ADVANTAGES OF REGULATING THE SPEED OF ROTATING MECHANISMS USING THYRISTOR VOLTAGE REGULATORS

ABSTRACT

Asynchronous motors are widely used in industry, agriculture, everyday life and other areas, due to the simplicity of their design, reliability in operation and relatively low cost. The consumption of scarce non-ferrous metals and the total mass per unit of power for asynchronous motors is 1.5-2.5 times less than for DC motors. In addition, in the vast majority of cases, converter installations are not required to power asynchronous motors, since they receive energy directly from industrial-frequency AC networks. When operating various machines and mechanisms, to ensure the rational progress of the technological process, it often becomes necessary to regulate the rotation speed of the working bodies. To regulate the rotation speed of the working body, in principle, there are two possibilities: changing the rotation speed of the electric motor and changing the parameters of the kinematic chain of the mechanical part of the drive. The last option has been known for a long time. Its technical implementation is associated with relatively complex and insufficiently reliable structures (gearbox, mechanical variators, etc.) [11, 12].

АННОТАЦИЯ

Асинхронные двигатели широко используются в промышленности, сельском хозяйстве, быту и других сферах благодаря простоте конструкции, надежности в эксплуатации и относительно невысокой стоимости. Расход дефицитных цветных металлов и общая масса на единицу мощности у асинхронных двигателей в 1,5-2,5 раза меньше, чем у двигателей постоянного тока. Кроме того, в подавляющем большинстве случаев преобразовательные установки для питания асинхронных двигателей не требуются, поскольку они получают энергию непосредственно от сетей переменного тока промышленной частоты. При эксплуатации различных машин и механизмов для обеспечения рационального хода технологического процесса часто возникает необходимость регулирования скорости вращения рабочих органов. Для регулирования скорости вращения рабочего органа в принципе имеются две возможности: изменение скорости вращения электродвигателя и изменение параметров кинематической цепи механической части привода. Последний вариант известен давно. Его техническая реализация связана с относительно сложными и недостаточно надежными конструкциями (коробками передач, механическими вариаторами и т.п.) [11, 12].

Ключевые слова: асинхронные двигатели, скорость вращения, тиристорный регулятор напряжения, регуляторы частоты, диапазон настройки.

Keywords: asynchronous motors, rotation speed, thyristor voltage regulator, frequency regulators, tuning range.

Introduction. By regulating the rotation speed of electric motors, which is currently an actual system, we mean its targeted change, regardless of the torque on the shaft, in accordance with the requirements for the law of motion of the working body of the mechanism. Analyzes of the mechanical characteristics of various motors have shown that their frequent rotation can change both when the parameters of the electrical circuits (resistances) or the power source (frequency voltage) change, and when the moment of resistance changes. Often the task arises of expanding the rotation speed control zone and thereby increasing the control range - D=ω_max/ω_min. However, the expansion of this zone cannot be unlimited. An increase in the upper limit of rotation speed ω_max is usually limited by the mechanical strength of the rotor [13, 14].

The designed asynchronous motor is necessary in everyday life, automatically controlled technical devices, control systems and for drive devices with speed control. The proposed design with its positive properties will put an end to the use of asynchronous motors with speed control with increased values of efficiency and operating parameters, which ultimately makes it possible to accelerate the use in important production devices [2].

The technical result of the design can be accepted as follows: simplicity of design; possibility of use in a wide power range; production of rotor winding from copper; transferring most of the heat created from the losses of the working rotor past the active part of the stator and, in connection with this, increasing the torque on the shaft and reducing the active weight of the engine [1].

The stator, consisting of windings and a magnetic core to intensify the cooling process, is pressed into the motor housing, equipped with ribs. The magnetic core of the working rotor is assembled from electrical steel 0.5 mm thick, the rotor windings are made of cast copper and it has short-circuited rings - on the side of the additional rotor made of copper, on the reverse end of metal with high active resistance, and all elements are placed on the shaft of the working rotor. The working rotor is equipped with ventilation ducts. The machine body is secured on both sides with a bearing shield. The first bearing shield sat on the main shaft using a bearing. A second bearing is located at the other end of the main shaft. The rotor, equipped with ventilation blades, is attached to an additional shaft. The bearing of the second bearing shield is attached to the additional shaft on one side, and the bearing of the third bearing shield on the other. A centrifugal fan is mounted between the second and third bearing shields on the shaft of the additional rotor. The third bearing shield is made with ventilation windows. To circulate internal air, ventilation ducts are installed between the stator core and the frame casing [15].

The electric motor works as follows:

When voltage is applied to the stator winding, a rotating magnetic field is created in the magnetic circuit. The created rotating magnetic flux penetrates both the working and additional rotors. Between the magnetic field and the currents created in the windings from the rotating magnetic field, a torque is created in both rotors. Since the working and additional rotors have a mechanical connection with each other through only one bearing located on the main shaft, they are completely free. The working and additional rotors have a magnetic connection with each other, which is created using internal parameters.

