SU1487121A1 - Device for protection of three-phase electric motor against overloads and phase breaks - Google Patents

Description

The invention relates to electrical engineering and can be used to protect against overloads and unacceptable asymmetry of the currents of three-phase electric motors. The purpose of the invention is to improve the reliability and reduction of weight and dimensions. Device to protect the motor 1 co2

holds the magnetic starter with power contacts 2, button 3 "Stop", button 4 "Start", resistor 5, auxiliary contact 6 magnetic starter, magnetic starter coil, triac 8, phase-shifting capacitor 9, threshold element 10, variable resistor 11, diode 12, current stabilizer 13, contacts 14 of button 4 “Start” and Zener diode 15. When overloading or phase failure, asymmetry of currents occurs in the phases of the electric motor and asymmetry current flows through the zero wire. The latter charges the capacitor 9 to the breakdown voltage of the threshold element 10, which opens and a current is applied to the control circuit of the thyristor switch. At this, the thyristor key opens and short-circuits the starter coil, which is triggered and switches off the electric motor 1 with its contacts. 1 sludge.

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The invention relates to electrical engineering and can be used to protect against overloads and unacceptable asymmetry of the currents of three-phase electric motors.

The purpose of the invention is to improve the reliability and reduction of weight and dimensions.

The drawing shows the principal-_ scheme of the described device.

The device for protection of the motor 1 against overload and phase failure contains a magnetic starter with power contacts 2 connected between the terminals of the windings of the motor 1, a control circuit consisting of successively connected buttons 3 "Stop", button 4 "Start", connected by series-connected resistor 5 and a block contact 6 of the magnetic starter, a magnetic starter coil 7, shunted by a triac 8, with the control circuit connected between the supply phase and neutral wires of the network, phase-shifting capacitor 9, soy Dininny through the threshold element 10 with the control circuit of the triac 8, while the first terminal of the variable resistor 11 is connected to the common connection point of the threshold element and the phase-shifting capacitor 9, the second terminal of which is connected to the cathode of the diode 12, the anode of which is connected to the zero point of the stator winding of the electric motor, stabilizer 13 current (barette) is connected in series to the power wire of one of the phases of the electric motor 1 and is bridged by the second closing contact 14 of the “Start” button, the zener diode 15 is connected parallel to the phase shifting th capacitor 9.

The device works as follows.

When the “Start” button 4 is pressed, the magnetic starter coil 7 is connected between the phase and the neutral wire of the supply mains through the button 3 “Stop” and switches on the electric motor 1 with its closing contacts 2. When the button 4 “Starting” is released, the magnetic starter 7 remains on a resistor 5 and its own block 6 of the self-pickup circuit. When the start button is pressed, its closing contact 14 shunts the current regulator (barette), which prevents the protection (opening of the triac 8) from starting currents of the electric motor 1. Since the stabilization current of the boretteter is equal to the operating current of the electric motor, the baretteter does not introduce asymmetry during normal operation of the load.

With overload, the current in the phases increases due to the fact that the resistance to the passage of the magnetic flux P4 increases (due to the reaction of the rotor flux), which leads to a decrease in

inductive resistance of motor windings. However, in the phase in which barretter is connected in series with it, this decrease in resistance is compensated for by an increase in the resistance of barretter, the current in it increases, that is, it stabilizes. This leads to the asymmetry of the current in the phases of the engine. In the neutral wire of an electric motor, a current of asymmetry arises. The diode 12 - resistor 11 - capacitor 9 goes through the current pulses, the amplitude of which is sufficient to charge the capacitor 9 to the breakdown voltage of the threshold element 10. The latter opens in the control circuit of the triac 8 and the capacitor 9 a current pulse is given. Triac 8 at the same time opens and short-circuits the coil of the magnetic starter 7, which is triggered and by its contacts 2 turns off the electric motor 1.

When a phase break or unacceptable asymmetry of the load phase currents occurs, an asymmetry current occurs in the neutral wire of the electric motor. A diode 12 - resistor 11 - capacitor 9 goes through the current pulses, the amplitude of which is sufficient to charge the capacitor 9 to the breakdown voltage of the threshold element 10. Shutdown of the electric motor 1 happens as th when overloaded.

In the event of a phase break, the greatest asymmetry of the phase currents is created and, consequently, the greatest asymmetry current in the neutral conductor of the load. The voltage on the capacitor 9 reaches the breakdown voltage of the Zener diode 15, the latter breaks through and thereby protects the triac control circuit from overvoltage.

Connecting a diode 12 and a resistor 11 to the charge circuit of the capacitor 5 provides a slowdown of its charge and creates a time delay for disconnecting, which prevents the device from tripping during short-term power failures of one of the phases and impulse noise in the motor circuit. Resistor 5 limits the current through the triac 8 when the device is triggered, provides a more economical power mode for the coil of the magnetic starter 7 and contributes to its clearer falling off when the neutral wire is broken.

The proposed device is simpler known, has less weight and dimensions, since it does not contain a current transformer and a voltage divider.

When overload occurs, the asymmetry of the currents in the phases of the electric motor, leading to the triac and tripping of the motor.

The asymmetry of the currents during overload occurs due to the introduction of a barette in one of the phases. In the event of failure of the boretteter, the protection is triggered as if one of the phases was broken. Therefore, an electric motor without overload protection element (baretteter)

1487121

will not work, and this is achieved by improving the reliability of protection.

All the protection elements in the neutral wire of the electric motor and the thyristor control circuit in the normal mode of the electric motor are not energized, which increases the reliability of the proposed device.

Claims (1)

Θ claims A device for protection of a three-phase electric motor against overload and phase failure, containing a magnetic starter with power contacts designed to connect the motor windings to the power supply network phases 15, a control circuit of the magnetic starter, made of the series-connected “Stop” and “Start” buttons, connected by series connected a resistor and a blocking contact of the magnetic starter, and a magnetic coil of the 20 starter, shunted by a triac, the above control circuit is connected to the terminals: - For connection with one of the phase conductors and the neutral conductor of the network, the phase-shifting capacitor connected through the threshold element to the control circuit of the triac, while the common AC terminal connects to the common connection point of the threshold element and phase-shifting capacitor a resistor, the second terminal of which is connected to the cathode of the diode, characterized in that, in order to increase reliability and reduce weight and size parameters, it is additionally equipped with a current stabilizer, the zener diode and the second closing contacts of the “Start” button shunting the current stabilizer with leads to be connected in series to the circuit of one of the phase drives of the electric motor, the zener diode is connected parallel to the phase-shifting capacitor, and the anode of the diode is connected to the terminal for connection to the zero point of the stator winding of the electric motor. ^four