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Umarov Shukhrat Badreddinovich, Associate Professor, "Electromechanics and Electrotechnologies" Department Tashkent State Technical University named after Islam Karimov
E-mail: shumarov1951@mail.ru
COMPARATIVE ANALYSIS OF POSSIBILITIES OF USING THE VALVE FREQUENCY CONVERTERS FOR SPEED CONTROL OF INDUCTION MOTORS WITH SQUIRREL-CAGE ROTOR
Abstract. The article presents the results of a comparative analysis of the possibilities of using valve frequency converters to control the speed of induction motors with a squirrel-cage rotor.
Keywords: valve frequency converters, induction motor, rotation speed of the electromagnetic field of the stator, frequency control method, autonomous inverter.
frequency, voltage and current, which is carried out by frequency converters performed on the basis of power semiconductor switches - thyristors and transistors. It should be noted that this method has its advantages and disadvantages. The main advantages include the possibility of regulation in a wide range due to the smoothness of regulation and high rigidity of mechanical characteristics, as well as energy-saving operation due to the fact that the motor operates with small values of absolute slip. Disadvantages are the complexity and therefore the high cost of frequency converters (especially for drives with high power, as well as difficulties in implementing regenerative braking for most circuits.
The frequency converters according to the type of connection with the mains supply to the semiconductor elements are divided into two broad classes:
- frequency converters with direct connection;
- frequency converters with DC link. Frequency converters with direct connection The basis of the power circuit of frequency converters with direct connection is a reversible thyristor converter. The multiphase output of the frequency converter with direct connection is achieved by using multiple reversible converters with single-phase
output. In the scheme of the frequency converter with direct connection, the DC link is expressed
As is known [1, 166-167; 2, 62-64], the speed of induction motors is determined by two parameters: the speed of rotation of the electromagnetic field of the stator o>Q and a slip s, i.e., there are two methods of speed control: control speed of rotation of the electromagnetic field of the stator and the regulation of the constant value uQ. In turn, the rotation speed of the electromagnetic field of the stator is determined by two parameters: the frequency of the voltage supplied to the stator windings/ and the number of pairs ofpoles of the motor Zp. Thus, two methods ofspeed control are possible: by changing the frequency of the supply voltage with the help of frequency converters included in the motor stator circuit, and by changing the number of pairs of motor poles.
The method of controlling the speed of the induction motor by changing the number of pairs of poles Zp allows you to get several fixed values of the operating speed. For example, induction motors of 4A-6A series produced two -, three - and four-speed, which are used for crane electric drives. As a rule, smooth speed control for such electric machines is not used, that is, multi-speed induction motors are not used for regulated electric drive systems [3, 99-100].
The frequency control method is based on the conversion of the alternating voltage of the supply network into an alternating voltage with a controlled
implicitly due to the simultaneous combination of two functions in one device - rectification and inversion. Hence, there are the following advantages and disadvantages of frequency converters with direct connection.
Advantages:
- single energy conversion, which determines the high efficiency of the converter;
- the ability to pass reactive power from the network to the load, and back.
Disadvantages:
- complexity of control devices. A large number of thyristors requires a large number of pulse-phase control systems;
- power factor converter substantially below one (cos^ ~ 0,15);
- significantly distorted form of supply voltage;
- difficulties in obtaining frequencies close to the frequency of the supply network. For a zero circuit, the maximum output voltage frequency is usually limited to f = 16 Hz. The transition to the
' J 1max
bridge circuit extends the operating range to f =
O COO J 1max
25 Hz. Therefore, induction motor drives with frequency converters with direct connection are used for low-speed without gear electric drives of medium and high power.
Therefore, frequency converters with direct connection are mainly used to power low-speed motors in medium and high power drives that operate in dynamically stressed modes accompanied by frequent reverse, braking with recuperation [4, 8-9].
Frequency converters with DC link, in turn, are divided into:
- autonomous current inverters (ACI);
- autonomous voltage inverter (AVI).
In its structure, autonomous inverters contain a rectifier, a power filter and an inverter that converts a constant voltage (current) into an alternating voltage (current) of a given frequency.
