Three-phase motor protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Two variants of a device that disconnects a three-phase electric motor from the network when a change in the supply voltage is dangerous for it are considered - a simple relay and a relatively complex one based on integrated circuits. The devices react not only to a general increase or decrease in the mains voltage, but also to a “phase imbalance” dangerous for the engine - a change in voltage of only one of them.

The problem of protecting a three-phase electric motor from mains voltage fluctuations that are dangerous for it is especially relevant if the engine is running in the absence of a person constantly watching it (for example, driving a water pump), as well as in rural areas where the quality of electrical networks leaves much to be desired.

It is equally important to continuously monitor the temperature of the motor housing, there are many reasons why it can overheat. The most common are mechanical overload of the engine or jamming of its shaft in bearings.

The simplest way to protect against loss or a significant decrease in voltage in one of the phases is illustrated by the circuit familiar to many electricians, shown in Fig. 1.

The winding of the starter KM1 is connected to the phase (for example, C) and the neutral of the network through the normally open contacts of the relay K1.1 and K2.1. The relay windings are connected to two other phases. As a result, the loss of any phase voltage will cause the KM1 starter to disconnect the electric motor from the network.

Starter and relay windings must be rated for AC voltage 220 V, 50 Hz. If there is a starter with a 380 V winding, its right output according to the diagram is connected not to the neutral (N), but to one of the phase wires (A or B). Relays with windings designed for a voltage of 12 ... 24 V can be used by connecting them according to the circuit shown in fig. 2.

Capacitor C1 - K73-17. Its capacity is indicated for the RSC52 relay (passport RS4.523.205, winding resistance 220 Ohm). If another is applied, the capacitor is chosen so (usually with a nominal value of 0,47 ... 1,5 μF) that the current necessary for its operation flows through the relay winding. The oxide capacitor C2, shown in the diagram with a dashed line, is installed only if the triggered relay "buzzes". Capacitor capacitance (several microfarads) is chosen to be as small as possible to eliminate buzz.

A diagram of a more advanced protection device is shown in fig. 3. It reacts not only to the deviation of the mains voltage from the nominal and to the "skew" of the phases, but is also equipped with a temperature sensor of the motor housing.

(click to enlarge)

The three phase voltage control channels are identical according to the scheme. Therefore, we will consider the operation of only one of them, which controls the voltage of phase A. The circuit R1, R4, VD2, R10, R17, C4 forms a constant proportional to it from an alternating phase voltage. The latter is fed to the inputs of two op-amps of the DA3 chip, which serve as comparators. A voltage is applied to the inverting input of the lower comparator according to the R8R12 resistive divider, which sets the protection threshold when the phase voltage exceeds the permissible value. The voltage of the "lower" threshold (from the resistive divider R7R11) is applied to the inverting input of the second (upper) comparator. The outputs of the comparators are connected to the inputs of the element OR-NOT DD1.1. The logic level at the output of this element is high as long as the controlled phase voltage remains within the limits set by trimmers R11 and R12.

Element DD2.1 combines the output signals of the three control channels. Until none of them has worked, the level at the output of this element is low. The HL2 LED is on, signaling the health of the three-phase network. Similarly to element DD2.1, element DD2.2 operates, but one of its inputs is additionally supplied with a signal for actuation of the temperature control unit. Therefore, the transistor VT1, the base circuit of which is connected to the output of the element DD2.2 through the integrating circuit R22C7 and the inverter DD2.3, is open only if the network is working and the temperature of the motor case is below the permissible value.

The coil of relay K1 is included in the collector circuit of transistor VT1. If everything is in order, relay K1 and contactor KM1 are in the activated state and the electric motor is connected to the network. In an emergency, the transistor will be closed and the open contacts of relay K1.1 will de-energize the winding of the KM1 starter, which will turn off the electric motor. The R22C7 circuit mentioned above, delaying the protection operation by 2 ... 4 s, prevents the reaction to short-term surges in the mains voltage.

Thermistor RK1 serves as the temperature sensor of the motor housing. With the help of op-amp DA6, the voltage falling on the thermistor is compared with the exemplary one supplied to the inverting input of the op-amp from the resistive divider R9R16. In case of overheating of the motor, the resistance of the thermistor and the voltage drop across it decrease so much that the high logic level at the output of DA6 is replaced by a low one, leading to the HL1 LED turning off and the KM 1 starter turning off the motor.