Depending on the voltage value (to obtain the required rotation speed, a full or less voltage value can be applied to the stator winding), the working rotor will rotate at a certain rotation speed. To regulate the rotation speed, this voltage varies over a wide range. Assembled from sheets of electrical steel with a thickness of 0.5 mm, the working flow is supplied with a short-circuited winding made of cast copper. On the side of the additional rotor, the winding inputs are closed using a short-circuited copper ring. On the other hand, this winding has a short-circuit ring with a special design made of a material with high electrical resistance, so that when the voltage value supplied to the stator winding changes, the rotation speed of the working rotor can change within a range of 1:10. Accordingly, due to the fact that the rotor winding is made of a material with low resistance and the magnetic circuit is made of sheet electrical steel, losses at low speeds are not particularly large, which allows an increase in engine torque [8, 9].

It is necessary to change the air flow of the ventilation system, control the performance of pumps, regulate the speeds of individual moving parts of the unit, and regulate the rotation speed of engines to the required level of performance.

Although frequency controllers are widely used in manufacturing for the purpose of frequently regulating the speed of rotation of mechanisms, there are  certain disadvantages in the operation of transmission governors of this type.

In the presence of a non-sinusoidal supply voltage, high harmonic losses occur in the motor. To reduce these losses, frequency regulators use special filters and wide pulse modulation control. When using control with wide pulse modulation, the number of pulses increases as a result of a large number of switching operations, and excess voltages are generated in the stator and rotor windings. The listed processes lead to rapid failure of the regulators. Another significant disadvantage of the frequency converter is that semiconductor switches quickly fail when operating at high speeds. And this leads to high voltages between the switching elements. High voltages, in turn, lead to the formation of sparks in the pads and then to their failure. Breaks of additional windings, noise and vibration in the motor also occur. Thus, the use of frequency regulators requires a special approach. However, the fact that these devices are very expensive from an economic point of view limits their use in some controlled machinery (trucks, pumps, ventilation systems, etc.).

In such cases, the speed of rotation of asynchronous motors can be controlled in a more economical way by changing the amount of voltage supplied to the stator. For this, an autotransformer, a thyristor voltage regulator - TGT, etc. are used. you can use tools [10].

TGTs have a number of advantages over other regulators (autotransformer, saturation choke, etc.): speed, high FIE, low cost, characteristics of hard and smooth regulation, etc. [4]. In a three-phase circuit, two thyristor, which allows the load current to flow in two half-cycles at the network voltage U1 (Fiq. 1).

Figure 1. Connecting a thyristor voltage regulator to a three-phase load

Using the TGT, you can regulate the voltage to U_nom-0. With a non-sinusoidal stator voltage, the use of TGT is more advantageous in machines with a large critical slip value [5, 7]. The use of regulators of the TGT RST series on freight transport (crane transport) allows you to adjust the engine in the range of 10:1, and in dynamic modes - to adjust the starting and braking torque. TGT type PCT are manufactured for continuous current of 100, 160 and 320 A and alternating current voltage of 220 and 380 V.

Three-phase AC crane motors operate primarily in repetitive short-term modes, with a wide range of adjustment. Their work is already accompanied by loading processes, often starting, reversing and braking. In addition, electric motors with crane mechanisms operate in conditions of increased shaking and vibration. In a number of metallurgical shops they are exposed to vapors and gases with temperatures of 60⁰-70⁰C. Therefore, crane electric motors differ from other electric motors of general industrial production in their technical and economic indicators and characteristics [6]. These include:

- play connected

- heat resistance class F and H of insulating materials

- the moment of inertia of the rotor is minimal, rotation speeds are relatively low (to reduce energy losses during transient processes)

- minimum mode 1-1.5 hours.

A torque-vector asynchronous motor is usually used as a control device in the transmissions of crane mechanisms. The tuning range of this motor (1:3-1:4) is stepped, starting torque and tuning are limited.

In mechanisms of this type, asynchronous motors with a squirrel cage rotor are rarely used as a regulator due to the low starting torque and high starting currents, although their weight is 8% less than motors with the same powerful wound rotor, and the cost is 1.3 times cheaper. These types of motors are mainly suitable for low-duty and low-speed cranes.

Conclusions

The new design of the two-rotor squirrel-cage induction motor overcomes the above-mentioned limitations.

This type of motor has a high starting torque, low starting currents, a wide adjustment range of 1:10 and a high cooling system regardless of conditions, which makes its application wider and more appropriate than other motors (phase rotor and DC). This type of engine is also favored by its low price, ease and simplicity of maintenance and repair, as well as the low use of non-ferrous metals. If we compare the operating costs, the presented motor is 5 times smaller than a vector motor and 10 times smaller than a DC motor.

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