Protection of three-phase motors from unbalanced operating modes. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protection of three-phase motors from unbalanced operating modes. 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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In the literature [1,2], a description is given of simple circuits for protecting electric motors (EM) using a current relay, which is connected between the zero point of the motor and the network neutral.

But these schemes are not new among ED protection devices and have already been repeatedly considered on the pages of magazines, for example, in [3]. One of the disadvantages of these schemes is that they require access to the zero point of the EM windings, i.e. the motor must have six leads. Since ED series 4A with a power of 0,06 ... 0,37 and 0,55 ... 11 kW have only three outputs (C1, C2, C3) when connecting the windings with a star or a triangle [4], the scope of the proposed protection schemes limited. In addition, the circuits do not have a time delay for the protection to operate.

(click to enlarge)

The ED protection circuit (see figure) is free from these shortcomings. An artificial neutral is created using three capacitors C1 ... C3 with a capacity of 0,01 ... 0,033 μF with a voltage of 400 ... 600 V, which are connected to the input terminals of the ED or to the output terminals of the magnetic starter (contactor).

Resistors R3, R4 are used as a voltage divider. Potentiometer R4 can be used to adjust the protection operation setting. Diode VD3 and filter capacitor C4 perform the function of a half-wave rectifier, with the output of which a ballast resistor R5 and a zener diode VD4 are connected, designed for parametric voltage stabilization.

Transistor VT1 performs the function of amplifying the bias voltage in asymmetric modes of operation of the ED. It is powered by a parametric voltage regulator based on a ballast resistor R8 and a zener diode VD5, which, together with a storage capacitor C5, provides the necessary supply voltage to the amplifier in the event of a break in any phase of the supply network. Protection of the base-emitter junction of the transistor VT1 from an increased neutral bias voltage is carried out by the zener diode VD4.

The voltage stabilizer of the transistor amplifier is powered by a two-phase rectifier assembled on diodes VD1, VD2, which are connected to phases A and B of the ED power supply network through resistors R1, R2 and fuses .U. Resistors R1 and R2 facilitate the operation of the diodes VD1, VD2 and increase their reliability. An electromagnetic relay K1 (actuating element) is connected to a two-phase rectifier through a thyristor key VS1.

The K1.1 relay contacts are introduced into the KM magnetic starter coil circuit in series with the KM1.1 blocking contacts. The VD6 diode, connected in parallel with the K1 coil, facilitates the operation of the K1 relay and protects the VS1 thyristor from self-induction EMF, which is induced in the relay circuit when it is turned off. The coil of the KM magnetic starter is connected through the "Stop" and "Start" buttons to phases A and C of the ED power supply to increase the reliability of the diodes VD1, VD2 of the rectifier.

The ED protection device from asymmetric modes operates as follows. Let us assume that the voltage is present on all three phases of the ED power supply network. During the start of the EM, when the "Start" button is pressed, a three-phase supply voltage is supplied to the capacitors C1 ... C3 of an artificial neutral. In this case, there is no bias voltage between the artificial and grounded neutral, the voltage across the resistor R4 is practically zero, the transistor VT1 and the thyristor VS1 remain closed, the relay K1 is de-energized, and its contacts K1.1. in the circuit of the coil of the magnetic starter KM remain closed, which ensures blocking of the "Start" button after the start of the EM. At the same time, capacitor C8 is charged through resistor R5 to the stabilization voltage of the zener diode VD5, which ensures that the protection circuit is ready for operation. Since the circuit elements VT1, VS1 and relay K1 are de-energized, and the resistance of the resistor R8 is quite large and amounts to 51 kOhm, the ED consumes practically no energy during operation.

In the event of a break in one of the phases of the power supply network of the ED or a violation of the symmetry of the linear voltages between the artificial neutral of the capacitors C1 ... C3 and the grounded neutral of the three-phase network, a neutral bias voltage occurs, part of which is removed from the potentiometer R4, rectified by the VD3 diode, stabilized by a parametric stabilizer and fed to the base transistor VT1. At the same time, transistor VT1 and thyristor VS1 open, relay K1 is activated and opens its contacts K1.1 in the EM control circuit, the coil of the magnetic starter (contactor) is de-energized, and the EM is disconnected from the power supply by linear contacts K. After opening the linear contacts of the magnetic starter, the bias voltage the neutral becomes almost equal to zero, the transistor VT1 and the thyristor VS1 are closed, the relay K1 is de-energized and returns to standby mode.

The device also provides blocking the start of the EM in the event of a break in one of the phases or an unacceptable value of the voltage unbalance of the three-phase network.

Capacitor C4 is used to delay the response time of the device in case of non-simultaneous closing of the linear contacts of the magnetic starter (contactor) KM, as well as in the event of asymmetric short circuits in the mains. To improve the reliability of the circuit operation, the setting for the protection against asymmetric modes is set for the non-working stroke mode of the EM by potentiometer R4.

Details. The device uses resistors of the MLT type, a variable resistor of the type SP3-16a or SP5-2, diodes VD1, VD2 of the D211 series for 600 V can be replaced with KD105V, G. Instead of a VD4 stabistor of the KS119A type with a stabilization voltage of 1,9 V, you can use the stabistor KS107A (2S107A at 0,7 V) and KS113A (2S113A at 1,3 V), connecting them in series. Zener diode VD5 type D814A with a stabilization voltage of 7 ... 8,5 V can be replaced by two series-connected zener diodes of the type KS139A. Diode VD6 series D226B or KD105 with any letter index.

Thyristor VS1 type KU202N can be replaced with KU202M, KU202L, KU202K, KU201L or unified type TI2-10 class 4, which has a housing almost identical to the housing of the KD202 diode. Transistor VT1 (npn) type KT315 with any letter index, it can be replaced with germanium type MP38 or MP37 with a current gain of 20 ... 30. Capacitors C1 ... C3 type MBM or ceramic with the same deviation from the nominal value. Electrolytic capacitors C4 and C5 - type K50-6 for a voltage of 10 V. Relay K1 small-sized type RP-21 for a voltage of 220 V AC.

Elements of the EM protection circuit are mounted on a printed circuit board, which is placed in the housing of an intermediate relay of the RP-24 or RP-25 type and connected to the EM circuit by input terminals 1...8.

The device has been tested in laboratory and production conditions. Its practical use will make it possible to significantly increase the reliability of the ED operation during operation.

References:

Semenov I. All phases flock to zero / / Radio amateur. - 2000. - No. 9. - P.14.
Semenov I.P. Current relay//Radioamator-Electric. - 2001. - No. 5. P.21.
To help the radio amateur: Issue 19. - M.: DOSAAF, 1964. - 80s.
Stokolov V.E., Usyshkin G.S. and others. Electrical equipment of forging and pressing machines: Ref. 2nd ed. - M.: Mashinostroenie, 1981. - 304 p.

Authors: K.V. Kolomoytsev, Yu.F. Romanyuk, R.M. Kolomoytseva

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