The length of the wires connecting the thermistor RK1 with a protective device can reach 2 ... 3 m. Capacitor C1 eliminates the interference induced on these wires. If a thermistor with a nominal resistance that differs from that indicated in the diagram is used, it is necessary to select resistor R15 so that when the thermistor is heated to the operating temperature, the voltage at the inverting input DA6 does not drop below 2 V. At a lower value, the parameters of the KR140UD608 op-amp switched on according to the above diagram noticeably deteriorate . The same applies to the voltage supplied to the inputs of the op-amp of the DA3-DA5 microcircuits.

The power unit of the protective device consists of a step-down transformer T1, a diode bridge VD1, a filter capacitor C2 and two integral stabilizers - DA1 and DA2. A voltage of 9 V from the output of the first stabilizer is fed by microcircuits DA3-DA6, DD1, DD2. The current consumption does not exceed 30 mA, so the heat sink of the DA1 chip is not required. From the voltage of 5 V, stabilized by the DA2 chip, exemplary voltage levels are obtained for setting the protection operation thresholds.

The device is assembled on a printed circuit board (Fig. 4) with dimensions of 80x75 mm from double-sided foil fiberglass. All the elements are located on it, with the exception of the T1 transformer, the K1 relay with the VD5 diode connected directly to the terminals, and, of course, the KM1 starter.

(click to enlarge)

Resistors R1-R3 - MLT-0,5, the rest are constant - C2-23 0,125 W or MLT-0,125. Trimmer resistors R9, R11, R12 -SPZ-19a. They can be replaced with other small ones. Thermistor - MMT-4, ST1 or TR-4. Oxide capacitors - K50-35 or similar imported ones. Instead of the KT972A transistor, KT972B or imported 2SD1111 will do.

Dual KM140UD20 op amps can be replaced with KR140UD20A, KR140UD20B, as well as LM358N, KR574UD2A or (by changing the printed circuit board) various modifications of single op amps K140UD6, K140UD7. Replacement of the stabilizer 7809 - KR142EN8A, KR142EN8G

Relay K1 - imported KR8S company "Elesta". Another one with an operating voltage of not more than 24 V and contacts capable of switching a voltage of 380 V is also suitable. Transformer T1 - any with a secondary winding for a voltage of 18 ... 20 V, providing the current necessary to power the relay.

Establishing a protective device is reduced to setting the thresholds for comparators. Temporarily connecting the inputs A-C, an alternating voltage is applied to them relative to the circuit N from an adjustable autotransformer. Having set 180 V here, alternately measure the voltage values ​​​​on the capacitors C4-C6 with a DC voltmeter. If they differ by more than 0,1 V, it is necessary to eliminate the spread by selecting the values ​​​​of the resistors R1-R3 or R4-R6.

By rotating the engine of the tuned resistor R11, the HL2 LED is ignited. If this fails, change the position of the slider of the tuned resistor R12 and try again. Then, using an autotransformer, the voltage is increased to 250 V at the connected inputs of the protective device. LED HL2 should go out. By moving the engine of the tuned resistor R12, it is lit again. It remains to make sure that the HL2 LED is lit while the input voltage is within 180 ... 250 V, and goes out if it is outside this interval. If necessary, repeat the adjustment.

If it is not possible to use an autotransformer, the protection thresholds can be set approximately. The voltage measured by a high-resistance voltmeter on the engine of the tuning resistor R11 should be 3,16 V, and on the engine R12 - 4,44 V. The given values ​​are valid if the resistance of each of the resistors R1-R6, R10, R13, R14, R17-R19 in accuracy is equal to the nominal value indicated on the diagram.

Before adjusting the temperature control channel, move the trimmer resistor R9 to the left position according to the diagram. As a result, the HL1 LED should light up. Heating the thermistor RK1 to the required temperature, rotate the slider of the mentioned resistor until the LED goes out. As soon as the thermistor cools down a bit, the LED should light up again. If both LEDs (HL1 and HL2) are lit, relay K1 and starter KM1 should work.

Author: I.Korotkov, Bucha village, Kyiv region, Ukraine

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