.

2.4 Application of Soft Starter

(1) The production equipment is precise, and it is not allowed to start the impact, otherwise it will cause adverse consequences to the production equipment and products;

(2) The motor has high power, so if it is started directly, the capacity of the main transformer needs to be increased;

(3) The power supply system with strict requirements on grid voltage fluctuation and voltage drop ≤10% un;

(4) Equipment that does not require high starting torque and can be started with no load or light load.

Strictly speaking, the starting torque should be less than 50% of the rated torque, so it is suitable to use soft starter to solve the starting shock problem. For the equipment that needs heavy load or full load starting, if the soft starter is used for starting, not only can the starting current not be reduced, but the capacity of the thyristor of the soft starter needs to be increased, which will increase the cost; If it is not operated properly, it may burn out the thyristor. At this time, only variable frequency soft start can be used. Because the soft starter does not adjust the voltage and frequency, slip power always exists, and it is inevitable that the starting current will be too large; The frequency converter adopts frequency modulation and voltage regulation, which can realize soft starting without overcurrent and provide starting torque of 1.2~2 times the rated torque, especially suitable for heavy-duty starting equipment. But the price of inverter is much higher than that of soft starter.

3 Voltage regulation and speed regulation of asynchronous motor

Voltage regulation and speed regulation of asynchronous motor is an inefficient speed regulation method, because there is always slip loss in the speed regulation process, so voltage regulation and speed regulation is very limited. It is impossible for any ordinary cage motor to realize voltage regulation and speed regulation with a set of thyristor voltage regulation device.

First of all, the external characteristics of the motor must be changed, and the new external characteristics must make the motor have a wide and stable speed regulation range. Usually, a high slip motor, an AC torque motor or a method of connecting resistors in series in the rotor winding of a wound-rotor motor, together with closed-loop speed control, can be used for stable speed adjustment.

Secondly, the rotor temperature rise caused by differential power during speed regulation should be well led out of the machine to realize long-term stable work. Rotating heat pipe structure or special air duct cooling structure can be used here, which are all effective methods.

Today, with the high development of power electronics technology, it doesn't seem to have much practical significance to consider voltage regulation and speed regulation when the frequency conversion speed regulation device is no longer expensive.

4 conclusion

(1) The electronic soft starter has a simple structure. Compared with the traditional △/Y starter, the autotransformer starter has the advantages of no contact, no noise, light weight, small volume, controllable starting current and starting time, stable starting process and less maintenance workload. When the motor is unloaded or lightly loaded, the energy-saving effect is remarkable, especially suitable for short-term full load and long-term no-load load.

(2) For high slip motors, solid rotor motors and torque motors. Especially with fans and water pumps, it has good speed regulation performance, but it is not suitable for ordinary cage motors.

(3) Intelligent controller is adopted, which has perfect motor protection function, convenient setting of protection setting value and reliable protection performance.

(4) Its biggest disadvantage is that it has harmonic interference to power grid and motor due to the phase shift control of crystal A grid tube.