Device for protecting a three-phase asynchronous motor in the event of a fuse failure. Scheme, description

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Device for protecting a three-phase asynchronous motor in the event of a fuse failure. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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The article describes a simple device for protecting a three-phase asynchronous motor in case of a blown fuse link. The principle of operation of the device is based on the fact that it controls the integrity of the circuit of fuse links in the phases of the engine during its operation.

The operation of a three-phase asynchronous electric motor (IM) in two phases is a common occurrence during its operation. The reason for this is a break in the phase wire in the power line or a broken contact in one of the phases in switching devices, for example, in magnetic starters, contactors, circuit breakers, fuses.

Most often, a break occurs when the fuse fuse burns out due to a short circuit in the blood pressure circuit or due to its incorrect choice. Sometimes the desire to make the protection of blood pressure more sensitive leads to the installation of undersized fuses in the fuse. As a result, when the IM is under load, the fusible links overheat, the material from which they are made oxidizes, the contact resistance increases, the insert burns out, the IM operates in two phases and overheats. Bad contact in the fuses leads to the same result. The latter is especially characteristic of old cork fuses, which are still found in practice.

In open-phase operation, both the motor current and the voltage on it change. It is characteristic that the voltage on the broken phase of the AD at idle is 90% of the nominal, and when working under load 65 ... 75%. And only when the engine is overturned does it become equal to zero. It is because of this that the closest known device [1] does not provide reliable protection of the IM in the event of a fuse failure, since the blown fuse in any of the phases does not lead to the disappearance of the voltage on the broken phase, therefore, the signal to turn off the engine is not received from the reacting body. In addition, the device is not economical due to the constant flow of current around all elements of the circuit, which also reduces the reliability of their operation.

The figure shows a schematic diagram of a device that provides more reliable protection of AD in the event of a fuse failure. The essence of the proposed technical solution [2] lies in the fact that it controls the integrity of the circuit of fuse links during IM operation.

(click to enlarge)

The device contains thyristors VS1VS3 connected by cathodes to the corresponding phases of the network after the power fuses FU1-FU3 of the engine. The anodes of the thyristors are connected together and connected to an artificial zero point formed by the diodes VD1-VD3, through the auxiliary contact K1 of the magnetic starter HELL and the relay winding P of the reacting body.

In this case, the anodes of the diodes VD1-VD3 are connected to the corresponding phases of the network before the fuses. The control electrode of each of the thyristors is connected to the corresponding phase of the network through the resistor R1 (R2, R3) and the diode VD4 (VD5, VD6) to the power fuse.

Opening contact P1 of relay P is included in the control circuit of the coil K of the magnetic starter HELL in series with the blocking contact K2 of the start button "Start". Diodes VD4VD6 connected to each phase perform the functions of a half-wave rectifier, the load of which is the resistor R1 (R2, R3) and the np-junction of the thyristor cathode VS1 (VS2, VS3).

Serviceable fuses FU1-FU3 shunt the input of half-wave rectifiers in the AM phases, as a result of which the thyristors are closed, the relay P is de-energized, its contact P1 in the circuit of the coil K of the magnetic starter is closed.

The device works as follows. The "Start" button is pressed, thereby supplying power to the coil K of the magnetic starter, which turns on, while contact K1 and blocking contact K2 of the starter close and power contacts K3-K5 in the BP circuit, which starts.

Burnout of the fusible link of any fuse leads to the fact that the series chain of the elements VD4 (VD5, VD6) and R1 (R2, R3) and the np-junction of the thyristor VS1 (VS2, VS3) is energized. This leads to the opening of the thyristor VS1 (VS2, VS3) and the operation of the relay P, which opens its contact P1 in the circuit of the coil K of the magnetic starter. In this case, the power contacts K3-K5 disconnect the motor from the network, the contacts K1 and K2 of the magnetic starter open, the relay P is de-energized.

Thus, the described device is a consumer of electricity only during the disconnection of the engine from the network due to a fuse failure. With serviceable fuses, all elements of the circuit are not flowed with current and the device does not consume electricity from the network.

In the normal mode of operation of the device, the engine is turned off by the "Stop" button.

Details. Thyristors VS1-VS3 type T6 not lower than class 6 or KU221A. They can be replaced with unified type T112-10 and class not lower than 6, having approximately the same housing as diodes of the KD202 type. Diodes VD1-VD3 type KD105V for a current of 0,3 A and a voltage of 600 V or even better type KD105G for 800 V with a red dot on a plastic case. It is possible to use diodes of old types: D211, D217, D218 or two series-connected diodes of the D226B type, shunted with resistors of the MLT-0,5 type with a resistance of 100 ... 200 kOhm. Diodes VD4VD6 in the control circuits of thyristors of the KD105V type with a green dot on a plastic case or KD105G and the like. Resistors R1-R3 type MLT-1, their resistance is selected during commissioning. Relay R DC for 220 V type RP-23 or RP-24.

Adjustment. In series with fuses FU1-FU3 or instead of them, single-phase circuit breakers are switched on, designed for the load current of HELL, and the resistors R1-R3 in each phase are replaced by a series circuit consisting of a variable resistor with a resistance of 100 ... 200 kOhm, 2 W and a constant type MLT -2 300 ohm resistance. Turn on the AD and load it up to the rated load. Then the circuit breaker in the circuit of the first phase of the motor is turned off and, by rotating the knob of the variable resistor, the thyristor VS1 is opened, that is, the protection device is activated, which leads to the disconnection of the AD from the network. At the same time, it must be remembered that turning off the circuit breaker in the motor phase leads to its overload, and therefore operation in this mode should be short-term, that is, the selection of the resistance value R1 by turning the variable resistor knob should not be long.

After stopping the engine, it is necessary to disconnect the device from the mains and measure the resistance of the series circuit, select the closest constant resistor in the decreasing direction and solder it into the circuit. Check the operation of the protection device when the circuit breaker is turned off. Similarly, the device is adjusted for the 2nd and 3rd phases.

References:

Author's certificate of the USSR No. 639077, class. H02H 7/08, 1977.
Author's certificate of the USSR No. 851607, class. H02H 7/08, 1979.

Author: K.V. Kolomoitsev

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