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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Three-phase motor protection device against phase failure. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies The article provides a description of the device for protecting a three-phase asynchronous motor from a phase failure of the supply network. The device circuit provides for automatic control of currents in the motor power line using transformer-type sensors. The device provides a delay in disconnecting the motor from the supply network in case of short circuits in neighboring sections of the network, as well as in the event of a short-term disappearance of the power source phase and blocks the start of the engine in open-phase operation. One of the common causes of damage to asynchronous three-phase electric motors (IM) are open-phase modes of their operation, which occur due to phase failures, broken contacts in switching or protective devices. Thermal relays, which are designed to protect the AD from overload, do not always work during phase failures, as a result of which the motors overheat and fail due to insulation damage. Below is a description of the IM protection device from operation in two phases, which differs from [1] in the presence of separate transformer-type current sensors, which allows it to be used with small magnetic starters that do not have thermal relays. Therefore, the scope of use of the device is wider compared to the previous development. The block diagram of the protection device is shown in Fig.1.
The device consists of a power supply unit BP, three independent channels for monitoring the currents of the phases of the supply line A, B, C, each of which contains a current sensor DT, an amplifier U and a detector D, an OR logic element, an EZ delay element, a threshold device PU, an electronic key EK, magnetic starter MP, control buttons PS asynchronous motor HELL. Schematic diagram of the device is shown in Fig.2.
The power supply is assembled according to a transformerless circuit. Voltage is supplied to it directly from one of the phases of the three-phase supply network through contacts 1-2 of the SA1 switch, which ensures the choice of the IM operation mode: normal without phase failure control (contacts 3-4 closed) or automatic with current control in the IM phases (contacts 12 closed, and 3-4 are open). Figure 2 shows the automatic mode. The power supply rectifier is assembled according to a half-wave circuit on a VD13 diode. Zener diode VD14 provides recharging of the quenching capacitor C12, shunted by resistor R27. This resistor ensures the discharge of the capacitor C12 after the protection circuit is turned off. The ballast resistor R29 reduces the inrush current through the capacitors C10, C12 when voltage is applied to the power supply. The phase failure protection device consists of three independent identical channels for monitoring the currents of the phases of the supply line, which work on a common actuating element - triac VS1. All control channels have sensors - current transformers ТТ1-ТТ3. When current flows through the primary winding of the transformer, which occurs during normal operation of the IM, an EMF is induced in the secondary winding, which is fed to the input of a single-stage amplifier assembled on the transistor VT1. From the output of the amplifier, the voltage through the capacitor C4 is fed to the input of the detector with a doubling of the voltage VD4, VD7, the load of which is the capacitor C7. The constant component of the signal from the capacitor C7 through the limiting resistor R13 is fed to the input of the transistor VT4. The second and third channels (transistors VT5 and VT6) work similarly. Transistors VT4-VT6 and diodes VD10-VD12 form a logic element "OR". During normal operation of the AD, the voltage on the collectors of any of the transistors is zero, respectively, the voltage at the output of the logic element "OR" is equal to zero. The EZ delay element consists of resistors R19, R20 and capacitor C11, the capacitance of which determines the delay time for the operation of the BP protection device. If there is no voltage at the output of the "OR" element, there is no voltage at the input of the threshold device PU, assembled on transistors VT7-VT9. In this case, the transistors VT7, VT8 form a Schmitt trigger, which ensures the precise operation of the actuator - the triac VS1 in the magnetic starter circuit. During normal operation, the transistor VT7 is closed, and VT8 is open, therefore the transistor VT9 is open, the triac VS1 is open, and it bypasses the start button S2 of the magnetic starter. Diodes VD1-VD3 in the input circuits of transistors VT1-VT3 provide protection for transistors during transients in the power line of the IM motor, which occurs when connected to the network and short circuits. To reduce the rate of voltage rise across the triac, a capacitor C13 is connected in parallel with it. Resistor R28 limits the discharge current of capacitor C13. The device works as follows. Assume that voltage is present on all three phases