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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Soft starter for asynchronous motor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors The interest of radio amateurs in the development of soft starters for asynchronous electric motors is not weakening. There are all new designs. One of them is offered to readers. Soft starters based on the KR1182PM1 chip, for example, described in [1], have gained quite a lot of popularity. But this microcircuit has features that do not allow achieving the desired results without forced complication of the circuit. The first of them is the maximum mains voltage of not more than 276 V. This is clearly not enough for a three-phase electric motor. It is necessary to zero the midpoint of the "star" of its stator so that the current does not flow between the phases, but between each phase and neutral. But in this case, it is required to regulate the current of all three phases, otherwise, a current that many times exceeds the rated current will flow through one of the windings during the entire start-up time. And when turning on the "star" windings with an isolated midpoint, it is enough to regulate the current in only two phases. The second feature is the need for an external circuit to force the discharge of the timing capacitor, since the current of its discharge through the KR1182PM1 chip itself is very small and the device will be ready to restart the engine only after a rather long time. I recently decided to develop my own soft starter. I immediately decided not to use a microcontroller in it, to do without a node for determining the passage of current through zero (for example, such as in [2]) and make it insensitive to the phase sequence.
The scheme of the proposed device is shown in fig. 1. It consists of three functional blocks. Two of them are the same and are triac regulators of the effective value of the voltage on the load, controlled by optocouplers. The use of symmetrical dinistors VS3 and VS4 in them (more precisely, analogues of such dinistors - KR1167KP1B microcircuits) made it possible to significantly simplify the regulators. The third block simultaneously controls both regulators, forming the necessary law of change in the effective value of the voltage applied to the motor during the start-up process. To do this, it appropriately changes the current flowing through the emitting diodes of the optocouplers U1-U4 that control the regulators. The photodiodes of these optocouplers operate in the photovoltaic mode, the voltage they generate gradually opens the transistors VT1 and VT2. At the same time, the resistance of the transistors decreases, due to which, in each half-cycle of the mains voltage, the capacitors C7 and C8 have time to charge up to the opening voltage of the dinistors VS3 and VS4 in less and less time. Accordingly, the triacs VS1 and VS2 in each half-cycle open earlier and more and more of the half-cycles enter the windings of the motor M1. Unfortunately, the maximum voltage on the motor windings when using such regulators is 20 ... 25 V less than the voltage in the network. Therefore, relay K1 is provided, which operates at the end of the start-up process and connects electrodes 1 and 2 of triacs VS1 and VS2 with its contacts. This also achieves a reduction in the heat generation of the soft starter in the operating mode of the engine. The control unit is powered from one of the phases of a three-phase network through a quenching capacitor C1 and a rectifier on a diode bridge VD2-VD5. Given that the voltage at the bridge output is negligible compared to the mains voltage, the rectifier can be considered a current source, the value of which is about 20 mA and is set by the reactance of the capacitor C1 and is practically independent of the load. Resistor R5 limits the charging current pulse of capacitor C1 at the moment the device is connected to the network. I recommend installing this resistor at a height of 5.7 mm above the surface of the circuit board, so that if it burns out (for example, as a result of a breakdown of the Cl capacitor), the board will not be damaged. Resistor R6 is needed to discharge the capacitor C1 after disconnection from the network. Capacitor C5 smooths out the ripple. Two circuits consisting of series emitting diodes of optocouplers U1, U2 and U3, U4 are connected to the positive terminal of this capacitor through a constant resistor R2 and trimmer R1. The current through the radiating diodes depends on the resistance of these resistors and the value of the voltage rectified by the diode bridge VD2-VD5, which, with a constant rectified current, depends on the load resistance of the rectifier. The first part of this load is the emitting diode circuit. The second part is formed by two series-connected parallel integral stabilizers DA1 and DA2. The more of the available 20 mA flows through the integrated regulators, the less is left for the emitting diodes. The stabilizer DA1 is included in such a way that as the capacitor C4 charges, the resistance of its cathode-anode section increases smoothly and the current through it decreases. In this case, the rectified voltage and current through the emitting diodes of the optocouplers gradually increase. Stabilizer DA2 sets the initial value of this voltage (set by a trimmer resistor R9), which is achieved very quickly after closing the contacts of switch SA1. A further increase in voltage occurs smoothly at a rate set by the resistance of the tuning resistor R7 and the capacitance of the capacitor C4. Why is it necessary to set the initial voltage? The fact is that if the voltage on the motor windings is too low, the current flows through