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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Questions of protection of three-phase electric motors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors The literature has already considered the protection of asynchronous three-phase electric motors, but basically it is protection against phase failure [1, 2]. Less common is the protection of the electric motor from the so-called phase imbalance, i.e. when the voltage in one or two phases at once decreases (or increases) to an unacceptable value for any reason. In such cases, the phase failure protection usually does not work, since the voltage in the phase remains, but a decrease in the voltage in the phase to 150 ... 160 V has a deplorable result on the motor: after a while, the motor overheats and burns out. The same can be said about increasing voltage. A winding designed for 220 V does not tolerate voltage increases above 250 V very well. This problem is especially relevant in cases where the engines operate in the absence of a person (for example, water pumps, elevators, etc.), as well as in rural areas, where the quality of electrical networks leaves much to be desired. Still relevant is the issue of controlling the temperature of the electric motor itself, since there are many reasons due to which the engine can overheat. For example, an increase in the load on the shaft or jamming. In the end, in our difficult time, we have to deal with cases of installing an engine whose power is insufficient for this equipment, due to the lack of an electric motor of the required power. In these cases, the overheating protection has a positive result. Bimetal thermal relays installed in starters often do not work when necessary. Therefore, taking into account the foregoing, I propose to once again consider some ways to protect electric motors. The easiest way is to install two relays with 220 V windings (Fig. 1).
Such protection is familiar to many electricians and helps protect the motor from phase failure. The starter winding is switched on through the normally open relay contacts K1 and K2. Thus, in the absence of any of the phases, the starter opens. In [1], a device is described that, in my opinion, is too complex for the function it performs. The circuit shown in Fig. 1 is quite capable of almost completely replacing it. If a starter with a 380 V winding is used, then the upper relay contact K1 must be disconnected from the ground wire and connected to phase A or phase B. In the absence of a relay with 220 V windings, you can use a 12 ... 24 V relay, as well as add a phase loss indication to the circuit. Such a scheme is shown in Fig.2.
Indicators in some cases allow you to quickly notice a phase break and facilitate troubleshooting. This circuit allows a wide variety of relays to be used. It is enough just to select the capacitors C2, C4 in such a way as to obtain the required voltage on the winding of the relay used. Typically, the capacitance of capacitors is selected in the range of 0,47 ... 1,5 μF. The diagram shown in Fig. 2 shows the capacitance of capacitors C2, C4 when using relays K1 and K2 of the RSCH-52 type, passport RS4.52 3.205 with a winding resistance of 220 ohms. The LEDs in the circuit can be taken of the AL307 type or any other, normally glowing at a current of 5 ... 10 mA. The diode bridge VD1, VD2 can be used for any voltage above 200 V and the permissible current required for the type of relay used. Capacitors of type K7317, resistors of type MLT-0,125. The above phase loss protection circuits are simple and reliable in operation, their assembly does not require high qualifications, but they do not protect electric motors from phase imbalance. Figure 3 shows a diagram of a device for protecting three-phase motors from phase imbalance, phase failure, includes monitoring the temperature of the motor using a temperature sensor mounted on the motor housing.
The device consists of three channels, each of which controls the voltage in its corresponding phase, and a temperature control channel on the motor housing. The outputs of all channels are combined using the "AND-NOT" scheme and fed to the actuator. All three channels for monitoring the phase voltage level are similar and consist of a controlled voltage generation circuit, two comparators and an "OR-NOT" combining element. Consider the operation of one of the channels that controls the voltage in phase A. The phase voltage is reduced and rectified to 3,5 ... 4 V by the circuit R15, R16, VD2, R1, R2, C2. As a result, a voltage is obtained at the positive terminal of capacitor C2, which is directly proportional to the voltage in the controlled phase. This voltage is supplied to the inputs of the DA1 comparators, made on the dual op-amp KR140UD20, one of the inputs is inverting, and the second is non-inverting. An exemplary voltage is applied to the corresponding second inputs of the op-amp, which is taken from the resistors KR1 and KR2. At the same time, an exemplary voltage is applied to the non-inverting input DA1 (pin 2), which corresponds to the minimum voltage on the capacitor C2, and an exemplary voltage corresponding to the maximum voltage on the capacitor C1 is applied to the inverting input OA7 (pin 2). As a result, the terminals 10 and 12 of the op-amp DA1 will be low if the voltage across the capacitor C2 is within the limits set by the potentiometers KR1, KR2, and the output of the OR-NOT cell DD1.1 will be correspondingly high. As soon as the voltage goes beyond these limits, one of the comparators will switch and the unit level will be set at its output, which will change the level at the DD1.1 output to low. All three outputs of the voltage control channels are fed to the combining cell DD2.1. The unity level from the comparator made on the DA6 op-amp, which controls the temperature of the RT1 sensor, also comes here. When the thermistor RT1 is heated, its resistance decreases and, accordingly, the voltage at pin 3 DA6 decreases. This leads to a change in the level at the output of DA6 to zero when the input voltage at the non-inverting input of the op-amp reaches the level set by the potentiometer RP2 at the inverting input of DA6. Capacitor C5 smooths out interference that may occur on the wire coming from the temperature sensor, since its length