of the network. With the SA1 switch, we apply voltage to the power supply of the device using contacts 1-2. We start HELL by pressing the S2 ("Start") button. In this case, the magnetic starter is activated, and through the contacts K1.2 a three-phase voltage is supplied to the terminals C1-C3 of the motor. EMF is induced in all three current transformers, as a result, all channels of the device are open, there is no voltage at the output of the "OR" element, the triac VS1 is open and shunts the start button S1.1 through the closed contact K2 of the magnetic starter. The start of the AD is completed. If any phase is broken, for example "A", the current in the primary winding of TT1 disappears, and the phase "A" protection channel closes (on the collectors VT1 and VT4 - high voltage). A signal appears at the output of the "OR" element, the Schmitt trigger goes into another stable state, the transistor VT9 closes, and hence the triac VS1. The coil of the magnetic starter is de-energized, and the HELL is disconnected from the network. Details. The device uses resistors R1-R24 type MLT-0,25; R25-R29 type MLT-0,5; diodes VD1-VD12 of type D9G can be replaced with diodes of type D9D, D9B, D310-D312, and diode VD13 of type D226 can be replaced with a diode of type KD105 with any letter index. Instead of the zener diode VD14 type D815D, you can use D815G. Capacitors C1-C11 of type K50-6 for a voltage of 25 V. Capacitor C12 consists of two parallel-connected capacitors of type K73-17, 2 μF, 400 V, they can be replaced by corresponding capacitors of the MBGO-2 type. Transistors VT1-VT8 type KT361 can be used with any letter index. The VT9 transistor of the KT315G series can be replaced with a KT312 series transistor. Instead of a triac VS1 type KU208G, you can use a unified type TS112-10-4 for 10 A, 400 V with any last digit not lower than 4, they have almost the same housing as the KD202 diodes. Current sensors ТТ1-ТТ3 are made on a ferrite core of M2000NM1 grade and K33Ch16Ch9 size. For an IM with a power of 1,1 kW, the primary windings of the sensors contain 2 turns of the wire of the line supplying the motor, the secondary windings - 25-50 turns of the PELSHO wire with a diameter of 0,18 mm. All parts of each channel of the device, including the "OR" element, are mounted on a separate printed circuit board 90x50 mm in size, 1 mm thick. Similarly, the power supply and the threshold device are mounted on separate boards along with the delay element. All printed circuit boards are installed in the housing of a conventional intermediate AC relay type RP23 one above the other and are attached to the base of the relay with three studs. Adjustment. With the circuit breaker AB turned off, the control electrode of the triac VS26 is disconnected from the resistor R1, and the triac itself is shunted with a wire jumper. Then, turning on the AB, switch SA1 with contacts 1-2, turn on the device in the network. An avometer measures the voltage at the output of the power supply, which should be in the range of 9 ... 13 V, depending on the type of zener diode used. The S2 button starts the IM motor and checks for the presence of voltage at the output of the current sensors, which should be 1 ... 1,5 V at the nominal load of the IM. If the voltage goes beyond the specified limits, then it is corrected by changing the number of turns of the primary winding of the current sensors, after which the open state of the transistors of each channel (VT1, VT4; VT2, VT5; VT3, VT6) and the absence of a signal at the output of the "OR" element are checked. In this case, transistors VT8 and VT9 must be open. After that, the HELL and AV are turned off, the operation of the protection circuit is restored by removing the wire jumper from the triac VS1, a single-pole circuit breaker is installed in each of the phases of the supply line and the HELL is started using the S2 button. In this case, the start button S2 by opening the triac VS1 and closing the contacts K1.1 of the magnetic starter should be shunted. If shunting did not take place (BP stops when the S2 button is released), it is necessary to select the appropriate value of resistance R26. Having reached the bypass of the S2 button, the operation of the device is checked when each phase of the supply line is turned off in turn using a single-band circuit breaker. At the same time, it should be remembered that the shutdown of the BP by the protection does not occur immediately after the circuit breaker is turned off, but with a delay of 0,5 ... 1 s. The device was tested in laboratory conditions with a 4A series motor with a power of 1,1 kW, a voltage of 220/380 V at a mains voltage of 380 V. It showed reliable protection of IM in the event of a phase failure at different IM loads. The introduction of this device in production will make it possible to significantly reduce the number of cases of IM failure due to phase failure, which, according to recent data, reaches 40-50%, for example, in agriculture. References:
Authors: K.V. Kolomoitsev, I.V. Surface, Yu.F. Romanyuk
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