its windings, and the shaft still remains motionless. In this case, the motor hums, and the windings heat up. To prevent such an undesirable mode, the initial voltage setting is provided, which ensures the immediate start of the shaft rotation. The required value of this voltage is highly dependent on the mechanical load on the shaft, so its adjustment with a tuning resistor R9 should be carried out in real engine operating conditions. Upon completion of the process of starting the engine, the third part of the rectifier load on the diode bridge VD2-VD5 begins to operate - the zener diode VD1 and the radiating diode of the optocoupler U5 connected in series. When the voltage at the output of the bridge reaches the stabilization voltage of the zener diode (24 V), the resistance of the latter decreases sharply. Through it and the emitting diode of the optocoupler U5, current begins to flow. The photodistor of the optocoupler opens, and the relay K1 is activated, shunting the triacs VS1 and VS2 with its contacts. From now on, motor M1 receives full mains voltage. 3OD101V optocouplers were used as U1-U4 optocouplers only because I had them in stock. Since the voltage generated by the photodiode of one optocoupler was insufficient to open the transistor, the number of optocouplers was doubled. Both the emitting diodes and the photodiodes of each pair are connected in series. Experiments with other diode optocouplers have not been carried out. It is possible that they will fit too. There are dual diode optocouplers (for example, AOD134AS), as well as those that contain two photodiodes illuminated by one emitting diode (for example, AOD176A). Might be worth trying them too. When selecting a replacement for the 2SC4517 transistors, attention should be paid to the maximum collector-emitter voltage. It should not be less than 600 V. The same applies to the maximum voltage in the off state of triacs VS1 and VS2. Transistors 2SC4517 in this device can be used without heat sinks. Whether it is necessary to remove heat from triacs depends on the power of the electric motor and on how often it is planned to turn it on. Relay K1 - RP-64 [3] with a coil for 220 V, 50 Hz. It can be replaced, for example, with a relay R20-3022-96-5230 [4] with two groups of normally open contacts and a 230 V AC coil. Capacitors C2 and C3 are film capacitors. KR1167KP1B microcircuits can be replaced with imported DB3 symmetrical dinistors.
Establishing a soft starter should begin with balancing the two regulators. To do this, as shown in Fig. 2, apply a single-phase voltage of 220 V to it by connecting two 1 V 220 W incandescent lamps instead of the M40.60 electric motor. The terminals of the capacitor C4 must be closed with a jumper. After applying the supply voltage, set the trimmer resistor R9 to the minimum brightness of the glow of the lamps, and with the trimmer resistor R1 achieve the same intensity of their glow. After turning off the power, remove the jumper from the capacitor and turn on the device again, monitoring the voltage on the capacitor C5. When it reaches 25.26 V, relay K1 should operate. If everything is in order with this, you can check the voltage on the lamps. Before the relay K1 is activated, it must be at least 190 V. If the voltage on the lamps is less, you can reduce the resistance of the resistor R2, but only so that the maximum allowable control current of the optocouplers U1-U4 is not exceeded. Now you can connect an electric motor to the device and apply three-phase voltage. In my opinion, it is better to start the selection of the desired acceleration duration from the minimum voltage rise rate on the motor (the R7 trimming resistor engine in the upper position according to the diagram) and the minimum starting voltage (the R9 trimming resistor engine in the lower position according to the diagram). I want to draw your attention to the fact that it is technically easy to abandon the DA2 stabilizer by simply excluding it and the elements related to it from the circuit and connecting together the wires that went to the anode and cathode of the stabilizer. To adjust the starting voltage, in this case, trimming resistors R1' and R2' are installed, shown in the diagram in Fig. 1 with dashed lines. Noah wouldn't recommend doing that. Firstly, this is inconvenient, since you will have to operate with two tuning resistors in turn, trying not to violate the equality of the voltage values on the motor windings. Secondly, not all tuning resistors are able to withstand a voltage of about 400 V applied to them. Thirdly, in the device under consideration, the resistors R1 'and R2 ', unlike other tuning resistors, will be under high voltage relative to the neutral of the three-phase network, which may be dangerous if accidentally touched. In conclusion, I want to say that a soft starter cannot replace a frequency speed controller and maintain a reduced motor shaft speed for a long time. With it, you can only increase the acceleration time to rated speed and reduce the starting current. Staying the motor in acceleration mode longer than necessary will lead to overheating of the windings, because the current flowing through them in this mode, although much less than the standard starting current, still exceeds the rated current. In this mode, the motor is very sensitive to the load on the shaft and can stop when it is slightly increased. Some analogy for a soft starter for an electric motor can be considered a clutch mechanism in a car. The constant operation of an asynchronous electric motor in acceleration mode is similar to driving a car with a clutch not fully engaged. Literature
Author: P. Galashevsky
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