A device for braking a three-phase asynchronous electric motor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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The article describes a simple device for electrodynamic braking of a three-phase asynchronous electric motor with a squirrel-cage rotor, which provides automatic braking when disconnected from the network by short-term flow of a pulsating current of the supply network through its windings.

The proposed device relates to electrical engineering and can be used in electric drives of general industrial mechanisms.

Devices for braking three-phase asynchronous electric motors with a squirrel-cage rotor (IM) are known, containing diodes and capacitors, resistors and magnetic starters that connect two phases of the AD to the network, and the third phase of the electric motor is connected directly to one of the windings of its stator [1,2].

Closest to the proposed device in terms of technical essence and the achieved result is the device described in [3]. However, the known device is distinguished by the relative complexity of the primary switching circuit and increased weight and size due to the presence of four power valves.

The proposed device, the schematic diagram of which is shown in the figure, is distinguished by a simpler primary switching circuit and, accordingly, improved weight and size indicators.

The device for braking HELL [4J] contains power contacts 1K1 and 1K2 of the magnetic starter in the first and third phases of the static winding of the HELL. The first thyristor VS1, the cathode of which is connected to the third phase of the stator winding of the IM, the first VD1 and the second VD2 diodes, the anodes of which are connected to the first and third phases of the network, respectively, and the cathodes are combined and connected through the SA1 switch and the resistor R1 to one of the outputs of the adjustable resistor R2 . Another output R2 through capacitor C, shunted by a series circuit of resistor R3 (not shown in the diagram) and the closing auxiliary contact K1 of the magnetic starter, is connected through the opening auxiliary contacts K2 of the same starter to the anode of the third diode VD3, the cathode of which is connected to the control electrode first thyristor VS1. Power diode VD4, the anode of which is connected to the second phase of the stator winding of the IM, and the cathode is connected to the third phase of the stator winding of the IM through breaking power contacts 1 short circuit of the magnetic starter. The second thyristor VS2 and the fifth diode VD5, the cathode of which is connected to the control electrode of the thyristor VS2, and the anode to the anode of the third diode VD3, the cathode of the thyristor VS2 is combined with the cathode of the thyristor VS1 and is connected to the third phase of the stator winding HELL. The anodes of the thyristors VS1 and VS2 are combined with the anodes of the diodes VD1 and VD2, respectively, and connected to the corresponding phases of the network.

The device works as follows. In the initial pre-start position, the switch SA1 of the IM braking control circuit is open. An automatic switch in the engine circuit supplies voltage to the IM control circuit and starts it by pressing the start button (not shown in the diagram). The magnetic starter works and connects the AD to the network with its power contacts 1K1 and 1K2, the latter starts up while the power contacts 1 K3 and the block contacts K2 of the magnetic starter open, and the block contacts K1 close, which leads to the discharge of the capacitor C through these contacts to the resistor R3 (not shown in the diagram). Capacitor C could have been charged during the previous start-up and braking of the IM. After starting the IM, the engine braking control circuit is prepared for operation by turning on the SA1 switch. Thyristors VS1 and VS2 are in a non-conducting state.

When the HELL is disconnected from the network by pressing the "Stop" button, the power contacts 1K1, 1K2 and auxiliary contacts K1 of the magnetic starter open, and the contacts 1K3 and K2 close. The positive half-wave of the network phases is fed to the anodes of the thyristors and current flows through the circuit of their control electrodes through the diodes VD1 and VD2, resistors R1 and ^capacitor C, breaking contacts K2, diodes VD3 and VD5. As a result, the thyristors break off and the stator windings of the second and third phases are flowed around by the rectified current of the network. In non-conductive periods, current continues to flow through the stator windings in the same direction, which closes through the diode VD4 and contacts 1K3 of the magnetic starter due to the action of the EMF of electromagnetic induction. The engine is heavily braked.

At the end of the charge of the capacitor C, the current in the circuit of the control electrodes of the thyristors stops, the thyristors close, respectively, the current flow through the windings of the second and third phases of the AD stops. The braking process is over. In this case, the capacitor is in a charged state. The subsequent start of the IM leads to its automatic discharge, and the device is ready for a repeated braking cycle.

Details. For electrodynamic braking of electric motors, for example, with a power of 4 ... 7,5 kW, the following elements can be used: thyristors VS1, VS2 type T14-160 or TL-160, class 8 (160 A, 800 V); diode VD4 type B50, class 6 (50 A, 600 V); diodes VD1 and VD2 of the KD105G type can be replaced by diodes of the D226B type (0,3 A, 400 V), two in series in the arm, by shunting each of them with a resistor of 100 ... 200 kOhm of the MLT-0,5 type; diodes VD3, VD5 type KD105V or KD202 (1 A, 600 V), as well as diodes D226B; switch any suitable flow and voltage; resistor R1 type PEV15 (10 ... 15 W; 1 ... 1,5 kOhm); resistor R2 type PPB-25D (25 W; 2,2 ... 10 kOhm); capacitor C type MBGO-600-10 (10 ... 20 μF; 600 V); any magnetic starter suitable for current and voltage, for example, type PML of the third magnitude for a current of 40 A or PME-312.

Setting. The duration of braking of AD is determined by the charge time of the capacitor C, i.e. depends on the value of its capacitance, and the braking efficiency depends on the opening angle of the thyristors, which is determined by the value of the resistance R2. Therefore, setting up the device mainly consists in selecting the required value of the variable resistor R2. If the braking duration is insufficient (when the rotor runs out), it is necessary to slightly increase the capacitance of the charging capacitor C. After tuning, the variable resistor R2 can be replaced by a constant of the same power.

A simpler primary switching circuit of the device increases the reliability of its operation, reduces the cost, reduces the cost of installation, adjustment and operation. The device does not consume electricity when the IM is in operation.

References:

1. Petrov L.P. and others. Automatic control of the braking of machine electric motors. - M.: Mashinostroenie, 1978.
2. Author's certificate of the USSR No. 754621, class N02r 3/24, 07.08.80/29/XNUMX, bul. No. XNUMX.
3. Author's certificate of the USSR No. 1022276, class N02r 3/24, 07.06.83/21/XNUMX, bul. No. XNUMX.
4. Application No. 4044572/07(045740) dated March 27.03.86, XNUMX.

Authors: K.V. Kolomoitsev, R.M. Kolomoitsev

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