Powerful semiconductor devices, used in power converter devices, operate only in key modes for which there are two steady states:
- open state, the maximum electrical conductivity;
- closed state - minimum electrical conductivity.
When operating in the key mode, the loss of active power P=UI in semiconductor devices is small, since one of the current and voltage output factors (current I or voltage U) has the minimum possible value. This ensures high efficiency of electrical energy converters.
In the process of switching from the closed state of semiconductor devices to the open state and vice versa, the voltage and current vary along the D C load line. Current and voltage output increases significantly. Therefore, it is important that these switches occur in the shortest possible time. This condition has now been realized in two types of semiconductor devices with internal positive feedback, accelerating the switching of semiconductors - IGBT transistors or bipolar transistors with isolated gate and thyristors.
Two-stage converter devices are based on the rectifier of three-phase alternating voltage of the network and an autonomous inverter that converts rectified voltage into three-phase alternating voltage with adjustable frequency and amplitude. Despite the twofold energy conversion and due to this slight decrease, the efficiency of the frequency converters with DC link has received the greatest distribution in the regulated electric drive.
At the transition of the motor, supplied from an autonomous current inverter, to the generator mode, the direction of the counter EMF of the inverter changes, which switches to the mode of operation of the rectifier, which could cause an increase in current in the DC link. However, due to the strong negative feedback on the current, which covers the rectifier, the current in the DC link is maintained at the same level, and the rectifier is transferred to the inverter mode, driven by the network. As a result, energy is recovered into the supply network without changing the direction of current in the DC link.
Thus, in autonomous current inverters, braking modes of the motor with energy recovery into the
network are easily realized, which makes its use in reversible electric drives preferable.
A more perfect scheme of this class is a circuit of an autonomous current inverter with cut-off diodes.
In the scheme of an autonomous current inverter with cut-off diodes, capacitors are separated from the load by diodes, so that capacitors participate in the operation of the inverter only in a relatively short switching time, and the rest of the current does not flow through them. This can significantly reduce the capacitance of the capacitors.
The main advantages of the frequency converter with a stand-alone current inverter:
- possibility of energy recovery in the network;
- the output voltage is close in form to a sine wave;
- failure-free mode of short circuit in the load.
Disadvantages:
- output frequency limit at 100 ^ 125 Hz;
- switching overvoltage on thyristors, which makes the power circuit more complicated;
- inability to work on the group load;
- substantial weight and dimensions of the induction filter.
The second type of frequency converters with a DC link is based on an autonomous voltage inverter (AVI), the DC link of which contains an LC filter (or only a capacitance C). In the AVI, there is a direct dependence of the voltage in the DC link on the voltage at the load, so it is a source of voltage [5, 40-41]. If the current inverters at each time operates one valve in the anode and cathode groups, the voltage inverters more appropriate simultaneous operation of two valves in one group and one
in the other. Due to the presence of capacitance C during operation of the inverter as a voltage source for the active-inductive load (induction motor), the exchange of reactive energy between the motor and the DC link is provided.
The output voltage of the inverter can be regulated in two ways:
- regulation of the input voltage in the DC link for which a controlled rectifier is used, whereas the inverter performing the function of a phase switch forms the required frequency. Such frequency converters called AVI with pulse-amplitude modulation;
- pulse-width voltage regulation in the inverter, which is carried out by the carrier frequency voltage (key switching frequency) signal of the fundamental frequency. Such frequency converters called AVI with pulse-width modulation. In them, the rectifier can be adopted unmanageable.
Given that the first method has such drawbacks as a step form of the output voltage and a low power factor of the converter, the second method is more promising. It should be noted that the pulse-width modulation is possible not only to regulate the average value of the voltage for the period, but also to adjust the form of the output voltage.
Currently, in the regulated electric drives on the system of "induction motors with squirrel-cage rotor - frequency converter based on autonomous inverters" in the power range up to 50 kW, transistor voltage inverters are mainly used, and in the higher power ranges and for all applications with active load braking (centrifuges, test equipment, cranes), the main preference is given to current inverters [6, 64-65; 7, 3-5].
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4. Khashimov A. A., Aripov N. M. Frequency-regulated induction motor drive of silk reeling.- Fergana: FerPI Publishing House, 2000.- 92 p.
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