is usually 2 ... XNUMX m. The resistance of the thermistor may differ from that indicated in the diagram. It is only necessary to check that the voltage at the connection point RT1, R9 with a heated thermistor is above 2 V, since the comparator on the op-amp with a unipolar supply and an input voltage below 1,5 V is unstable. The same applies to the voltages on the capacitors C2-C4, which are supplied to the OS DA1-DAZ, as well as the exemplary voltage on the engine of the resistor RP1. Their minimum value should not be set below 2 V. A change in the state of any of the comparators that control the voltage, or the comparator that controls the temperature, is indicated by the LEDs HL1 and HL2, respectively. From the output of the cell DD1.1 through the smoothing chain C7, R21 and DD2.3, inverting it, the signal is fed to the transistor VT1, loaded on the relay K1. The smoothing circuit eliminates the possible relay rattling during short surges in one of the phases that are not dangerous for the motor, and also gives a protection response delay of about 2...4 seconds. If necessary, this time can be increased by increasing the capacitance of the capacitor C7 accordingly. The relay contacts, closing, supply voltage to the starter. The circuit allows you to use a starter of any size and with a winding voltage of not only 380 V, but also 220 V. To do this, it is enough to connect the upper output of the starter winding according to the circuit not to the phase wire, but to the ground wire. The device is powered by a stabilized voltage of 9 V, obtained using a DA5 stabilizer. The exemplary voltage that is applied to the potentiometers RP1, RP2 and resistors R9, R10 is taken from the DA4 stabilizer. The maximum current consumed by the circuit when relay K1 is open does not exceed 30 mA, so a radiator for the DA5 stabilizer is not required. As a TR1 transformer, you can use almost any transformer with a secondary winding for a voltage of 18 ... 20 V and capable of providing current to power the relay used. Figure 4 shows the circuit board of the device. It is made on double-sided foil fiberglass. The board contains all the elements from Fig. 3, except for the transformer TK1, relay K1, diode VD5 (soldered directly to the relay outputs) and, of course, starter K2.
Details. The resistors used in the circuit can be of the C2-23 or MLT-0,125 type, except for R15, R17, R19. The latter should be 0,5 watts. It is advisable to select resistors R1-R6, R15-R20 in each channel with a minimum spread across the channels. Since the exemplary voltage is supplied in parallel to all three channels, then with a large spread of these resistances there will be a large spread in the levels of operation of the comparators. The applied tuning resistors of the SPZ-19AV type can be replaced with resistors of the SP516VV, SP5-16VA types. The electrolytic capacitors used in the circuit are K50-35 type, but it is better to use imported K10-17 type capacitors. The 2SD1111 transistor can be replaced with a domestic KT972 with any letter index. Op-amp KR140UD20 can be replaced by LM358N, KR574UD2A or single KR140UD6, UD7 (subject to a change in the printed circuit board). The thermistor can be used in almost any type, such as MMT-4, ST1, TR-4. As BA5, you can use the stabilizer KR142EN8A, B, G, D. I used the K1 relay (Elesta KR8S), but you can use any other with a 24 V winding and contacts capable of switching a voltage of 380 V. Setting up the device is simple and consists mainly in setting the limits for the operation of the comparators. To do this, you can temporarily connect all three inputs of the device and apply voltage to them through an autotransformer relative to the "ground". First, a voltage of 180 V is set on the autotransformer and, using a voltmeter with an input resistance of at least 1 MΩ, measure the voltage at the positive terminals of capacitors C2-C4. It should be almost the same. If it differs by more than 0,1 V, then it is necessary, using a slight change in the resistance of resistors, for example, R4, R6, to equate the voltage on capacitors C3, C4 to the voltage on capacitor C2. Next, a voltmeter is connected to the engine of the potentiometer RP1 and the same voltage is set on it as on the capacitors C2-C4. Then, a voltage of 250 V is set on the autotransformer, the voltage on the capacitors C2-C4 is measured and the same is set on the RP2 engine. After that, a voltage of 220 V is set on the autotransformer, while the HL1 LED should light up. Next, you need to configure the temperature sensor. To do this, the RP2 potentiometer slider is set to the upper position according to the diagram, the thermistor is heated to the required temperature and, by rotating the potentiometer slider, the HL2 LED goes out. As soon as the thermistor cools down a bit, HL2 should light up again. When both LEDs are lit, relay K1 should operate. At the end of the settings, the operation of the protection is checked for each channel separately. To do this, connect the device to a three-phase network in accordance with the diagram and turn on the autotransformer in turn in the circuit of each channel. By decreasing and increasing the voltage on the autotransformer, they control the extinction of the HL1 LED when the input voltage reaches the set limits. This completes the setup. In the absence of an autotransformer, the voltage control channels can be configured using the table, provided that the values of the resistors R1-R6, R15-R20 correspond to the values \u3b\u1bspecified in the diagram in Fig. 2. To do this, on the engines of the potentiometers RPXNUMX, RPXNUMX, the voltages of the minimum and maximum levels of operation of the comparators selected from this table are set.
If there is no need to use a thermal protection sensor, then you can not connect a thermistor to the circuit. In this case, the DA6 output will always be high, and the device will be fully operational. References:
Author: I.A. Korotkov
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Comments on the article: Passer The circuit in Fig. 1 is simple, but alas, it is useless. If a phase is lost while the engine is running, it continues to rotate in emergency mode. Scheme 2 is almost the same. ![[cry]](https://diagram.com.ua/comments/smilies/cry.gif){kind=small}
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