RU2465714C1 - Method to control variable frequency drive - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: frequency converter (FC) comprises a rectifier (2), a capacitor (3) and an inverter (4) and serves to speed up an electric motor (6). In these modes switchboards (1) and (5) are on, and (7) is open, and the electric motor (6) receives supply from the FC. After speed-up and during operation of the motor (6) with the fixed grid frequency the switchboard (7) is on. If required to stop the motor 6, control pulses are picked from the FC, and its outlet is connected to the motor (6) supplied from the grid. At the same time the capacitor (3) is charged due to "on" condition of the switchboard (1). Afterwards the switchboard (7) switches off. The capacitor (3), joined via a return diode bridge of an inverter (4) with the converter outlet reduces overvoltage on motor windings. Energy stored in inductances of the electric motor (6) is sent to the capacitor (3), which serves as a damper circuit.

EFFECT: reliability improvement.

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Description

The invention relates to the field of electrical engineering and is used in electric drives.

A widely known method / 1 / control of a variable frequency drive containing an electric motor associated with a frequency converter, which consists in controlling the frequency of the motor depending on the required performance.

The disadvantage of this method is the relatively high power consumption, since the motor is constantly powered by a converter.

The closest in technical essence and the achieved result is the prototype / 2 / - a method of controlling a variable frequency drive containing an electric motor and a frequency converter, which consists of a series-connected rectifier, capacitor and bridge voltage inverter, composed of fully controlled valves, shunted by reverse diodes, as well as input, network and starting switches, so that the inputs of the converter and the motor are connected to the network through the input and network comm respectively utensils, and the output of the converter is connected to the engine through the starting switch, and the control of the switches and the converter is carried out from a controller that provides frequency control of the engine, connecting it to the network or to the converter, depending on the required performance, so that after the converter is brought to the maximum or minimum value frequency, the motor is respectively connected to the network or disconnected from it. The disadvantage of this method is its low reliability, because when the engine is disconnected from the network, overvoltages occur. Interruption of current by mechanical switching devices causes an arc in the devices themselves and overvoltage on the motor windings. In installations with a voltage of 3-10 kV with sealed circuit breakers (vacuum, gas-insulated), such overvoltages can be significant (several times higher than the nominal value) and lead to damage to the motor windings. Dangerous and therefore undesirable switching overvoltages in low voltage motors.

The aim of the invention is to increase reliability.

This goal is achieved due to the fact that before disconnecting the motor from the network, the converter output with the control signals disabled is connected to it, and after the motor is disconnected from the network, the converter is disconnected from the engine.

The drawing shows a single-line wiring diagram for implementing the method. The input of the rectifier 2 is connected to the input switch 1, the poles of which are connected to the filter-capacitor 3 and the voltage inverter 4. The inverter 4 is made bridge. Each arm of such a bridge has a fully controllable valve (transistor), shunted by a counter-parallel diode. The rectifier 2, the capacitor 3 and the inverter 4 form a frequency converter (IF), the output of which through the start switch 5 is connected to an electric motor 6, which is connected through a network switch 7 to the network 8. The inverter 4 and the switches 1, 5, 7 are controlled from the microprocessor 9 (or manually).

The claimed method is as follows. The inverter serves to accelerate the electric motor 6 and ensure the operation of the latter with a frequency different from the network one. In these modes, switches 1 and 5 are turned on, and 7 is open and motor 6 is powered by the inverter. After acceleration and during operation of the engine 6 with a fixed network frequency, switch 7 is turned on, and switch 5 is turned off and the inverter can be used for other purposes (for example, to disperse other electric motors). If it is necessary to stop the motor 6 from the inverter, the control pulses are removed and its output is connected to the mains-powered motor 6. In this case, the capacitor 3 is charged due to the switched on state of the switch 1. After that, the switch 7 is turned off. The capacitor 3 is connected through the reverse diode bridge of the inverter 4 to the output converter, reduces overvoltage on the motor windings. The energy stored in the inductances of the electric motor 6 is transferred to the capacitor 3. Since the capacitance of the capacitor is hundreds to thousands of microfarads, the voltage across the capacitor will increase slightly.

Information sources

1. DE patent No. 3918246 A1, cl. EB03 11/16.

2. Patent RU No. 2154891 C1, cl. H02P 1/54, 1999.

Claims (1)

The method of controlling a variable frequency drive containing an electric motor and a frequency converter, which consists of a series-connected rectifier, capacitor and a bridge voltage inverter, composed of fully controllable valves, shunted by reverse diodes, as well as input, network and starting switches, so that the inputs of the converter and the motor are connected to the network, respectively, through the input and network switches, and the output of the converter is connected to the motor through the start switch, p Moreover, the control of the switch and the converter is carried out from the controller, which regulates the frequency of the motor, connects it to the network or to the converter depending on the required performance, so that after the converter has reached the maximum or minimum value of the frequency, the motor is respectively connected to or disconnected from the network, characterized in that before disconnecting the motor from the network, the output of the frequency converter with the control signals turned off is connected to it, and after it is turned off The motor is disconnected from the mains by the inverter.